Type species

Ceratiocaris solenoides M’Coy, Reference M’Coy1849

Ceratiocaris winneshiekensis new species

Figures 1.1–1.15, 2.1–2.11, 3

Reference Liu, McKay, Young, Witzke, McVey and Liu2006 Phyllocarid Liu et al., fig. 2G.

Reference Liu, McKay, Witzke and Briggs2009 Phyllocarid Liu et al., pl. 1, figs. 1,2.

Figure 1

Ceratiocaris winneshiekensis n. sp.: (1, 2) holotype, SUI 102858, left lateral, preserving appendages; (1) part, and (2) explanatory drawing; (3, 4) SUI 138443, left lateral, preserving appendages; (3) part, and (4) explanatory drawing; (5, 6) SUI 138433, dorsoventrally flattened showing gut; (5) part, and (6) explanatory drawing; (7) SUI 138437b, oblique, shield foreshortened exposing thorax wider than pleon; (8, 9) SUI 138436, right lateral; (8) specimen, and (9) explanatory drawing; (10, 11) SUI 138442, left lateral; (10) specimen, and (11) explanatory drawing; (12, 13) SUI 138437a, left lateral; (12) incomplete left valve; (13) exfoliated surface showing internal polygonal structure; (14, 15) SUI 138435, right lateral oblique showing thorax; (14) specimen, and (15) explanatory drawing. cr, caudal rami; e, eye; g, gut trace, lv, left valve; m, mouth; mu, trunk muscles; p1 etc., limbs of pleon; rv, right valve; te, telson; t1 etc., thoracic limbs; T, thoracic somites; T-P, boundary between thorax and pleon; 1–7 somites of pleon.

Figure 2

Ceratiocaris winneshiekensis n. sp.: (1, 2) SUI 138447, right lateral; (1) specimen, and (2) explanatory drawing; (3, 4) SUI 138448, left lateral; (3) specimen, and (4) explanatory drawing; (5, 6) SUI 138439, right lateral; (5) part, and (6) explanatory drawing; (7–9) SUI 138444, left lateral, valves offset; (7) specimen; (8) explanatory drawing; (9) isolated structure with denticles, shaded in Figure 2.8 where it overlies the rear of the right valve and somite 3 of the pleon; (10, 11) SUI 138437c; (10) disarticulated left valve, partial pleon and associated structure with denticles; (11) denticulated structure. cr, caudal rami; g, gut trace, lv, left valve; rv, right valve; te, telson; 1–7 somites of pleon.

Figure 3

Ceratiocaris winneshiekensis n. sp.: SUI 138446. Portion of a coprolite (maker unknown) containing phyllocarid fragments.

Holotype

SUI 102858 (WL).

Other material

SUI 138433-138442 (WL); SUI 138443-138451 (WS). SUI 138446 is a coprolite containing the remains of phyllocarids.

Diagnosis

Small Ceratiocaris with smooth shield, ventral ridge, and telson approximately three times the length of the furca.

Description

Most specimens of this small phyllocarid preserve the shield and trunk somites, but evidence of the appendages has only been observed on a few specimens (e.g., Fig. 1.1–1.4).

The majority of specimens are flattened in lateral or near lateral aspect (Fig. 1.1–1.4); dorso-ventral flattening is rare (Fig. 1.5–1.7) and a butterflied configuration (i.e., valves symmetrical about the hinge line and preserved with the outline more or less entire) has not been observed. The cuticle of the exoskeleton displays crumpling and folding, implying some flexibility. Adjacent specimens are preserved in different orientations to bedding indicating that the light delicate carcasses settled on the sediment surface in different attitudes. SUI 138433 (Fig. 1.5, 1.6) is a dorso-ventrally flattened specimen (see also Fig. 1.7): the gut trace runs along the mid line and the telson is flanked symmetrically by the furcal rami (Fig. 1.5, 1.6). It is clear that the valves were not wide (i.e., inflated laterally) when viewed in dorsal or ventral aspect. The pleon is usually inclined to the hinge line at an angle of approximately 40° (Fig. 1.8–1.11) but some specimens preserve the valves flipped upward to varying degrees (e.g., Fig. 1.1, 1.2) in an attitude reminiscent of molt configurations. The prevalence of gut traces extending anteriorly well beyond the telson, however, indicates that the specimens are carcasses (Rolfe Reference Rolfe1962b, p. 913), and the attitude may be a result of decay. Where the shield is exfoliated (Fig. 1.12, 1.13) it reveals a reticulate structure below the surface made up of polygons of the order of 30 to 50 µm across which may reflect a prismatic structure of the cuticle.

The valves are fusiform or lemon shaped in lateral view (Fig. 1.1, 1.2, 1.8–1.11). The hinge line is gently convex, the ventral margin more strongly so. The hinge-line projects anteriorly into a triangle or horn that tapers to a narrow rounded apex (Fig. 1.1, 1.2, 1.10, 1.11). There is no evidence of a separate rostrum, nor has it been identified in isolation. Posteriorly the hinge terminates in a sharp spine-like projection (Fig. 1.1, 1.8–1.11). The maximum height of the valves is anterior of the mid-length (preplete in ostracod terminology) (Fig. 1.8–1.11, 1.14, 1.15). The outline of the valves varies depending on the attitude of the specimen to bedding. This is illustrated by comparing the more usual outline with the maximum height anterior of the mid length (Figs. 1.8–1.11, 1.14, 1.15, 2.1, 2.2) with SUI 138448 (Fig. 2.3, 2.4) where the maximum height is behind the mid length. There is some evidence that the valves became more elongate as they increased in size (Fig. 2.5, 2.6) but in other specimens they are foreshortened by flattening (Fig. 2.7, 2.8). A narrow marginal ridge runs along the edge of the valves (Fig. 1.8, 1.9). A ventral ridge runs close to this margin converging with it anteriorly and posteriorly (Fig. 1.8–1.11). The ratio of length to height of the valves increases in larger specimens but this can only be measured in a small number of examples and is influenced by attitude to bedding: compare Figure 2.1, 2.2 at ~2.75 mm long with Figure 2.5, 2.6 at ~9.2 mm long, both with a height of ~1 mm (SUI 138442, not figured, is 6.7 mm long and 1.4 mm high).

Details of the head are rarely preserved, but extrapolation based on the distribution of appendages or segments (Fig. 1.1, 1.2, 1.14, 1.15) indicates that it was similar in length to the thorax; it appears to be shorter than the thorax in SUI 138443 (Fig. 1.3, 1.4) although the boundary is difficult to discern. The gut trace in SUI 138435 extends posterodorsally from the presumed position of the mouth and curves posteriorly to run the length of the trunk (Fig. 1.14, 1.15). A small dark circular structure overlapping the antero-ventral margin of the right valve in this specimen may represent the eye (Fig. 1.14, 1.15). The posterior appendages of the head may be preserved in outline in SUI 102858 and SUI 138443 (Fig. 1.1–1.4); the boundary between head and thorax in these specimens is not clear.

The thorax consists of eight somites that are normally largely concealed by the shield (Fig. 1.15). Where they are exposed they are not as well preserved as those of the pleon indicating that the cuticle was thinner, particularly in the more anterior part of the thorax, where they were protected by the shield (Fig. 1.7, 1.14, 1.15). Where traces of divisions are preserved (Figs. 1.14, 1.15, 2.3, 2.4) it is clear that the somites of the thorax are shorter than those of the pleon. Such traces rarely indicate the full height/width of the thorax but where the somites are exposed beneath the valves in SUI 138437b (Fig. 1.7) it appears that the thorax was more than two times wider than the pleon; likewise, the traces of intersomite divisions in SUI 138442 show that the thorax was higher than the pleon (Fig. 1.10, 1.11). Traces of the divisions between the 8 thoracic somites are evident in SUI 138435 (Fig. 1.14, 1.15).

Most specimens are preserved with the pleon, and sometimes part of the thorax (Fig. 1.1, 1.2, 1.14, 1.15), extending beyond the valves. The somites of the pleon are cylindrical in cross-section and similar in length to each other, apart from the seventh, which is approximately twice as long as the rest (Figs. 1.14, 1.15, 2.3, 2.4). SUI 138443 is unusual in preserving traces of ventral longitudinal muscles, arranged in segmental bundles, in the posterior somites of the thorax and anterior somites of the pleon (Fig. 1.3, 1.4). This segmental arrangement is reminiscent of that in living Nebalia (see Hessler, Reference Hessler1964, fig. 46). Investigation of these structures with the environmental scanning electron microscope revealed them to be 3–6 µm wide with the appearance of myofibrils and arranged in open bundles. Neither ultrastructural details nor microbes were observed. EDAX revealed significant calcium and phosphate (presumably apatite) in these muscles within a signature dominated by clay components. Higher peaks of calcium and phosphorus were obtained from the gut contents whereas the adjacent matrix, in contrast, contains negligible calcium or phosphorus.

An isolated incomplete subtriangular structure bearing rows of closely spaced denticles was found in association with two specimens. The larger example (Fig. 2.7–2.9), which overlies the anterior part of the pleon in SUI 138444, shows a row of larger denticles overlying a series of smaller ones (Fig. 2.9); the structure does not appear to belong to the specimen itself. The smaller example is associated with the disarticulated shield and pleon of SUI 138437c (Fig. 2.10, 2.11). These structures appear delicate compared to phyllocarid mandibles (Dzik, Reference Dzik1980). They might represent the furcal ramus of another taxon.

The gut trace runs through the ventral part of the pleon, terminating at the posterior extremity of the seventh somite, indicating the position of the anus (Fig. 1.14, 1.15). The telson extends beyond the posterior margin of the seventh somite a distance equivalent to more than the length of the preceding four somites of the pleon (Fig. 1.8–1.11, 1.14, 1.15). At least two ridges separated by a groove are evident running along the telson and there is a line of closely spaced setae at the proximal end (Fig. 1.14, 1.15). In some cases, the posterior part of the telson is flexed (Fig. 2.1, 2.2) or almost rope-like (Fig. 1.1, 1.2) presumably as a result of degradation. In specimens where the telson appears relatively short, the termination has been lost or is concealed in the matrix (e.g., Fig. 2.3, 2.4). Two short spine-like rami make up the caudal furca; a groove is evident running along the right one in SUI 138435 (Fig. 1.14, 1.15). The length of the caudal rami is approximately one-third that of the telson itself (Fig. 1.1, 1.2, 1.8–1.11, 1.14, 1.15).

Evidence of appendages has only been observed in a few specimens, including SUI 102858 (Fig. 1.1, 1.2) and SUI 138443 (Fig. 1.3, 1.4). The limbs of at least eight, maybe nine somites are evident in SUI 102858 in the thorax and posterior part of the head (Fig. 1.1, 1.2). Some of these appear paired or biramous. Elongate pleopods are present on the first six somites of the pleon, becoming smaller posteriorly. These are also paired or biramous. SUI 138443 preserves traces of limbs apparently corresponding to nine or ten somites in the head and thorax (Fig. 1.3, 1.4), most of them extending beyond the ventral margin of the valves; the pleopods are not preserved. Neither specimen preserves the antennae. No details of the limb morphology are evident.

The length of the shield of Ceratiocaris winneshiekensis varies from approximately 2.5 mm to approximately 10 mm.

SUI 138446 (Fig. 3) is a two-dimensional coprolite containing fragments of phyllocarid cuticle.

Etymology

After the name of the formation in which the fossils occur.

Remarks

Ceratiocaris winneshiekensis is the most abundant macrofossil (~16% if bromalites are included and individual conodont elements are excluded) in the Winneshiek assemblage, and by far the most abundant bivalved arthropod in the fauna.

In the most recent review of Paleozoic phyllocarids Collette and Hagadorn (Reference Collette and Hagadorn2010c, p. 802) diagnosed the genus Ceratiocaris, within Ceratiocaridina, by ‘sinuous posterior carapace margin; carapace cuticle thin. Carapace ornamented with raised ridge ornamentation, roughly parallel to the ventral carapace margin.’ The outline of Ceratiocaris winneshiekensis is consistent with this diagnosis, although it varies with attitude to the bedding as evidenced particularly by the height-length ratio. It differs from the bivalved shield of Caryocaris where the posterior margin is usually straight to oblique and typically fringed with spines (Collette and Hagadorn, Reference Collette and Hagadorn2010b). In contrast to other species of Ceratiocaris, however, some specimens of C. winneshiekensis show a ventral ridge, and the cuticle is smooth. The polygonal structure evident on some valves is an internal structure that emerges with exfoliation (‘decortication’ of Rolfe, Reference Rolfe1962b, p. 916) and is similar in style and scale to that recorded in some specimens of the much larger Ceratiocaris papilio from the Silurian of Scotland (Rolfe, Reference Rolfe1962c). The presence of a ventral ridge and absence of striae on C. winneshiekensis might be argued to provide a basis for erecting a new genus, but cuticle ornament varies with size even within species (Rolfe, Reference Rolfe1962b).

The ratio of telson to furcal length is difficult to determine accurately in C. winneshiekensis but ranges from about 2.6 to 2.75, higher than that in other species of Ceratiocaris figured by Collette and Hagadorn (Reference Collette and Hagadorn2010c, fig. 3) apart from C. macroura Collette and Rudkin, Reference Collette and Rudkin2010a, and much higher than in Caryocaris, where the telson is equal to or shorter than the furcal rami (Collette and Hagadorn, Reference Collette and Hagadorn2010b). C. winneshiekensis and C. macroura also share the presence of lateral setae on the telson. C. winneshiekensis differs from C. macroura, however, not only in the presence of a ventral ridge and absence of cuticle ornament, but also in the more pronounced and pointed anterior projection of the valves and the greater width/height of the thoracic divisions relative to those of the pleon. We cannot discern any evidence of a rostral plate in C. winneshiekensis but this may be taphonomic. Collette and Hagadorn (Reference Collette and Hagadorn2010c) regarded Ceratiocaris silicula from the Martinsburg Shale of Virginia (Caradoc-Ashgill – i.e., Sandbian and younger) as the oldest valid species of Ceratiocaris. Thus, the Winneshiek example is the earliest well documented representative of the Ceratiocarididae. Collette and Hagadorn (Reference Collette and Hagadorn2010c) followed Rode and Lieberman (Reference Rode and Lieberman2002) in confining Ceratiocaridina to the family Ceratiocarididae.

Hannibal and Feldmann (Reference Hannibal and Feldmann1997) described two taxa of phyllocarids from core from the likely coeval Ames impact structure in Oklahoma. The first of these, which Hannibal and Feldmann (Reference Hannibal and Feldmann1997) referred to as ‘resembling Ceratiocaris,’ shares the valve outline, the presence of more robust cuticle on the pleon than the thorax, and the elongate telson, with the Winneshiek form. The telson of the Ames ?Ceratiocaris (Hannibal and Feldmann, Reference Hannibal and Feldmann1997, figure 1), however, appears longer relative to the pleon than that of C. winneshiekensis. Racheboeuf et al. (Reference Racheboeuf, Crasquin and Brussa2009, p. 402) regarded this Ames phyllocarid as a probable new archaeostracan (possible ceratiocarid). Phyllocarid taxonomy, at least at the species level, has been complicated by the erection of large numbers of species based on fragmentary material and inadequate criteria (Collette and Hagadorn, Reference Collette and Hagadorn2010c). Thus, pending a detailed description of the Ames ceratiocarid, we note that it is closely similar to the Winneshiek form. The second taxon from the Ames structure appears to be Caryocaris.

Only two other Paleozoic Lagerstätten preserve phyllocarids with traces of the limbs: the Silurian Herefordshire locality (Cinerocaris magnifica—Briggs et al., Reference Briggs, Sutton and Siveter2004) and the Devonian Hunsrück Slate (Nahecaris stuertzi—Bergström et al., Reference Bergström, Briggs, Dahl, Rolfe and Stürmer1987, and Oryctocaris balssi—Bergmann and Rust, Reference Bergmann and Rust2014). In both cases, the taxa are larger and more details are available. In C. magnifica, the endopod of the last cephalic appendage is similar to those of the thorax, a condition that may also prevail in Ceratiocaris winneshiekensis. The Winneshiek phyllocarid does not preserve sufficient detail of the limbs to allow a comparison of their structure.

The phylogeny of phyllocarids has been subjected to a number of cladistic analyses, initially by Rode and Lieberman (Reference Rode and Lieberman2002), and subsequently by Briggs et al. (Reference Briggs, Sutton and Siveter2004), Collette and Hagadorn (Reference Collette and Hagadorn2010c), and Bergmann and Rust (Reference Bergmann and Rust2014). Rode and Lieberman (Reference Rode and Lieberman2002) demonstrated that traditional taxa were paraphyletic and redefined them to correspond to major clades: Ceratiocaridina, Pephricaridina, Rhinocaridina, and Echinocaridina. Briggs et al. (Reference Briggs, Sutton and Siveter2004) obtained a similar result, but Pephricaridina fell out as a sister taxon to Echinocaridina. Collette and Hagadorn (Reference Collette and Hagadorn2010c) extended these analyses with a comprehensive review of Ceratiocaridina and obtained a separate Caryocaridina. The phyllocarid phylogeny of Collette and Hagadorn (Reference Collette and Hagadorn2010c) is similar to those obtained in the less comprehensive studies of Rode and Lieberman (Reference Rode and Lieberman2002) and Briggs et al. (Reference Briggs, Sutton and Siveter2004) except that Pephricaridina fell out basal to Ceratiocaridina and Rhinocaridina rather than with Echinocaridina. Bergmann and Rust (Reference Bergmann and Rust2014) analysed a smaller number of taxa, including five genera not included in previous cladistic analyses, and used TNT rather than PAUP. They recovered Caryocaridina and Rhinocaridina but, in contrast to previous analyses, obtained paraphyletic Ceratiocaridina and Echinocaridina. The Winneshiek specimens do not provide sufficient new information to justify revisiting phyllocarid phylogeny.

The evidence of a coprolite containing fragments of C. winneshiekensis (Fig. 3) indicates that it was preyed upon, probably by larger arthropods or fishes.

Crustacea Brünnich, Reference Brünnich1772

Family, genus and species indeterminate

Figure 4.1–4.3

Figure 4

Undetermined trunk and telson: (1–3) SUI 1384552, right lateral; (1) specimen showing gut trace, rear abdominal somites and telson; (2) explanatory drawing; (3) telson showing multisegmented furcal rami. cr, caudal rami; g, gut trace, te, telson.

Material

SUI 138452 (WS).

Description

A single tiny specimen preserves a gut trace, five abdominal somites, and a telson with caudal furca only (Fig. 4.1, 4.2). The telson does not project beyond the insertion of the furcal rami and the latter are long and multisegmented (Fig. 4.3).

Remarks

The telson of this incomplete specimen reveals that it is not a phyllocarid. Multisegmented furcal rami are rare among living crustaceans where they tend to be diagnostic of notostracan branchiopods. Young et al. (Reference Young, Rudkin, Dobrzanski, Robson and Nowlan2007, fig. 2) recorded a ?notostracan from the late Ordovician William Lake Lagerstätte in Manitoba but it has yet to be described. The oldest notostracan presently known is from the Upper Devonian Strud locality in Belgium (Lagebro et al., Reference Lagebro, Gueriau, Hegna, Rabet, Butler and Budd2015) although molecular considerations indicate that notostracans could have originated by the Ordovician (e.g., Oakley et al., Reference Oakley, Wolfe, Lindgren and Zaharoff2012). The telson of the unusual Early Ordovician arthropod Douglasocaris collinsi Caster and Brooks, Reference Caster and Brooks1956, from Tennessee is similar. Although Caster and Brooks (Reference Caster and Brooks1956) assigned Douglasocaris to Phyllocarida, Rolfe (Reference Rolfe1969) thought it might belong with the notostracan branchiopods. Without evidence of the rest of the Winneshiek arthropod, it is not possible to determine its affinities.

?Crustacea Brünnich, Reference Brünnich1772

?Thylacocephala Pinna, Arduini, Pesarini, and Teruzzi, Reference Pinna, Arduini, Pesarini and Terruzzi1982

Genus Decoracaris new genus

Type species

Decoracaris hildebrandi new genus. new species

Diagnosis

As for species.

Etymology

From the locality Decorah combined with caris (L.), shrimp.

Decoracaris hildebrandi new genus, new species

Figure 5.1–5.5

Figure 5

Decoracaris hildebrandi n. gen. n. sp.: (1, 2) holotype, SUI 138453, right lateral; (1) right valve; (2) explanatory drawing of anterior horn, with insert showing cuticle ornament; (3, 4) SUI 138458, right lateral; (3) portion of right valve; (4) explanatory drawing; (5) SUI 138456, right lateral, large incomplete specimen.

Holotype

SUI 138453 (WS).

Other material

SUI 138454-138456 (WL); SUI 138457-138462 (WS) (SUI 138459 shows no obvious diagnostic ornament), and other fragmentary specimens.

Diagnosis

Suboval valve more than two times longer than high, pronounced rounded anterior horn overlying shallow notch; fine anastomosing ridges around valve margin.

Description

This large taxon is represented only by valves, which have a thick phosphatized cuticle. Flattening resulted in crushing and fragmentation, and the shale rarely split along the plane of the specimen. Usually only part of the valve outline is evident. For this reason, the description is based mostly on a single complete right valve (Fig. 5.1).

The valve of SUI 138453 is elongate suboval in outline, with an anterior horn (Fig. 5.1, 5.2). The maximum height of the valve, which occurs about the mid-length, is approximately 40% of the length. A straight hinge line runs about 75% the length of the valve. Beyond the hinge, the margin slopes down anteriorly to form a rounded anterior projection (horn) beneath which is a shallow notch (Fig. 5.1, 5.2). The ventral margin of the valve is gently convex, joining the short posterior margin, which makes an angle of approximately 115° with the hinge. The lack of distortion with flattening indicates that the relief on the valve was not very high; the vertical fold indicates that the maximum convexity was about the mid-length.

A marginal ridge, rounded on both outer and inner side, runs around the edge of the valve. Flanking this ridge, on the outer surface of the valve, is a rim about 1 mm wide which displays a sculpture of fine anastomosing ridges (Fig. 5.2). The remainder of the valve surface appears smooth.

The characteristic anastomosing sculpture allows other specimens to be identified with this taxon. SUI 138458 preserves part of the anterior of a right valve (Fig. 5.3, 5.4). It shows that the sculpted rim may be bounded on its inner margin by a shallow groove. This specimen also preserves a ventro-lateral ridge running near-parallel to a short length of the margin (Fig. 5.3, 5.4), but this is not evident on all specimens (Fig. 5.1, SUI 138457, SUI 138461). SUI 138461 (not figured) preserves part of both right and left valves superimposed laterally, but partly offset, both showing marginal ornament and giving the impression of a ridge.

SUI 138453 (Fig. 5.1) is 88 mm long (maximum parallel to the hinge). The preserved portion of SUI 138456 (Fig. 5.5) is ~85 mm; the complete valve was slightly longer. None of the other specimens provides an indication of the complete length of the shield.

Etymology

After Hildebrand, in honor of the landowner who kindly allowed us access to the site.

Remarks

Only the bivalved shield of this taxon is known. Its morphology differs sufficiently from that of other shields to justify the erection of a new genus and species. In the absence of soft tissues, including appendages, the affinities of bivalved arthropods are difficult to establish (Briggs, Reference Briggs1983; Siveter et al., Reference Siveter, Briggs, Sutton and Joomun2013). This is emphasized by the range of morphology in Cambrian bivalved arthropods (Nereocaris, Jugatacaris, Pectocaris, Odaraia, Branchiocaris, Perspicaris, Canadaspis) that fall on the arthropod stem (Legg et al., Reference Legg, Sutton, Edgecombe and Caron2012); the bivalved shield alone provides little constraint on affinity. The diversity of bivalved stem arthropods in Cambrian Lagerstätten does not appear to persist into the Lower Ordovician—they are rare, for example, in the Fezouata Lagerstätten of Morocco (Van Roy and Briggs, Reference Van Roy and Briggs2011; Van Roy et al., 2015).

Anomalocaridids, however, persisted at least to the Lower Ordovician, where they occur in the Fezouata Lagerstätten. The outline of the valves of Decoracaris hildebrandi differs from known lateral elements from the Fezouata, but it is similar to that of some examples of the P-element of the Cambrian Burgess Shale anomalocaridid Hurdia (originally referred to Proboscicaris, see Rolfe, Reference Rolfe1962a, fig. 1). The Hurdia shield, however, included a median triangular H-element (Daley et al., Reference Daley, Budd and Caron2013) and no such structure has been found at Winneshiek. Furthermore the shield of D. hildebrandi lacks any posterior notch such as accommodates the eye in Hurdia (Daley et al., Reference Daley, Budd and Caron2013) and a number of specimens of D. hildebrandi, albeit incomplete, show evidence of overlapping valves, favoring a bivalved nature.

The affinity of Decoracaris hildebrandi is likely to lie with forms characteristic of the post Cambrian. Its valve morphology is similar in general outline to that of phyllocarids assigned to Ceratiocaris or Warneticaris (Collette and Hagadorn, Reference Collette and Hagadorn2010c) but none of these shows the shallow anteroventral notch. Aristozoe also shows a rounded horn, but this phyllocarid has a much wider marginal rim, and pronounced anterodorsal nodes (Rolfe, Reference Rolfe1969; Chlupáč, Reference Chlupáč1992). A phyllocarid of the size of Decoracaris would likely have robust mandibles, and although these would not necessarily be preserved in association with the valves, there is no sign of isolated examples in the Winneshiek assemblage in spite of detailed searching of bedding planes for conodont elements.

Bivalved thylacocephalans, like phyllocarids, are known from restricted environments, but the earliest known representatives of this group are Silurian (Mikulic et al., Reference Mikulic, Briggs and Kluessendorf1985; Haug et al., Reference Haug, Briggs, Mikulic, Kluessendorf and Haug2014), although Vannier et al. (Reference Vannier, Chen, Huang, Charbonnier and Wang2006) identified possible representatives in the Cambrian. The anterodorsal projection of the valve of Decoracaris hildebrandi is larger and more rounded, and the notch shallower and more ventral than that in most thylacocephalans, but the variation in the anterior morphology of the valves in the group as a whole is considerable (see e.g., Briggs and Rolfe, Reference Briggs and Rolfe1983, text-fig. 6; Schram, Reference Schram2014, figs. 2, 3). Although the depth of the notch in D. hildebrandi is less pronounced than that accommodating the eye of most later thylacocephalans, it is comparable to that in the Silurian form Thylacares brandonensis (Haug et al., Reference Haug, Briggs, Mikulic, Kluessendorf and Haug2014). Thus, we tentatively assign D. hildebrandi to Thylacocephala, pending the discovery of more of its morphology. If this attribution were confirmed, it would make the Winneshiek taxon the earliest known example of the group (Haug et al., Reference Haug, Briggs, Mikulic, Kluessendorf and Haug2014).

?Crustacea Brünnich, Reference Brünnich1772

Genus Iosuperstes new genus

Type species

Iosuperstes collisionis new genus, new species

Diagnosis

As for species.

Etymology

Abbreviation of Iowa (Io was also a priestess in Greek mythology) combined with superstes (L.), witness, survivor.

Iosuperstes collisionis new genus, new species

Figure 6.1–6.9

Figure 6

Iosuperstes collisionis n. gen. n. sp.: (1, 2) holotype, SUI 138463; (1) right valve; (2) explanatory drawing of marginal structures in posteroventral area with position circled in insert; (3) SUI 138471, left valve (specimen is ‘butterflied’, attached right valve dorsal of hinge line is incomplete); (4) SUI 138465, right valve; (5) SUI 138464, right valve; (6) SUI 138476, left valve; (7) SUI 138467, left valve; (8) SUI 138473, left lateral, left valve overlying and offset on right; (9) SUI 138479, incomplete left valve.

Holotype

SUI 138463 (WS).

Other material

SUI 138464-138472 (WL); SUI 138473-138479 (WS).

Diagnosis

Shield of two suboval valves with narrow defined margin and unevenly spaced pits <50 µm in diameter revealed when the outer surface of the cuticle is exfoliated.

Description

Iosuperstes collisionis is represented only by valves, which have a mineralized cuticle. The valves are suboval in outline with a near-straight hingeline (Fig. 6.1–6.7). Most specimens are single isolated valves but examples articulated and splayed in a ‘butterfly’ fashion are also known (Fig. 6.3). The rounded posterior margin makes an angle of approximately 115° with the hinge; the anterior margin makes a shallower angle of approximately 150°. The maximum height lies to one side of the midlength (Fig. 6.1–6.7); the valves are assumed to be postplete, i.e., expanded posteriorly. The height:length ratio varies from ~50% to 67%, but this variation is, at least in part, a function of orientation to bedding. Original relief is difficult to detect due to flattening of the valves. An indication of original convexity is provided by concentric and transverse crumpling (Fig. 6.1, 6.2, 6.4–6.7). There is no evidence that the pattern of flattening reflects structures within or between the valves; the trunk segments are not preserved. Variation in the degree of crumpling presumably reflects differences in diagenesis including, perhaps, demineralization of the cuticle (compare Fig. 6.1 and 6.7 with 6.3).

A narrow ridge flanked by a gently convex rim ~0.5–0.75 mm wide follows the margin of the valves (Fig. 6.1, 6.2). This convex rim is mirrored on the interior surface of the valve. In some specimens, an inner ridge is evident running near parallel to the anterior margin of the valve (Fig. 6.3) and in others there is an indication of raised structures in the postero-dorsal part of the valves (e.g., SUI 138463, Fig. 6.1). However, neither of these features is consistently preserved.

The surface of the valve is covered by unevenly spaced pits less than 50 µm in diameter (Fig. 6.1–6.3, 6.5, 6.8, 6.9). The pits are not always evident (e.g., on SUI 138465: Fig. 6.4). Their expression appears to be determined by the degree of exfoliation of the cuticle; i.e., they are evident when the outer surface of the cuticle is absent. The pits cover most of the central part of the valve of SUI 138471 (Fig. 6.3). They are evident on the marginal rim in SUI 138464 and SUI 138476 (Fig. 6.5, 6.6) and are also present, but less densely distributed, inside the rim in SUI 138464 (Fig. 6.5). In SUI 138463, they are not evident on the rim, but only inside it where the outermost layer of cuticle has been lost (Fig. 6.1, 6.2). In other specimens, they are only evident on part of the valve (e.g., Fig. 6.7).

The valves range in length from ~10 mm (SUI 138469: not figured) to 23 mm (Fig. 6.1, 6.2). SUI 138473, preserved in lateral aspect, is more heavily pitted (Fig. 6.8). The hinge and part of the anterior outline were lost when the shale was split. The left valve is offset anteriorly and overlies the right. Flattening of the valves obscures any larger scale structures (nodes, tubercles) that might be present on the surface. The major part of the left valve is covered with small pits. Where these are closely spaced, as in the center, they create the appearance of fine scale reticulation on the surface. Toward the margin of the valve, the pits are more widely spaced and discrete, and in places, particularly along the antero-ventral margin and at the posterior, the valve surface is largely smooth. The pits are evident as tubercles on the interior surface of the underlying right valve where it is exposed on the edge of the shale fragment.

Larger fragmentary specimens are thought to belong to this taxon, although this cannot be confirmed without a complete valve outline. SUI 138479 (Fig. 6.9), which is 22 mm high and at least 32 mm long (based on a minimum ratio), preserves unevenly preserved larger pits, about 150 µm in diameter, on the outer surface, as well as pits about one-third this size, similar to those on the smaller specimens. A fragment of a third specimen (SUI 138475, not figured: largest dimension 21 mm parallel to the hinge, but clearly much larger) also preserves pits of two sizes as well as traces of an anastomosing ornament along part of the margin. Although the marginal ornament is reminiscent of that in Decoracaris hildebrandi, the surface of the valves in that taxon is smooth and these smaller pitted specimens show no evidence of an anterior projection.

Etymology

Collisio (L.), impact, referring to the arthropod’s occurrence in a meteorite crater.

Remarks

The specimens we assign to Iosuperstes collisionis include considerable variation in the degree and distribution of ornamentation, from none visible (Fig. 6.4, 6.6, 6.7), through pitting on the rim (Fig. 6.5), on part of the surface (Fig. 6.3), to dense pitting over most of the surface (Fig. 6.8). While some of this variation is real, in other cases it is likely to be taphonomic (i.e., the result of processes such as exfoliation or decortication (Rolfe, Reference Rolfe1962b, p. 916). While the idea of using this variation to distinguish separate species is attractive, ornamentation can vary significantly even within species of bivalved arthropod (Rolfe, Reference Rolfe1962b) and other crustaceans (Waugh et al., Reference Waugh, Feldmann, Burrell, Hull, Hein and Schweitzer2009) particularly with growth and molting. Thus, we have grouped shields of otherwise similar morphology together in the one taxon pending evidence to the contrary.

Iosuperstes collisionis is similar in size and valve outline to the phyllocarid Arenosicaris from the late Cambrian of Wisconsin (Collette and Hagadorn Reference Collette and Hagadorn2010b), but it lacks the concentric ridges that parallel the valve margins in that taxon. The outline of the valves is also similar to that in a diversity of Cambrian arthropods occupying different positions on the stem (Legg et al., Reference Legg, Sutton, Edgecombe and Caron2012; Legg and Caron, Reference Legg and Caron2014) such as Canadaspis laevigata (Hou and Bergström, Reference Hou and Bergström1997), Pectocaris eurypetala (Hou et al., Reference Hou, Bergström and Xu2004) and Jugatacaris agilis (Fu and Zhang, Reference Fu and Zhang2011) from the Chengjiang biota, and Canadaspis perfecta (Briggs, Reference Briggs1978), Perspicaris dictynna (Briggs, Reference Briggs1977), and Loricicaris spinocaudatus (Legg and Caron, Reference Legg and Caron2014) from the Burgess Shale. Some of these forms, however, show a somewhat more pronounced posterior projection of the hinge and none of them preserve the pitting characteristic of the Winneshiek form. On this basis we assign it to a new genus and species here. The comparisons with other bivalved forms simply serve to emphasize the difficulty of assigning Iosuperstes collisionis to a particular group.

?Crustacea Brünnich, Reference Brünnich1772

?Order Leperditicopida Scott, Reference Scott1961

?Leperditiidae Jones, Reference Jones1856

Genus and species indeterminate

Figure 7.1–7.10

Figure 7

Undetermined ?leperditiid: (1, 2) SUI 138488; (1) internal view; (2) explanatory drawing; (3, 4) SUI 138482; (3) valves incomplete anterolaterally; (4) explanatory drawing, pitted areas are poorly defined; (5, 6) SUI 138489; (5) left valve fold underneath; (6) explanatory drawing; (7, 8) SUI 138486; (7) specimen; (8) explanatory drawing, circular areas are poorly defined; (9, 10) SUI 138480; (9) specimen; (10) explanatory drawing.

Material

SUI 138480-138483 (WL), SUI 138484-138489 (WS).

Description

This taxon is represented only by the bivalved shield, which has a mineralized cuticle. The majority of the specimens are preserved with the valves articulated but splayed in a ‘butterfly’ fashion (Fig. 7.1–7.10). The valves are sub-oval in outline, with a very narrow ridge along the margin. In most cases the ventral margin is folded beneath the valve itself (e.g., Fig. 7.5, 7.6). Only when the paired valves are flattened in a somewhat oblique attitude to bedding is the outline of one of the valves evident (Fig. 7.1, 7.2 right valve, on left in figure; Fig. 7.5, 7.6 right valve). The maximum height lies to one side of the midlength (Fig. 7.5, 7.6); the valves are presumably postplete; i.e., the more expanded part is posterior. The fact that one valve generally appears higher (the dimension orthogonal to the hinge line) than the other may reflect different dimensions, as in leperditicopid arthropods (Berdan, Reference Berdan1976, Reference Berdan1984). Inside the marginal ridge is a rim (‘brim’ in the terminology of Berdan, Reference Berdan1976, text fig. 3) which narrows to a point at the hinge line (Fig. 7.1–7.10). The rim appears to taper out along the ventral margin. Some specimens preserve possible nodes (perhaps even eye tubercles: Fig. 7.1, 7.2), in some cases with hints of pitting in subcircular areas (Fig. 7.3, 7.4, 7.7, 7.8), but node-like features are not present in a consistent position from specimen to specimen. The valves are irregularly crumpled (e.g., Fig. 7.9) making such relief difficult to characterize. The surface of the valves is smooth. Due to the flattened preservation, it is not possible distinguish any features of the ventral margin (see Vannier et al., Reference Vannier, Wang and Coen2001, figs. 1, 13).

The valves range in length from 8.8 mm (SUI 138489, Fig. 7.5, 7.6) to 17.5 mm (SUI 138485, not figured).

Remarks

This bivalved arthropod is similar to leperditicopes in the size and external appearance of the valves (Berdan, Reference Berdan1984; Vannier et al., Reference Vannier, Wang and Coen2001). Vannier et al. (Reference Vannier, Wang and Coen2001) noted that the earliest leperditiid is Eoleperditia ambigua Berdan, Reference Berdan1976 from the early Llanvirn of Utah—a similar age to the Winneshiek occurrence. The best preserved leperditicopes from other localities are uncrushed and in the round, preserving significant relief (Williams and Siveter, Reference Williams and Siveter2008). Silicified specimens can be released from limestone by dissolution; calcitic specimens have to be cracked out of limestones. The Winneshiek shields, in contrast, are flattened and diagenetically altered, eliminating relief, and features of the internal surface of the valves are not evident, all of which make diagnostic characters difficult to observe (Berdan, Reference Berdan1984, p. 3). Even though leperditicopid arthropods range from Ordovician to late Devonian (Vannier et al., Reference Vannier, Wang and Coen2001) unequivocal flattened specimens (requiring demineralization and preservation in shale) are essentially unknown but this may be, in part, because they are difficult to identify. The absence of the rim around the ventral margin in the Winneshiek form indicates an affinity with Leperditiidae rather than Isochilinidae (of the two major groups recognized by Berdan, Reference Berdan1984) but the lack of diagnostic features prevents the identification of this taxon with a genus (it is similar to a number of smooth forms of leperditiids with a wide marginal rim).

The affinities of leperditicopes are uncertain and will only be resolved with the discovery of preserved soft parts (Becker, Reference Becker2005). Although traditionally assigned to ostracods (e.g., Berdan, Reference Berdan1984), such a relationship is controversial (Becker, Reference Becker2005) and most authors now refer them only to arthropods (Vannier et al., Reference Vannier, Wang and Coen2001; Williams and Siveter, Reference Williams and Siveter2008). Frustratingly (in contrast to the small phyllocarids), no soft parts are preserved in the Winneshiek specimens.

Vannier et al. (Reference Vannier, Wang and Coen2001, p. 75) observed that leperditicopes ‘preferentially occupied very shallow marginal habitats … that were subjected to environmental stress,’ including salinity (see also Knox and Gordon, Reference Knox and Gordon1999; Knox, Reference Knox2001). Berdan (Reference Berdan1984, p. 17) considered them to be euryhaline and shallow water. They are generally considered to be benthic (Vannier et al., Reference Vannier, Wang and Coen2001, p. 91). The low oxygen bottom conditions within the Winneshiek basin would not have suited them, implying that they lived beyond the crater and were washed in following death (this would be consistent with their ‘butterflied’ configuration and the absence of preserved soft parts).

Flattened leperditicopes occur elsewhere where they are similarly difficult to identify. Large numbers of leperditicopids are known from the Lower Silurian Waukesha lagerstätte (Telychian) of Wisconsin, some of them ‘butterflied’ (Mikulic et al., Reference Mikulic, Briggs and Kluessendorf1985). Large numbers of Leperditia also occur in association with eurypterids in the Upper Silurian Tonoloway Formation (Upper Ludlow-Přídoli) of Pennsylvania (Vrazo et al., Reference Vrazo, Trop and Brett2014) and leperditicopes are associated with eurypterids from the Bertie Group (Přídoli) of New York State in the collections of the Yale Peabody Museum of Natural History.

Crustacea Brünnich, Reference Brünnich1772

Ostracoda Latreille, Reference Latreille1802

?Palaeocopida Henningsmoen, Reference Henningsmoen1953

cf. Lomatopisthia (Guber and Jaanusson, Reference Guber and Jaanusson1964)

Figure 8.1–8.3

Figure 8

Ostracods: (1–3) cf. Lomatopisthia; (1, 2) SUI 138490; (1) internal view; (2) explanatory drawing, shaded areas approximate positive relief on the exterior of the valve; (3) SUI 138491, external view, much of the valves lost posteriorly; (4, 5) unidentified ?palaeocopid, SUI 138492; (4) internal view; (5) ?posterior showing radiating structures; (6–8) unidentified ?podocopid; (6) SUI 138497 showing some relief; (7) SUI 138498 shell demineralized, relief lost; (8) SUI 138499, oblique compaction, ?trunk and limbs projecting ventrally

Material

SUI 138490 (WS), SUI 138491 (WL).

Description

SUI 138490 (Fig. 8.1, 8.2) is preserved as two articulated valves, opened in butterfly fashion and affording a view of the interior. The right valve (the lower one in Fig. 8.1, 8.2) shows the suboval outline, with some loss of the anterior margin. The maximum height lies at or slightly anterior of the mid length, i.e., the valve is assumed to be preplete. The left valve is folded on itself longitudinally exposing the exterior of the ventral part. A narrow ridge runs around the margin, which is particularly evident on the right valve. The relief on the valves is somewhat obscured by flattening and the features appear negative. The right valve preserves evidence of four lobes (shaded in Fig. 8.2). Lobe 1 is anterodorsal, subcircular in outline, and appears discrete; a similar somewhat larger structure lies ventral to it. Lobe 2 is poorly defined, similar in height to Lobe 1, but extends dorsally to merge with Lobe 3. Lobes 3 and 4 extend to the dorsal margin and merge ventrally into a raised area along the ventral margin (?ventral lobe). The surface of the valves is granular, with a covering of unevenly spaced tubercles that are 30–40 µm in diameter and appear as pits on the interior surface; they may, in part, be evident due to exfoliation. SUI 138491 (Fig. 8.3) is slightly larger than the better-preserved specimen. It is flattened, obscuring the relief, and much of the posterior part of the valves is lost.

The length of the valves is ~4 mm in SUI 138490 (Fig. 8.1, 8.2) and 5.5 mm in SUI 138491 (Fig. 8.3).

Remarks

The long straight dorsal margin and pronounced lobes identify this form as an ostracod of the Order Palaeocopida. It appears comparable to species of Lomatopisthia that occur in the Sandbian/Katian of Oklahoma (Harris, Reference Harris1957; Kraft, Reference Kraft1962; Williams, Reference Williams1991; Williams and Siveter, Reference Williams and Siveter1996). Lomatopisthia shows a pattern of four dorsoventral lobes (Guber and Jaanusson, Reference Guber and Jaanusson1964) although the described species are much smaller than the Winneshiek form. Lomatopisthia is unusual among ostracods from the middle Ordovician Bromide Formation in that its ecological range is not confined to open marine but extends to shore face (Williams and Siveter, Reference Williams and Siveter1996). The ostracods of Late (and presumably mid) Ordovician Laurentia tend to be highly endemic (Mohibullah et al., Reference Mohibullah, Williams, Vandenbroucke, Sabbe and Zalasiewicz2012). Better-preserved material of the Winneshiek taxon would likely allow its status as a separate species (and possibly genus) to be confirmed. Hairapetian et al. (Reference Hairapetian, Mohibullah, Tilley, Williams, Miller, Afzal, Pour and Hejazi2011) extended the range of Lomatopisthia to the early Silurian of Iran.

Ostracoda Latreille, Reference Latreille1802

?Palaeocopida Henningsmoen, Reference Henningsmoen1953

Genus and species indeterminate

Figure 8.4–8.5

Material

SUI 138492, SUI 138493 (WS).

Description

SUI 138492 preserves both valves in a butterflied configuration (Fig. 8.4). The valves are suboval in outline and there is little relief except on the periphery. The margins are marked by a narrow ridge and in places a marginal rim is evident. The surface of the valves (Fig. 8.5) is unusual in displaying arrays of radiating straight lines, the sets converging at high angles. The cuticle appears absent over much of the valves and there is evidence of wrinkling implying flexibility, perhaps because of demineralization. SUI 138492 is about 4 mm long. A second, somewhat larger specimen (SUI 138493), is incomplete anteriorly.

Remarks

The radiating structures are reminiscent of ridges on the surface of Bulbosulculus fragilis (Kanygin, Reference Kanygin1971) although the latter are coarser and more robust. The structures on the Winneshiek specimens also resemble the striae on the fringes of some palaeocopes, but such striae are usually oriented normal to the valve margin and the valves show no evidence of a fringe. The radiating structures are too straight to represent the anastomosing ridges that occur within the valves of some leperditicopids and living myodocopes (Vannier et al., Reference Vannier, Wang and Coen2001). Nor is their arrangement like that of the calcified radiate microstructures, polygonal to circular in outline, which are characteristic of Recent myodocopes where they represent platelets of calcium carbonate growing in the valves. Such platelets have also been described in Ordovician, Silurian and Carboniferous myodocopes (Siveter et al., Reference Siveter, Vannier and Palmer1987, Reference Siveter, Tanaka, Farrell, Martin, Siveter and Briggs2014; Briggs et al., Reference Briggs, Clark and Clarkson1991a) where they may be modified by diagenesis. Although the radiating structures in this Winneshiek specimen differ in appearance, they may be similar in nature.

Ostracoda Latreille, Reference Latreille1802

?Podocopida Henningsmoen, Reference Henningsmoen1953

Genus and species indeterminate

Figure 8.6–8.8

Material

SUI 138494-138495 (WL), SUI 138496-138502(WS) (138500 may not belong here).

Description

A number of small bivalved specimens are characterized by a straight hinge line, inflated oval outline and narrow marginal ridge. SUI 138497 (Fig. 8.6) preserves two valves preserved in a butterflied configuration with a smooth surface and narrow marginal rim. The valves appear carbonaceous but there are traces of mineralized structures. Most of these have the appearance of polygonal cracks but in places there are hints of a radial arrangement similar to that in the calcified platelets of myodocopes. Other specimens show hints of such a radial arrangement even where the cuticle is largely lost, and there is little or no relief (Fig. 8.7). Yet other specimens preserve the mineralized valves with little relief and a smooth surface. SUI 138496 (not figured), a small distorted specimen, shows short projections beyond the hinge line which are reminiscent of antennae, but their nature is uncertain. SUI 138499, preserved in an oblique attitude to bedding, also shows traces of structures that may represent part of the trunk and some thoracic limbs (Fig. 8.8).

Remarks

Bivalved taxa, including ostracods, are difficult to assign taxonomically in the absence of appendages (Siveter et al., Reference Siveter, Briggs, Sutton and Joomun2013). We assign this provisionally to Podocopida based on the long straight hinge line, valve outline, and smooth surface.