1. Introduction
Following their appearance approximately halfway through the Cambrian (Landing et al. Reference Landing, Bowring, Davidek, Westrop, Geyer and Heldmaier1998), trilobites underwent a number of diversifications followed by major extinction events on the Laurentian platform. Palmer (Reference Palmer1965, pp. 149, 150) proposed an iterative pattern of Late Cambrian trilobite evolution, and introduced the term ‘biomere’ as a ‘regional biostratigraphic unit bounded by diachronous, non-evolutionary changes (i.e. extinctions) in the dominant elements of a single phylum’. Subsequent work showed that ‘biomere’ boundaries are conventional zonal boundaries defined by the appearances of new species (Ludvigsen & Westrop, Reference Ludvigsen and Westrop1985), and that previous claims of a unique, iterative macroevolutionary pattern (Stitt, Reference Stitt1971a, Reference Stitt1975) are dubious (Westrop & Ludvigsen, Reference Westrop and Ludvigsen1987). In the absence of any novelty, ‘biomeres’ were equated to the bases of newly proposed stages in a new Cambrian chronostratigraphic nomenclature for Laurentia (Ludvigsen & Westrop, Reference Ludvigsen and Westrop1985; Palmer, Reference Palmer1998). Global stages have yet to be formally proposed for the highest part of the Upper Cambrian Furongian Series (however, note proposal by Landing, Westrop & Miller, Reference Landing, Westrop, Miller, Fatka and Budil2010), and we will discuss the Cambrian–Ordovician boundary interval extinction in the terms of the current Laurentian nomenclature. This discussion focuses on the youngest Cambrian trilobite extinction (Ludvigsen & Westrop, Reference Ludvigsen and Westrop1983b; Westrop & Cuggy, Reference Westrop and Cuggy1999) at the end of the Sunwaptan Stage (=end of the ‘Ptychaspid Biomere’; Fig. 1).
Correlation of trilobite and conodont zonal schemes for the Sunwaptan–Skullrockian boundary interval of Laurentia (modified from Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003). Tangshanaspis and Parakoldinioidia Zones are the Missisquoia depressa and M. typicalis subzones, respectively of earlier reports (e.g. Stitt, Reference Stitt1977; Westrop, Reference Westrop1986b). Abbreviations: Clavo. – Clavohamulus; Hirsuto. – Hirsutodontus.
The term ‘Cordylodus proavus Zone’ (Fig. 1) follows Miller's (Reference Miller1969, Reference Miller1980) original definition of this interval as a unit composed of five successive subzones (i.e. Hirsutodontus hirsutus, Fryxellodontus inornatus, Clavohamulus elongatus, H. simplex, and C. hintzei Subzones). As detailed in Landing, Westrop & Keppie (Reference Landing, Westrop and Keppie2007, p. 932), the proposal in a number of reports to separate off the upper two subzones of the C. proavus Zone as a ‘Cordylodus intermedius Zone’ and retain ‘C. proavus Zone’ for the lower three subzones must be rejected. The ‘lower Fauna B interval’ (see Ethington & Clark, Reference Ethington, Clark, Sweet and Bergström1971) describes the terminal Cambrian–lowest Ordovician strata that are regionally missing at the Little Falls–Tribes Hill formational boundary (described in Section 4.b). The lower Fauna B interval is divided into three zones in some publications: an uppermost Cambrian Cordylodus lindstromi Zone (defined by an eponymous ‘species’ that is a morphological variant of all early Cordylodus species; e.g. Landing, Ludvigsen & von Bitter, Reference Landing, Ludvigsen and von Bitter1980), lowest Ordovician Iapetognathus Zone, and a lower Cordylodus angulatus Zone (e.g. Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003).
As Lee, Lee & Choi (Reference Lee, Lee and Choi2008), we restrict Parakoldinioidia stitti Fortey, Reference Fortey1983 (a replacement name for Missisquoia typicalis Shaw, Reference Shaw1951), to the type material from Vermont. This has biostratigraphic implications as this species is the name-bearer of a biostratigraphic unit that has been identified across Laurentia as the ‘Missisquoia typicalis Zone’. Lee, Lee & Choi (Reference Lee, Lee and Choi2008) also noted that the putative occurrences of P. stitti at various localities likely represent different species. Restudy, with preparation of additional sclerites, from Stitt's (Reference Stitt1971b, Reference Stitt1977) collections from southern Oklahoma (now in the Oklahoma Museum of Natural History) by S. R. Westrop (unpub. data) reveals an even more complex taxonomic pattern, with multiple species of Parakoldinioidia present in the ‘M. typicalis Subzone’. Thus, an ‘M. typicalis Subzone’ based on the eponymous species is unrecognizable. We agree with Lee, Lee & Choi (Reference Lee, Lee and Choi2008) that Tangshanaspis Zhou and Zhang, Reference Zhou and Zhang1978 is a valid taxon (contra Ludvigsen, Reference Ludvigsen1982), and that Missiquoia depressa Stitt, Reference Stitt1971b belongs to this genus, rather than to Parakoldinioidia. Tangshanaspis depressa is the name-bearer of the lower subzone of the Parakoldinioidia Zone (Stitt, Reference Stitt1971b). In this paper, we restrict the Parakoldinioidia Zone to the former M. typicalis Subzone (defined by the lowest appearance of the eponymous genus), and refer to the ‘M. depressa Subzone’ as the Tangshanaspis Zone (Fig. 1). Finally, the Symphysurina Zone begins at the lowest occurrence of the eponymous genus, and the range of Parakoldinioidia extends up into this zone (e.g. Stitt, Reference Stitt1977, p. 53).
2. Geological setting and collecting localities
Two types of passive margin successions are present in the Cambrian–Ordovician boundary interval of east New York. Continuous successions exist in continental slope facies of the black mudstone-dominated Hatch Hill Formation in the Taconic allochthon (Landing, Reference Landing1993, Reference Landing, McLelland and Karabinos2002, Reference Landing and Landing2007; Landing et al. Reference Landing, Franzi, Hagadorn, Westrop, Kröger, Dawson and Landing2007). However, the Cambrian–Ordovician boundary is a biostratigraphically resolvable, type-1 sequence boundary between carbonate platform units (Little Falls Formation and overlying Tribes Hill Formation) on the east Laurentian craton and in the parautochthonous Appalachian Mountains of east New York and Vermont (Fig. 2). The uppermost Little Falls has lower Skullrockian Stage (Furongian) biotas, and the Tribes Hill has upper Skullrockian (lower Tremadoc) faunas (Westrop, Knox & Landing, Reference Westrop, Knox and Landing1993; Landing, Westrop & Knox, Reference Landing, Westrop and Knox1996; Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003; Kröger & Landing, Reference Kröger and Landing2007; Landing & Kröger, Reference Landing and Kröger2009) (Fig. 3).
General locality map. Carbonate platform units form the terminal Cambrian (Little Falls Formation) and lowest Ordovician (Tribes Hill Formation) of northeast Laurentian craton and western parautochthonous Appalachian slices. Arrows with asterisks mark Crossman quarry and Skene Mountain in Washington County and the Ritchie and Gailor quarry localities in Saratoga County. Arrows without asterisks show Little Falls and Tribes Hill Formation localities of earlier publications (Landing, Reference Landing and Landing1988, Reference Landing, McLelland and Karabinos2002, Reference Landing and Landing2007; Westrop, Knox & Landing, Reference Westrop, Knox and Landing1993; Landing, Westrop & Knox, Reference Landing, Westrop and Knox1996; Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003; Landing & Westrop, Reference Landing and Westrop2006; Landing et al. Reference Landing, Franzi, Hagadorn, Westrop, Kröger, Dawson and Landing2007; Kröger & Landing, Reference Kröger and Landing2007). Abbreviations: ARC – Amsterdam Road cut; Beek. – Beekmantown village; BR – Borden Road; Can – Canajoharie quarry; CCr – Canajoharie Creek; Com – Comstock section; Conn. – Connecticut; DurQ – Durand Road quarry; FC – Flat Creek; Gai – Gailor quarry; HL – Rte 82 roadcut east of Halcyon Lake; Hof – Hoffmans roadcut; Mass. – Massachusetts; NY-67 – quarry north of NY Rte 67; Rbv – Rathbunville Road section; Rit – Ritchie Limestone; SB – Smith Basin section; SFr – Steves Farm section; Sh – Shoreham village; SR – Sanders Road; Tom – Tompkins Point; TrQ – Tristates quarry.
Sections in Little Falls and Tribes Hill formations; see Figure 2 for locations.
Hydrothermal dolomitization is pervasive, and this means that primary sedimentary structures and carbonate macrofossils typically exist in relatively small outcrop areas (Landing, Westrop & Knox, Reference Landing, Westrop and Knox1996; Landing, Reference Landing and Landing2007). The reflux/evaporitic shelf/meteoritic models earlier applied to explain this dolomitization (e.g. Friedman, Reference Friedman, Zenger, Dunham and Ethington1980) are not an appropriate explanation of these dolostones because ‘windows’ of fossiliferous limestone deposited under relatively normal marine salinity can often be traced laterally into massive, sucrosic dolostone.
Undolomitized terminal Cambrian successions in the Little Falls Formation occur at eastern platform localities in Washington County, eastern New York (Fisher, Reference Fisher1984; Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003; Crossman quarry and Skene localities in Fig. 2). This project now also shows that the top of the Little Falls is undolomitized northwest of Saratoga Springs, where this interval was termed the ‘Ritchie Limestone’ and given a formation-level status by Fisher and Hanson (Reference Fisher and Hanson1951) (locality Rit in Fig. 2).
2.a. Washington County
The terminal Cambrian Little Falls Formation is an ~ 100 m thick, carbonate-dominated interval in the gently E-dipping sections near Whitehall village (see maps in Taylor & Halley, Reference Taylor and Halley1974, fig. 1 and Landing et al. Reference Landing, Franzi, Hagadorn, Westrop, Kröger, Dawson and Landing2007, fig. 4). Taylor and Halley (Reference Taylor and Halley1974) described Saukia Zone and Parakoldinioidia Zone (their Missisquoia Zone) faunas from the ‘Whitehall Formation’ in this area (‘Whitehall’ is a junior synonym of the Little Falls Formation, see Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003, p. 87; Landing, Reference Landing and Landing2007, in press). Heavy vegetation and low topography mean that outcrops are relatively small and isolated in this area. The abandoned Crossman quarry immediately north of Washington County Route 10 (Fig. 2) provides one of the best undolomitized Upper Cambrian sections (Fig. 3).
Taylor and Halley's (Reference Taylor and Halley1974) sample H-1 from Crossman quarry (our 10–10.4 horizon at the east end of the quarry) yielded an upper Saukia Zone fauna that they assigned to the Prosaukia serotina Subzone, based on the presence of ‘Calvinella’ prethoparia Longacre, Reference Longacre1970. Taylor & Halley's material is most likely not conspecific with the types of the latter from the Wilberns Formation of Texas (see Section 8). Similarly, their identification of Acheilops masonensis Winston & Nicholls, Reference Winston and Nicholls1967 does not stand up to critical scrutiny (see Section 8), and their Euptychaspis typicalis Ulrich in Bridge (Reference Bridge1931) is assigned to the correct genus but represents an undescribed species (Adrain and Westrop, Reference Adrain and Westrop2004, p. 17). From stratigraphic position and associated conodonts, the fauna is likely high in the Sunwaptan, but high-resolution, species-based correlation with the zonal successions of Texas (Longacre, Reference Longacre1970), Oklahoma (Stitt, Reference Stitt1971b, Reference Stitt1977) and Alberta (Westrop, Reference Westrop1986b) is not possible. We resampled Crossman quarry to determine if the upper Sunwaptan trilobites were succeeded by trilobites or conodonts of the overlying Skullrockian Stage, or if the end-Sunwaptan extinction event was associated with lithofacies changes.
An attempt was made to record a Sunwaptan–Skullrockian boundary succession on the west slope of Skene Mountain, approximately 1.9 km southeast of Crossman quarry. Taylor & Halley's (Reference Taylor and Halley1974) samples 7098-CO and 7099-CO included only Parakoldinioidia (identified as Missisquoia typicalis) from the Skullrockian Stage at the top of Skene Mountain. Unfortunately, the Little Falls Formation is largely covered by heavy second growth on Skene Mountain, and this precluded measuring a section or expanding the known trilobite faunas. Light grey trilobite packstone beds (to 20 cm thick) that correspond to Taylor & Halley's (Reference Taylor and Halley1974) samples are interbedded with buff weathering, slaty dolostone in 1.0 m thick slabs that are scattered around a microwave tower on the top of Skene Mountain. A sample (Skene-0.65) was taken for conodonts from the middle of the slabs (Table 1).
Uppermost Cambrian conodonts and phosphatic problematica from productive samples from the Little Falls Formation, Crossman quarry, Washington County, eastern New York
6 kg samples were disaggregated. Designations: ‘el.’ – element; P – present.
2.b. Saratoga County
The uppermost Cambrian and lowest Ordovician carbonate platform succession in Saratoga County on the south side of the middle Proterozoic rock of the Adirondack Mountains massif has had an unstable, local stratigraphic nomenclature. This might be expected given the limited exposures due to low topographic relief and Pleistocene cover. Numerous normal block faults also obscure stratigraphic continuity. Hydrothermal dolomitization is widespread and further complicates litho- and biostratigraphic study.
Ulrich & Cushing (Reference Ulrich and Cushing1910) recognized that the Little Falls Formation could be traced north into the Saratoga area, and placed the Hoyt Limestone with its abundant mid-Sunwaptan trilobites (e.g. Walcott, Reference Walcott1912; Ludvigsen & Westrop, Reference Ludvigsen and Westrop1983a) as a member-level unit at its base (Landing, Reference Landing1979). However, they believed the lowest Ordovician Tribes Hill Formation was absent in the area. Fisher & Hanson (Reference Fisher and Hanson1951) later proposed the ‘Gailor Dolomite’ for strata that they correlated, on the basis of rare cephalopods at the Gailor quarry in the northeast of Saratoga Springs (Fig. 2, locality Gai; Fig. 3), with the typically less dolomitized Tribes Hill Formation.
Fisher & Hanson (Reference Fisher and Hanson1951) found a locality where the ‘Gailor Dolomite’ base is exposed, and where it unconformably overlies an undolomitized, massive, light grey weathering limestone that they initially regarded as a formation-level unit. This unit was termed the ‘Ritchie Limestone’ (Fisher & Hanson, Reference Fisher and Hanson1951). The Ritchie is exposed only at its type locality northwest of Saratoga (Fig. 2, locality Rit; see detail map in Landing et al. Reference Landing, Franzi, Hagadorn, Westrop, Kröger, Dawson and Landing2007, fig. 2). We were interested in the bio- and lithostratigraphy of this locality because the eroded Ritchie–‘Gailor’ boundary (2.1 m of relief) suggests the type-1 sequence boundary between the terminal Cambrian Little Falls and lowest Ordovician Tribes Hill Formation known elsewhere on the east Laurentian platform (e.g. Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003; Landing, Reference Landing and Landing2007).
Limited fossil recovery long precluded a precise correlation of the Ritchie Limestone. Fisher & Hanson (Reference Fisher and Hanson1951) noted snails with large whorls as evidence of a Late Cambrian or Early Ordovician age. Later, Fisher (in Mazzullo et al. Reference Mazzulo, Agostino, Seitz and Fisher1978) reported a cephalopod (Fig. 5), which he interpreted to be an Early Ordovician ellesmeroceroid, and used it to reinterpret the Ritchie as an undolomitized lens of the lower ‘Gailor Dolomite’ (Fisher & Mazzullo, Reference Fisher and Mazzullo1976; Mazzullo et al. Reference Mazzulo, Agostino, Seitz and Fisher1978). This interpretation made the Ritchie a member-level, Lower Ordovician unit. Our interest in the age of the Ritchie Limestone, the duration of the Ritchie–‘Gailor’ unconformity and the relationship of the ‘Gailor’ to the well-studied Tribes Hill Formation (e.g. Landing, Reference Landing and Landing2007) prompted our examination of the carbonate succession in Saratoga County.
3. Bio- and lithostratigraphic results
3.a. Crossman quarry and Skene Mountain
Taylor and Halley's (Reference Taylor and Halley1974, fig. 2) Sunwaptan assemblage occurs in a trilobite pack- to grainstone lens in an ooid and intraclast packstone at 10.4 m at Crossman quarry (Fig. 3, sample 10–10.4). Sparse trilobite sclerites occur at many levels (Fig. 3, note ‘trilobed’ symbols), but stylolites and neomorphism preclude their crack-out except from rare grainstone lenses. Massive packstones with ooids and intraclasts in lime mud are prominent in Crossman quarry, and suggest rapid burrow-homogenization of lime mud-rich and higher energy, ooid-, intraclast- and trilobite sclerite-rich layers.
Conodont samples were taken through Crossman quarry (Fig. 3), and elements were recovered from sample 10–10.4 to the quarry top (Table 1). They are thermally altered and have a Colour Alteration Index (CAI) of 3.5 (Epstein, Epstein & Harris, Reference Epstein, Epstein and Harris1977). Not unexpectedly, Proconodontus serratus Miller, Reference Miller1969, a characteristic upper Eoconodontus Zone form (Fig. 6a), was found with the trilobite fauna in sample 10–10.4 (Table 1).
Cordylodus proavus Zone conodonts characteristic of the Skullrockian Stage first appear in a thin intraclast pebble bed at 11.4 m, although no identifiable trilobites were recovered (Table 1; Fig. 3). Cordylodus andresi Viira & Sergeeva in Kaljo et al. (Reference Kaljo, Borovko, Heinsalu, Khasanovich, Mens, Popov, Sergeeva, Sobolevskaya and Viira1986) (Fig. 6d), a form that appears on the west Laurentian platform in the lower C. proavus Zone (Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003) but occurs in earlier strata in offshore and high-latitude environments (Landing, Westrop & Keppie, Reference Landing, Westrop and Keppie2007; Landing et al. Reference Landing, Franzi, Hagadorn, Westrop, Kröger, Dawson and Landing2007, pp. 51, 52), appears with Hirsutodontus rarus Miller, Reference Miller1969 (Fig. 7c, d). This suggests the lowest subzone (Hirsutodontus hirsutus Subzone) of the C. proavus Zone. However, trilobites of the basal Skullrockian Eurekia apopsis Zone and succeeding Tangshanaspis Zone are unknown at this or other sites in New York.
Immediately overlying strata include a white-weathering, coarse-grained, calcareous quartz arenite overlain by 50 cm wide, low SH-V stromatolites. These distinctive beds divide the Crossman quarry succession into lower beds characterized by intraclast and ooid packstones with minor thrombolites, and an upper interval where thrombolites are dominant (Fig. 3). Thrombolites characterize the onset of the highstand facies through the Lower Ordovician carbonate formations of east New York (Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003; Kröger & Landing, Reference Kröger and Landing2007, Reference Kröger and Landing2008, Reference Kröger and Landing2009; Landing, Reference Landing and Landing2007, in press). However, the abundance of ooid and intraclast pack- to grainstone beds through the Crossman quarry section (Fig. 3) does not suggest significant changes in depth or ambient energy in this part of the Little Falls Formation.
Post-extinction, Parakoldinioidia Zone trilobites (P. maddowae Westrop sp. nov.) occur in a pack- to grainstone lens higher in Crossman quarry (Fig. 3, sample 10–15.4). An association of the conodonts Fryxellodontus inornatus Miller, Reference Miller1969 (Fig. 6i, k) and F. aff. lineatus Miller, Reference Miller1969 (Fig. 6j), the latter with elements with vertical to radiating, not horizontal, ridges, suggests the upper part of the second subzone (F. inornatus) of the Cordylodus proavus Zone (Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003, p. 36) and, hence, the upper half of the Parakoldinioidia Zone. That the Parakoldinioidia Zone assemblage on the top of Skene Mountain (Taylor and Halley, Reference Taylor and Halley1974; our sample Skene 0.65) is approximately coeval with the 15.4 m level at Crossman quarry is suggested by the presence of Teridontus? francisi Landing sp. nov. (Fig. 6l–p) in both horizons.
Semiacontiodus nogamii Miller, Reference Miller1969, reported as first appearing in the overlying Hirsutodontus simplex Subzone of the middle C. proavus Zone and with lower Symphysurina Zone trilobites in west Laurentia, appears in 10–15.4 (Miller, Reference Miller1980; Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003). However, as discussed above, the other conodonts from 10–15.4 are best interpreted as indicating a somewhat older first-appearance of S. nogamii on the east Laurentian platform.
Higher conodont faunas emphasize how condensed the Little Falls Formation is at Crossman quarry. Clavohamulus elongatus Miller, Reference Miller1969, the eponymous species of the third subzone of the C. proavus Zone, appears in 10–18.0. Only slightly higher, Monocostodus sevierensis (Miller, Reference Miller1969) and Hirsutodontus simplex (Druce & Jones, Reference Druce and Jones1971) in 10–24.0 indicate the fourth subzone (H. simplex) of the C. proavus Zone, and show that the Crossman quarry succession ranges into the lower part of the trilobite-based Symphysurina Zone as defined in other regions.
The 13.6 m thick interval from the lowest Cordylodus proavus Zone (10–11.4) to the (probably lower) Hirsutodontus simplex Subzone at Crossman quarry is thin by comparison with coeval intervals in the western United States. In the Ibex area, western Utah, this interval is about 42 m thick, and coeval strata in the Llano Uplift, central Texas, are 30 m thick (Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003).
The thinness of the Crossman quarry interval reflects the long-term, slow subsidence rates of Cambrian–Middle Ordovician platform and continental slope strata along the New York Promontory (Landing, Reference Landing and Landing2007, in press). The lack of Tangshanaspis Zone trilobites could simply record sample failure in a sparsely fossiliferous succession, but the zone may be missing, as in central Texas (Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003). A disconformity is suggested by the calcareous sandstone and stromatolite horizon in Crossman quarry and could be responsible for the short stratigraphic distance (4.0 m) that separates the lowest C. proavus Zone (10–11.4) from Parakoldinioidia Zone trilobites (10–15.4).
3.b. Ritchie Limestone and Gailor quarry
Whereas Crossman quarry has a typical, albeit thin, Sunwaptan–Skullrockian boundary interval, unanticipated bio- and lithostratigraphic information was obtained in Saratoga County. Conodonts provide the first biostratigraphic control of the thin (10.2 m) Ritchie Limestone (Fig. 3). Our samples yield a conventional succession of faunas and show that the Ritchie brackets the three upper subzones of the Cordylodus proavus Zone (Fig. 1; Table 2). The conodont elements have a CAI of 3.5.
Uppermost Cambrian and lowest Ordovician conodonts from productive samples from the upper Little Falls Formation (samples Rit-0.2–Rit-18.5) and Gailor Formation (samples Rit-20 and Gai-series), Saratoga County, east-central New York
6 kg samples were disaggregated. Designations: ‘el.’ – element; P – present. Rit-12.3–12.8 processed from cuttings from sample that yielded relict ‘Sunwaptan’ trilobites.
The eponymous species of the Clavohamulus elongatus Subzone appears in sample Rit-5.4, below the cherty, sucrosic dolostone that underlies the Ritchie Limestone. Clavohamulus simplex of the overlying C. simplex Subzone appears in Rit-17.0; and abundant Clavohamulus hintzei Miller, Reference Miller1969 (Fig. 7g, h) specimens of the C. hintzei Subzone of the uppermost C. proavus Zone appear almost at the top of the Ritchie (Rit-18.5). The H. simplex Subzone base probably lies at least as low as Rit-10.0, based on the presence of multielement Cordylodus caseyi Druce & Jones, Reference Druce and Jones1971, a form often reported as Cordylodus intermedius Furnish, Reference Furnish1938 in many publications (see discussion in Landing, Reference Landing1993, pp.13, 15, 16), and which has its lowest occurrence in the H. simplex Subzone (Miller, Reference Miller1980; Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003).
In terms of trilobite biostratigraphy, the conodont succession indicates that the Ritchie Limestone is referable to the lower Symphysurina Zone. Trilobites were recovered from a well exposed, trough cross-bedded, intraclast granule and ooid pack- to grainstone dune that shows eastward transport (Rit 12.0–12.8 m, Fig. 3). In view of the condont-based age of the Ritchie, the trilobite assemblage composition was entirely unexpected. As discussed in Section 8, sclerites of Acheilops and dikelocephalid trilobites occur in the dune. Elsewhere in Laurentia, these trilobites are characteristic of Sunwaptan to basal Skullrockian (Eurekia apopsis Zone) strata, and are unknown higher in the Skullrockian.
To corroborate that Sunwaptan-aspect trilobites occur well into the Skullrockian, a 6.0 kg sample of the matrix left over from trilobite preparation was disaggregated for conodonts (Table 2, sample Rit-12.3–12.8). This sample yielded Clavohamulus elongatus Miller, Reference Miller1969, a characteristic taxon of the lower Symphysurina Zone (e.g. Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003). The trilobite fauna might be viewed as recording an ‘early’ occurrence of the conodonts. However, the fact that the conodont faunas comprise a ‘normal’ sequence of three successive subzones comparable to that elsewhere in Laurentia indicates that a ‘late’ occurrence of typical Sunwaptan trilobites is the most parsimonious interpretation. That is, our age assignment for the trilobites indicated by the associated conodonts in sample Rit-12.3–12.8 is also supported by the conodont faunas that underlie and overlie this horizon.
The top of the Ritchie Limestone is an unconformity with 2.1 m of relief (Fisher & Hanson, Reference Fisher and Hanson1951; Fig. 3). A coarse-grained, dolomitic quartz arenite with pebbles and cobbles of quartz arenite and dolostone fills the relief, and is succeeded by 0.6 m of dolomitic quartz arenite. Variabiloconus bassleri (Furnish, Reference Furnish1938) from the overlying silty dolostone (Rit-20.0; Fig. 7n) shows that the Ritchie–‘Gailor Dolomite’ unconformity corresponds to the Cambrian–Ordovician boundary. Across Laurentia, V. bassleri first appears in upper Fauna B and persists through much of the Rossodus manitouensis Zone (Landing, Westrop & Knox, Reference Landing, Westrop and Knox1996; Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003; Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003).
More information on the ‘Gailor Dolomite’ was obtained at its type section in the Gailor quarry in the backyard of 46 East Street, northeast Saratoga Springs. Description of the type ‘Gailor Dolomite’ was aided by the fact that the quarry wall and top were cleaned in 2006 for construction of a waterfall and splash pond. This allowed a determination that the massive lower carbonates and upper thrombolites of the type ‘Gailor’ are identical to those of Fisher's (Reference Fisher1954) Wolf Hollow Member of the middle Tribes Hill Formation. Rossodus manitouensis Zone conodonts of the ‘Gailor Dolomite’ (Table 2) include middle–upper Tribes Hill taxa, i.e. Chosonodina herfurthi (Müller, Reference Müller1964) (Fig. 7q); Laurentoscandodus triangulodus (Furnish, Reference Furnish1938) (Fig. 7o); ‘Leukorhinion’ sp. Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003 (Fig. 7t); Scalpellodus longpinnatus (Ji & Barnes, Reference Ji and Barnes1994); Semiacontiodus iowensis (Furnish, Reference Furnish1938) (Fig. 7p, r) (see Landing, Westrop & Knox, Reference Landing, Westrop and Knox1996; Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003). The conodont elements show strong heating (CAI = 5), probably due to hydrothermal activity along a fault at the east end of the quarry that once was a collecting site for quartz and dolomite (Rowley, Reference Rowley1951).
These data emphasize that the Tribes Hill Formation and its members extend from the Mohawk valley northeast to Washington County and through the Lake Champlain lowlands to southern Quebec, as well as south to Dutchess County (Fig. 2, locality HL) (Landing & Westrop, Reference Landing and Westrop2006; Kröger & Landing, Reference Kröger and Landing2007; Landing, Reference Landing and Landing2007, in press). The ‘Gailor Dolomite’ of Saratoga County is a junior synonym of the Tribes Hill Formation. Our study also documented the lower sandy beds and silty dolostones of the Sprakers Member of the Tribes Hill Formation (Landing, Westrop & Knox, Reference Landing, Westrop and Knox1996; Fig. 3) at the top of the Ritchie section.
4. Discussion
4.a. Lithostratigraphy
Shallow-marine platform carbonates comprise the terminal Cambrian and lowest Ordovician at localities in Washington and Saratoga Counties, New York. Slow rock accumulation rates through the Cordylodus proavus Zone in both areas are consistent with long-term slow subsidence of the New York Promontory region (Landing, Reference Landing and Landing2007, in press). The rate of sedimentary rock accumulation was two to three times faster in the Llano area, central Texas, and the Ibex area, western Utah. The ooid and intraclast packstone and rare grainstone beds in Washington County indicate episodic high energy conditions close to the platform margin, but the massive packstone beds in Crossman quarry suggest rapid burrow-homogenization of mud- and intraclast-dominated intervals.
The eroded, type-1 sequence boundary at the top of an uppermost Cambrian unit (Ritchie Limestone) and the presence of the overlying, lowest Ordovician Tribes Hill Formation in Saratoga County are identical to Cambrian–Ordovician boundary unconformity features elsewhere across the northeast Laurentian platform (e.g. Landing, Reference Landing and Landing2007). The Ritchie Limestone is not a lower member of the Tribes Hill/’Gailor’ (latter designation abandoned) Formation as proposed by Zenger (Reference Zenger1971), Fisher & Mazzullo (Reference Fisher and Mazzullo1976), Fisher (Reference Fisher1977) and Mazzullo et al. (Reference Mazzulo, Agostino, Seitz and Fisher1978). Rather, the Ritchie Limestone is lithologically comparable to thick bedded, burrow-homogenized, non-dolomitized limestones in the upper Little Falls Formation in Washington County such as the Steves Farm and overlying Rathbunville School Limestone members (e.g. Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003; Fig. 4).
Platform stratigraphy on northeast Laurentian craton (three columns on left) and in parautochthonous succession of Appalachian New York (see locality HL in Fig. 2).
The Ritchie and uppermost Little Falls in Washington County are the same age. Conodonts show the Ritchie Limestone correlates with the lower Symphysurina Zone, while the conodonts of the Steves Farm and Rathbunville School Limestones in the uppermost Little Falls Formation in Washington County show a comparable age (Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003). The lower range of the trilobite Millardicurus Adrain & Westrop, Reference Adrain and Westrop2006 (NYSM 13925) from the Rathbunville School Limestone (Landing & Kröger, Reference Landing and Kröger2009) is consistent with a terminal Cambrian, lower Symphysurina Zone correlation of the top Little Falls Formation in Washington County.
The Ritchie Limestone is best regarded as the upper member of the Little Falls Formation, and the Hoyt Limestone is the formation's basal member. The thickness of the Little Falls Formation is probably less in Saratoga County than in Washington County, but remains only approximately known: Fisher & Hanson (Reference Fisher and Hanson1951) estimated 29 m, but Mazzullo et al. (Reference Mazzulo, Agostino, Seitz and Fisher1978) reported 40 m for the thickness of the Hoyt and Ritchie in a core. Our attempt to locate this core was not successful, and it is now apparently lost. The core was drilled at the Pallette Stone Company just south of locality Rit (Fig. 2).
4.b. Relative sea levels
The relative sea-level changes in the Cambrian–Ordovician boundary interval of the northeast Laurentian platform suggested by this study and our earlier work correspond, only in part, to interpreted sea levels proposed on passive margins in the west and south-central United States (Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003) and globally (e.g. Haq & Schutter, Reference Haq and Schutter2008).
Miller et al. (Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003) figured two strong latest Cambrian sea-level fall–rise couplets in the latest Cambrian in west Laurentia. The lower, the Lange Ranch Eustatic Event (Miller, Reference Miller and Clark1984) or Lange Ranch Lowstand (Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003), is equated with trilobite extinctions characteristically thought to define the Sunwaptan–Skullrockian Stage boundary, and with the lowest Cordylodus proavus Zone conodont assemblage appearing in shoaling, regressive facies. At Crossman quarry, no lithologic change or shoaling from the underlying succession is associated with the lowest C. proavus Zone, although the overlying calcareous quartz sandstone is compatible with offlap of nearshore sands and shoaling during later stages of a Lange Ranch Lowstand. The minor eustatic fall–rise couplet that Haq & Schutter (Reference Haq and Schutter2008) show at the base of the Australian Datsonian Stage probably equates to the Lange Range Lowstand.
A second eustatic fall–rise couplet in the terminal Cambrian is reported in the lower Hirsutodontus simplex Subzone of the Cordylodus proavus Zone of west and southern Laurentia (Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003). Lower H. simplex Zone shoaling, which may equate to a minor eustatic fall in the middle Datsonian (Haq & Schutter, Reference Haq and Schutter2008, fig. 1) is not evident in the Crossman quarry or Ritchie sections, which show relatively unchanged carbonate facies and no evidence for unconformity/stratigraphic truncation in this interval.
The Little Falls–Tribes Hill unconformity, or Furongian–Tremadocian unconformity, records a strong offlap–onlap couplet. Shorelines withdrew from the upper Mississippi River valley of the upper Midwest. The top of the Little Falls was exposed and eroded, after which sea levels advanced again into the upper Midwest (e.g. Landing, Reference Landing and Landing1988, in press; Landing, Westrop, & Keppie, Reference Landing, Westrop and Keppie2007). Haq & Schutter's (Reference Haq and Schutter2008, fig. 1) model shows minimal sea-level fall through this interval, which suggests that a prominent unconformity should not have developed, while they show that sea levels remained perhaps 100 m above present through the Cambrian–Ordovician boundary interval. However, Miller et al. (Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003) show a strong sea-level fall through this interval in west and south Laurentia, which they referred to the Acerocare Regressive Event (Nielsen, Reference Nielsen, Webby, Paris, Droser and Percival2004), that was first proposed on the Baltic palaeocontinent (Erdtmann, Reference Erdtmann1986). Thus, Miller et al. (Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003) record a strong sea-level fall–rise couplet in the Cambrian–Ordovician boundary interval that is not evident in Haq & Schutter's (Reference Haq and Schutter2008) eustatic curve.
The Tribes Hill Formation and correlatives, such as the Oneota Dolostone in the upper Mississippi River valley, would have been deposited during the strong early Tremadoc eustatic rise shown by Haq & Schutter (Reference Haq and Schutter2008). Miller et al.'s (Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003, fig. 3) gradual early Tremadoc sea-level rise is punctuated by strong sea-level fall in the lower Rossodus manitouensis Zone and correlatives (i.e. the Peltocare Regression of Erdtmann, Reference Erdtmann1986; see also Nielsen, Reference Nielsen, Webby, Paris, Droser and Percival2004); a fall that is not recorded in the deepening–shoaling cycle of the Tribes Hill Formation (Landing, Westrop & Knox, Reference Landing, Westrop and Knox1996; Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003).
4.c. Biostratigraphic implications of the Ritchie trilobite fauna
The Ritchie trilobite fauna includes Acheilops Ulrich and several species of dikelocephalids. Without associated conodont data, there would be no doubt that the trilobites indicate a late Sunwaptan or earliest Skullrockian (Eurekia apopsis Zone) age. However, we emphasize that the trilobite-bearing horizon lies within a ‘normal’ succession of Skullrockian conodont zones, and the most parsimonious interpretation of the biostratigraphic data is a ‘late’ occurrence of the trilobites, and not an anomalously ‘early’ appearance of an entire succession of conodont faunas.
The oldest species of Acheilops from a well-documented section is A. montis Westrop (Reference Westrop1986b) from the Proricephalus wilcoxensis Fauna (upper Sunwaptan) of the Mistaya Formation in Alberta. The P. wilcoxensis Fauna correlates with the Saukiella junia Zone of central Texas (Winston & Nicholls, Reference Winston and Nicholls1967; Longacre, Reference Longacre1970) and Oklahoma (Stitt, Reference Stitt1971b, Reference Stitt1977) (Westrop, Reference Westrop1986b). Prior to this report, the youngest known occurrences of Acheilops were in the basal Skullrockian E. apopsis Zone (Longacre, Reference Longacre1970; Stitt, Reference Stitt1977).
In Laurentia, dikelocephalid trilobites (‘saukiids’ of some earlier reports; see Ludvigsen & Westrop in Ludvigsen, Westrop & Kindle, Reference Ludvigsen, Westrop and Kindle1989, pp. 27, 28 for discussion) appear in middle Sunwaptan strata (e.g. Ulrich & Resser, Reference Ulrich and Resser1933; Ludvigsen & Westrop, Reference Ludvigsen and Westrop1983b) and diversify during the remainder of the stage (e.g. Ulrich and Resser, Reference Ulrich and Resser1933; Longacre, Reference Longacre1970; Stitt, Reference Stitt1971b, Reference Stitt1977; Westrop, Reference Westrop1986b). The youngest representatives of the family occur in the lowest Skullrockian of northern Canada (Elkanaspis corrugata Zone; Westrop, Reference Westrop1995). Dikelocephalidae is pandemic, and its distribution outside Laurentia includes various parts of Gondwana (e.g. Robison and Pantoja-Alor, Reference Robison and Pantoja-Alor1968; Shergold, Reference Shergold1971, Reference Shergold1975), the South China (e.g. Peng, Reference Peng1992) and Sino-Korean blocks (e.g. Zhou & Zhang, Reference Zhou and Zhang1978; Choi et al. Reference Choi, Kim, Sohn and Lee2003; Sohn & Choi, Reference Sohn and Choi2007), and Central Asian microplates (e.g. Ergaliev, Reference Ergaliev1980). Opinions on the age of the youngest members of the family in these regions are varied, but recent assessments of international Cambrian correlation (e.g. Geyer & Shergold, Reference Geyer and Shergold2000; Choi et al. Reference Choi, Kim, Sohn and Lee2003; Landing, Westrop & Keppie, Reference Landing, Westrop and Keppie2007) indicate they range no higher than basal Skullrockian (lowermost Cordylodus proavus Zone; H. hirsutus Subzone and correlatives). Thus, the Ritchie Limestone records the youngest occurrence of dikelocephalids known worldwide.
The range extensions of characteristically Sunwaptan to basal Skullrockian trilobite taxa documented herein caution against the simple use of assemblages of genera for biostratigraphy in the Cambrian–Ordovician boundary interval. The possibility of relict faunas surviving the end-Sunwaptan extinction could lead to significant errors in correlation. However, the species composition of the Ritchie trilobite fauna is distinct from similar assemblages in older strata of New York (Taylor & Halley, Reference Taylor and Halley1974; this paper). Thus, it should be possible to develop a species-based zonation that will permit accurate correlation. As our study demonstrates, accuracy of correlation is enhanced by the combination of trilobite and conodont biostratigraphy.
5. Implications for the end-Sunwaptan extinction
The pattern of trilobite turnover at the end-Sunwaptan extinction has been documented at several localities in Laurentia. These include successions of shallow- (Stitt, Reference Stitt1971b, Reference Stitt1977) and deep-water (Ludvigsen, Reference Ludvigsen1982) carbonates, and shallow subtidal, mixed carbonate and fine siliciclastic facies (Westrop, Reference Westrop1986b). The signature of faunal change is similar at all sites, although it is recorded through a 6–26 m stratal interval (Ludvigsen & Westrop, Reference Ludvigsen and Westrop1983b).
The extinction occurs with biofacies replacements that reflect an increasing proportion of immigrant taxa as alpha diversity declines (Ludvigsen & Westrop, Reference Ludvigsen and Westrop1983b; Westrop & Cuggy, Reference Westrop and Cuggy1999). Relict pre-extinction taxa typical of sites inboard of the shelf margin are confined largely to the lower part (Eurekia apopsis Zone) of the extinction interval. In the upper extinction interval (Tangshanaspis [formerly Missisquoia depressa Subzone] Zone), diversity is reduced in shallow and deep subtidal settings (Westrop & Cuggy, Reference Westrop and Cuggy1999, fig. 6), and the plethopeltid trilobite Plethopeltis Raymond, Reference Raymond1913 is the sole survivor from pre-extinction shelf faunas at most localities (e.g. Stitt, Reference Stitt1971b; Westrop, Reference Westrop1986b). By the end of the extinction interval, turnover is complete, aside from isolated occurrences of the ptychaspidid Proricephalus? Westrop, Reference Westrop1986a in the Parakoldinioidia Zone of Texas (Winston and Nicholls, Reference Winston and Nicholls1967; their ‘Genus and species undetermined’) and the Symphysurina Zone of Oklahoma (Stitt, Reference Stitt1977; his ‘Genus and species undet. no. 3’). Another holdover is the catillicephalid Theodenisia Clark, Reference Clark1948 from shelf margin-derived boulders of the Symphysurina Zone of west Newfoundland (Fortey, Reference Fortey1983).
Despite the general uniformity of faunal change at various localities, regional or environmental variation in extinction patterns, including refugia, is implied by the fact that some families, including the Remopleurididae, Hungaiidae, Loganopeltidae and Norwoodiidae, are Lazarus taxa (Jablonski, Reference Jablonski and Elliot1986). These taxa disappear in the extinction interval at all sites presently documented, only to reappear at higher stratigraphic levels (Westrop & Ludvigsen, Reference Westrop and Ludvigsen1987). Indeed, refugia must occur at even the most catastrophic extinction events (e.g. Schulte et al. Reference Schulte, Alegret, Arenillas, Arz, Barton, Bown, Bralower, Christeson, Claeys, Cockell, Collins, Deutsch, Goldin, Goto, Grajales-Nishimura, Grieve, Gulick, Johnson, Kiessling, Koeberl, Kring, Macleod, Matsui, Melosh, Montanari, Morgan, Neal, Nichols, Norris, Pierazzo, Ravizza, Rebolledo-Vieyra, Reimold, Robin, Salge, Speijer, Sweet, Urrutia-Fucugauchi, Vajda, Whalen and Willumsen2010), and failure to discover them is a sampling issue. In the case of the end-Sunwaptan event, only a fraction of the geographic extent of marine sedimentary rocks (and associated trilobite faunas) of this age in Laurentia (e.g. Lochman-Balk, Reference Lochman-Balk1970, figs. 6, 7) has been sampled. Published accounts of complete successions through the extinction interval are restricted to sections in south Oklahoma (Stitt, Reference Stitt1971b, Reference Stitt1977), west-central Utah (Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003), western Alberta (Westrop, Reference Westrop1986b; Loch, Stitt & Derby, Reference Loch, Stitt and Derby1993) and the Mackenzie Mountains of northern Canada (Ludvigsen, Reference Ludvigsen1982). The boundary succession in central Texas includes a disconformity (Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003), and the entire continent to the east of the Mississippi is represented by incomplete records in New York (Taylor & Halley, Reference Taylor and Halley1974) and Newfoundland (Fortey, Landing & Skevington, Reference Fortey, Landing, Skevington, Bassett and Dean1982; Fortey, Reference Fortey1983; Ludvigsen, Westrop & Kindle, Reference Ludvigsen, Westrop and Kindle1989).
The persistence of clades in Texas and Newfoundland as noted above involves isolated Lazarus taxa as minor components of post-extinction biofacies dominated by immigrant taxa. In contrast, the Ritchie records an entire assemblage of relict, pre-extinction taxa that resembles older biofacies in the same region. As is the case with pre-extinction assemblages of the upper Little Falls Formation of the Whitehall area (Taylor & Halley, Reference Taylor and Halley1974), the Ritchie relict fauna is dominated by dikelocephalid and catillicephalid trilobites, although there are no species in common.
The Whitehall fauna, which was assigned to the Dikelocephalid Biofacies by Westrop & Cuggy (Reference Westrop and Cuggy1999, fig. 3, collection NY1), includes plethopeltid and entomaspidid trilobites that are missing from the Ritchie; plethopeltids were a significant component (about 10 %; Ludvigsen & Westrop, Reference Ludvigsen and Westrop1983b, fig. 2) of the Whitehall biofacies. Further comparisons between the Whitehall area and the Ritchie are hindered by small sample sizes. Even though the number of genera appears to be reduced in the Ritchie, at least six species are represented, five of which are dikelocephalids.
The Ritchie fauna implies a mosaic pattern to extinction and survival at the end-Sunwaptan event, with dwindling population sizes and contracting geographic ranges as contributing factors. Emerging data on modern faunas and floras implicate habitat destruction or alteration, and reduction in geographic ranges, perhaps in concert with the role of invasive species, as important mechanisms underlying extinction (e.g. Pimm & Askins, Reference Pimm and Askins1995; Didham et al. Reference Didham, Tylianikis, Gemmell, Rand and Ewers2007; Hanski et al. Reference Hanski, Koivulehto, Cameron and Rahagalala2007; Brook, Sodhi & Bradshaw, Reference Brook, Sodhi and Bradshaw2008; Harris & Pimm, Reference Harris and Pimm2008).
The forcing mechanisms for the end-Sunwaptan and other Late Cambrian trilobite extinctions remain uncertain (Westrop & Cuggy, Reference Westrop and Cuggy1999). However, there is evidence for at least regional environmental change and shifts in geographic distribution, including immigration (Ludvigsen & Westrop, Reference Ludvigsen and Westrop1983b; Westrop & Ludvigsen, Reference Westrop and Ludvigsen1987), which indicate that the end-Sunwaptan event is consistent with many of the processes that influence modern extinction.
6. Systematic palaeontology
Kröger is responsible for the treatment of the cephalopod, Landing for the conodonts, and Westrop for the trilobites. All material is housed at the New York State Museum (NYSM). In order to maximize depth of field, images in Figures 8–13 were rendered from stacks of images focused at 200 micron intervals using Helicon Focus 4.0 for the Macintosh (http://www.heliconsoft.com). Proportions expressed in percentages in descriptions and diagnoses of the trilobites are means, with numbers in parentheses indicating the range of values. Measurements were made on digital images to the nearest tenth of a millimetre using the Measure Tool of Adobe Photoshop™.
Class CEPHALOPODA Cuvier, Reference Cuvier1797
Cephalopod order, family, genus and species indet.
Figure 5
Material and occurrence. Hypotype NYSM 178816 collected by D. W. Fisher (in Mazzullo et al. Reference Mazzulo, Agostino, Seitz and Fisher1978) from an unrecorded level in the uppermost Cambrian (Clavohamulus elongatus–C. hintzei Subzones of the Cordylodus proavus Zone), Ritchie Limestone member, uppermost Little Falls Formation, Saratoga County, New York.
Upper Cambrian cephalopod gen. et sp. indet., polished section from the Ritchie Limestone member of the Little Falls Formation, Saratoga County, New York; black scale bar is 1.0 cm. Smooth outer shell visible on left of figure (left tip of white line); outer shell not preserved on right margin. NYSM 17816, collected by D. W. Fisher (in Mazzullo et al. Reference Mazzulo, Agostino, Seitz and Fisher1978).
Description. Partial phragmocone with approximately 25 chambers, maximum diameter greater than 26 mm, total length of 32 mm. Because of its fragmentary character, the conch shape is not completely known. Based on septal shape and remnants of outer shell on the left side of the phragmocone (at left tip of white line in Fig. 5), the conch was moderately curved. Shape and position of siphuncle unknown. Fragment consists of two separate parts that are slightly displaced against each other (note slight displacement of chambers above horizontal white bar in figure). Septa in contact with smooth, slightly convex conch surface towards the apical part of the specimen. Phragmocone chambers filled with a micritic matrix, which differs from the surrounding limestone by its lighter colour. As outer shell (preserved at end of horizontal white bar in figure) is eroded from large parts of specimen, this matrix is in contact with the darker surrounding limestone.
Interpretation and age. The specimen is a clast which was fragmented and eroded before final burial. As the siphuncle of the specimen is unknown, a highly resolved taxonomic determination is impossible. Despite this, the conch shape, septal spacing and size of the specimen of more than 25 mm are characteristic of the balkocerids of the Plectronocerida Flower, Reference Flower1964 and of the Protactinocerida Chen & Qi (in Chen et al. Reference Chen, Tsou, Chen and Qi1979). Both orders are known from the Upper Cambrian Fengshan Formation of South China (Chen et al. Reference Chen, Tsou, Chen and Qi1979; Chen and Teichert, Reference Chen and Teichert1983).
The preservation of the specimen is similar to approximately coeval, latest Cambrian cephalopods from the Rathbunville School Limestone member to the northeast in the upper Little Falls Formation of Washington County, New York (Landing & Kröger, Reference Landing and Kröger2009). The Ritchie Limestone specimen differs in having an only slightly curved conch and a much larger diameter that make it the largest known Cambrian cephalopod from Laurentia. The fragments from the Rathbunville School Limestone and the San Saba Limestone cephalopods from south Texas (Flower, Reference Flower1954) have diameters of less than 15 mm.
The Ritchie Limestone is only the fourth horizon in Laurentia that has yielded Upper Cambrian cephalopods. The other three include the Rathbunville School Limestone of the Little Falls Formation in nearby Washington County, New York; the San Saba Limestone member in central Texas; and the unidentified, poorly preserved specimens from the Whipple Cave Formation of Nevada (Landing & Kröger, Reference Landing and Kröger2009).
Class CONODONTA Eichenberg, Reference Eichenberg1930
Order CONODONTOPHORIDA Eichenberg, Reference Eichenberg1930
Genus Cordylodus Pander, Reference Pander1856
Figure 6d–h
Type species. Cordylodus angulatus Pander, Reference Pander1856 from the Lower Ordovician of Estonia.
Upper Cambrian conodonts from the Little Falls Formation, eastern New York, hypotypes unless otherwise indicated; white scale bars are 0.05 mm. (a) Proconodontus serratus Miller, Reference Miller1969, 10–10.4, NYSM 13881; very low denticles on posterior oral edge and in convex area posterior to broken (upper) end of element. (b, c) Eoconodontus notchpeakensis (Miller, Reference Miller1969), 10–11.4; (b) flat (nondentate cyrtoniodiform) element, NYSM 13882; (c) round (nondentate cordylodiform) element, NYSM 13883. (d) Cordylodus andresi Viira & Sergeeva in Kaljo et al. (Reference Kaljo, Borovko, Heinsalu, Khasanovich, Mens, Popov, Sergeeva, Sobolevskaya and Viira1986), rounded element, 10–11.4, NYSM 13884. (e, f) Cordylodus proavus Müller, Reference Müller1959, 10–15.4; (e) flat, weakly laterally deflected (cyrtoniodiform) element, NYSM 13885; (f) round (cordylodiform) element, NYSM 13886. (g, h) Cordylodus caseyi Druce & Jones (Reference Druce and Jones1971) emend. Landing (Reference Landing1993); (g) flat (cyrtoniodiform) element, Rit-10.0, NYSM 13887; (h) laterally deflected, round (cordylodiform) element, Rit-12.8, NYSM 13888. (i, k) Fryxellodontus inornatus Miller, Reference Miller1969, 10–15.4; (i), laterally compressed (palmate) element, NYSM 13889; (k) posterior view of tip of intermedius element with posterolateral costa broken off, NYSM 13890. (j) Fryxellodontus aff. lineatus Miller, Reference Miller1969, 10–15.4, palmate element, NYSM 13891. (l–p) Teridontus? francisi Landing sp. nov., 10–15.4; (l, m) proclined and erect paratypes with circular basal outline, NYSM 13892 and NYSM 13993; (n), proclined holotype with circular basal outline, NYSM 13894; (o), erect paratype with laterally compressed, elliptical basal outline, NYSM 13895; (p) proclined paratype with acontiodiform-like, rounded triangular basal outline, NYSM 13896. (q–s) Teridontus nakamurai (Nogami, Reference Nogami1967), Skene 0.65, proclined, erect, and reclined elements, respectively, NYSM 13896–13898. (t, u) Clavohamulus elongatus Miller, Reference Miller1969, 10–21.5; basal and lateral views of NYSM 13899 and NYSM 13900. (v, w) Semiacontiodus nogamii Miller, Reference Miller1969, 10–25.5; (v) posterior view of symmetrical (acontiodiform) element, NYSM 13901; (w) lateral view of asymmetrical (laterally unisulcate) element, NYSM 13902.
Upper Cambrian and lowest Ordovician conodonts and phosphatic problematicum from the Upper Cambrian Little Falls Formation and lowest Ordovician Tribes Hill Formation, eastern New York; all hypotypes; white scale bars are 0.05 mm. (a, b) Monocostodus sevierensis (Miller, Reference Miller1969), 10–24.0, reclined and proclined elements with sharp costa on right side of elements, NYSM 13903 and NYSM 13904. (c, d) Hirsutodontus rarus Miller, Reference Miller1969, 10–11.4. Lateral view of NYSM 13905 and NYSM 13906. (e, f) Hirsutodontus simplex (Druce & Jones, Reference Druce and Jones1971), 10–24.0. Lateral view of NYSM 13923 and NYSM 13924. (g, h) Clavohamulus hintzei Miller, Reference Miller1969, Rit-18.5. Posterior and anterior view, respectively, of NYSM 13909 and NYSM 13910. (i–k) Semiacontiodus lavadamensis (Miller, Reference Miller1969), Rit-18.5; (i) posterior view of weakly torted acontiodiform, NYSM 13911; (j) posterior view of scandodiform, NYSM 13912; (k) lateral view of weakly laterally compressed scolopodiform, NYSM 13913. (l, m) Phosphannulus universalis Müller, Nogami & Lenz, Reference Müller, Nogami and Lenz1974, 10–15.4; (l) basal (i.e. attachment) surface, NYSM 13914; (m) internal surface showing faint concentric lamellar lines, NYSM 13915. (n) Variabiloconus bassleri (Furnish, Reference Furnish1938), Rit-20.0. Lateral view of asymmetrical, laterally unicostate element, NYSM 13916. (o) Laurentoscandodus triangulodus (Furnish, Reference Furnish1938), Gai-1.0, NYSM 13917. (p, r) Semiacontiodus iowensis (Furnish, Reference Furnish1938); (p) acontiodiform (symmetrical) element, Gai-9.0, NYSM 13918; (r) paltodiform with costae on inner side, Gai-3.5, NYSM 13919. (q) Chosonodina herfurthi Müller, Reference Müller1964, Gai-1.0, NYSM 13920. (s) Scalpellodus longpinnatus (Ji & Barnes, Reference Ji and Barnes1994). Scandodiform, Gai-1.0, NYSM 13921. (t) ‘Leukorhinion’ sp. Landing, Westrop & Van Aller Hernick, Reference Landing, Westrop and Van Aller Hernick2003, Gai-1.0. Posterior view of broken element, NYSM 13922.
Discussion. Three Cordylodus species are recorded from the upper Little Falls Formation in this report. Their taxonomy and fundamentally bi-elemental apparatus (i.e. rounded, cordylodiform elements and flattened, laterally deflected cyrtoniodiform elements) reconstruction follows Landing (Reference Landing1993), Szaniawski & Bengtson (Reference Szaniawski and Bengtson1998), Landing et al. (Reference Landing, Franzi, Hagadorn, Westrop, Kröger, Dawson and Landing2007) and to a large degree Miller (Reference Miller1980). Cordylodus andersi Viira & Sergeeva in Kaljo et al. (Reference Kaljo, Borovko, Heinsalu, Khasanovich, Mens, Popov, Sergeeva, Sobolevskaya and Viira1986) is known from a rounded element (Fig. 6d), in which the large basal cavity almost reaches the cusp tip (see Szaniawski & Bengtson, Reference Szaniawski and Bengtson1998; Landing, Westrop & Keppie, Reference Landing, Westrop and Keppie2007). Cordylodus proavus Müller, Reference Müller1959 (Fig. 6e, f) has rounded (cordylodiform) and laterally compressed (cyrtoniodiform) elements in which the tip of the basal cavity reaches into the zone of maximum curvature of the cusp, and the anterior margin of the basal cavity is convex (e.g. Miller, Reference Miller1980). Secondary basal tips occur in some elements (i.e. Cordylodus lindstromi Druce & Jones, Reference Druce and Jones1971 sensu formo, as recognized by Müller, Reference Müller1959, p. 449, fig. 3). The third species, Cordylodus caseyi (Druce & Jones, Reference Druce and Jones1971) emend. Landing (Reference Landing1993) (Fig. 6g, h), has symmetrical cordylodiform elements with a basal cavity with a straight to weakly concave anterior margin and a centrally to anteriorly located basal tip that extends to or slightly higher than the posterior process. It has similar asymmetrical cordylodiforms with an inner lateral flare (i.e. Cordylodus drucei Miller sensu formo), which were not recovered in the relatively small assemblages from the uppermost Little Falls Formation. The cyrtoniodiform elements of C. caseyi have a shallow basal cavity that extends into the base of the cusp and a straight to slightly concave anterior margin of the basal cavity. ‘Lindstromi’ variants with secondary basal cavities occur in C. caseyi elements. As discussed by Landing (Reference Landing1993, p. 16) and Landing, Westrop & Keppie (Reference Landing, Westrop and Keppie2007, p. 916), all reports of Cordylodus intermedius from the uppermost Cambrian represent C. caseyi, and C. intermedius sensu formo is an element of the earliest Ordovician (Fauna B interval and Rossodus manitouensis Zone) Cordylodus angulatus Pander apparatus.
Genus Teridontus Miller, Reference Miller1980
Type species. Oneotodus nakamurai Nogami, Reference Nogami1967; Upper Cambrian of South China.
Teridontus? francisi Landing sp. nov.
Figure 6l–p
1993 Teridontus sp. Fåhraeus & Roy, p. 36, text-fig. 5.1.
Holotype. NYSM 13894, sample 10–15.4, middle Little Falls Formation, Crossman quarry north of Washington County Rte 10, Washington County, New York (Fig. 6n).
Paratypes. NYSM 13892, 13893, 13895, 13896 from sample 10–15.4.
Diagnosis. Probable Teridontus species characterized by cone-like elements with rapidly tapering base with circular, triangular, to laterally compressed cross-section and a small, proclined to erect albid cusp.
Etymology. francisi sp. nov., named for E. Landing's beloved white cat with a blue and a green eye (1998–).
Description. Species has rapidly tapering conical elements. Base hyaline, rate of expansion 40–60°; stubby albid cusp proclined to erect, tapers 30–40°; basal cavity fills base, extends to base of cusp; base cross-section variable, includes circular forms, laterally compressed ovals that may be asymmetrical with one lateral margin more flattened, and triangular (acontiodiform) shapes with rounded anterolateral and posterior corners; antero-basal margin convex.
Discussion. The distinctive, small elements of Teridontus? francisi sp. nov. were recovered from the lower, but not lowermost, Cordylodus proavus Zone at Crossman quarry and on the top of Skene Mountain, and its complete stratigraphic range is unknown. The species is tentatively referred to Teridontus as epithelial mineralization overgrowths have obscured the surfaces of all available elements. If the 1.0 μm wide, longitudinal striae of the type species Teridontus nakamurai (see Nicoll, Reference Nicoll1994, figs 10–12) are present in the new species, then T.? francisi can confidently be referred to Teridontus. A longer stratigraphic range of the new species is suggested by the morphologically identical elements of Teridontus sp. Fåhraeus & Roy, Reference Fåhraeus and Roy1993 in lowest Ordovician continental slope deposits of west Newfoundland. Teridontus? francisi resembles the pseudoconodont Fomitchella infundabiliforma Missarzhevsky, Reference Missarzhevsky and Raaben1969 in basal and lateral outline (see Landing, Reference Landing and Landing1988; Landing & Murphy, Reference Landing and Murphy1991), but has an opaquely white, not transparent, colourless cusp. Teridontus expansus Chen & Gong, Reference Chen, Gong and Chen1986 from South China also has an expanded base, but the aboral-basal outline is strongly concave. Elements of the younger genus Pseudooneotodus Drygant, Reference Drygant1974 have a shallower basal cavity and very blunt cusp. Teridontus? erectus (Druce and Jones, Reference Druce and Jones1971) has a similarly broad base, but a long, gently tapering cusp that is not albid, but has a rapidly expanding growth axis.
Teridontus nakamurai (Nogami, Reference Nogami1967) emended
Figure 6q–s
1967 Oneotodus nakamurai Nogami, pp. 216, 217, pl. 1, figs 9a–13, text-figs 3A–E.
1980 Teridontus nakamurai (Nogami); Miller, pp. 34, 35, pl. 2, figs 15, 16, text-fig. 4O (includes synonymy).
2007 Teridontus nakamurai (Nogami); Landing, Westrop & Keppie, fig. 6i–l.
2008 Teridontus gallicus Serpagli et al., pp. 614, 608, figs 1, 3–5 (author's synonymy of ‘T. gallicus’ is that of T. nakamurai).
Material and occurrence. 760 elements from upper Little Falls Formation, Washington and Saratoga Counties, New York (Tables 1, 2).
Emended diagnosis. Teridontus species with gently tapering (5–8°), conoidal elements with proclined, erect and reclined variants; base hyaline, cusp opaquely albid; basal cavity large, fills base, extends to zone of maximum curvature of elements; cusp approximately circular in cross-section; basal outline laterally compressed (acontiodiform) with rounded lateral margins to circular to vertically compressed oval; elements symmetrical (drepanodiform- to acontiodiform-like) to asymmetrical with lateral deflection of base or slight helicoid torsion of base relative to cusp; longitudinal, fine (approximately 1.0 μm wide) striae run from near basal margin to element tip; very shallow, posterior or posterolateral sulcus may occur on some elements.
Discussion. This diagnosis differs from Nicoll's (Reference Nicoll1994) emendation of the species by specifying the wide variation in inclination of the cusp (proclined to reclined) and in not attempting to fit the species’ elements into P, S or M morphologies. Nicoll (Reference Nicoll1994, p. 372) reported on a thousand elements from one Upper Cambrian sample and concluded that the cusp–base transition ‘is essentially a right angle in all elements’. However, it should be noted that he illustrated strongly reclined elements (Nicoll, Reference Nicoll1994, figs 4.2, 5.2, 6). He also showed a shallow posterolateral or posterior sulcus in the zone of greatest curvature in some short-based elements (Nicoll, Reference Nicoll1994, figs 22.2d, 12.2a). Serpagli et al. (Reference Serpagli, Ferretti, Nicoll and Serventi2008) used Nicoll's (Reference Nicoll1994) statement on the purported right angle cusp–base transition in Teridontus nakamurai to define their T. gallicus; a form which they stated includes proclined, erect and reclined elements. They concluded that the proclined elements in all earlier reports on the Upper Cambrian–lowest Ordovician T. nakamurai were referable to their T. gallicus (i.e. Serpagli et al. Reference Serpagli, Ferretti, Nicoll and Serventi2008, fig. 2 and their synonymy of T. gallicus), which apparently meant that many of the regularly associated erect to reclined elements reported in earlier publications were T. nakamurai.
The problem with the Teridontus gallicus proposal is that the holotype of T. nakamurai is proclined, as Nogami (Reference Nogami1967, fig. 9a, b) showed in lateral views of the holotype. The associated paratypes include reclined forms. The regular association of proclined, erect and reclined elements in almost all Cambrian and Early Ordovician reports cited in Serpagli et al.’s. (Reference Serpagli, Ferretti, Nicoll and Serventi2008) synonymy of Teridontus gallicus (see also Landing, Westrop & Keppie, Reference Landing, Westrop and Keppie2007, fig. 6i–l and this report, Fig. 6q–s) emphasizes that T. gallicus is best regarded as a junior synonym of T. nakamurai.
An attempt to distinguish P and S element morphologies in Teridontus nakamurai and in other apparatuses of euconodonts with conoidal elements (e.g. Nicoll, Reference Nicoll1994; Serpagli et al. Reference Serpagli, Ferretti, Nicoll and Serventi2008) is premature, and presumes that these forms actually had six- or septimembrate apparatuses that are homologous with species with much more strongly differentiated elements. For example, Nicoll's (Reference Nicoll1994, p. 371 and elsewhere) conclusion that an M element is absent in Teridontus was not reached by Ji & Barnes (Reference Ji and Barnes1994), who recognized an M/oistodiform element. Nicoll's (Reference Nicoll1994, figs 11, 12) P elements with laterally deflected, short bases resemble characteristic M elements in other conoidal element apparatuses, and Ji & Barnes's (Reference Ji and Barnes1994) proposal that Teridontus can be interpreted to have M elements may be more acceptable than Nicoll's (Reference Nicoll1994) and Serpagli et al.'s (Reference Serpagli, Ferretti, Nicoll and Serventi2008) conclusion.
Class TRILOBITA Walch, Reference Walch1771
Superfamily Dikelocephaloidea Miller, Reference Miller1899
Family Dikelocephalidae Miller, Reference Miller1899
Discussion. Dikelocephalid genera need comprehensive revision. We follow the provisional diagnoses of Prosaukia Ulrich & Resser, Reference Ulrich and Resser1933 and Calvinella Walcott, Reference Walcott1914 as proposed by Ludvigsen & Westrop (1983a; also Adrain & Westrop, Reference Adrain and Westrop2004, pp. 10, 11) and Westrop, Palmer & Runkel (Reference Westrop, Palmer and Runkel2005), respectively, for the species treated herein.
Genus Prosaukia Ulrich & Resser, Reference Ulrich and Resser1930
Type species. Dikelocephalus misa Hall, Reference Hall1863 from the Upper Cambrian Lone Rock Formation of Wisconsin (by original designation).
Prosaukia spinula Taylor in Taylor & Halley, Reference Taylor and Halley1974
Figure 8d, e
1974 ‘Prosaukia’ spinula Taylor in Taylor & Halley, pp. 29– 31, pl. 2, figs 15–20.
Upper Cambrian (Sunwaptan) trilobites from the upper Little Falls Formation, Crossman quarry (collection 10–10.4), Whitehall, New York. (a–c) Acheilops cf. A. masonensis Winston & Nicholls, Reference Winston and Nicholls1967. Cranidium, NYSM 17898, dorsal, lateral and anterior views, ×12. (d, e) Prosaukia spinula Taylor & Halley, Reference Taylor and Halley1974. Pygidium, NYSM 17899, dorsal and lateral views, ×6. (f–h) ‘Calvinella’ cf. ‘C.’ prethoparia Longacre, Reference Longacre1970. Cranidium, NYSM 17900, dorsal, lateral and anterior views, ×12.
Material and occurrence. A pygidium from the upper Little Falls Formation, Crossman quarry, Saratoga County, collection 10–10.4.
Discussion. The pygidium illustrated herein is from the type locality of Prosaukia spinula Taylor, a species that is known from only six sclerites. Our specimen is larger than the two pygidia figured by Taylor & Halley (Reference Taylor and Halley1974, pl. 2, figs 18, 20) and shows that the median spine becomes relatively smaller and more tapered during holaspid ontogeny.
Assignment of P. spinula to Prosaukia rests on a frontal area that is divided into an anterior border and a distinct preglabellar field. However, the pygidial outline and division of the pleural field by pleural and interpleural furrows clearly differs from the type species P. misa (Hall, Reference Hall1863; Westrop. Reference Westrop1986b, pl. 4, fig. 14) and other Laurentian species currently included in the genus (e.g. Adrain & Westrop, Reference Adrain and Westrop2004, pl. 2, figs 1–32, 34–40). Prosaukia spinula has a relatively longer and narrower pygidium than other species, with closest similarities to the poorly known P.? absona Shergold (Reference Shergold1975, pl. 15, fig. 3) from the Chatsworth Limestone of Australia, although the latter differs in the expression of the pleural bands. Prosaukia spinula is characterized by anterior pleural bands that are noticeably shorter (exsag.) and more convex than the posterior bands and, aside from the anteriormost, do not extend to the pygidial margin. This differs from other Laurentian species, in which the pleural bands are equal in length, are similar in convexity and extend to the pygidial margin.
Prosaukia? spp.
Figures 9a–j, 10d
Discussion. Although represented by limited material, there is considerable variation in the cranidia from Rit-12.3–12.8 that are assigned to Prosaukia, albeit questionably as they lack preglabellar fields. All are superficially similar in having an abbreviated anterior border and a firmly impressed preglabellar furrow. Each morph has an occipital spine. The most obvious differences lie in the sculpture. All have coarse granules to fine tubercles on the glabella that may (Fig. 9a) or may not (Fig. 9h) be expressed on the external mould. Some cranidia have granulose to tuberculate sculpture on the fixigenae (Fig. 9a, d); others are nearly smooth (Fig. 9e, f), but others have terrace ridges rather than tubercles on the fixigenae (Fig. 9h, j). These sculptural differences covary with other characters so that, for example, cranidia with terrace ridges on the fixigenae (Fig. 9h) have broader palpebral lobes and longer, more convex borders than those with almost smooth fixigenae (Fig. 9f). There are likely three species represented by the cranidial morphotypes. They are almost certainly new, but they cannot be diagnosed fully with the available sample, and we prefer to leave them in open nomenclature.
Upper Cambrian (Skullrockian) trilobites from the Ritchie Limestone, upper Little Falls Formation, Saratoga County (collection Rit-12.3–12.8), New York. Scale bars are 1 mm. (a–j) Prosaukia? spp. (a–c) cranidium, NYSM 17901, dorsal, lateral and anterior views, ×16; (d) cranidium, NYSM 17902, dorsal view, ×16; (e–g) cranidium, NYSM 17903, lateral, dorsal and anterior views, ×16; (h–j) cranidium, NYSM 17904, dorsal, lateral and anterior views, ×16.
A librigena with fine tuberculate sculpture on the genal field (Fig. 10d) likely belongs to the species with cranidia that have tubercles on the internal mould (Fig. 9a–d).
Upper Cambrian (Skullrockian) trilobites from the Ritchie Limestone, upper Little Falls Formation, Saratoga County (collection Rit-12.3–12.8), New York. Scale bars are 1 mm. (a–c, e, f) indet. dikelocephalids; (a, b) pygidium, NYSM 17905, dorsal and posterior views, ×14; (c) pygidium, NYSM 17906, dorsal view, ×14; (e) cranidium, NYSM 17907, dorsal view, ×14; (f) cranidium, NYSM 17908, dorsal view, ×16. (d) Prosaukia? sp. Librigena, NYSM 17909, dorsal view, ×14.
Calvinella Walcott, Reference Walcott1914 and Tellerina Ulrich & Resser, Reference Ulrich and Resser1933 resemble these cranidia in lacking a preglabellar field. However, the anterior border and, consequently, frontal area is more than twice the length in these genera than in the Crossman quarry cranidia, and the anterior border furrows proceed obliquely forward from the anterior corners of the glabella (e.g. Westrop, Palmer & Runkel, Reference Westrop, Palmer and Runkel2005, fig. 10; Ulrich & Resser, Reference Ulrich and Resser1933, pl. 41, figs 1, 3, 8). At the current state of knowledge, a relationship with Prosaukia seems more likely. The earliest Laurentian representatives of the genus are from mid-Sunwaptan strata in the Upper Mississippi Valley (Ulrich & Resser, Reference Ulrich and Resser1930) and New York (Ludvigsen & Westrop, Reference Ludvigsen and Westrop1983a), but younger Sunwaptan species occur in Pennsylvania (Rasetti, Reference Rasetti1959), New York (Taylor & Halley, Reference Taylor and Halley1974; this report), Nevada (Adrain & Westrop, Reference Adrain and Westrop2004), and Oklahoma and Texas (Longacre, Reference Longacre1970; Stitt, Reference Stitt1971b; see Adrain and Westrop, Reference Adrain and Westrop2004 for discussion). The occurrence in the Ritchie Limestone represents a significant range extension well into the Skullrockian Stage.
Genus Calvinella Walcott, Reference Walcott1914
Type species. Dikelocephalus spiniger Hall, Reference Hall1863 from the Upper Cambrian sandstone, Trempealeau, Wisconsin (by original designation).
‘Calvinella’ cf. ‘C.’ prethoparia Longacre, Reference Longacre1970
Figure 8f–h
cf. 1967 Calvinella ozarkensis Walcott; Winston & Nicholls, p. 80, pl. 11, figs 5, 9.
cf. 1970 Calvinella prethoparia Longacre, pp. 45, 46, pl. 6, figs 7–12.
1974 ‘Calvinella’ prethoparia Longacre; Taylor & Halley, pp. 27, 28, pl. 2, figs 1–3.
?1995 Calvinella prethoparia Longacre; Westrop, pp. 23, 24, pl. 8, figs 3–6.
Material and occurrence. One cranidium from the upper Little Falls Formation, Crossman quarry, Saratoga County, collection 10–10.4.
Discussion. Taylor (in Taylor & Halley, Reference Taylor and Halley1974, p. 29) catalogued the differences between Calvinella prethoparia Longacre, Reference Longacre1970 and the genotype C. spiniger (Hall, Reference Hall1863; Westrop, Palmer & Runkel, Reference Westrop, Palmer and Runkel2005, fig. 10.10), including the shorter palpebral lobe and, consequently, longer posterior area of the fixigena in the latter. We agree with Taylor that this species may represent a distinct dikelocephalid genus, but also note with some dismay that the knowledge of the family has not progressed sufficiently in the 36 years since his suggestion to allow this genus to be diagnosed and formally named.
Taylor (in Taylor & Halley, Reference Taylor and Halley1974, p. 28) considered differences between the Little Falls Formation cranidia and the types of Calvinella prethoparia Longacre to be of no taxonomic significance. The holotype and other holaspid cranidia from Texas (Longacre Reference Longacre1970, pl. 6, figs 7–9) have coarse granulose sculpture on the glabellae, but those from the Little Falls (Taylor & Halley, Reference Taylor and Halley1974, pl. 2, figs 2, 3; Fig. 8f–h) have terrace ridges. Taylor described the larger of his two cranidia as having coarse granules, but these appear to be the intersections of short terrace ridges on the crest of the glabella, and are likely not homologous with the more conventional, domical tubercles of Longacre's types.
Westrop (Reference Westrop1995, pl. 8, figs 3–5) followed Taylor's lead in assigning cranidia with terrace ridges and smooth internal moulds to C. prethoparia. However, this broad species concept cannot be justified with current knowledge. It is not possible to demonstrate that populations of C. prethoparia vary significantly in cranidial sculpture. Rather, very small samples from individual regions possess either granular sculpture or terrace ridges, but not both. Accordingly, only the type material can be assigned confidently to C. prethoparia, and the identities of other sclerites attributed to this species are uncertain.
Cranidia from the Rabbitkettle Formation of the Mackenzie Mountains, Canada (Westrop, Reference Westrop1995), are generally similar to those from New York, but further assessment must await discovery of additional material. The largest specimen illustrated by Westrop (Reference Westrop1995, pl. 8, fig. 3) has somewhat longer palpebral lobes, and may not be conspecific with the others (Westrop, Reference Westrop1995, pl. 8, figs 4–6).
A detailed description of ‘C.’ cf. ‘C.’ prethoparia is in Taylor & Halley (Reference Taylor and Halley1974, pp. 27, 28), and there is little to add. Our cranidium has a tubercle just in front of the occipital spine, and there is a very weakly inflated baccula on the fixigena opposite L1. One cranidium illustrated by Westrop (Reference Westrop1995, pl. 8, fig. 6) is also weakly bacculate.
Indet. dikelocephalid spp.
Figure 10a–c, e, f
Discussion. Two incomplete cranidia appear to represent distinct species, and increase the dikelocephalid diversity in collection Rit-12.3–12.8 from the Symphysurina Zone fauna. The smaller of the two (Fig. 10f) is an internal mould with faint, finely tuberculate sculpture on the glabella. It differs from all other cranidia illustrated herein in having a faint anterior border that is best expressed on the preocular area. It may represent an additional species of Prosaukia.
The larger cranidium (Fig. 10e) retains a small area of exoskeleton on LO with very coarse terrace ridges and large palpebral lobes. These features are shared with ‘P.’ spinula Taylor from late Sunwaptan strata of the Little Falls Formation (Taylor and Halley, Reference Taylor and Halley1974, pl. 2, figs 15, 17), which differs in having a distinct preglabellar field in smaller and larger holaspids.
A single type of dikelocephalid pygidium (Fig. 10a–c) is associated with the range of cranidia (Figs 8, 9e, f) documented herein. It is uncertain as to which of these, if any, it belongs. The axis has four well-defined axial rings with at least two segments incorporated into a terminal piece. The pleural furrows curve backwards to intersect the adjacent interpleural furrow no more than halfway across the pleural field, so that the posterior pleural band is narrower (tr.) and shorter (exsag.) than the anterior band, which expands abaxially. Pygidia of this type belong to species of Calvinella (Ulrich & Resser, Reference Ulrich and Resser1933, pl. 38–40; Westrop, Palmer & Runkel, Reference Westrop, Palmer and Runkel2005, figs 9.7, 9.8) and Tellerina (e.g. Ulrich & Resser, Reference Ulrich and Resser1933, pl. 41, fig. 7, pl. 43, fig. 7), and have also been attributed to some species of Prosaukia (Westrop, Reference Westrop1986b, pl. 3, fig. 15).
Superfamily Leiosteioidea Bradley, Reference Bradley1925
Family Missisquoidae Hupé, Reference Hupé1955
Genus Parakoldinioidia Endo, Reference Endo1937
Type species. Parakoldinioidia typicalis Endo, Reference Endo1937 from the Yenchou Formation, Liaoning, China (by original designation).
Discussion. Synonymy of Parakoldinioidia and Missisquoia Shaw, Reference Shaw1951 follows Fortey (Reference Fortey1983) and Lee, Lee & Choi (Reference Lee, Lee and Choi2008). The latter resurrected Tangshanaspis Zhou & Zhang (Reference Zhou and Zhang1978), which we regard as including, at minimum, the type species, T. zhaogezshuangensis Zhou & Zhang, Reference Zhou and Zhang1978 and M. depressa Stitt, Reference Stitt1971b.
Parakoldinioidia maddowae Westrop sp. nov.
Figures 11, 12
1974 Missisquoia typicalis Shaw; Taylor & Halley, pp. 22, 23, pl. 3, figs 1–9.
Upper Cambrian (Skullrockian) trilobites from the Little Falls Formation, Crossman quarry (collection 10–15.4), Whitehall, New York. Scale bars are 1 mm. (a–o) Parakoldinioidia maddowae Westrop sp. nov. (a–c) cranidium, NYSM 17910 (holotype), anterior, dorsal and lateral views, ×20; (d) cranidium, NYSM 17911, dorsal view, ×20; (e) cranidium, NYSM 17912, dorsal view, ×20; (f) cranidium, NYSM 17913, dorsal view, ×20; (g, k, l) cranidium, NYSM 17914, lateral, anterior and dorsal views, ×20; (h–j) cranidium, NYSM 17915, lateral, dorsal and anterior views; (m–o) cranidium NYSM 17916, lateral, anterior and dorsal views.
Upper Cambrian (Skullrockian) trilobites from the Little Falls Formation, Crossman quarry (collection 10–15.4), Whitehall, New York. Scale bars are 1 mm. (a–m) Parakoldinioidia maddowae Westrop sp. nov. (a, b) pygidium, NYSM 17917, dorsal and posterior views, ×20; (c–e) pygidium, NYSM 17918, dorsal, posterior and lateral views, ×20; (f) librigena, NYSM 17919, dorsal view, ×20; (g) librigena, NYSM 17920, dorsal view, ×20; (h–j) pygidium, NYSM 17921, dorsal, posterior and lateral views, ×20; (k–m) pygidium, NYSM 17922, posterior, dorsal and lateral views, ×20.
Types. All types are from the upper Little Falls Formation, Crossman quarry, Washington County, collection 10–15.4. The holotype (NYSM 17910) is a cranidium (Fig. 11a–c); paratypes are six cranidia (NYSM 17911–17916; Fig. 11), two librigena (NYSM 17919, 17920; Fig. 12), and four pygidia (NYSM 17917, 17918, 17921, 17922; Fig. 12). Additional, non-type material is illustrated in Taylor & Halley (Reference Taylor and Halley1974, pl. 3, figs 1–9) from localities 470b and H3, which are located about 0.7 km south and 2.2 km north of Crossman quarry, respectively.
Diagnosis. Parakoldinioidia species with anterior border of even width, uninterrupted by glabella. Palpebral area of fixigena gently convex in lateral view, stands well below crest of glabella. Palpebral lobe extends from S1 to S3, so that postoccular field is relatively long, equal to about 22 % (range 22–23 %) of cranidial length (sag.). Posterolateral projection relatively narrow (tr.), accounting for about 18 % (range 16–19 %) of maximum cranidial width and equal to about half (about 49 %; range 44–55 %) of glabellar width at SO. Sculpture of fine, closely packed granules over entire cranidium, except for furrows.
Etymology. Named for Rachel Maddow.
Description. Cranidium strongly arched in longitudinal profile (Fig. 11c, m) and anterior view (Fig. 11a, k, n); width across palpebral lobes at posterior equal to about 73 % (range 70–76 %) of maximum width across posterolateral projections; cranidial length roughly equal (97 %; range 93–100 %) to cranidial width across palpebral lobes. Axial and preglabellar furrows are shallow but clearly defined. Convex glabella long, occupies about 92 % (range 90–94 %) of cranidial length, and narrow, accounting for less than half (about 44 %; range 41–46 %) of cranidial width across palpebral lobes; tapers slightly forward, so that width at S3 is about 93 % (range 91–95 %) of width at SO, and rounded anteriorly; short median notch well expressed on larger individuals (Fig. 11b) but barely perceptible on some smaller specimens (Fig. 11d, i). SO well incised, nearly transverse medially but curved forward abaxially. LO wide medially, occupies about 18 % (range 16–20 %) of glabellar length, but shortens conspicuously abaxially; with ill-defined median tubercle. S1 firmly impressed, oblique, extends inward for less than one third of glabella width (tr.). L1 about 15 % (range 13–17 %) of glabellar length. S2 nearly transverse, similar in depth to S1, but slightly narrower (tr.). L2 shorter than L1, equal to about 10 % (range 9–13 %) of glabellar length. S3 shallow, nearly transverse; does not reach axial furrow, extends inward to same level as S2; L3 short, roughly half length of L1, and occupies about 7 % (range 5–9 %) of glabellar length. Some small cranidia display faint, transverse S4 (Fig. 11e, i) not expressed on larger individuals (Fig. 11b, f). Short, flat anterior border curved gently and evenly forward; narrows slightly abaxially but not in front of glabella; anterior border furrow shallow, joins preglabellar furrow adaxially. Preocular field of fixigena broad, equal to a little less than half (about 45 %; range 38–52 %) of glabellar width at S3, dips steeply forward. Palpebral area gently convex in anterior view, stands well below crest of glabella, width equal to about 58 % (range 48–65 %) of glabellar width at SO. Palpebral lobe evenly curved, flat, centred opposite anterior tip of L2, extends from S1 to S3, length about 24 % (range 22–27 %) of glabellar length; palpebral furrow is very finely etched, shallow groove, barely perceptible on larger cranidia. Postocular area gently sloping near glabella, length (exsag.), is about 22 % (range 22–23 %) of cranidial length. Posterolateral projection relatively narrow (tr.), is about 18 % (range16–19 %) of maximum cranidial width and equal to about half (49 %; range 44–55 %) of glabellar width at SO. Posterior border furrow firmly impressed, nearly transverse, reaches sutural margin. Posterior border expands abaxially, with length (exsag.) at axial furrow about half (about 45 %; range 39–51 %) of maximum length, so that posterior cranidial margin deflected gently backward as it approaches sutural margin. Anterior branches of facial suture nearly parallel between palpebral lobe and anterior border furrow, then swing abruptly inward along anterior cranidial margin. Posterior branches follow weakly sigmoid course, initially diverge rapidly along a curved path before swinging backward to intersect the posterior cranidial margin. Entire surface, apart from furrows, with sculpture of closely spaced coarse granules.
Librigena weakly convex, extends into short, stout genal spine with bluntly rounded termination. Lateral border furrow shallow, well-defined anteriorly but terminates short of base of genal spine. Lateral border gently convex, of uniform width (tr.). Librigenal field with low eye socle; narrow anteriorly, roughly equal in width to border, expands posteriorly. Sculpture of closely spaced, coarse to fine granules, augmented by terrace ridges along outer edge of border.
Pygidium subtriangular, length about 73 % (range 69–75 %) of maximum pygidial width, strongly convex in posterior (Fig. 12b, d, i, k) and lateral profiles (Fig. 12e, j, m). Strongly arched axis long, occupies about 90 % (range 87–93 %) of pygidial length, with width at first ring about 38 % (range 37–40 %) of maximum pygidial width; tapered gently backward, width at seventh ring about 59 % (range 56–62 %) of width at first ring and rounded posteriorly; weak postaxial ridge present. Axial furrows are shallow grooves. Axial ring furrows transverse, well incised over anterior half of axis, become shallower posteriorly. Eight axial rings plus terminal piece of at least two segments present; articulating half-ring shorter than first axial ring, with semielliptical outline. Pleural field flat near axis, quickly bends steeply downward, flattens slightly at pygidial margin to produce very narrow, ill-defined border where marginal spines are absent. Pleural furrows curved gently backward; firmly impressed on anterior pleura, become shallower towards the rear; divide pleural field into convex, roughly equal anterior and pleural bands; terminate short of tips of marginal spines, where present. Interpleural furrows shallow, fainter towards rear. Marginal spines on first three pairs of pleurae of largest specimen (Fig. 12a, b), up to five pairs on smaller individuals (Fig. 12c–e, k, m). External surface with fine to coarse, closely spaced granules.
Discussion. Lee, Lee & Choi (Reference Lee, Lee and Choi2008, p. 324) commented on the wide variability of sclerites assigned by various authors to P. stitti Fortey, Reference Fortey1983 (a replacement name for P. typicalis (Shaw, Reference Shaw1951), non Endo, Reference Endo1937 preoccupied) and concluded that more than one species is represented. We agree, and note that a number of problems, including small sample sizes, inadequate photographic documentation and, in most cases, limited information on sclerite associations, render published records of Parakoldinioidia (e.g. Winston & Nicholls, Reference Winston and Nicholls1967; Stitt, Reference Stitt1971b; Taylor & Halley, Reference Taylor and Halley1974; Westrop, Reference Westrop1986b; Miller et al. Reference Miller, Evans, Loch, Ethington, Stitt, Holmer and Popov2003) essentially uninterpretable. This is unfortunate as the ‘Missisquoia’ Zone is a significant biostratigraphic unit in the Cambrian–Ordovician boundary interval in Laurentia. We have taken a step towards revising Laurentian species of Parakoldinioidia with a re-evaluation of the species from the Little Falls Formation that was identified as ‘M. typicalis’ by Taylor (in Taylor & Halley, Reference Taylor and Halley1974).
Shaw's types of P. stitti have undergone some tectonic deformation (compare Shaw, Reference Shaw1951, pl. 23, figs 5, 6 and pl. 23, figs 8. 9), and an evaluation of biological variation is difficult at best. Lee, Lee & Choi (Reference Lee, Lee and Choi2008) restricted this species to the types, and we agree that this is the best approach. We propose a new species, Parakoldinioidia maddowae, from the upper Little Falls Formation in Washington County that is well documented from Taylor & Halley's (Reference Taylor and Halley1974, pl. 3, figs 1–9) specimens and our new material from the Crossman quarry (Figs 11, 12). It is possible to make only limited comparisons between P. maddowae sp. nov. and the types of P. stitti because of the effects of deformation in cranidial and pygidial proportions in the latter. The anterior borders of cranidia of P. stitti and the relatively narrow posterolateral projection appear to be similar to those of P. maddowae sp. nov. Pygidia of P. maddowae sp. nov. lack the ‘terminal spike’ described by Shaw (Reference Shaw1951, p. 109) on larger specimens of P. stitti, and also have shallower pleural furrows.
Material from Texas and Oklahoma figured prominently in formulating species concepts of Laurentian Parakoldinioidia. Winston & Nicholls (Reference Winston and Nicholls1967) assigned sclerites from the Wilberns Formation of Texas to ‘M. typicalis’, and described the new species ‘M.’ inflata and ‘M.’ nasuta. Sclerites of ‘M. typicalis’ include two small cranidia (Winston & Nicholls, Reference Winston and Nicholls1967, pl. 13, figs 5, 6) with shorter anterior borders and wider, more strongly upsloping palebral areas of the fixigenae than those of cranidia of P. maddowae sp. nov. A larger cranidium (Winston & Nicholls, Reference Winston and Nicholls1967, pl. 13, fig. 10) is closer to P. maddowae sp. nov. in border length and fixigenal width, but its posterior third is not preserved, and a full evaluation is impossible. Pygidia illustrated by Winston & Nicholls (Reference Winston and Nicholls1967, pl. 13, figs 15, 18) are from one collection and include two specimens that clearly differ from P. maddowae sp. nov. in their sharply triangular outlines, relatively long axes with more than a dozen segments, and well-incised pleural furrows over almost all of their pleural fields. A third, smaller pygidium (Winston & Nicholls, Reference Winston and Nicholls1967, pl. 13, fig. 13) is well rounded posteriorly and, in this respect, is unlike P. maddowae sp. nov.
Lee, Lee & Choi (Reference Lee, Lee and Choi2008, fig. 5) assigned ‘M.’ nasuta provisionally to Lunacrania Kobayashi, Reference Kobayashi1955, although their cladograms render this genus paraphyletic. In our view, the few published images (Winston & Nicholls, Reference Winston and Nicholls1967, pl. 13, figs 1, 3, 9) of this species are inadequate as coding sources, and its inclusion in a phylogenetic analysis is premature. Differences in the relative size and shape of the anterior border between large and small cranidia (Winston & Nicholls, Reference Winston and Nicholls1967, pl. 13, fig. 9 and figs 1, 3, respectively) imply significant changes during holaspid ontogeny that were not discussed or documented by Winston & Nicholls. The larger specimen may represent a distinct species (implicit in Lee, Lee & Choi's, Reference Lee, Lee and Choi2008 Appendix 1, where this specimen is excluded from their analysis), but this cannot be evaluated with the available data. In any event, none of the three Texas cranidia is similar to those of P. maddowae sp. nov. The two smaller specimens, including the holotype (Winston & Nicholls, Reference Winston and Nicholls1967, pl. 13, fig. 3) have a distinctly triangular anterior border and, hence, anterior margin, as well as broader, upsloping fixigenae; the largest has a very short anterior border that resembles that in Parakoldinioidia sp. A of Fortey (Reference Fortey1983; see below).
Winston & Nicholls's (Reference Winston and Nicholls1967, pl. 13, figs 4, 7) other new species, ‘M.’ inflata, is known in its type area from two figured cranidia. A third cranidium from the Survey Peak Formation of Alberta was assigned to the species by Loch, Stitt & Derby (Reference Loch, Stitt and Derby1993, fig. 9.11). Material from Texas is characterized by a suboval, strongly inflated glabella with deeply incised S1 and S2 furrows. The anterior border is difficult to discern in Winston & Nicholls's images, but is apparently much shorter than in P. maddowae sp. nov. The Alberta cranidium has a short anterior border, but does not resemble Winston & Nicholls's types in the configuration of the glabella furrows. Loch, Stitt & Derby (Reference Loch, Stitt and Derby1993, fig. 12) also assigned a pygidium to ‘M.’ inflata, although, curiously, illustrated it only in lateral view. This sclerite has a long terminal axial spine that clearly differentiates it from pygidia of P. maddowae sp. nov. Loch, Stitt & Derby (Reference Loch, Stitt and Derby1993, p. 509) note that similar pygidia are associated with ‘M.’ inflata-type cranidia in Texas, Oklahoma and Utah.
Stitt (Reference Stitt1971b) figured four sclerites from four different collections as Missisquoia typicalis. The collections span a 22 m thick interval in the Signal Mountain Formation of Oklahoma. Preparation of more material from Stitt's collections, now at the Oklahoma Museum of Natural History, indicates that several species are present in the Missisquoia Zone of the Joins Ranch section; these will be documented elsewhere. Among published illustrations, a cranidium from collection JoR 1142 (Stitt, Reference Stitt1971b, pl. 8, fig. 2) is closest to P. maddowae sp. nov., but has wider, upsloping fixigenae.
Pygidia from the Survey Peak Formation at Wilcox Pass, Alberta, figured as ‘M. typicalis’ (Dean, Reference Dean1977, pl. 1, fig. 5 [the pygidium in pl. 1, figs 4, 7 was assigned tentatively to ‘M. enigmatica’ by Westrop, Reference Westrop1986b], 1989, pl. 13, figs 1, 2, 5, 7) clearly differ from those of Parakoldinioidia maddowae sp. nov. The pleurae are much longer, with at least seven pairs of spinose tips. Although there is not a true border, there is a well-defined paradoublural furrow that is absent in P. maddowae sp. nov. All of the characters listed above differentiate Dean's material from the type material of P. stitti. Cranidia illustrated by Dean are poorly preserved; one of them (Dean, Reference Dean1989, pl. 13, fig. 3) apparently lacks lateral glabellar furrows, has well-defined, nearly transverse palpebral ridges, and is likely misassigned to the genus. A second cranidium (Dean, Reference Dean1989, pl. 13, fig. 4) appears to have a wider palpebral area of the fixigena than any specimen illustrated herein or by Taylor & Halley (1974, pl. 3, figs 1–4, 7, 9). It is from a different collection than any of the pygidia illustrated by Dean. A different cranidial morphotype (Dean, Reference Dean1977, pl. 1, figs 12, 15) occurs in the same collection (GSC loc. 92223) as the pygidia. It differs from P. maddowae sp. nov. in having steeply upsloping fixigenae and a wider (tr.) posterolateral projection; the S1 and S2 are deeply incised and contrast with the shallower furrows of P. maddowae sp. nov. A second, smaller cranidium from GSC loc. 92223 is incomplete, but may not be conspecific with the other figured cranidium as it appears to differ both in glabellar outline and incision of the lateral glabellar furrows. We conclude that Dean's pygidia represent a distinct Parakoldinioidia species, but the correct cranidial assignment is unclear. It is uncertain whether the cranidium and pygidium illustrated by Westrop (Reference Westrop1986b) from the Survey Peak Formation at Wilcox Peak, about 1.5 km along strike from Dean's sections, is conspecific with this new species. The pygidium (Westrop, Reference Westrop1986b, pl. 1, fig. 35) is similar in size to one of Dean's (Reference Dean1977, pl. 1, fig. 5) specimens, but appears relatively narrower, and the pleural furrows become noticeably shallower posteriorly. The cranidium (Westrop, Reference Westrop1986b, pl. 1, figs 36, 37) is from a different collection and is the most similar to P. maddowae sp. nov. of any sclerite described from Alberta. It differs from the latter in having a shorter anterior border and well-defined palpebral ridges. The status of a cranidium from the Survey Peak Formation at Mt Wilson, Alberta, attributed to M. typicalis by Loch, Stitt & Derby (Reference Loch, Stitt and Derby1993, fig. 7.26) is uncertain. It is similar to P. maddowae sp. nov. in glabellar outline, but the palpebral lobes appear to be smaller.
Parakoldinioidia is known from the Shallow Bay Formation of west Newfoundland from a few cranidia (Fortey, Landing & Skevington, Reference Fortey, Landing, Skevington, Bassett and Dean1982; Fortey, Reference Fortey1983). One of the two cranidia figured as M. typicalis (Fortey, Landing & Skevington, Reference Fortey, Landing, Skevington, Bassett and Dean1982, pl. 2, fig. 4) is too poorly preserved to compare it to P. maddowae sp. nov. The other (Fortey, Landing & Skevington, Reference Fortey, Landing, Skevington, Bassett and Dean1982, pl. 3, fig. 2) likely belongs to P. sp. A of Fortey (Reference Fortey1983, pl. 25, fig. 4), and differs from P. maddowae sp. nov. by its extremely short anterior border that lengthens slightly in front of the preocular field (see Fortey, Reference Fortey1983, p. 197, for discussion).
Superfamily Uncertain
Family Catillicephalidae Raymond, Reference Raymond1937
Genus Acheilops Ulrich in Bridge (Reference Bridge1931)
Type species. Acheilus dilatus Ulrich in Bridge (Reference Bridge1931) from the Eminence Formation of Missouri (by original designation).
Acheilops cf. A. masonensis Winston & Nicholls, Reference Winston and Nicholls1967
Figure 8a–c
cf. 1967 Acheilops masonensis Winston & Nicholls, pp. 77, 78, pl. 11, figs 23–25.
non 1970 Acheilops masonensis Winston & Nicholls; Longacre, p. 15, pl. 6, fig. 19.
cf. 1971b Acheilops masonensis Winston & Nicholls; Stitt, p. 15, pl. 7, fig. 5.
1974 Acheilops masonensis Winston & Nicholls; Taylor & Halley, pp. 19, 20, pl. 1, figs 4, 5.
Material and occurrence. A cranidium from the Little Falls Formation, Crossman quarry, Washington County, collection 10–10.4. Material from the Little Falls Formation figured by Taylor & Halley (Reference Taylor and Halley1974) came from localities 470b and H3, 0.8 km south and 2.25 km north, respectively, of Crossman quarry (Taylor & Halley, Reference Taylor and Halley1974, fig. 1).
Discussion. Taylor & Halley (Reference Taylor and Halley1974, pl. 1, figs 4, 5) figured a cranidium and librigena from the Little Falls Formation that they identified as Acheilops masonensis. The type material of this species (Winston & Nicholls, Reference Winston and Nicholls1967, pl. 11, figs 23–25) consists of two cranidia and a pygidium from collection LCS 55.5 (Eurekia apopsis Zone), Wilberns Formation, central Texas. A third cranidium (Longacre, Reference Longacre1970, pl. 6, fig. 19) from a different section (SS 411; E. apopsis Zone) has an occipital spine base that is not evident in the types, which have a simple, aspinose LO. A cranidium from Oklahoma that was referred to the species (Stitt, Reference Stitt1971b, pl. 7, fig. 5) has an incomplete LO and anterior cranidial margin; in our view, it is inadequate for identification at the species level.
In our opinion, A. masonensis is too poorly known to permit confident application of the name beyond Winston & Nicholls's types, and Taylor & Halley's (Reference Taylor and Halley1974) material is best placed in open nomenclature. A large cranidium from Crossman quarry (Fig. 8a–c) has the coarse granulose sculpture of Taylor & Halley's illustrated specimen. The new cranidium shows details of the glabellar furrow morphology that are not clearly visible in Taylor & Halley's (Reference Taylor and Halley1974, pl. 1, fig. 5) figure. The furrows are shallower than those of other species, and are expressed primarily as narrow, weakly depressed bands of exoskeleton that lack sculpture. As in other cases (e.g. Westrop, Reference Westrop1986b, pl. 39, fig. 1), S1 is deflected backward adaxially. S2 has a similar configuration, and this differs from the condition in most other species, in which S2 is transverse (e.g. Winston & Nicholls, Reference Winston and Nicholls1967, p. 77, pl. 11, fig. 24; Westrop, Reference Westrop1986b, pl. 39, fig. 1). S3 is barely perceptible, transverse, and terminates well short of the glabellar margin. S4 is also very weakly defined, and proceeds obliquely forward to terminate at an apodemal pit near the anterior corner of the glabella. In other respects, the Little Falls cranidium is comparable to A. masonensis, although the granulose sculpture is coarser and, unlike the specimen attributed to the species by Longacre (Reference Longacre1970, pl. 6, fig. 19), the palpebral furrow and lobe are ill-defined.
Acheilops olbermanni Westrop sp. nov.
Figure 13
Types. All are from the upper Little Falls Formation (Ritchie Limestone member), Saratoga County, collection Rit-12.3–12.8. Holotype (NYSM 17925) is a nearly complete cranidium (Fig. 13e); paratypes are five cranidia (NYSM 17923, 17924, 17928, 17930, 17931), one librigena (NYSM 17926) and three pygidia (NYSM 17927, 17929, 17932).
Upper Cambrian (Skullrockian) trilobites from the Ritchie Limestone, upper Little Falls Formation, Saratoga County (collection Rit-12.3–12.8), New York. Scale bars are 1 mm. (a–o) Acheilops olbermanni Westrop sp. nov. (a) cranidium, NYSM 17923, dorsal view, ×20; (b–d) cranidium, NYSM 17924, dorsal, lateral, anterior views, ×20; (e) cranidium, NYSM 17925 (holotype), dorsal view, ×20; (f–h) librigena, NYSM 17926, lateral, dorsal and anterior views, ×20; (i) pygidium, NYSM 17927, dorsal view, ×20; (j) cranidium, NYSM 17928, dorsal view, ×20; (k) pygidium, NYSM 17929, dorsal view, ×20; (l) cranidium, NYSM 17930, dorsal view, ×20; (m) cranidium, NYSM 17931, dorsal view, ×20; (n, o) pygidium, NYSM 17932, dorsal and posterior views, ×20.
Diagnosis. Acheilops with occipital spine. Glabellar furrows gently impressed, expressed largely as exoskeleton bands that lack sculpture. Palpebral furrow barely visible as very finely etched groove. Pygidium weakly furrowed, gently impressed pleural furrows extend across about half of pleural width (tr.); axial ring furrows shallow. Pleural field broad (tr.), equal to about 86 % (range 84–88 %) of maximum axis width.
Etymology. Named for Keith Olbermann.
Description. Cranidium strongly arched in lateral (Fig. 13c) and anterior (Fig. 13d) view; width across palpebral lobes about 105 % (range 101–112 %) of cranidial length. Glabella convex, stands well above level of palpebral lobe, occupies entire cranidial length and about 58 % (range 55–60 %) of cranidial width opposite palpebral lobe midpoint. Glabella bulb-shaped, changes little in width from SO to S2 (may be slightly expanded at L1; Fig. 13e, j), expands rapidly in front of S2 so that maximum width (opposite L3) is about 72 % (range 70–74 %) of glabellar width at LO. Axial furrows firmly impressed, terminate at intersection with anterior tips of palpebral lobes; preglabellar furrow obsolete. SO well incised, short (sag., exsag.) groove; transverse medially but curved gently forward abaxially. LO (excluding spine) is about 17 % (range 15–18 %) of glabellar length; extended into short, stout, subtriangular spine, length at least 140 % of length of remainder of LO. S1 shallow, defined largely as exoskeleton strip that lacks sculpture, nearly transverse over most of width but deflected sharply backward adaxially; L1 about18 % (range 16–20 %) of glabellar length. S2 similar in depth and definition as S1, transverse but for slight posterior deflection adaxially; L2 is about 13 % (range 11–16 %) of glabellar length. S3 ill-defined, evident only as transverse band that lacks sculpture (Fig. 13e); S4 terminates short of lateral glabellar margin, barely perceptible as oblique band of exoskeleton without sculpture (e.g. Fig. 13a); may deepen at anterior termination to form weak apodemal pit (Fig. 13b, d). Palpebral lobe is long, arcuate band, equal to about 39 % (range 37–40 %) of cranidial length; separated from glabella posteriorly by narrow band of fixigena but abuts glabella anteriorly. Palpebral furrow barely perceptible as finely etched groove (Fig. 13e, j). Palpebral area of fixigena broad, semielliptical in outline; curves steeply upward from axial furrow, becomes nearly flat abaxially. Anterior branches of facial suture run along glabellar margin, swing outward before converging inward anteriorly; posterior area of fixigena not preserved, but librigena (Fig. 13g) shows that posterior branches initially abruptly divergent but then swing sharply back to become almost parallel. Exterior of cranidium, apart from furrows, with closely spaced fine to coarse granules; frontal lobe of glabella with terrace ridges that run subparallel to anterior glabellar margin.
Librigena unfurrowed except for shallow groove that defines narrow (tr.), weakly convex extension of posterior border. Conspicuous genal spine tapers rapidly to sharply pointed tip, length about 90 % of visual surface of eye. Visual surface of eye large, strongly curved in dorsal view, and mounted on rim-like eye socle; socle furrow finely etched groove. External surface of librigena, apart from eye and eye socle, with coarse terrace ridges that run roughly parallel to lateral librigenal margin.
Pygidium strongly arched in posterior view, subelliptical in outline, maximum width just in front of mid-length of terminal piece of axis; length equal to about 61 % (range 59–63 %) of maximum width. Axis convex, tapered gently backward, well rounded posteriorly, occupies about 85 % (range 85–87 %) of pygidial length and about 41 % (range 39–43 %) of maximum pygidial width. Axis with three well-defined axial rings and terminal piece that incorporates two segments. Axial ring furrows well-incised, deflected backward to varying extent around median nodes on posterior edges of axial rings 1–3. Axial rings 1–3 curve backward medially, are equal in length (sag.); terminal piece accounts for 40 % of axis length (range 38–43 %), partly divided by shallow, backwardly curved furrow that does not reach axial furrows. Pleural field dips abruptly downward from shallow axial furrows. Four pairs of pleurae defined by shallow, oblique interpleural furrows that extend to pygidial margin. Equally shallow pleural furrows extend across about half of width of at least first two pairs of pleurae; anterior pleural band roughly half of length (exsag.) of posterior pleural band. Pygidial margin mostly entire, may be interrupted by minute marginal spines at first two pairs of pleurae. External surface of pygidium smooth (Fig. 13k).
Discussion. Acheilops olbermanni sp. nov. has a well-defined, triangular occipital spine (Fig. 13b–d, l) that differentiates it from other members of the genus. In naming A. masonensis from the Wilberns Formation of Texas, Winston & Nicholls (Reference Winston and Nicholls1967, p. 77) stated that the species has an occipital ring whose posterior margin ‘may be curved or with a suggestion of a spine’. The cranidium attributed to A. masonensis by Longacre (Reference Longacre1970, pl. 6, fig. 19) preserves a narrow (tr., sag.) base of what must be a more slender occipital spine than that of A. olbermanni sp. nov. As noted above, the type cranidia are from a different locality and are aspinose, and there is no compelling evidence (i.e. photographs of spinose and aspinose cranidia from a single collection) to suggest that A. masonensis may be polymorphic. Thus, we consider the specific identity of Longacre's specimen to be uncertain.
Acheilops olbermanni sp. nov. is also distinct from A. masonensis in pygidial morphology. The latter (Winston & Nicholls, Reference Winston and Nicholls1967, pl. 11, fig. 25) has well-incised pleural furrows on the anterior two pairs of pleurae that terminate close to the pygidial margin. The axial ring furrows are also firmly impressed. In contrast, the pygidia of A. olbermanni sp. nov. (Fig. 13i, k, n, o) are weakly furrowed with gently impressed pleural furrows that extend across barely half of the pleural width (tr.), and has shallow axial ring furrows. Acheilops masonensis has a narrower (tr.) pleural field, equal to 66 % of the axis width, versus 86 % of axis width in A. olbermanni sp. nov. Finally, Winston & Nicholls (Reference Winston and Nicholls1967, p. 77) described the pygidial margin of A. masonensis as spinose, although the spines are difficult to see on their photograph. The pygidial margin is not fully preserved in any of the specimens available of A. olbermanni sp. nov., but taken together, they indicate that the margin was mostly entire and perhaps interrupted by minute spines on the first two segments (e.g. second segment on left side of Fig. 13n).
Acheilops montis Westrop (Reference Westrop1986b, pl. 39, figs 1–5) from the Mistaya Formation of Alberta is clearly different from A. olbermanni sp. nov. in having preocular fixigenae and, consequently, wider palpebral areas of the fixigenae. The Alberta species also lacks an occipital spine, as do the poorly known A. dilatus Ulrich in Bridge (1930, pl. 19, figs 21, 22) from the Eminence Dolostone of Missouri and A. norwalkensis Ulrich & Resser (1930, pl. 21, fig. 8) from the Jordan Sandstone of Wisconsin. In addition, A. dilatus apparently possesses well-incised palpebral furrows, whereas those of A. olbermanni sp. nov. are barely perceptible (Fig. 13e, j).
Acknowledgements
EL and BK gratefully acknowledge field and laboratory support from the New York State Museum. BK was supported by the DFG during his New York work. S. Bowser arranged access to scanning microscopy at the Wadsworth Center for Laboratories and Research, New York State Department of Health (under National Science Foundation grant 0116551). A. Andreas assisted with scanning microscopy. F. Mannolini picked the conodont residues. SRW was supported by the National Science Foundation through grant EAR 0308685. Reviewers C. E. Brett and N. C. Hughes contributed a number of helpful comments.
