Octoberaspis ushakovi, Dunlop, 2002

Dunlop, Jason A., 2002, Arthropods from the Lower Devonian Severnaya Zemlya Formation of October Revolution Island (Russia), Geodiversitas 24 (2), pp. 349-379 : 357-377

Octoberaspis ushakovi n. sp. ( Figs 6-10 )

Eurypteridae – Novilskaya et al. 1983: 94, fig. 5.

HOLOTYPE. — GIL No. 35/336 (oc 67/bd 12).

ETYMOLOGY. — In honour of G. A. Ushakov, an Arctic explorer and the leader of the first expedition to Severnaya Zemlya in 1930-1932.

ADDITIONAL MATERIAL. — GIL Nos. 35/324, 35/337, 35/338 (the latter two on the same block as the holotype), 35/339, 35/379, 35/707 (all oc 67/bd 12), 35/711, 35/712 (both oc 1/bd 21), 35/713, 35/714 (both oc 67/bd 12), 35/719 (oc 1/bd 21), 35/735 (oc 67/bd 12).

ADDITIONAL MATERIAL (NOT FIGURED). — GIL Nos. 35/708-710, 35/715-717 (all oc 67/bd 12); 35/718, 35/720 (both oc 41/bd 12), 35/721-734, 35/736-749 (all oc 67/bd 12), 35/750-768 (all oc 1/bd 21); 35/769- 777 (all oc 41/bd 12); 35/778 (oc 40/bd 6).

GEOGRAPHICAL AND STRATIGRAPHICAL DISTRIBU- TION. — All material is from the Severnaya Zemlya Formation (Lower Devonian [lower Lochkovian]), October Revolution Island.

DIAGNOSIS. — As for the genus.

DESCRIPTION

No. 35/336 (holotype)

Specimen Nos. 35/336 and 35/337 are preserved together in one nodule ( Figs 6B ; 7B ) along with a poorer specimen, 35/338, not described here. No. 35/336 very well preserved as external mould showing dorsal surface. Total length 25, carapace length 7, preabdomen length 8, postabdomen length 10, including telson. Maximum width 14. Carapace subquadrate with approximately straight posterior margin merging into a genal spine on the right side. Middle of carapace somewhat deformed, but with evidence for at least one median eye in the centre of the carapace. Reniform lateral eye tubercles present. All 13 opisthosomal segments preserved. Fragments of three appendages preserved beyond margin of left side of carapace. Opisthosomal tergite 1 short, tuberculate and occupying full width of carapace. Preabdominal tergites 2-4 large with curving posterior margins and distinct ornament of tubercles. Postabdominal segments taper smoothly towards telson. Segment 5 longest, but successive tergites strongly telescoped together. Postabdominal segments with slight posterior curvature; ornamentation absent. Pretelson with pair of short, styliform epimera. Telson short and styliform.

No. 35/324

Excellent specimen preserved as external mould in dorsal view ( Figs 6A ; 7A ). Total length 34, carapace length 9, preabdomen length 7, postabdomen length 18. Maximum width 13. Carapace well preserved, rounded anteriorly and showing both median and lateral eyes. Median eyes small and located in slight triangular depressions on posteriorly tapering tubercle in centre of carapace. Broad depressions present either side of median eyes. Lateral eye tubercles reniform, each located at the posterior end of what appears to be an ophthalmic ridge. Slight ridge preserved close to anterior margin of the carapace where it follows the carapace curvature. Small prosomal limb fragment preserved on left side of specimen beyond carapace margin. All 13 opisthosomal segments preserved. Tergite 1 short, but lateral tuberculation evident. Tergites 2-4 large, forming the preabdomen, with tergite 4 longest. Preabdomen with strong sculpture of tubercles, with most tubercles concentrated at the lateral margins. Postabdomen elongate with all segments approximately the same length and narrowing gradually posteriorly. Postabdominal segments lack ornamentation. Terminal segment with poorly preserved pair of epimera surrounding a short telson. Telson with slight median depression.

No. 35/337

Specimen preserved in dorsal view as an internal mould ( Figs 6B ; 7B ). Total preserved length 18, carapace length 8, preabdomen length 5, postabdomen length 5, though terminal end of postabdomen missing. Maximum width 12. Carapace rounded anteriorly, highest in the centre where the median eyes would be expected, but morphological details lacking. Preabdomen distorted and lacks detail, though tuberculation at the lateral margins well preserved. Postabdomen telescoped with short segments tapering posteriorly. Terminal end of postabdomen and telson absent.

No. 35/339

Specimen preserved as internal mould in dorsal view ( Figs 8C ; 9C ). Total length 20, carapace length 8, preabdomen length 7, postabdomen length 5. Maximum width 12. Carapace rounded anteriorly and with straight posterior margin, but shows few morphological details. Two appendages, possibly prosomal limbs 3 or 4, preserved beyond carapace margin. Limbs slender with evidence for division into short podomeres. All 13 opisthosomal segments preserved. Tergite 1 distinct and short. Preabdomen poorly preserved with much of tergites 2-4 missing to reveal the featureless ventral plate beneath them. Postabdomen highly telescoped and bends down towards the matrix at the distal end such that posterior tip is absent. First postabdominal segment (segment 5) broken to reveal full length of next segment tucked under it.

No. 35/379

Specimen probably in ventral view, but with prosoma missing ( Figs 8E ; 9E ). Total preserved length 27, preabdomen length 9, postabdomen length 18. Maximum width 12. Preabdomen composed of broad, mostly featureless, but slightly folded area (the ventral plate) overlying tuberculated tergites visible on right margin of preabdomen. Postabdomen well preserved, though segments have become slightly disarticulated and separated by thin areas of matrix. Postabdomen tapers posteriorly with all segments approximately the same length. Postabdominal segments lack ornamentation. Thirteenth segment with clear pair of short, styliform epimera. Short, styliform telson lies between these epimera.

No. 35/707

Specimen preserved in dorsal view ( Figs 8B ; 9B ). Total length 26, carapace length 9, preabdomen length 7, postabdomen length 11. Maximum width 12. Carapace rounded anteriorly with distinct ridge close to carapace margin and median eyes clearly visible as a pair of tiny circles in the centre of the carapace set into slight triangular depressions. Lateral eyes not preserved. Appendage fragments preserved on right side of specimen, including a poorly preserved paddle. Tergite 1 not preserved. Preabdomen poorly preserved with much of the segmentation missing to reveal the ventral plate underneath. Postabdomen quite well preserved, though segmentation indistinct. Postabdomen tapers towards the fragmentory pretelson segment and telson.

No. 35/711

Specimen preserved in ventral view, mostly as an external mould, but with some structures in positive relief ( Figs 6C ; 7C ). Total length 33, prosoma length 7, opisthosoma length 26, tagmosis into preabdomen and postabdomen not clearly demarcated. Maximum width 9. Prosoma poorly preserved, but may show areas corresponding to coxae of prosomal appendages. Prosomal appendages not preserved. Opisthosoma without characteristic broad preabdomen visible in dorsal view. Segment 1 (the metastoma) not clearly preserved. Segments 2-4 expressed as sclerites (probably operculae). Distinct genital appendage preserved on the midline in positive relief, length 4, originating from segment 2 and extending down to posterior margin of segment 3. Genital appendage relatively squat, apparently consisting of a single section, widening posteriorly with a distinctly trilobed terminal end. Postabdomen tapers posteriorly with all segments approximately same length. Epimera on segment 13 poorly preserved, but short, styliform telson present.

No. 35/712

Specimen preserved in ventral view ( Figs 6D ; 7D ). Total length 30, prosoma length 6, preabdomen length 5, postabdomen length 19. Prosoma fairly poorly preserved, but with distinct shield-shaped metastoma representing opisthosomal segment 1. Opisthosomal segments 2-4 expressed as sclerites (probably opercula). Anterior two sclerites strongly curved posteriorly and sculptured with a small number of tubercles. Sclerites 2 and 3 approximately same length, sclerite 4 longer. Sclerite 2 with indeterminate, squat median structure reaching entire length of plate. Median structure apparently composed of rectangular basal section and triangular distal section, again interpreted as a genital appendage. Postabdomen tapers posteriorly, curving slightly to the right, with all segments approximately the same length. Segment 13 with poorly preserved pair of short, styliform epimera. Telson short and styliform.

No. 35/713

Small, probably juvenile specimen in dorsal view ( Figs 8F ; 9F ). Length 11, carapace length 4, opisthosoma length 7. Differentiation into preabdomen and postabdomen not so distinct as in adults. Maximum width 4. Carapace rounded with proportionally large median and lateral eye tubercles. Opisthosoma apparently with full complement of 13 segments, postabdomen tapering posteriorly, though posteriormost segments difficult to distinguish. Telson slender and proportionally longer than in adults.

No. 35/719

Small, probably juvenile, specimen preserved mostly in outline and in dorsal view ( Figs 8G ; 9G ). Total length 8, carapace length 3, preabdomen length 3, postabdomen length 3. Maximum width 3.5. Carapace rounded anteriorly but lacks detail. Eyes and appendages not preserved. Preabdomen rectangular and segmentation not evident. Postabdomen tapers distally where segment boundaries are faintly visible. Telson not preserved.

No. 35/735

Relatively poor specimen in dorsal view in centre of nodule surrounded by a number of additional fragments ( Figs 8A ; 9A ). Total preserved length 27, carapace length 8, preabdomen length 8, postabdomen length 11, though terminal end missing. Maximum width 12. Carapace broadly rounded anteriorly with genal spine on right side, but details lacking. Small paddle clearly preserved on left side at the posterior left corner of the carapace. Terminal three podomeres of paddle preserved, a broad proximal segment, length 3, widening distally and an oval distal segment, length 3. Small diamond-shaped podomere articulates between them. Preabdomen with somewhat indistinct segmentation. Tergites on left side removed during preparation to reveal ventral plate underlying the tergites. Scratches on ventral plate are preparation artefacts. Left side of ventral plate itself further removed to reveal ventral surface of preabdomen at its lateral margins where it forms a doublure tucking under to form ventro-lateral region of opisthosoma. This ventral surface matches outline of dorsal surface and has marginal, ventral tuberculation similar to the marginal dorsal tuberculation. Ventral opercula not revealed during preparation. Postabdomen telescoped, but poorly preserved and segmentation indistinct. Telson faintly preserved.

Chasmataspida incertae sedis

No. 35/714

Poor specimen lacking details, but appears to show different proportions of the postabdomen in comparison to the other material ( Figs 8D ; 9D ). Total preserved length 29, carapace length 8, preabdomen length 7, postabdomen length 14, though not all postabdominal segments preserved. Maximum width 12. Carapace rounded anteriorly, but lacking detail. Appendages absent. Preabdomen appears broader than long but segmentation and ornamentation not preserved. At least seven postabdominal segments preserved, all very narrow compared to preabdomen, average width 3. These segments do not clearly taper posteriorly.

MORPHOLOGICAL RECONSTRUCTION

Carapace

The carapace is broadly rounded anteriorly with a distinct ridge, or rim, close to the anterior margin (No. 35/324, Figs 6A ; 7A ). The posterior margin of the carapace is approximately straight, though in the more complete carapaces small genal spines are present at the postero-lateral corners of the carapace (e.g., No. 35/336, Figs 6B ; 7B ). Genal spines are also known from Chasmataspis and though not explicitly described from Diploaspis and Heteroaspis , appear to be present based on Størmer’s (1972) plates and figures of the Alken an der Mosel fossils. The carapace lacks any obvious ornament, like that seen on the preabdomen. The detailed shape of the carapace in life is difficult to reconstruct since there is evident compression of the material, though there appears to be slight lobes on the carapace between the median and lateral eyes. This, however, would be difficult to interpret as a xiphosuran-like cardiac lobe. There are lateral depressions of the carapace delineated by curving lines posterior to the lateral eyes (No. 35/324, Figs 6A ; 7A , No. 35/713, Figs 8F ; 9F ; 11 ). A similar morphology has been reconstructed for some eurypterid carapaces (e.g., Størmer 1955: fig. 17).

Eyes

The median eyes, or ocelli, are tiny structures located in V-shaped depressions on a small, posteriorly tapering tubercle in the centre of the carapace (Nos. 35/324, 35/707, Figs 6A, B ; 7A, B ). Lateral eye tubercles can be clearly seen in 35/324 as well ( Figs 6A ; 7A ). The lateral eyes are fairly small, kidney-shaped and located close to the lateral margins of the carapace at about the same level as the median eyes. The lateral eyes appear to be situated on slight ridges, at least anterior to the eye tubercle itself, where a short ridge curves laterally towards the anterior carapace rim (No. 35/324, Figs 6A ; 7A ). These are similar to the ophthalmic ridges of xiphosurans, though the presence of this character should be treated with caution as it has been considered an autapomorphy of the latter group ( Anderson & Selden 1997) and is not obvious in Chasmataspis , Diploaspis or Heteroaspis .

Prosomal appendages

The prosomal appendages are poorly preserved. Apart from the metastoma (see below), the morphology of the coxal region and chelicerae (if present) are unknown. However, a number of specimens (e.g., Nos. 35/324, 35/336, 35/707, 35/339, Figs 6A, B ; 7A, B ; 8B, C ; 9B, C ) show fragments of narrow, pediform appendages projecting beyond the carapace. Similar, though dissociated, appendages were described in Diploaspis by Størmer (1972). The present material, with the legs in situ, suggests they were somewhat eurypterid-like, since in xiphosurans the appendages are held entirely beneath the carapace. Unfortunately the detailed morphology of these pediform chasmataspid appendages remains unknown and the fossils do not show the number of podomeres, whether or not they bore spines and whether or not they were chelate (like at least one leg described by Caster & Brooks [1956] from Chasmataspis ).

In addition to these pediform appendages, this new material includes two examples of paddles (Nos. 35/735, 35/707, Figs 8A, B ; 19A, B) similar in size and position to those figured by Størmer (1972) in Diploaspis . These paddles probably represent the modified sixth prosomal appendage, though with an incomplete coxal region this cannot be proven here. Three podomeres of the paddle can be seen in this material, two larger podomeres with a smaller, diamond-shaped podomere in between them (No. 35/735, Figs 8A ; 9A ) which appears to act as some sort of articulation between the “blades” of the paddle. Though strongly resembling the paddles of many eurypterids, and Recent portunid crabs, this morphology differs slightly to the eurypterid paddle (e.g., Størmer 1955), which does not have a diamond-shaped podomere in this somewhat lateral position. Furthermore there is no obvious distal “claw” in the chasmataspid paddle, something that is seen in eurypterids.

Opisthosomal segmentation

This new material, along with an ongoing revision of the Chasmataspis type material, indicates that chasmataspids had 13 opisthosomal segments (e.g., Anderson & Selden 1997), not 12 as reported by Caster & Brooks (1956) and most subsequent authors. The extra-segment comes from the recognition of a short tergite between the carapace and the preabdomen which can be seen in Nos. 35/324 and 35/336 ( Figs 6A, B ; 7A, B ). This is a genuine sclerite, and not just a space where the prosoma and opisthosoma have disarticulated, because in both specimens there are traces of tuberculation on this first tergite. Following this, tergites 2-4 are broad sclerites that form the dorsal surface of the preabdomen (e.g., No. 35/324, Figs 6A ; 7A ), or “buckler” to use Caster & Brooks’ (1956) term. These preabdominal tergites are associated with three ventral opercula (see below). Following the preabdomen, segments 5-13 are interpreted as ringlike segments with fused tergites and sternites, and form the nine-segmented postabdomen (e.g., Nos. 35/324, 35/379, Figs 6A ; 7A ; 8E ; 9E ). The short telson is not considered a true body segment.

Preabdomen

Dorsally the preabdomen is slightly wider than long, somewhat rounded in appearance and is divided into three tergites (tergites 2-4) with slightly curved posterior margins (e.g., Nos. 35/324, 35/336, Figs 6A, B ; 7A, B ). Tergite 4 is the longest and has a pair of broad projections forming the postero-lateral corners. In Chasmataspis the “buckler” of tergites appears to be a fully fused structure similar to the xiphosurid thoracetron, but fusion appears less likely in this new material as there is a distinct overlap between the tergites. The preabdomen has a distinct ornamentation of tubercles (see also Systematics) with rows of larger, oval median tubules merging into a higher concentration of smaller tubules at the lateral margins of the tergites.

Ventrally, there are three preabdominal plates (Nos. 35/711, 35/712, Figs 6C, D ; 7C, D ), presumably corresponding to tergites 2-4 (see also below). This is a significant observation. Both Caster & Brooks (1956) and Størmer (1972) described their chasmataspids as having a preabdomen with a single, fused ventral plate and interpreted this as having a pair of long, posterior slits opening respectively into a pair of pouches containing the gills. This would be very different to what is known from other aquatic chelicerates, i.e. xiphosurans and eurypterids, where there are a series of ventral, plate-like operculae derived from the opisthosomal appendages. Simonetta & Delle Cave (1981) noted the obvious problems of functional morphology in getting water through narrow slits without some sort of pumping mechanism. Their suggestions about how the chasmataspid gills were oxygenated were ingenious, though not entirely convincing, while both they and Størmer (1972) further speculated that the large ventral plate might protect the gills from drying and imply that chasmataspids were at least partially terrestrial.

The new Severnaya Zemlya material suggests that chasmataspids were in fact more typical chelicerates and did have a series of plate-like opercula on the ventral surface of the opisthosoma. Furthermore, unpublished studies by the author in collaboration with Simon Braddy and Lyall Anderson have interpreted some late Cambrian material from Texas as ventral impressions of a Chasmataspis -like animal which also appears to have had a series of preabdominal operculae (see also Dunlop et al. 1999). Unfortunately chasmataspid respiratory organs, and which segments they belonged to, are unknown. Meanwhile, two questions remain from the preabdomen of the fossils described here. Firstly, the ventral operculae are not as broad as the corresponding tergites. This implies a similar situation to Recent xiphosurans in which the tergites (here fused into a thoracetron) continue round onto the ventral surface as a broad doublure (e.g., Størmer 1955), while the gill operculae occupy a trapezoidal space set into this doublure. This interpretation is supported by No. 35/735 ( Figs 8A ; 9A ), which shows essentially a dorsal view but where successive layers of the preabdomen have been removed by preparation. This specimen reveals the ventral surface of the preabdomen at its lateral margins which retains the outline of the dorsal surface, and was likewise tuberculated. This interpretation is made with some reservations as no single specimen shows the entire ventral preabdomen, but the latter feature is shown in the reconstruction ( Fig. 10 ).

Secondly, this specimen, along with Nos. 35/707, 35/339 and 35/379 ( Figs 8B, C, E ; 9B, C, E ) preserves a broad, featureless area within the postabdomen that lacks obvious segmentation. Preparing down through a specimen, removing first the tergites and then this ventral plate, indicated that the plate is sandwiched between the dorsal tergites and the ventral preabdomen (see No. 35/735, Figs 8A ; 9A ). This plate is unusual for chelicerates and furthermore it is probably the same structure as the single ventral plate described by both Caster & Brooks (1956) and Størmer (1972) in their chasmataspids. Its presence is confirmed in this new material, but its interpretation remains difficult. The simplest explanation is that it is the ventral wall of the preabdomen from which the operculae hang. Nothing directly comparable is known from fossil eurypterids or xiphosurans. However examination of Recent xiphosuran material from which the gill operculae were removed showed that these operculae do overly a single plate, essentially the fused true sternites of the opisthosoma, although unlike the chasmataspid plate observed here this has a relatively large space anteriorly where the operculae attach. Re-examination of both Caster & Brooks’ (1956) and

Severnaya Zemlya arthropods

Størmer’s (1972) material will probably be need- ed before this structure can be fully explained.

Genital appendage

One specimen, No. 35/711 ( Figs 6C ; 7C ), shows a structure on the midline of the ventral preabdomen which originates on the anteriormost operculum and extends down to the posterior margin of the next sclerite. Here it widens and divides into three parts. This structure is very similar in size, shape and position to the longer, type A, genital appendage of eurypterids, a character previously thought to be unique (i.e. autapomorphic) for this group. A second specimen, No. 35/712 ( Figs 6D ; 7D ), shows a short, squat structure, again on the midline and located on the anteriormost preabdominal operculum. The structure here consists of a rectangular basal region and a triangular distal region pointing posteriorly. This is not unlike the shorter, type B, genital appendage in eurypterids; see Braddy & Dunlop (1997) for a recent study of eurypterid genital appendages.

A genital appendage in chasmataspids would be a significant discovery, but we must exclude the possibility that Nos. 35/711 and 35/712 are simply misidentified eurypterids. Firstly, there are no other specimens from this locality which can unequivocally be interpreted as eurypterids. Secondly, these two specimens do not show the eurypterid body plan of a preabdomen of seven segments and a postabdomen of five segments. They are much more consistent in overall appearance with chasmataspids and their ninesegmented postabdomen plus very short telson; the telsons of most eurypterids are elongate by comparison. Finally, the genital appendage of eurypterids is associated with a large genital operculum, probably a fusion of two operculae ( Braddy & Dunlop 1997). There is no large genital operculum in these Severnaya Zemlya Devonian specimens, which can therefore be regarded with some confidence as chasmataspids having a genital appendage. This structure also supports the pattern of opisthosomal segmentation presented herein. If the metastoma belongs to opisthosomal segment 1 (see below) then the next segment should be the genital segment in chelicerates (number 2), and indeed the structure interpreted as the genital appendage of chasmataspids originates on this segment.

Metastoma

As well as the genital appendage, No. 35/712 ( Figs 6D ; 7D ) has a shield-shaped plate on the ventral prosoma. This is pointed posteriorly and has clear margins preserved on the anterior and right sides. This plate is very similar in size, position and appearance to the eurypterid metastoma, a plate covering the posterior gnathobases in these animals. Like the genital appendage, a metastoma was thought to be an eurypterid autapomorphy (e.g., Dunlop & Selden 1997), but this new material shows that it was present in chasmataspids too. The metastoma is believed by many authors (e.g., Størmer 1955) to represent the fused appendages of opisthosomal segment 1 and this interpretation would fit the pattern of chasmataspid opisthosomal segmentation presented above.

Postabdomen

The postabdomen comprises nine ring-like segments which originate beneath the preabdominal tergites (e.g., No. 35/379, Figs 8E ; 9E ). The first postabdominal segment, segment 5, is usually longest, with successive segments approximately the same length. The whole postabdomen tapers posteriorly towards the telson. However, there are also a number of “short” specimens in which there is considerable telescoping of the postabdomen (e.g., Nos. 336-7, Figs 6B ; 7B ). Initially, it was suspected that the “short” morphotype represented sexual dimorphism, or perhaps even a different species, but careful observation of these “short” forms in which overlying segments were either removed by preparation or naturally broken revealed complete, “long” segments underneath (e.g., No. 35/339, Figs 8C ; 9C ). Furthermore, the degree of shortening is not consistent between specimens. This type of postabdominal telescoping is known from eurypterids (Simon Braddy pers. comm.), and is presumably taphonomic in origin.

The postabdominal segments lack the preabdominal ornamentation of large tubercles and have a slight curve to their posterior margins. Where the full length of the postabdomen is preserved there is a slight bend in some examples (e.g., No. 35/712, Figs 6D ; 7D ), which suggests that there was some flexibility in this structure. The thirteenth (pretelsonic) opisthosomal segment has a pair of short, tapering, posteriorly directed, epimera (No. 35/379, Figs 8E ; 9E ). The telson originates between these epimera and is similarly short and tapering, though a little longer than the adjacent epimera. This gives the end of the opisthosoma a trifurcate appearance. The telson itself has a slight median dorsal depression (No. 35/324, Figs 6A ; 7A ).

Juveniles

Specimen No. 35/719 ( Figs 7G ; 9G ) is a very small, probably juvenile example, about 8 mm long, preserved with a truncated postabdomen. More interesting is No. 35/708 ( Figs 8F ; 9F ), another small specimen, about 11 mm long. Its status as a juvenile is suggested by the proportionally large median and lateral eye tubercles relative to the rest of the carapace. It also shows that at least some juveniles had the full complement of 13 opisthosomal segments (it has been reported [e.g., Størmer 1955], probably erroneously, that juvenile eurypterids had less segments than adults) and that the telson was relatively long in early instars. This suggests that the short telson of the adult form is somewhat derived relative to a plesiomorphic condition still expressed in juveniles. A number of other small, but poorly preserved specimens in this material probably represent juveniles too.

Incertae sedis

One curious specimen remains among the Severnaya Zemlya material (No. 35/714, Figs 8D ; 9D ). Although preserved in outline, there are few morphological details. However, there are at least seven postabdominal segments preserved that appear to be rather narrower than the postabdomens of the other chasmataspid material from this locality. In fact, it is similar in overall appearance to Chasmataspis . Since the specimen is not well preserved I am reluctant to create a new taxon for what may be a preservational variant, but the specimen is included here for completeness.

PHYLOGENY

Four chasmataspids have been previously described: Chasmataspis , from the Ordovician of the USA ( Caster & Brooks 1956), Diploaspis and Heteroaspis , both from the Lower Devonian of Germany ( Størmer 1972), and Forfarella from the Lower Devonian of Scotland (Dunlop et al. 1999). A Middle Devonian example has also been recognised from Scotland ( Anderson et al. 2000). Two additional taxa, Borchgrevinkium taimyrensis Novojilov, 1959 and “ Eurypterus ” stoermeri Novjilov, 1959, originally described as eurypterids, may also be chasmataspids ( Størmer 1972), but this requires confirmation from the original material.

As a group, chasmataspids are undoubtedly chelicerates ( Dunlop & Selden 1997), but until now their position within Chelicerata was less certain. Chasmataspids were originally interpreted as an order of the Xiphosura ( Caster & Brooks 1956; Stormer 1972; Selden & Siveter 1987). Subsequently, Eldredge (1974) suggested chasmataspids were related to eurypterids based on similar appendages. Though not strictly an Eldredge character, the present study suggests that the paddles of chasmataspids and eurypterids are probably a convergent development in both groups (see also Bergström 1975). Meanwhile, Bergström (1975, 1979, 1980) questioned chasmataspid monophyly, restricting Chasmataspida to Chasmataspis , placing Heteroaspis as Chasmataspida incertae sedis and Diploaspis , with its appendages unlike those of other xiphosurans, close to the ancestry of arachnids. Simonetta & Delle Cave (1981) and Delle Cave & Simonetta (1991) regarded chasmataspids as one of the chelicerate branches radiating from among the “emeraldellids”, i.e. Emeraldella and similar looking “Burgess Shale” type arthropods.

Anderson & Selden (1997) excluded chasmataspids from Xiphosura due to differences in the segmentation and tagmosis of the opisthosoma, correctly recognising the presence of 13 opisthosomal segments. Dunlop & Selden (1997) recognised Chasmataspida as a monophyletic group within Chelicerata, defined by their autapomorphic opisthosomal tagmosis, i.e. a preabdomen of three segments and a postabdomen of nine. Dunlop & Selden (1997) further recognised the clade (Chasmataspida ( Eurypterida + Arachnida)), with xiphosurans as the outgroup. However, this clade was based on a synapomorphy of an opisthosoma of 12 segments, a character we now know to be incorrect for chasmataspids ( Anderson & Selden 1997; Dunlop et al. 1999; this study), and which may be incorrect for eurypterids and scorpions too ( Dunlop & Webster 1999).

The new Severnaya Zemlya fossils are significant in resolving the phylogenetic position of chasmataspids. The genital appendage and metastoma seen in this material represent two strong synapomorphies for Eurypterida and Chasmataspida. These characters were previously autapomorphic for eurypterids, and their presence, and significance, in chasmataspids was noted by Dunlop (1997) and Dunlop et al. (1999). The genital appendage and metastoma could be seen as grounds for referring Chasmataspida to Eurypterida , although chasmataspids have genal spines and the material described here has lateral eyes associated with poorly-defined carapace ridges (both absent in eurypterids). Chasmataspids also have a different pattern of opisthosomal tagmosis. The position of the scorpions, which have been considered as the sister group of eurypterids by some authors (see Dunlop & Webster 1999), may complicate this issue. Chasmataspids share a number of characters both with xiphosurans (e.g., genal spines, opisthosomal doublure,?ophthalmic ridges) and eurypterids (genital appendage, metastoma,?longer legs) and will eventually need to be placed as part of a broader revision of chelicerate phylogeny.

MODE OF LIFE

Størmer (1972) considered Diploaspis and Heteroaspis to be primarily aquatic, especially given the paddles in Diploaspis which suggest an animal that could swim. This interpretation seems reasonable for the Severnaya Zemlya fossils which have paddles too. Like these new Russian forms, Diploaspis and Heteroapsis are thought to have been deposited in quiet, possibly lagoonal waters. Both Størmer (1972) and Simonetta & Delle Cave (1981) speculated that chasmataspids were capable of terrestrial activity, though this proposal was

st

FIG. 12. — Elymocaris urvantsevi n. sp., interpretative drawings of the specimens shown in Fig. 11 ; A, No. 35/377; B, No. 35/373; C, No. 35/372 (holotype). Abbreviations: ab, abdomen; at, anterior tubercle; cp, carapace pleural fold; fu, furca; st, striae; tl, telson. Scale bars: 5 mm.

partly based on the belief that these animals had a ventral opisthosoma covered by a single plate which could protect the gills on land (see discussion above). There is no convincing evidence for chasmataspids having special opisthosomal structures allowing them to be habitually terrestrial; although an amphibious mode of life has been proposed for eurypterids (see Manning & Dunlop 1995 for a review) and even xiphosurans emerge onto land to breed.

With regards to feeding, most chelicerates are predators and though we have no details of the chasmataspid mouthparts it seems likely that they were essentially benthic predators with a mode of life analogous to xiphosurans. Small phyllocarids and the ostracodes recorded from the same formation as the chasmataspids represent a possible source of food, although xiphosurans also dig soft-bodied prey such as worms out of the substrate. Paddles suggest these chasmataspids could swim, although whether they swam to catch prey or to avoid predators (i.e. acanthodian fish) remains speculation. Paddles might also be used to help bury themselves in the substrate.

Subclass PHYLLOCARIDA Packard, 1879

REMARKS

Approximately 70 phyllocarid crustaceans are present among this Severnaya Zemlya material and these were provisionally referred to Ceratiocaridae by Novilskaya et al. (1983). Some additional arthropod fragments also probably represent crustaceans. The fossils are evidently phyllocarids, having a large, bivalved carapace comprising a cephalothoracic shield formed from two pleural folds and an anteriorly projecting rostral plate, and an abdomen projecting from this carapace ending in a telson with a pair of furcal rami. The best specimens are described and figured here (Nos. 35/372-3, 35/375, 35/377, 35/778-9, 35/781-2) ( Figs 11-14 ).

Order ARCHAEOSTRACA Claus, 1888

REMARKS

Rolfe (1969) diagnosed this order on the presence of a hinge line on the carapace, an elongate seventh abdominal (i.e. pretelsonic) segment and a telson produced between two furcal rami, all of which are present in this new Severnaya Zemlya material.

Suborder RHINOCARINA Clarke in Eastman & Zittel, 1900 DIAGNOSIS. — Carapace with a median dorsal plate separating the valves behind rostral plate; last abdominal somite elongated ( Rolfe 1969).

REMARKS

Although tentatively referred to Ceratiocarididae (a family of suborder Ceratiocarina ) by Novilskaya et al. (1983), the new Severnaya Zemlya material appears to have a median dorsal plate and so can be referred to the suborder Rhinocarina sensu Rolfe 1969 .

Family RHINOCARIDIDAE Hall & Clarke, 1888 DIAGNOSIS. — Carapace valves elongate, subovate; median dorsal plate narrow and with chevron ornament; rostral plate and median dorsal plate slightly bent along median carina ( Rolfe 1969).

REMARKS

The other rhinocaridid family mentioned by Rolfe (1969), Ohiocarididae , was diagnosed by him as having subcircular carapace valves, and as such this new Severnaya Zemlya material can be referred to Rhinocarididae with some confidence.

Genus Elymocaris Beecher, 1884

TYPE SPECIES. — Elymocaris siliqua Beecher, 1884 .

ADDITIONAL SPECIES. — Elymocaris urvantsevi n. sp. DIAGNOSIS. — Like Rhinocaris but carapace valves without posteroventral spine; no mesolateral carina, anterior tubercle present; rim with oblique, posteriorly imbricating ridges; rostral plate folded ventrally along two lateral anteriorly converging carinae; telson with broad median ridge ( Rolfe 1969).

REMARKS

The Severnaya Zemlya material is referred to Elymocaris with some reservations. It lacks posteroventral spines on the carapace, which excludes it from Rhinocaris Hall & Clarke, 1888 , and Dithyrocaris Scouler, 1835 , and it has an anterior tubercle, which was reported absent by Rolfe (1969) in Tropidocaris Beecher, 1884 , the other rhinocaridid genus lacking posteroventral spines. There is also a ridge on the telson in this new material. However, some specimens shows structures which could be interpreted as lateral carina, but could equally well be taphonomic folding of the carapace. Furthermore, the folding of the rostral plate, a diagnostic character of Elymocaris , is not obvious in this Severnaya Zemlya material. I am reluctant to create a new genus unnecessarily and so have referred this material to the genus which it most closely resembles, Elymocaris , though I note Rolfe’s (1969) and Racheboeuf’s (1994) comments that many supposed generic characters may only be of specific value. As such, considerable revision of the family may be required.

Elymocaris urvantsevi n. sp.

( Figs 11-15 )

Ceratiocaridae – Novilskaya et al. 1983: 94, fig. 4.

HOLOTYPE. — GIL No. 35/372 (oc 1/bd 21).

ADDITIONAL MATERIAL. — GIL Nos. 35/373 (oc 1/bd 21), 35/375 (oc 41/bd 12), 35/377-378, 35/779 (all oc 67/bd 12), 35/781 (oc 1/bd 21), 35/782.

ADDITIONAL MATERIAL (NOT FIGURED). — GIL Nos. 35/374, 35/376 (both oc 41/bd 12), 35/780 (oc 1/bd 21), 35/783-790 (all oc 67/bd 12), 35/791-817 (all oc 1/bd 21), 35/818-837 (all oc 41/bd 12).

ETYMOLOGY. — In honour of N. N. Urvantsev, a geologist who participated in the first expedition to Severnaya Zemlya in 1930-1932.

GEOGRAPHICAL AND STRATIGRAPHICAL DISTRIBU- TION. — All material is from the Severnaya Zemlya Formation (Lower Devonian [lower Lochkovian]), October Revolution Island.

DIAGNOSIS. — Elymocarid with a downwardcurving, hook-shaped rostral plate.

DESCRIPTION

No. 35/372 (holotype)

Almost complete specimen in lateral view ( Figs 11C ; 12C). Total length 51, carapace length 29, abdomen length, including telson, 22. Maximum carapace width 13. Rostral plate faint, hook-shaped. Cephalothoracic shield preserved as impression of left pleural fold, broadly oval, straighter along the dorsal surface and with a distinct, narrow margin; posterior end of shield obscured. Pleural fold with broad, oblique folds (?carina) below which is a large depression in the antero-ventral region. Pleural fold ornamented with fine antero-posterior striae, closely packed near the rostal plate, more widely spaced in the posterior ventral region of the shield. Abdomen tapers posteriorly, individual pleomeres difficult to distinguish. Styliform telson preserved along with one shorter furcal ramus. Appendages not preserved.

No. 35/373

Disarticulated carapace and abdomen ( Figs 13A ; 14A ). Maximum carapace length 14, though posterior end of carapace not preserved. Maximum carapace width 8. Rostral plate apparently hook-shaped, but detached from rest of carapace and surrounded by other cuticle fragments. Right pleural fold of cephalothoracic shield preserved, broadly oval in outline with narrow margin, but details of morphology poor. Abdomen detached from carapace, maximum length including telson 17. Five abdominal pleomeres preserved, possibly in ventral view, plus telson. Pleomere lengths: 3, 2, 1.5, 2, 3.5, average width 4, but narrowing posteriorly. First pleomere poorly preserved, second with faint ridge, third with large tubercle, fourth and fifth with distinct ventral ridges. Telson styliform with median groove, length 6. Both styliform furcal rami preserved, length 3. Appendages not preserved.

No. 35/375

Terminal end of abdomen in dorsal view ( Figs 11B ; 12B). Maximum length 23. Division of abdomen into pleomeres indistinct, but abdomen tapers posteriorly. Telson styliform, length 15, with broad median depression. Furcal rami also styliform and with median depression, but shorter than telson, length 8. Right ramus better preserved than left ramus.

No. 35/377

Large carapace preserved in positive relief ( Figs 11A ; 12A). Maximum preserved length 40, though posterior end of carapace absent. Maximum width 19. Rostral plate absent. Cephalothoracic shield preserved as internal mould of right pleural fold. Cephalothoracic shield broadly oval in outline though straighter along dorsal margin where a slight fold indicates a median dorsal plate was present, visible more clearly when specimen is examined from above. Pleural fold with narrow margin and oblique fold running from near where the rostal plate would be to the ventral margin about two thirds along the length of the carapace. Depression occurs below this fold, but with rounded, raised anterior tubercle nearer the rostral end of the cephalothoracic shield. Posterior margin of pleural fold broken to reveal underlying abdominal pleomeres. Rest of abdomen and appendages not preserved.

No. 35/378

Almost complete specimen in lateral view ( Figs 13D ; 14D ). Maximum preserved length 31, carapace length 17, abdomen length, including telson, 14. Maximum carapace width 9. Rostral plate obscured. Cephalothoracic shield preserved as cast of left pleural fold, but with the ventral half restricted to an impression of the carapace. Large area dorsal to specimen probably deformed right pleural fold and joined to left pleural fold by median dorsal plate. Pleural fold broadly oval, straighter along the dorsal surface with a distinct, narrow margin; ornamentation not preserved, but broad, slightly oblique ridge runs dorso-ventrally. Posterior margin of shield slightly broken to reveal at least three underlying pleomeres; anterior two short, posterior one longer. Abdomen tapers posteriorly. Styliform telson and one poor furcal ramus preserved. Appendages not preserved.

No. 35/779

Carapace preserved in lateral view ( Figs 13E ; 14E ). Maximum preserved carapace length 25. Maximum carapace width 14. Rostral plate hook-shaped, curving ventrally, but slightly displaced. Cephalothoracic shield preserved as crystaline internal mould of right pleural fold, though crystaline part missing from ventral region. Pleural fold broadly oval, straighter along the dorsal surface and with a distinct, narrow margin. Part of median dorsal plate clearly preserved above the specimen. Oblique line originating close to the rostral plate creates a depression in the anterior ventral part of the cephalothoracic shield, though this effect may be exaggerated by the loss of the crystaline part in this region noted above. A second oblique line runs close to the margin of the cephalothoracic shield. Posterior part of pleural fold broken to reveal three underlying pleomeres. Remainder of abdomen and appendages not preserved. Additional, poorer specimen (not figured) preserved in same nodule.

No. 35/781

Specimen preserved in lateral view ( Figs 13C ; 14C ). Maximum preserved length 30, carapace length 19, abdomen length 11, though not all of abdomen preserved. Maximum carapace width 9. Rostral plate well preserved, hook-shaped, curving ventrally. Cephalothoracic shield preserved as impression of left pleural fold. Pleural fold broadly oval with distinct, narrow margin. Fine striae preserved near postero-dorsal region of the pleural fold, above where the abdomen originates. Anterior tubercle clearly preserved. Dark area below rostal plate, adjacent to margin of cephalothoracic shield, could be remains of an eye. Abdomen originates approximately a third of the way into the carapace, though individual pleomeres not discernable. Possible dark gut trace preserved within abdomen. Telson and furcal rami indistinct. Appendages not preserved.

No. 35/782

Relatively poor specimen in lateral view ( Figs 13B ; 14B ). Maximum length 33, carapace length 20, abdomen length 13. Maximum carapace width 10. Rostral plate hook-shaped, curving ventrally. Cephalothoracic shield preserved as impression of left pleural fold with narrow margin, but details of morphology poor. Pleural fold broadly oval with oblique striae present near dorsal margin. Grey area below rostral plate adjacent to margin of cephalothoracic shield may be remains of an eye. Abdomen possibly displaced, emerging rather ventrally from the carapace and individual pleomeres not discernable. Telson styliform, apparently with shorter, styliform furcal ramus preserved dorsal to the telson. Appendages not preserved.

MORPHOLOGICAL RECONSTRUCTION

The crustaceans range in size from very large, No. 35/789 (not figured) lacks detail but is at least 11 cm long, through to small specimens of about 1 cm. Morphological terminology is based on that used by Vannier et al. (1997) and Rolfe (1969). Appendages are not preserved in the Severnaya Zemlya Devonian material, but in phyllocarids they comprise two pairs of large antennae, thoracic appendages typically held within the carapace, and a series of abdominal pleopods projecting beyond the carapace and used in swimming. A reconstruction of the animal in life attitude is presented in Fig. 15 .

Carapace

The overall morphology of the carapace is best seen in specimen Nos. 35/377, 35/372 35/781, 35/378, 35/779 ( Figs 11A, C ; 12A, C; 13C-E; 14C-E). The paired pleural folds are broadly elongate and are curved both dorsally and ventrally, giving the whole carapace an oval shape in lateral view. The pleural folds have a distinct marginal rim (No. 35/781, Figs 13C ; 14C ) and are connected by a median dorsal plate behind the rostral plate (No. 35/779, Figs 13E ; 14E ). This plate appears to be strongly attached to the pleural folds; the division between the two is not well preserved in this material (Nos. 35/782,

rp

35/781, Figs 13B, C ; 14B, C ). The rostral plate has a distinct, downward-curving hook shape (see also Systematics). It is difficult to say whether or not lateral carina (i.e. strong ridges on the pleural folds) were present. Two specimens show what appear to be folds or carina on the carapace (Nos. 35/372, 35/375, Figs 11C ; 12C; 13A; 14A), but these are not in the same position and could be taphonomically induced as they are not seen at all in other material. Similarly, two specimens show strong evidence for an anterior tubercle on the pleural fold (Nos. 35/377, 35/781, Figs 11A ; 12A; 13C; 14C) although it is not unequivocally present in every specimen. Some specimens also suggest that the antero-ventral region of the carapace had a slight oblique fold below which it was slightly depressed (e.g., No. 35/377, Figs 11A ; 12A).

When examined in detail the carapaces of Nos. 35/372 and 35/781 ( Figs 11C ; 12C; 13C; 14C) clearly show a fine ornament of ridges, or striae. These striae are short, fine and closely packed near the rostral plate where they run approximately antero-dorsally ( Figs 11C ; 12C). Meanwhile longer, oblique striae running postero-dorsal to antero-ventral can be seen near

Severnaya Zemlya arthropods the median dorsal plate ( Figs 13C ; 14C ). Larger and more widely spaced striae are present in the ventral part of the carapace where they run longitudinally, following the ventral curvature of the pleural fold ( Figs 11C ; 12C). Similar striae have been reported in fossil phyllocarids by Racheboeuf (1994) and Vannier et al. (1997) where they have been interpreted as an aid to burrowing (see below).

Abdomen

The phyllocarid abdomen is composed of a series of seven segments, or pleomeres, the anteriormost of which originates beneath the carapace. The abdomen can be seen overlapped by the carapace, e.g., No. 35/781 ( Figs 13C ; 14C ) where the abdomen occupies the posterior third of the space beneath the carapace. Up to five of the seven pleomeres can be seen in this material (No. 35/375, Figs 14A ; 15A ). No. 35/781 ( Figs 13C ; 14C ) preserves a possible gut trace running along the length of the abdomen. The abdomen tapers posteriorly and in No. 35/375 ( Figs 13A ; 14A ), which appears to be in ventral view judging from the position of the emerging furcal rami, there is ornamentation in the form of ridges on the posterior abdominal segments. The pretelsonic pleomere (segment 7) is longer than the preceeding pleomeres (No. 35/375, Figs 13A ; 14A ). The telson is elongate and styliform (e.g., No. 35/373, Figs 11B ; 12B) with a pair of furcal rami either side. The furcal rami are also styliform, but are shorter and originate ventrally close to the base of the telson (No. 35/378, Figs 13D ; 14D ). Both the telson and furcal rami are ornamented with ridges (No. 35/373, Figs 11B ; 12B).

PHYLOGENY

There are morphological differences and inconsistencies among the specimens described here. However, since these morphological differences do not divide the material neatly into two or more groups, and since there has been taphonomic distortion, all the material is referred to a single species. The systematics and phylogeny of phyllocarids remain in a somewhat unsatisfactory state (see comments by Rolfe [1969]) with no clear indication of polarity for the characters used to differentiate phyllocarid taxa. Such a revision is beyond the scope of the present work.

MODE OF LIFE

Vannier et al. (1997) studied living phyllocarids in an attempt to determine the palaeobiology of the Palaeozoic forms. They concluded that most Palaeozoic phyllocarids were capable of swimming, both as pelagic and nekto-benthonic forms, but that they also showed adaptations for burrowing, such as the striae on the carapace observed in the new Severnaya Zemlya material. These striae may have helped channel the sediment past the carapace during burrowing. Vannier et al. (1997) speculated that some Palaeozoic phyllocarids burrowed during the day to avoid predatory fish, emerging at night to feed in the water column. Considering the predatory acanthodians present in the Severnaya Zemlya Formation, this mode of life seems appropriate for the new phyllocarid material too.

Figures

FIG. 6. — Octoberaspis ushakovi n. gen., n. sp.; A, No. 35/324, 34 × 13 mm; B, Nos. 35/336 (holotype) (above), 24 × 14 mm, and 35/337 (below), 18 × 12 mm; C, No. 35/711, 33 × 9 mm; D, No. 35/712, total length 30 mm.

FIG. 7. — Octoberaspis ushakovi n. gen., n. sp., interpretative drawings of the specimens shown in Fig. 6; A, No. 35/324; B, Nos. 35/336 (holotype) (above) and 35/337 (below); C, No. 35/711; D, No. 35/712. Abbreviations: ap, prosomal appendage; cp, carapace; cr, marginal carapace rim; ep, epimera of pretelsonic segment; ga, genital appendage; gs, genal spine; le, lateral eye tubercle; me, median eye tubercle; ms, metastoma; op, opisthosomal operculae; pra, preabdomen; poa, postabdomen; tb, preabdominal tuberculation; tl, telson, opisthosomal segments numbered. Scale bar: 5 mm.

FIG. 8. — A-C, E-G, Octoberaspis ushakovi n. gen., n. sp.; A, No. 35/735, 27 × 12 mm; B, No. 35/707, 26 × 12 mm; C, No. 35/339, 20 × 12 mm; E, No. 35/379, 27 × 12 mm; F, No. 35/713 (juvenile), 11 × 4 mm; G, No. 35/719 (juvenile), 8 × 3.5 mm; D, Chasmataspida incertae sedis, No. 35/174, 29 × 12 mm.

FIG. 9. — Interpretative drawings of the specimens shown in Fig. 8; A-C, E-G, Octoberaspis ushakovi n. gen., n. sp., A, No.35/735; B, No. 35/707; C, No. 35/339; E, No. 35/379; F, No. 35/713 (juvenile); G, No. 35/719 (juvenile); D, Chasmataspida incertae sedis, No. 35/714. Abbreviations: ap, prosomal appendage; cp, carapace; cr, marginal carapace rim; ep, epimera of pretelsonic segment; gs, genal spine; le, lateral eyes; me, median eyes; od, opisthosomal doublure where tergites tuck round onto ventral surface; pd, paddle (prosomal appendage VI?); pra, preabdomen; poa, postabdomen;tl, telson; vp, ventral plate underlying tergites. Scale bars:5 mm.

FIG. 10. — Reconstruction of Octoberaspis ushakovi n. gen., n. sp. in both dorsal and ventral view. Chelicerae and leg coxae hypo- thetical, based on comparisons with eurypterids and xiphosurans. Scale bar: 5 mm.

FIG. 11. — Elymocaris urvantsevi n. sp.; A, No. 35/377, 40 × 19 mm; B, No. 35/373, 14 × 8 mm; C, No. 35/372 (holotype), total length 51 mm.

FIG. 13. — Elymocaris urvantsevi n. sp.; A, No. 35/375, maximum length 23 mm; B, 35/782, maximum length 33 mm; C, 35/781, maximum preserved length 30 mm; D, No. 35/378, maximum preserved length 31 mm; E, No. 35/779, maximum preserved carapace length 25 mm, maximum carapace width 14 mm.

FIG. 14. — Elymocaris urvantsevi n. sp., interpretative drawings of the specimens shown in Fig. 13; A, No. 35/375; B, 35/782; C, 35/781; D, No. 35/378; E, No. 35/779. Abbreviations: ab, abdomen; at, anterior tubercle; cp, carapace pleural fold; gt, possible gut trace; fu, furca; mdp, median dorsal plate; rp, rostral plate; st, striae; tl, telson; abdominal segments numbered (note longer segment 7). Scale bars: 5 mm.

FIG. 15. — Reconstruction of Elymocaris urvantsevi n. sp. in lateral view. Appendages reconstructed by comparison to other phyllo- carids. Scale bar: 5 mm.