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Introduction
Paleontological and paleobiological research is hampered by the incomplete fossil record, and traditionally relies on anatomical information gained from fossilized hard parts. The fossil record does, however, contain examples of exceptional preservation and occasionally, labile and decay-prone soft tissues are preserved with a high degree of morphological fidelity. Such rare finds have the unique potential to yield information crucial for our understanding of evolutionary patterns, modes of life and locomotion, degree of adaptation, and the biology of extinct animals. The most important sources of such information for reconstructing long-extinct biotas are fossil Lagerstätten.
The fossil record of complete body fossils of polychaetes is rather meager, yet is continuously improving thanks almost exclusively to exceptionally preserved Lagerstätte material. It includes a wide range of preservational styles, ranging from body fossils with soft-tissue preservation, to dwelling tubes, trace fossils, and jaws (e.g. Briggs and Kear, Reference Briggs and Kear1993; Sutton et al., Reference Sutton, Briggs, Siveter and Siveter2001; Bracchi and Alessandrello, Reference Bracchi and Alessandrello2005; Briggs and Bartels, Reference Briggs and Bartels2010, and references therein). Yet despite the facts that most polychaete tissue decays readily, and whole body fossils are rare, there is a substantial amount of data from the fossil archive of these animals. Some polychaetes, particularly those belonging to the order Eunicida, are equipped with hard and resistant jaws—scolecodonts—that have good preservation potential. These structures can be extracted from their enclosing rocks in great abundance and diversity (e.g., Kielan-Jaworowska, Reference Kielan-Jaworowska1966; Bergman, Reference Bergman1989; Hints and Eriksson, Reference Hints and Eriksson2007), and have provided most information on extinct polychaetes. This situation is analogous to that found in the study of the unrelated conodonts, where nearly all studies over the last 150 years have been based solely on conodonts’ phosphatic mouth parts.
Whereas the oldest fossil polychaetes known are from the early Cambrian Sirius Passet fauna (Conway Morris and Peel, Reference Conway Morris and Peel2008; Vinther et al., Reference Vinther, Eibye-Jacobsen and Harper2011), the oldest known scolecodonts are of latest Cambrian age (Williams et al., Reference Williams, Nowlan, Barnes and Batten1999; Hints and Nõlvak, Reference Hints and Nõlvak2006). However, it is not until the Middle Ordovician that scolecodonts become more common faunal elements in the geological record, and in late Darriwilian times (late Middle Ordovician), eunicidan jaw-bearing polychaetes rapidly diversified at the global scale (Hints and Eriksson, Reference Hints and Eriksson2007). The diverse Silurian faunas are well known, and many of the genera were geographically wide spread (Eriksson et al., Reference Eriksson, Jeppsson and Bergman2004, Reference Eriksson, Hints, Paxton and Tonarová2013).
Albeit not as abundant in Lagerstätten as the Cambrian rock record (e.g., Zhu et al., Reference Zhu, Babcock and Peng2006), the Silurian has a number of famous localities that have allowed insights into the ecosystems of that age. Several authors have noted the occurrence of both polychaete body fossils and scolecodonts in Silurian Lagerstätten (e.g., Mikulic et al., Reference Mikulic, Briggs and Kluessendorf1985; Kluessendorf, Reference Kluessendorf1994; Sutton et al., Reference Sutton, Briggs, Siveter and Siveter2001; von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007), but few studies have dealt with these fossils in detail. In this paper, we focus on the relatively newly discovered Eramosa Lagerstätte of Ontario, Canada, which is becoming known for its rich and diverse faunas, different preservational styles and a combination of soft-body preservation associated with shelly body and trace fossils (von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007). Polychaete body fossils and scolecodonts from this Lagerstätte (Tetreault Reference Tetreault2001; von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007) comprise a significant part of the biota. We describe the preserved scolecodonts and jaw apparatuses of Eramosa Lagerstätte polychaetes, and discuss their faunal composition, state of preservation, as well as their paleoecological and paleobiogeographic significance.
Geological setting and stratigraphy
The Eramosa Lagerstätte is a biological association of unusually well-preserved middle Silurian (Wenlock) fossil organisms in the Eramosa Formation of the Bruce Peninsula, Ontario, Canada (Fig. 1). This Lagerstätte preserves taxonomically and taphonomically diverse biotas, including articulated conodont apparatuses, tolypelepid and corvaspid heterostracan fish, annelids and arthropods with soft body parts, and a diverse marine flora (von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007). Although large eurypterids, and rare marine scorpions from the Eramosa Lagerstätte have been known for many years (Copeland and Bolton, Reference Copeland and Bolton1985, and Waddington and Jeram, Reference Waddington and Jeram1997, respectively), the smaller, exceptionally preserved faunal and floral elements of the Eramosa Lagerstätte were recognized only relatively recently (Tetreault, Reference Tetreault2001; von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007). The study of these smaller floral and faunal elements, e.g., thallophyte or dasyclade algae (S.T. LoDuca, personal communication, 2014), conodonts (von Bitter and Purnell, Reference von Bitter and Purnell2005; Jones et al., Reference Jones, Purnell and von Bitter2009), phyllocarid crustaceans (Collette and Rudkin, Reference Colette and Rudkin2010), and others, including this study, is in its infancy. The Eramosa Lagerstätte was regarded as atypical by von Bitter et al. (Reference von Bitter, Purnell, Tetreault and Stott2007) and Collette and Rudkin (Reference Colette and Rudkin2010) when they compared its biota with that of other Silurian Lagerstätten, such as those described by Mikulic et al. (Reference Mikulic, Briggs and Kluessendorf1985), Kluessendorf (Reference Kluessendorf1994), and Sutton et al. (Reference Sutton, Briggs, Siveter and Siveter2001), because of its mixture of non-biomineralized and shelly organisms and rich evidence of bioturbation in the form of abundant trace fossils of Chondrites, Planolites, and phyllocarid burrows. However, the Eramosa Lagerstätte has some faunal elements, including scolecodonts, in common with the slightly older Llandovery (Early Silurian) Waukesha Lagerstätte of Wisconsin (Mikulic et al., Reference Mikulic, Briggs and Kluessendorf1985), as well as with the Late Ordovician Airport Cove Lagerstätte of Manitoba, Canada (Young et al., Reference Young, Rudkin, Dobrzanski, Robson and Nowlan2007).
Map of parts of southern Ontario, Canada, with an inset highlighting Eramosa Lagerstätte localities in the southern Bruce Peninsula discussed in the main text. (A) Park Head locality containing Biota 1. (B) and (C) Hepworth localities containing Biota 2. (D) Wiarton locality containing Biota 3. Only localities (A) and (D) have yielded polychaete material studied herein. Figure modified from von Bitter et al. (Reference von Bitter, Purnell, Tetreault and Stott2007).
Strata on the Bruce Peninsula containing the Eramosa Lagerstätte, as well as directly associated under- and overlying units, are herein referred to the Eramosa Formation, following Brett et al. (Reference Brett, Tepper, Goodman, LoDuca and Eckert1995), and subsequent usage by von Bitter et al. (Reference von Bitter, Purnell, Tetreault and Stott2007) and Collette and Rudkin (Reference Colette and Rudkin2010). These dolostone beds, as the Eramosa Member, were previously included in the Amabel Formation by Bolton (Reference Bolton1957) and Liberty and Bolton (Reference Liberty and Bolton1971), in both the Amabel and Guelph formations by Armstrong (Reference Armstrong1988) and Armstrong and Meadows (Reference Armstrong and Meadows1988), and in the Guelph Formation by Armstrong and Goodman (Reference Armstrong and Goodman1990) and Armstrong (Reference Armstrong1993a, Reference Armstrongb). Identification of strata on the Bruce Peninsula as the Eramosa Member, or, subsequently, the Eramosa Formation, by the preceding and other authors, is based on the correlation of these beds with the “Eramosa beds” of Williams (Reference Williams1915), which are bituminous, eurypterid-bearing Silurian dolostones that, according to Winder (Reference Winder1961), have their type locality on the banks of the Eramosa River between Rockwood and Guelph, Ontario. Using conodonts, Stott et al. (Reference Stott, von Bitter, Kleffner, Tetreault and Armstrong2001) determined a Wenlock age for the Eramosa Formation, which was further refined by Bancroft (Reference Bancroft2008) using carbonate carbon-isotope stratigraphy and the zonally diagnostic conodonts Ozarkodina sagitta rhenana (Walliser, Reference Walliser1964), Kockelella walliseri (Helfrich, Reference Helfrich1975), and Ozarkodina sagitta (Walliser, Reference Walliser1964) to conclude that the Eramosa is middle Sheinwoodian to perhaps earliest Homerian in age.
The Eramosa Lagerstätte was first identified at Wiarton, Ontario, in an unpublished study by Tetreault (Reference Tetreault2001); subsequently, its concept was broadened biologically and extended geographically by von Bitter et al. (Reference von Bitter, Purnell, Tetreault and Stott2007), when they recognized exceptionally preserved fossils occur at several other localities of the Eramosa Formation in Ontario in an outcrop belt extending approximately 16 km from Wiarton in the north, to Park Head in the south (Fig. 1). The Eramosa Lagerstätte appears to be confined to the Interbedded Unit of Armstrong and Meadows (Reference Armstrong and Meadows1988), a 7–9 m-thick sequence of alternating dolostone, limestone, and bituminous shale constituting the uppermost, shaly part of the Eramosa Formation. Using conodont associations described by Viira (Reference Viira1982) and Aldridge and Jeppsson (Reference Aldridge and Jeppsson1984, Reference Aldridge and Jeppsson1999), von Bitter et al. (Reference von Bitter, Purnell, Tetreault and Stott2007, Reference von Bitter, Bancroft and Purnell2012) and von Bitter and Purnell (Reference von Bitter and Purnell2009) interpreted the three biotas constituting the Lagerstätte to have come from several laterally intergrading environments, possibly a nearshore-offshore continuum as described by Aldridge and Jeppsson (Reference Aldridge and Jeppsson1999). These hypothesized environments with their characteristic biotas range from open marine at Wiarton and Hepworth (Fig. 1), to more restricted, shallower marine, possibly lagoonal west of Hepworth, to brackish, nearshore marine, possibly lacustrine, at Park Head (Fig. 1; see also von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007, and discussion below).
Materials and methods
The studied scolecodont material was collected by the second author, and includes specimens preserved on bedding-plane surfaces, as well as specimens recovered by acid digestion of carbonate rocks in the former micropaleontological laboratories of the Royal Ontario Museum, Toronto. As noted, scolecodonts have only been recognized and recovered from two localities so far from the northern and southern ends of the presently defined Lagerstätte belt, where they occur not only associated with distinctly different biotas, but apparently also within distinct paleoenvironments.
The more northerly of the scolecodont localities is just north of Wiarton, Ontario (Fig. 1) in the Owen Sound Ledgerock Quarry in Lot 9, Concession 24, Amabel Township, Town of South Bruce, Bruce Co., Ontario, and is owned and operated by the family of the late Harold Stobbe. The sampled locality is situated north of Bruce Co. Rd 13, NW of Wiarton, at the northeastern side of a long, narrow quarry. It produced abundant, but somewhat crushed, scolecodonts from shaly weathering bedding plane surfaces (ROM Accession Nos. 08PB10 and 08PB11). These specimens were collected from near the apparent top of the fossiliferous Interbedded Unit of Armstrong and Meadows (Reference Armstrong and Meadows1988), and were closely associated with both abundant thallophyte or dasyclade algae and finely disseminated carbonaceous debris. Only material collected from one or more bedding plane surfaces was studied, i.e., no scolecodonts were observed or recovered from samples from this locality broken down with acids.
The more southerly of the two scolecodont-yielding localities is immediately north of Park Head, Ontario (Fig. 1) in Lot 1, Concession 6, Amabel Township, Town of South Bruce, Bruce Co., and belongs to the family of William Morton of Hepworth. This locality is situated approximately 740 m north of the intersection of Park Head Rd. and Grey County Road 10, between and around excavations for two sheds on the west side of Grey Co. Road 10. In total, 4353 grams of carbonate from three combined samples, ROM Acc. No. 02PB9A, B, and C, from Park Head were dissolved in 10% buffered acetic acid. Residue was separated using a tetrabromoethane solution with a specific gravity of 2.875 into a heavy fraction, containing abundant phosphatic fish remains, and a light fraction, yielding scolecodonts and semi-articulated jaw apparatuses in great abundance. The latter were electrostatically handpicked and sorted into microfossil slides with a white-colored background. Representative specimens were selected and mounted on SEM stubs with double adhesive carbon tape and coated with gold before being studied and photographed with a Hitachi S-3400N scanning electron microscope at the Department of Geology in Lund, Sweden. Wiarton bedding-plane specimens were photographed there under a binocular light microscope fitted with a digital camera.
All figured specimens are housed in the type invertebrate fossil collections of the Palaeobiology Section, Department of Natural History, Royal Ontario Museum, Ontario, Toronto, Canada (repository abbreviation: ROM), as are additional unfigured scolecodonts from Wiarton and Park Head.
The Eramosa Lagerstätte polychaetes
Polychaete annelid fossils, including eunicidan polychaete jaws, were reported from the Eramosa Lagerstätte of Wiarton by Tetreault (Reference Tetreault2001), and from Wiarton and Park Head by von Bitter et al. (Reference von Bitter, Purnell, Tetreault and Stott2007). They also illustrated full annelid body fossils of Biota 3 that occur in the lower part of the Interbedded Unit of the Eramosa Formation at Wiarton. These fossils comprise spectacularly well-preserved specimens, including a possible annelid of Myoscolex-like form (von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007, figs. 2H, I) and aphroditid-like polychaetes with phosphatized muscle tissue (von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007, fig. 2G). None of these body fossils contains visible jaws, although some are expected to have had such, and if present, they are probably concealed in retracted position within the anterior one-third or so of the specimens. Extant scale-worms, Aphroditiformia, commonly referred to as “sea mice,” have an eversible pharynx equipped with jaws, although they may be lacking or poorly developed in some species (Day, Reference Day1967; Norlinder et al., Reference Norlinder, Nygren, Wiklund and Pleijel2012). Aphroditiformia belongs to the order Phyllodocida, which have other types of jaws in terms of composition and structural arrangement than those of the Eunicida. Phyllodocidan jaws also seem to have lesser preservation potential, and from what is now known, their fossil record is significantly shorter, possibly beginning in the Permian (e.g., Voss-Foucart et al., Reference Voss-Foucart, Fonze-Vignaux and Jeuniaux1973; Olive, Reference Olive1980; Nakrem et al., Reference Nakrem, Szaniawski and Mørk2001; Dutta et al., Reference Dutta, Hartkopf-Fröder, Mann, Wilkes, Brocke and Bertram2010), as compared to the first record of eunicidan scolecodonts in the latest Cambrian (Hints and Eriksson, Reference Hints and Eriksson2007). No isolated phyllodocidan scolecodonts were found in the studied Eramosa material.
The scolecodont material is generally well preserved and the relative number of full to partially articulated jaw apparatuses in the Eramosa Lagerstätte is estimated to represent approximately 6% of all specimens in the acid-digested collection from Park Head. Whereas well-preserved scolecodonts from elsewhere generally have smooth and glossy outer surfaces, individual elements from both Wiarton and Park Head have a relatively rough, corroded, and/or partly dissolved surface structure, likely due to dolomitization of the host rock. Ventral mandibles generally seem to be slightly better preserved than the dorsal maxillae (for a general guide to the composition and different jaw elements and architectural types of fossil eunicidan jaw apparatuses, see Kielan-Jaworowska, Reference Kielan-Jaworowska1966; Paxton, Reference Paxton2009).
The entire collection recovered from Park Head consists of several hundred specimens belonging to one new species, Hindenites parkheadensis n. sp. In addition, 26 bedding plane specimens, including isolated elements as well as partly articulated apparatuses, from Wiarton, are discussed. Scolecodonts preserved on bedding-plane surfaces are often difficult to unambiguously determine taxonomically because parts of the specimens are concealed. That, combined with the sometimes incomplete preservation and the relatively small collection studied from the Eramosa Lagerstätte, leads us to leave many forms in open nomenclature, particularly at the species level, pending additional material.
Among bedding-plane specimens, seven to nine different species can be identified. Two higher taxa dominate identifiable specimens: Polychaetaspidae and Paulinitidae, and both are very common polychaete families in Silurian strata from several regions of the world (e.g., Kielan-Jaworowska, Reference Kielan-Jaworowska1966; Bergman, Reference Bergman1989; Eriksson et al., Reference Eriksson, Jeppsson and Bergman2004, Reference Eriksson, Hints, Paxton and Tonarová2013; Hints and Eriksson, Reference Hints and Eriksson2007; Tonarová et al., Reference Tonarová, Eriksson and Hints2012).
Polychaetaspids
One common form (Fig. 2.1–2.6) belongs to Oenonites Hinde, Reference Hinde1879, a widespread genus in Middle Ordovician through Silurian strata (Eriksson et al., Reference Eriksson, Jeppsson and Bergman2004; Hints and Eriksson, Reference Hints and Eriksson2007). All Wiarton specimens recorded belong to one (or possibly two) species here assigned to Oenonites cf. O. hibbardi (Eller, Reference Eller1940). Originally described by Eller (Reference Eller1940) as Lumbriconereites hibbardi from the Silurian Albion Beds of the Niagara Gorge, New York (now the Power Glen Formation of the Medina Group, according to Bergman, Reference Bergman1991), Eller (Reference Eller1944, p. 745) subsequently reported that species also from a locality near Oxenden, approximately four miles northeast of Wiarton, Ontario. The O. cf. O. hibbardi morphotype is common in Silurian strata, and there are a number of species showing the same general morphology of the first maxillae (MI); sturdy elements with a prominent, club-like and rounded ramus at approximately mid-length. Jaws of the Wiarton species are morphologically similar to, for example, the corresponding jaws of O. eichwaldi Eriksson, Reference Eriksson1997 from the Silurian of Gotland, Sweden.
Light microscope photographs of scolecodonts and jaw apparatuses preserved on bedding plane surfaces. All specimens are from the Eramosa Lagerstätte, Eramosa Formation, immediately north of Wiarton, Ontario. Scale bar 0.5 mm. (1–6) Oenonites cf. O. hibbardi; (1) right MI in ventral view, ROM 59864a; (2) right MI in lateral view, ROM 59866a; (3) right MI in dorsal view, ROM 59865a; (4) Left MI in dorsal view, ROM 59864b; (5) Right MI in dorsal view, ROM 59864c; (6) right MI in dorsal view, associated with a cluster of possible paulinitid affinity, ROM 59865b. (7) A jaw apparatus in dorsal view belonging to the Polychaetaspidae or possibly Kalloprionidae, ROM 59866b. (8–15) Kettnerites cf. K. invisibilis; (8) left MI in ventral view, ROM 59864d; (9) right MI in ventral view, ROM 59864e; (10) right MI in dorsal view, ROM 59864f; (11) left MI in dorsal view, ROM 59863a; (12) left MI in dorsal view, ROM 59866c; (13) left MI in dorsal view, ROM 59858a; (14) left MI in dorsal view, ROM 59862a; (15) left MII in ventral view, ROM 59865c. (16–19) paulinitid (likely Kettnerites) MII; (16) right MII in ventral view, ROM 59860a; (17) right MII in dorsal view, ROM 59864g; (18) dasycladean or thallophytic algae with two entangled scolecodonts, ROM 59864h; (19) close-up of the scolecodont specimens in (18), two fused MII of probable paulinitid affinity. (20) Articulated paulinitid jaw apparatus with seemingly all elements preserved in situ except for the posterior-most carriers, which are absent, ROM 59862b. The paired mandibles are preserved on top of the maxillary apparatus that is seen in ventral view.
It remains uncertain if the specimens shown in Fig. 2.6 all belong to the same species; the poorly preserved cluster to the left in Fig. 2.6 may actually belong to a different taxon, possibly a paulinitid, than does the O. cf. O. hibbardi MI to the right. Moreover, the indistinct jaw apparatus shown in Fig. 2.7 could be a polychaetaspid or possibly a kalloprionid.
Paulinitids
Paulinitids are also common taxa, and based on the collection studied, seem to be represented by two species from Wiarton, and one, Hindenites parkheadensis n. sp., from Park Head. Although the collection at hand is limited, their abundance in the Park Head residues, together with the bedding plane specimens from Wiarton, indicates that the Paulinitidae was the most common polychaete family in the Eramosa environments. This is not surprising considering their abundance and diversity in Silurian strata globally (Bergman, Reference Bergman1989; Eriksson et al., Reference Eriksson, Jeppsson and Bergman2004, Reference Eriksson, Hints, Paxton and Tonarová2013; Tonarová et al., Reference Tonarová, Eriksson and Hints2012).
Of the Wiarton paulinitid bedding-plane specimens recorded, most are assigned to Kettnerites Žebera, Reference Žebera1935, the most common and species-rich genus known (Bergman, Reference Bergman1989; Eriksson et al., Reference Eriksson, Jeppsson and Bergman2004; Hints and Eriksson, Reference Hints and Eriksson2007). Although the specimens are only moderately well preserved, their external morphology fit well with those of Kettnerites. The first maxillae (MI) of these differ, for example, from those of Hindenites by having a smaller myocoele opening and lacking a pronounced differentiation between the hook and the remaining part of the jaw (see also below). Specimens shown in Fig. 2.8–2.15 are closely similar to, and may be conspecific with, Kettnerites invisibilis (Eller, Reference Eller1940) (see Bergman, Reference Bergman1991), to which they are tentatively assigned herein. Bergman (Reference Bergman1991) eliminated several redundant synonyms, and thoroughly revised and re-described K. invisibilis using modern multi-element taxonomy. Eller (Reference Eller1944, p. 745) also reported Nereidavus invisibilis (=K. invisibilis), similar to O. hibbardi (Eller) discussed above, from the Oxenden locality near Wiarton.
In addition to the specimens assigned to K. cf. K. invisibilis, there are a couple of smaller and less well-preserved second maxillae (MII) of paulinitid, and probable Kettnerites, affinity (Fig. 2.16–2.19) in the collection. Of these, two fused and elongated MII were found entangled in remains of thallophyte or dasycladalean algae (Fig. 2.18–2.19). Such algae are particularly common as articulated and disarticulated remains in Biota 3 (von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007), and to a lesser degree in Biota 2, of the Eramosa Lagerstätte.
A beautifully preserved, almost fully articulated jaw apparatus from Wiarton (complete, except for the posterior-most carriers and possibly some of the minor anterior-most maxillae) that includes the mandibles, is illustrated in Fig. 2.20. The specimen is preserved in lateral view, showing the myocoele openings of the maxillae and the ventral mandibles resting on top of the maxillae. The slender appearance of the jaws and the restricted myocoele openings of the first maxillae of this specimen are similar to those of Langeites Kielan-Jaworowska, Reference Kielan-Jaworowska1966, or Lanceolatites Bergman, Reference Bergman1987 (see, e.g., Kielan-Jaworowska, Reference Kielan-Jaworowska1966; Bergman, Reference Bergman1989).
Eunicidan scolecodont clusters, or jaw apparatuses, in which the mandibles are associated with the dorsal maxillary apparatus, are evidence of deceased individuals. The massive mandibles have growth lines and grew continuously throughout life of the worm (Olive, Reference Olive1980; Paxton, Reference Paxton2000). By contrast, the maxillae of most eunicidan polychaetes were shed, or molted, once or multiple times during ontogeny (Mierzejewski, Reference Mierzejewski1978; Bergman, Reference Bergman1989; Paxton Reference Paxton2005, Reference Paxton2006; Paxton and Safarik, Reference Paxton and Safarik2008; Paxton and Eriksson, Reference Paxton and Eriksson2012). Paxton (Reference Paxton2006) even showed that the maxillary apparatus of the extant onuphid Diopatra aciculata could be shed in its entirety. Thus, several of the isolated maxillae and the maxillary apparatuses recovered from the fossil record probably are the result of such shedding. This would also explain why complete jaw apparatuses that include both maxillae and mandibles, are rare in the fossil record (see additional examples in Kielan-Jaworowska, Reference Kielan-Jaworowska1966).
Other taxa
In addition to the polychaetaspid and paulinitid specimens recorded, several specimens, including semi-articulated apparatuses, are of unknown affinity. Most of these are mandibles, sometimes associated with poorly preserved maxillae. Although isolated mandibles are well known from the fossil record (e.g., Eisenack, Reference Eisenack1939; Lange, Reference Lange1949), they are typically difficult to reconstruct into multi-element apparatuses together with the maxillae, unless monospecific (or low diversity) scolecodont assemblages are found. This is because mandibles share very few, if any, morphological characteristics with the maxillae; also, as noted, few apparatuses are known in which all jaws are still preserved in situ that can be used for comparisons. Thus, apparatuses such as those from the Eramosa Lagerstätte are potentially very important for future taxonomic studies.
The bedding-plane specimen from Wiarton shown in Fig. 3.1 consists of two relatively well-preserved, fused mandibles. Underneath and slightly below these are putatively corresponding two first maxillae that are heavily dissolved. The mandibles grossly resemble those that Eisenack referred to as Palaeosigma (see Eisenack, Reference Eisenack1939, fig. C.17). The apparatus might belong to a polychaetaspid (Oenonites-type) polychaete, or possibly a placognath taxon, as the outlines of the maxillae are reminiscent also of those of Pistoprion. Another enigmatic specimen (Fig. 3.2) shows well-preserved mandibles, albeit partly concealed by the host rock. These mandibles, of unknown affinity, are associated with a sub-circular feature of dark organic matter that might belong to the soft part anatomy of the worm. Yet another relatively complete cluster includes the pair of mandibles of characteristic shape; long, slender and gently curved shafts that anteriorly terminate in paddle-like blades (Fig. 3.3). Below and slightly to the left of these are parts of the maxillary apparatus with the maxillae seen in ventral view and the elements in the posterior-most part might be the posteriormost carriers. In addition, a few mandibles both preserved as pairs (Fig. 3.4–3.5) and singles (Fig. 3.6), also of unknown affinity, were identified.
Light microscope photographs of scolecodonts and jaw apparatuses preserved on bedding plane surfaces. All specimens are from the Eramosa Lagerstätte, Eramosa Formation, north of Wiarton, Ontario. Scale bars (1–3) are 1 mm, (4–7) are 0.5 mm. (1) cluster with two fused mandibles similar to Palaeosigma of Eisenack on top of two first maxillae in ventral view, ROM 59858b; (2) mandibles and a sub-circular feature of unknown affinity, ROM 59859a; (3) one pair of mandibles preserved on top of a partial maxillary apparatus in ventral view, ROM 59862c; (4) one pair of partly broken mandibles, ROM 59864i; (5) one pair of partly broken mandibles, ROM 59864j; (6) a single mandible, ROM 59864k; (7) a comb-like element, similar to Lunoprionella, ROM 59864l.
Finally, one comb-like, curved jaw element with lamellae-like denticles was also found on a bedding plane surface (Fig. 3.7). The specimen is partly concealed by host rock matrix, but its general appearance resembles those of the elements of Lunoprionella Eisenack, Reference Eisenack1975 and related forms. This genus (or group of genera) has a wide distribution and is known from Ordovician and Silurian strata in several regions of the world. However, as noted by other authors (Hints, Reference Hints1998; Eriksson et al., Reference Eriksson, Jeppsson and Bergman2004; Tonarová et al., Reference Tonarová, Eriksson and Hints2012), their higher-level affinity and apparatus architecture remains currently unknown.
Systematic paleontology
Descriptive terminology is based on that of Kielan-Jaworowska (Reference Kielan-Jaworowska1966), Jansonius and Craig (Reference Jansonius and Craig1971), and Bergman (Reference Bergman1989). Type and figured specimens are housed in the Type Collection of Fossil Invertebrates, Palaeobiology Section, Department of Natural History, Royal Ontario Museum (ROM), Toronto.
Family Paulinitidae Lange, Reference Lange1947
Remarks
For discussion of the family, see Lange (Reference Lange1949), Kielan-Jaworowska (Reference Kielan-Jaworowska1966), Bergman (Reference Bergman1989), and Eriksson et al. (Reference Eriksson, Grahn, Bosetti and Vega2011, Reference Eriksson, Hints and Bergman2012a).
Genus Hindenites Bergman, Reference Bergman1987
Type species
Hindenites angustus (Hinde, Reference Hinde1879).
Remarks
Hindenites parkheadensis n. sp., recovered abundantly from the Eramosa Lagerstätte in the Eramosa Formation at Park Head, Ontario, fits best into the genus concept of Hindenites Bergman, Reference Bergman1987 (see Bergman, Reference Bergman1989), particularly with regards to the anteriorly tapering first maxillae (MI) that ends in a prominent fang (or hook) and the relatively big myocoele openings of those same jaws. Bergman (Reference Bergman1989, p. 45) noted that the large myocoele opening of the MI is characteristic of Hindenites and distinguishes the genus from other genera. However, the second maxillae (MII) and some aspects of the MI also make them similar to Gotlandites Bergman, Reference Bergman1987. Gotlandites and Hindenites are closely related and differ from other paulinitid genera (such as Kettnerites) particularly in having MI with a wide posterior part and more pronounced distinction between the fang and the rest of the jaw (Bergman, Reference Bergman1989, p. 39). Although some aspects of the Park Head specimens are intermediate between Gotlandites and Hindenites, the material fits best with the latter genus concept. This is the first report of Hindenites from outside Baltica.
Hindenites parkheadensis new species
Holotype
ROM 62853 (Figure 4.2), a semi-complete jaw apparatus. ROM 62852 and ROM 62854– 62885) are paratypes.
SEM micrographs of jaw apparatuses and fused elements of Hindenites parkheadensis n. sp., from the Eramosa Lagerstätte, Eramosa Formation, immediately north of Park Head, Ontario. Figure 4.2 is of holotype ROM 62853; remaining figures are of paratypes. Scale bars 200 μm. (1) a virtually complete dorsal maxillary apparatus, except for the carriers, ventral view, ROM 62852; (2) fused left MI, MII and MIII, dorsal view, ROM 62853, Holotype; (3) jaw apparatus with left and right MI, MII and right MIV, dorsal view, ROM 62854; (4) left MI and MII, ventral view, ROM 62855; (5) left MI and MII, dorsal view, ROM 62856; (6) right MII and MIV, dorsal view, ROM 62857; (7) left MII and single left MIII, dorsal view, ROM 62858; (8) close-up of the same specimen, ROM 62858.
Diagnosis
Paulinitid species with MI with a long and well pronounced tapering fang, bulky sub-rectangular posterior portion of jaw, paucidentate anterior dentary and slightly enclosed myocoele opening; right MI with distinct sub-rectangular fused basal plate; left MI with short, sub-rectangular inner wing. MII with long shank and tapering long ramus; most commonly single cusp in right MII preceeded by one to three precuspidal denticles and in left MII prominent cusp followed by one to four small intermediate denticles. Single left MIII concavo-convex, sickle-shaped with moderately sized denticles. MIV deep, concavo-convex elements with long denticles of which second and fourth are smaller than surrounding ones, small ramus. Carriers dorso-ventrally flattened, wide head abruptly truncated anteriorly.
Occurrence
Eramosa Lagerstätte, Eramosa Formation, Wenlock, town of South Bruce, Bruce Peninsula, Ontario; presently only known from immediately north of Park Head, Ontario.
Description
Right MI, dorsal view: length 0.55–1.67 mm, width 0.33–0.40 jaw length; maximum width in posterior portion of jaw (excluding fang which may protrude further laterally). Smaller specimens tend to have greater width/length ratio than larger ones. Sub-triangular jaw, inner and outer margins sub-parallel in posterior part, but jaw tapers anteriorly and terminates with very well-developed, more or less curved and dorso-laterally directed fang (or hook) sub-oval in cross-section and equipped with distinct cutting edges. Posterior one-third of jaw characteristically bulky compared to strongly tapering anterior portion. Inner margin straight to slightly curved and denticulated from posteriormost part of falcal arch to undenticulated ridge. Dentary, which consists of 12–15 (possibly up to 18) denticles spread along inner margin, occupies approximately 0.5–0.6 of total jaw length. Anteriormost 5–6 denticles of moderate to relatively large size and paucidentate; middle one or two largest. Posterior denticles tightly packed, generally minute and gently and evenly decrease in size posteriorly, ending at undenticulated ridge, the transition to which may be more or less distinct. Dentary considerably less differentiated in smaller specimens with smaller and more evenly sized denticles. Undenticulated ridge occupies approximately one-third of jaw length and is prominent and somewhat elevated reaching almost to posterior margin. Distinct inner wing, forming approximately one-fourth of jaw length, almost rhomboidal and sometimes with a curved outline and tapering posteriorly; inner margin parallel to sub-parallel to undenticulated ridge. Distinct, narrow basal furrow widening slightly anteriorly. Basal furrow partly covered by skew, sub-rectangular, fused basal plate. Basal plate undenticulated and confluent with MI on its anterior and inner margin. Posterior termination of jaw cut forming sigmoidally concave and posterior margin that measures approximately one-fourth of jaw length. Anterior to basal plate, in basal portion, ventral ligament rim forms narrow outer wing. At distance approximately three times the length of basal plate the outer wing drops, forming curved anterior termination seen in dorsal view. Basal angle 35–40°.
Ventral view: Slightly enclosed myocoele has a characteristic opening extending 0.4–0.45 jaw length. Smoothly rounded anteriorly with prominent ligament rim that narrows posteriorly.
Left MI, dorsal view: Jaw similar to mirror image of right MI with some small features differing. Length 0.63–1.63 mm, width 0.30–0.35 jaw length; maximum width in posterior portion of jaw (excluding fang which may protrude further laterally). Sub-triangular jaw, inner and outer margins sub-parallel in posterior part, but jaw tapers anteriorly and terminates with very well-developed, more or less curved and dorso-laterally directed, fang that is sub-oval in cross-section and equipped with cutting edges. Posterior one-third of jaw characteristically bulky compared to strongly tapering anterior portion. Inner margin straight to slightly curved and denticulated from posteriormost part of falcal arch to undenticulated ridge. Dentary consists of 14–23 denticles spread along inner margin. Anteriormost 5–7 denticles of moderate to relatively large size and paucidentate; middle two or three are largest. Posterior remaining denticles tightly packed and gently and evenly decrease in size posteriorly, ending at undenticulated ridge, transition to which may be indistinct. In some specimens dentary terminates above anterior end of inner wing whereas in other specimens it continues slightly further posteriorly. Dentary considerably less differentiated in smaller specimens with smaller and more evenly sized denticles. Undenticulated ridge occupies approximately one-third of jaw length (although it may be difficult to measure) and is prominent and somewhat elevated and almost reaches posterior margin. Distinct inner wing, forming slightly less than one-fourth of jaw length, sub-rectangular and sometimes with faintly curved outline and tapering posteriorly; inner margin parallel to sub-parallel to undenticulated ridge. Distinct, narrow basal furrow widening slightly anteriorly. Posterior termination of jaw transversely cut forming nearly straight posterior margin that measures approximately one-fourth of jaw length. Sub-rectangular to vaguely triangular, i.e., commonly tapering slightly posteriorly, inner wing extending 0.19–0.22 jaw length. Basal angle 20–25°. Thin and distinct ligament rim visible in dorsal view extending anteriorly on outer margin for 0.31–0.39 of jaw length, where outer wing bends smoothly to rather sharply and connects with outer margin. Anteriorly outer margin is smooth and continues to form part of cutting edge of fang.
Ventral view: Myocoele opening extends 0.33–0.39 jaw length. Opening crescent-shaped anteriorly and ligament rim thickened on anterior and inner part.
Right MII, dorsal view: Length 0.57–1.25 mm, width 0.54–0.62 jaw length. Sickle-shaped, rather slender single cusp of medium size equipped with cutting edges, forms anterior rounded termination of jaw. Dentary arranged along inner margin in gently and convexly curved fashion, altogether holding 11–13 sub-triangular and posteriorly slanted denticles. Inner face commonly concealed by dentary in dorsal view. Anterior part of dentary variable with either no clearly distinguishable cusp (Fig. 5.11) or, most commonly, a third (sometimes fourth) largest denticle representing cusp (Fig. 5.10, 5.12, 5.13). Cusp commonly followed by smaller, intermediate denticle. Thereafter, denticles increase very gently in size posteriorly and from approximately mid-length start decreasing and becoming more tightly packed. Shank with almost parallel sides large and occupies 0.65–0.83 jaw length; posterior extremity narrows slightly and terminates sub-angularly and usually bent slightly outwards. Undenticulated ridge rather insignificant and forming 0.08–0.15 jaw length. Bight is deep, anteriorly concave and with variable bight angle of c. 40–60°. Ramus long, narrow with posteriorly oriented, pointed extremity. Anterior outer margin has slight sinus or is straight.
SEM micrographs of scolecodonts of Hindenites parkheadensis n. sp., from the Eramosa Lagerstätte, Eramosa Formation, immediately N. of Park Head, Ontario. Scale bars 100 μm. All illustrated specimens are paratypes; all in dorsal view except (1) in lateral view and (4, 13, 17, 18, 22) in ventral view. (1) right MIV, ROM 62859; (2) single left MIII, ROM 62860; (3) single left MIII, ROM 62861; (4) left MII, ROM 62862; (5) left MII, ROM 62863; (6) left MII, ROM 62864; (7) left MII, ROM 62865; (8) left MII, ROM 62866; (9) left MII, ROM 62867; (10) right MII, ROM 62868; (11) right MII, ROM 62869; (12) right MII, ROM 62870; (13) right MII, ROM 62871; (14) right MII, ROM 62872; (15) carrier, ROM 62873; (16) left MI, ROM 62874; (17) left MI, ROM 62875; (18) left MI, ROM 62876; (19) left MI, ROM 62877; (20) left MI, ROM 62878; (21) left MI, ROM 62879; (22) left MI, ROM 62880; (23) left MI, ROM 62881; (24) right MI, ROM 62882; (25) right MI, ROM 62883; (26) right MI, ROM 62884; (27) right MI, ROM 62885.
Ventral view: Slightly enclosed myocoele has an opening extending for 0.78–0.82 jaw length. Delimited by a smooth and narrow ligament rim along curved anterior and outer margin.
Left MII, dorsal view: Length 0.42–1.26 mm, width 0.46–0.52 jaw length. Dentary arranged along inner margin in straight to gently and convexly curved fashion, altogether holding 11–16 sub-triangular and posteriorly slanted denticles (including cusp). Large sickle-shaped, single cusp equipped with distinct cutting edges, followed by one to four (commonly three) intermediate and commonly very small denticles. Following these are normal-sized denticles that gently decrease in size and become more tightly packed posteriorly. Shank, with sub-parallel sides and tapering slightly posteriorly, is large and occupies 0.58–0.68 jaw length; posterior extremity narrows slightly and terminates sub-angularly and usually bent slightly outwards. Bight deep, anteriorly concave; variable bight angle 30–55°. Ramus is variably long, narrow with posteriorly oriented, pointed extremity. Undenticulated ridge rather insignificant and forming 0.06–0.10 jaw length. Anterior outer margin straight to sigmoidal. Narrow inner wing sometimes developed and seen in dorsal view, extending approximately 0.5 jaw length.
Ventral view: Slightly enclosed myocoele has an opening extending for 0.67–0.77 jaw length. Delimited by smooth and narrow ligament rim along the curved anterior, extending out along ramus and outer margin.
Left MIII, dorsal view: Unpaired, single left element. Length 0.53–0.68 mm, width approximately 0.5 or slightly more of jaw length (measured from tip of ramus to inner margin, perpendicular to length axis). Arcuate elongate element with concave inner margin and convex outer margin. Inner margin sometimes equipped with very thin-walled flange that may extend considerably inwards, and usually more or less fragmented (this also affects width measurement). Transverse cross-section high particularly in anterior part of jaw and becomes lower posteriorly. Short and slightly dorso-ventrally flattened ramus; ramal extremity sub-triangular in outline and points inwards. Curved dentary consisting of 8–13 sub-triangular denticles slightly bent towards inner margin and posterior. Denticles gently and evenly decrease in size posteriorly, except for second denticle, which is approximately half-length of surrounding ones. Shank, with sub-parallel sides and tapering slightly posteriorly, is large; posterior extremity narrows slightly and terminates sub-angularly and usually bent slightly outwards, similar to that of left MII.
Ventral view: Myocoele opening gaping and surrounded by thin ligament rim.
Right MIV, dorsal view: Length 0.28–0.65 mm, width approximately one-third of jaw length (latter ratio difficult to measure). Arcuate and elongate element with concave inner margin and convex outer margin. Similar to MIII but even more pronounced in this element, inner margin sometimes equipped with thin flange extending inwards, and which is usually more or less fragmented (this also affects width measurement). Transverse cross-section high and triangular anteriorly and becomes lower posteriorly. Jaw sub-triangular in lateral view tapering significantly towards posterior. Dentary composed of 7–10 long denticles weakly slanting posteriorly and inwards. Denticulated ridge straight or slightly curved. Except for second denticle, which may be smaller than surrounding ones, denticles decrease evenly and gently in size posteriorly with largest denticle being anteriormost one. In some specimens it appears as if anteriormost denticle is loosely attached to the remainder of the jaw and a faint denticle boundary can be seen running down outer and inner face.
Ventral view: Gaping myocoele opening.
Left MIV, dorsal view: Nearly mirror reflection of right MIV.
Carriers: Sub-triangular, dorso-ventrally flattened and chisel-like element tapering posteriorly, 1.8–2.7 times as long as maximum width which is at anteriormost part of head. Anterior margin of head straight to gently convex and abruptly truncated. Head tapers posteriorly and extends into shaft, which in turn tapers gently posteriorly. In posterior half of element width of shaft is one-fifth or less of maximum element length and inner and outer margins run sub-parallel to each other. Inner margin straight. Posteriormost part slightly narrower than remainder of shaft and element terminates with bluntly sub-rounded posterior margin.
Etymology
Named after Park Head, Ontario, Canada.
Other material
Several hundred isolated maxillae and some dozen more or less articulated jaw apparatuses, all from same sample from immediately north of Park Head as the holotype and paratypes.
Remarks
The abundance of specimens of Hindenites parkheadensis n. sp. in the Park Head sample, considered to represent a single population, provides useful constraints on individual element morphologies and the intraspecific variability, including ontogenetic changes. It should be noted, however, that all elements recorded most likely represent variably sized adult or sub-adult forms and no larvae (cf. Paxton and Eriksson, Reference Paxton and Eriksson2012). Although ventral mandibles were not recorded for H. parkheadensis n. sp., the wealth of material allowed confident reconstruction of the entire maxillary apparatus. Based on the semi-articulated apparatuses recovered, the relative size of the different maxillae can also be assessed.
The intraspecific variability of H. parkheadensis n. sp. is expressed in a number of features including number of denticles and the architecture of the dentary that may be more or less paucidentate, particularly in the MI and MII. Smaller MI tend to have fewer and smaller denticles than larger specimens do. Moreover, inner and outer margins of the MI vary from being nearly straight to gently curved, concavely or convexly.
The jaw elements of H. parkheadensis n. sp. have a diagnostic morphology and there are no identical species previously described. There are, however, a few taxa with a similar morphology. As already indicated above, H. parkheadensis n. sp. show jaw characteristics in common with some species belonging to both Gotlandites and Hindenites, particularly with regards to the MI morphology; size and shape of the myocoele opening and inner wing, and denticulation. For example, some specimens (MI) of H. parkheadensis n. sp. particularly resemble those of H. naerensis (see Bergman, Reference Bergman1989, fig. 25A, B), H. gladiatus (see Bergman, Reference Bergman1989, fig. 24A1–A2, D1–D2), but also to some extent Gotlandites slitensis (see Bergman, Reference Bergman1989, fig. 22A, C, E, F) from the Silurian of Gotland, Sweden. However, the complete jaw apparatus architecture and combined characteristics of the different jaw elements of these taxa differ. In terms of the denticulation formula of the right MII sensu Bergman (Reference Bergman1989), H. parkheadensis has a variable anterior dentary with either no distinguishable cusp or, most commonly, a third (sometimes fourth) biggest denticle representing the cusp. Thus, this may (in specimens lacking a distinguishable cusp) resemble the right MII of H. naerensis, which has no pronounced cusp, but is unlike the double cusp of H. angustus and the single cusp with one small precuspidal denticle of H. gladiatus and G. slitensis (see Bergman, Reference Bergman1989, table 1).
Discussion
The presumably soft and soupy bottom substrate that once formed the strata of the present-day Eramosa Lagerstätte accommodated a diverse fauna of vertebrate and invertebrate organisms, including polychaete annelid worms. The polychaete fossils occur in a wide range of preservational styles, from articulated and disarticulated eunicidan jaw apparatuses to full body fossils (von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007), revealing that different taphonomic processes were acting in the different environmental settings (cf. Briggs and Kear, Reference Briggs and Kear1993), even at the same locality (e.g., the articulated and disarticulated eunicidan jaw apparatuses and soft-tissue polychaete body fossils grouped by von Bitter et al. (Reference von Bitter, Purnell, Tetreault and Stott2007) in Biota 3 at Wiarton occur separately in the upper and lower parts of the Interbedded Unit, respectively). What is remarkable about the scolecodonts of the Eramosa Lagerstätte is not the preservation of individual specimens, which is moderate, but rather the relative abundance of semi-articulated jaw apparatuses (cf. Eriksson et al., Reference Eriksson, Jeppsson and Bergman2004) that also suggests limited post mortem transport.
In addition to the full body fossils of polychaetes illustrated by von Bitter et al. (Reference von Bitter, Purnell, Tetreault and Stott2007), we record an additional eight to ten polychaete species based on the scolecodont material from the Eramosa Lagerstätte presently available to us. Our relatively limited collection almost certainly does not reveal the full taxonomic diversity of eunicidan polychaetes, and it seems likely that additional taxa will be discovered when more study material becomes available. The relative abundance of scolecodonts observed on bedding plane surfaces at Wiarton, and recovered from the acid resistant residues from Park Head, suggests that polychaete worms were among the most abundantly occurring invertebrates at these localities. Although extant free-living eunicidan polychaetes have variable modes of life, most taxa are infaunal burrowers, epifaunal crawlers or tube builders (e.g., Paxton, Reference Paxton2000). Thus it appears likely that at least some of the trace fossils preserved in the Eramosa Lagerstätte (von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007) were produced by burrowing worms. Study of these ichnofossils could consequently add to the understanding of the preservational styles represented, and further elucidate the significance of polychaetes in the Eramosa biotas.
The jawed polychaete fauna of the Eramosa Lagerstätte is of moderate diversity, with polychaetaspids (Oenonites), paulinitids (Kettnerites, Hindenites, and possibly Langeites or Lanceolatites), and a few enigmatic forms (e.g., Lunoprionella) identified. Thus, there are a number of well-known Silurian taxa, such as mochtyellids, kalloprionids, atraktoprionids, and symmetroprionids (see, e.g., Kielan-Jaworwoska, Reference Kielan-Jaworowska1966; Eriksson et al., Reference Eriksson, Jeppsson and Bergman2004; Hints et al., Reference Hints, Killing, Männik and Nestor2006), that have not been recorded from the Eramosa Lagerstätte, probably because of both limited sampling and environmental control on the distribution of polychaetes. Nonetheless, the genera identified are familiar from the Silurian of Baltoscandia and some also from Laurentia and other parts of the world (Eriksson et al., Reference Eriksson, Jeppsson and Bergman2004, Reference Eriksson, Hints, Paxton and Tonarová2013). To our knowledge, however, Hindenites has not previously been unambiguously identified in Silurian strata outside Baltoscandia. The presence of Kettnerites and Oenonites in the Eramosa Lagerstätte is a further testament to their status as two of the most common faunal elements in Silurian polychaete assemblages worldwide (Eriksson et al., Reference Eriksson, Hints, Paxton and Tonarová2013).
Among the eight North American Silurian Lagerstätten assessed by Kluessendorf (Reference Kluessendorf1994, fig. 3), the Brandon Bridge, Mississinewa, and Syracuse Lagerstätten were reported to include scolecodonts, whereas one additional (the Lecthaylus Lagerstätte) was also reported to contain “other worms” and papillate annelids (also recorded in the Mississinewa and Brandon Bridge Lagerstätten). Kluessendorf (Reference Kluessendorf1994) subdivided the eight Silurian Lagerstätten into two categories based on their biotic components and sedimentary features: eurypterid-phyllocarid-dominated (EP) biotas and dendroid-graptolite-characterized (DG) biotas. Although not precisely, the Eramosa Lagerstätte fits reasonably well into the EP biota type, containing eurypterids as well as phyllocarids. It is also clear that neither scolecodonts, nor other annelid remains, are exclusive to either one of these two types of Lagerstätten.
The apparent monospecific nature of the polychaete fauna from Park Head, with H. parkheadensis n. sp. as the sole representative, either reflects true colonization and that the environment simply was suitable to that species only, or it is a consequence of the less likely possibility of limited sampling. This locality contains Biota 1 sensu von Bitter et al. (Reference von Bitter, Purnell, Tetreault and Stott2007), a biota characterized by rare articulated skeletons and abundant isolated dermal elements of tolypelepid and corvaspid heterostracans, common erettopterid eurypterid molts, closed leperditiid and beyrichiid ostracode carapaces, and the remains of H. parkheadensis n. sp. Biota 1 is presently known only from Park Head, and is preserved in thin- to medium-bedded, shaly-weathering, gray microcrystalline limestone, and was recognized to contain a broad range of preservation styles, indicating a complex taphonomic history (cf. Briggs and Kear, Reference Briggs and Kear1993; von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007). Organisms of this biota are known to be tolerant of a broad range of salinities, and conversely, that the normal and expected indicators of fully marine conditions, such as brachiopods, corals, cephalopods, and conodonts, are absent (von Bitter et al., Reference von Bitter, Purnell, Tetreault and Stott2007). Thus, Biota 1 was interpreted to have lived in the shallowest, most restricted of the Eramosa environments, in a stable, hyposaline body of standing water, such as a lake or a brackish nearshore marine environment. Extant eunicidan polychaetes are known to range from intertidal zone to great depths (Paxton, Reference Paxton2000), and the presence of H. parkheadensis n. sp. at Park Head further supports the idea that it was a marine environment, albeit perhaps one with an anomalous salinity and/or oxygen level. The environment apparently was not suitable for supporting diverse polychaete faunas, suggesting that H. parkheadensis n. sp. may become a useful paleoenvironmental indicator.
Scolecodonts recovered from bedding planes at Wiarton, belonging to Biota 3U sensu von Bitter et al. (Reference von Bitter, Purnell, Tetreault and Stott2007), are with seven to nine taxa identified so far, a far more diverse polychaete fauna than is that recorded from Park Head. The abundant and diverse fossils at Wiarton, particularly the occurrence of the conodont Wurmiella excavata (Branson and Mehl, Reference Branson and Mehl1933), were used by von Bitter and Purnell (Reference von Bitter and Purnell2009) to interpret good water circulation in an open marine environment for that locality.
The paleonvironment at Hepworth in the central part of the Eramosa Lagerstätte belt was interpreted by von Bitter et al. (Reference von Bitter, Purnell, Tetreault and Stott2007) as having been an open marine environment because of the dominance of W. excavata. Immediately west of Hepworth, the relative dominance of the conodont Aldridgeodus Jeppsson, 2008 (in Calner et al., Reference Calner, Eriksson, Clarkson and Jeppsson2008) and Ctenognathodus Fay, Reference Fay1959, and the absence or near absence of W. excavata in identical lithologies as in Hepworth caused von Bitter and Purnell (Reference von Bitter and Purnell2009) and von Bitter et al. (Reference von Bitter, Bancroft and Purnell2012) to postulate a lagoonal environment for this locality. Scolecodonts or other annelid remains have not, so far, been found at or in the vicinity of Hepworth, i.e., in the central part of the Eramosa Lagerstätte belt, but the potential for finding them in light fractions from the heavy-liquid recovery of conodonts remains high. As a comparison to the Hepworth localities, Calner et al. (Reference Calner, Eriksson, Clarkson and Jeppsson2008) reported a slightly younger (lowermost Klinteberg Formation; early Ludlow) shale, denoted the A-shale, in the interior area of a carbonate platform of Gotland, Sweden. Not only does this shale yield the conodont Aldridgeodus but it also may be the same species, A. minimus Jeppsson, 2008 (in Calner et al., Reference Calner, Eriksson, Clarkson and Jeppsson2008), that is commonly present in the Eramosa Lagerstätte at Hepworth, and west of Hepworth. Although the Hepworth localities have yet to be carefully investigated for polychaetes, it is noteworthy that the A-shale of Gotland yielded a very high diversity and abundance of scolecodonts, including Hindenites spp.
Another analogue to the Eramosa Lagerstätte is the now-covered Lagerstätte bed, unit d of the Sheinwoodian (lower Wenlock) Högklint Formation on Gotland, Sweden, commonly referred to as the “Pterygotus Beds.” It contains, among other fossils, eurypterids, phyllocarids, a unique scorpion, Palaeophonus nuncius, and scolecodonts in exceptional abundance and diversity (see Jaanusson et al., Reference Jaanusson, Laufeld and Skoglund1979, and references therein). These argillaceous strata contain the equivalent of the EP Biota sensu Kluessendorf (Reference Kluessendorf1994) and have several fossil groups, including scorpions (Waddington and Jeram, Reference Waddington and Jeram1997), in common with the Eramosa Lagerstätte; they were interpreted as having been deposited in very shallow, warm, and tranquil sheltered waters by Jaanusson (Reference Jaanusson1979). Two species of Hindenites are very common in this interval (Bergman, Reference Bergman1979, Reference Bergman1989), that author noting (1989, p. 11–12) that this is the only locality on Gotland where Hindenites species are fairly common.
The record of Lagerstätten with polychaete remains (e.g., Thompson and Johnson, Reference Thompson and Johnson1977; Conway Morris, Reference Conway Morris1979; Schram, Reference Schram1979; Thompson, Reference Thompson1979; Mikulic et al., Reference Mikulic, Briggs and Kluessendorf1985; Briggs and Kear, Reference Briggs and Kear1993; Sutton et al., Reference Sutton, Briggs, Siveter and Siveter2001; Bracci and Alessandrello, Reference Bracchi and Alessandrello2005; Conway Morris and Peel, Reference Conway Morris and Peel2008; Whittle et al., Reference Whittle, Gabbott, Aldridge and Theron2008; Briggs and Bartels, Reference Briggs and Bartels2010) offers crucial information with regards to the fossil history of these animals. Primarily because much of the systematics of extant polychaetes is based on soft-body characters (e.g., Fauchald and Rouse, Reference Fauchald and Rouse1997; Rouse and Fauchald, Reference Rouse and Fauchald1997), Lagerstätte material provides an increased opportunity to integrate the systematics of fossil and extant taxa and to better understand polychaete phylogeny. Such material also offers vital information with regards to the anatomy, mode of life, paleoecology, and taphonomy of the extinct representatives. Moreover, from the Eramosa Lagerstätte alone, which comprises full body polychaete fossils and jaw apparatuses and scolecodonts, we may have the possibility of ultimately linking well-known Silurian scolecodont-based taxa to full body fossils in the future. One way of potentially resolving these matters and elucidating the extent of anatomical (and internal) preservation would be to analyze the findings of full-body polychaete fossils with the non-destructive Synchrotron X-ray Tomographic Microscopy (SRXTM) technique in order to possibly detect retracted jaws as well as other internal, soft-tissue structures. The SRXTM technique has proven to have wide applicability in the study of microscopic structures in exceptionally preserved fossil materials (e.g., Donoghue et al., Reference Donoghue, Bengtson, Dong, Gostling, Huldtgren, Cunningham, Yin, Yue, Peng and Stampanoni2006; Eriksson et al., Reference Eriksson, Terfelt, Elofsson and Marone2012b) and can provide high-resolution, three-dimensional data.
Acknowledgments
MEE thanks C. F. Bergman for discussions and the Swedish Research Council for funding. PvB thanks the Royal Ontario Museum for financial and other support, K. David and H. Choong, J. Kroezen for guiding him to the Park Head locality, and A. Bancroft and S. LoDuca for information on the Eramosa Formation. Finally, thanks to the William Morton family and the late H. Stobbe and his family, owners of the Owen Sound Ledgerock Quarry in Wiarton, for permission to collect. Journal editor B. Pratt and reviewers J. Kluessendorf and O. Hints made valuable suggestions for the improvement of the manuscript.
