1. Introduction
Exceptionally preserved biotas (EPBs) are important fossil assemblages that provide the most complete record of Earth’s life forms and palaeo-communities through time. Silurian EPBs have been less studied than those of other periods. They include the Herefordshire (e.g. Briggs et al. Reference Briggs, Siveter, Siveter, Sutton, Garwood and Legg2012), Leintwardine (also in Herefordshire, e.g. Gladwell, Reference Gladwell2018) and, in North America, the Waukesha (Kleussendorf & Mikulic, Reference Kluessendorf and Mikulic2017; Anderson et al. Reference Anderson, Schiffbauer, Jacquet, Lamsdell, Kluessendorf and Mikulic2021; Gass & Braddy, Reference Gass and Braddy2023), Scotch Grove (van Iten et al. Reference Van Iten, Hughes, John, Gaines and Colbert2023) and Eramosa (von Bitter et al. Reference Von Bitter, Purnell, Tetreault and Stott2007) EPBs.
These cases represent a range of palaeo-environments from marginal to open marine. Kleussendorf (Reference Kluessendorf2007), discussing North American Silurian EPBs, inferred that a eurypterid-phyllocarid association (in the Williamsville, Syracuse, Pointe-aux-Chenes, Kokomo and Waubakee biotas) represented hypersaline peritidal lagoons. Vrazo et al. (Reference Vrazo, Brett and Ciurca2017: 383) further discussed the North American eurypterid-bearing units, concluding that their occurrence was ‘controlled by the presence of an ecological–taphonomic window that recurred predictably in nearshore, marginal environments during transgressions’.
Recent collecting from the uppermost 5–10 m of the Ustya Formation along the Smotrych River in the Silurian of Podolia, in a geological context comparable with that of the North American eurypterid-bearing EPBs, has yielded several new arthropods, the first of which, a synziphosurine euchelicerate, is described here.
The suborder Synziphosurina was erected by Packard (Reference Packard1886) for a number of merostome arthropods which could be placed into neither Eurypterida nor Xiphosura, but appeared to be bridging forms between these groups. He included in the suborder Bunodes von Eichwald, Reference Eichwald1854, Hemiaspis Woodward, Reference Woodward1865, Pseudoniscus Nieszkowski, Reference Nieszkowski1859, Exapinurus Nieszkowski, Reference Nieszkowski1859 and (with reservation) Neolimulus Woodward, Reference Woodward1868. Of these, Hemiaspis species are now split between Bunodes and Limuloides Woodward, Reference Woodward1865, and Exapinurus is in Bunodes (see Howard, Reference Howard2024, for the complex nomenclature surrounding Limuloides). Eldredge (Reference Eldredge1974) and Bergström (Reference Bergström1975) revised the suborder; the former author included within it Weinbergina Richter and Richter, Reference Richter and Richter1929, Legrandella Eldredge, Reference Eldredge1974, Bunodes and Limuloides, whilst Bergström (who considered it an order) included the same genera except for Legrandella (which was mentioned in an addendum). Both of these authors included Pseudoniscus and Neolimulus within the xiphosurids. Anderson and Selden (Reference Anderson and Selden1997), in the first cladistic analysis of Palaeozoic Xiphosura, showed that the synziphosurines formed a paraphyletic grade of stem-group horseshoe crabs, rather than a monophyletic group. More recently, phylogenetic analyses including synziphosurines within a broader sample of Euchelicerata by Lamsdell (Reference Lamsdell2013, Reference Lamsdell2016), Lamsdell et al. (Reference Lamsdell, Briggs, Liu, Witzke and McKay2015) and Selden et al. (Reference Selden, Lamsdell and Qi2015) have shown synziphosurines to be polyphyletic. Half of synziphosurine taxa (e.g. Legrandella and Dibasterium Briggs et al. Reference Briggs, Siveter, Siveter, Sutton, Garwood and Legg2012) resolved in the euchelicerate stem group, while the rest, the more traditional synziphosurines such as Limuloides, Pseudoniscus and Bunodes, as well as Houia (Lamsdell et al. Reference Lamsdell, Xue and Selden2013) from China, resolved between Xiphosurida and Dekatriata Lamsdell, Reference Lamsdell2013 (Chasmataspida + Eurypterida + Arachnida) (Selden et al. Reference Selden, Lamsdell and Qi2015: figure 6). Some recent analyses of synziphosurine relationships, in the Supplemental Material of Lustri et al. (Reference Lustri, Antcliffe, Gueriau and Daley2024 a, b), located Bunodidae Packard, Reference Packard1886 (essentially Bembicosoma Laurie, Reference Laurie1899, Bunodes and Limuloides and probably also Pasternakevia Selden & Drygant, Reference Selden and Drygant1987) as sister to Pseudoniscidae Packard, Reference Packard1886 (Pseudoniscus and Cyamocephalus Currie, Reference Currie1927), chasmataspids, eurypterids and arachnids, but not Xiphosura (which occupied a more basal position as sister to this entire grouping). More recent phylogenetic analysis (Lamsdell, Reference Lamsdell2025, supplementary figure S1) resolved an expanded Bunodidae (including both bunodids and pseudoniscids) as sister to Xiphosura.
The new synziphosurine described here, Smotrychaspis kurtopleurae gen. et sp. nov., belongs to the traditional synziphosurine group, which contains the families Bunodidae and Pseudoniscidae. Whilst the new animal shows similarities to both these families, it cannot be housed in either, and so remains Euchelicerata incertae sedis.
2. Geological setting
Silurian rocks of Podolia, Ukraine, were deposited in an epicontinental basin on the East European Platform, part of Baltica, with water depth increasing in a WSW direction from the Ukrainian Shield towards the Teisseyre-Tornquist Zone (Skompski et al. Reference Skompski, Luczyjski, Drygant and Koziowski2008; Mazur et al. Reference Mazur, Mikolajczak, Krzywiec, Malinowski, Buffenmyer and Lewandowski2015; Skompski et al. Reference Skompski, Koziowska, Koziowski and Luczyjski2023). A belt of carbonate deposition 150–200 km wide extends along strike for nearly 2000 km with a central zone of crinoidal and stromatoporoid-coral shoals separating an inner shelf facies from outer shelf and slope settings.
The Silurian shallow-water inner shelf facies are represented in the Podolia region mainly by fine-grained laminated dolomicrites (see Reference MazzulloMazzullo, 2000, for discussion) containing common ostracodes and eurypterids. Desiccation cracks have been locally observed (e.g. Naugolnykh & Shpinev, Reference Naugolnykh and Shpinev2019), indicating coastal peritidal depositional environments. Laminated dolomicrites are commonly intercalated with levels rich in stromatoporoids (Skompski et al. Reference Skompski, Luczyjski, Drygant and Koziowski2008).
Recent collecting from the Ustya Formation (D. Polypenko, pers. comm) has yielded eurypterids, synziphosurine chelicerates including Pasternakevia podolica (Selden & Drygant, Reference Selden and Drygant1987; Krzeminski et al. Reference Krzeminski, Krzeminska and Wojciechowski2010) and several undescribed forms, ceratiocarids, leperditicopid ostracodes, orthoconic and breviconic nautiloid cephalopods, bivalves, lingulid brachiopods and marine algae.
Jarochowska et al. (Reference Jarochowska, Munnecke, Frisch, Ray and Castagner2016) reasoned that the Ustya Formation ranges in age from Homerian (Wenlock) to Gorstian (Ludlow) age, with the Wenlock-Ludlow boundary within the middle to upper Ustya Fm (Figure 1). While cautioning (Jarochowska et al. Reference Jarochowska, Munnecke, Frisch, Ray and Castagner2016: 189) that the “stratigraphic distribution of Ctenognathodus is not well-known, as this genus shows a strong affinity to shallow-water, lagoonal environments”, elements of Ctenognathodus sp. S found 7 m below top Ustya Formation were placed in the C. murchisoni Zone (uppermost Homerian). They also stated (Jarochowska et al. Reference Jarochowska, Munnecke, Frisch, Ray and Castagner2016: 193) that “Based on correlations with the conodont succession in Saaremaa, Estonia (Viira and Einasto, Reference Viira and Einasto2003), it is more likely that the uppermost Ustya Fm., particularly the interval bearing C. jeppssoni, is of early Ludlow age.” Since this is the interval containing the fauna described here (D. Polypenko, pers. comm), the age of the Smotrych EPB is most probably Lower Ludlow (Gorstian) to topmost Homerian.
Location map and stratigraphy of the fossil locality on the Smotrych river, near Velikozalissya, Podolia, Ukraine. Horizon: uppermost 5–10 m of the Ustya Formation (Silurian: topmost Wenlock: Homerian to Ludlow: Gorstian; ∼427 Ma).
3. Materials and methods
3.a. Preservation and morphological interpretation
The specimen is preserved as part and counterpart in very fine-grained dolomicrite. The slab bearing the specimen in convex relief, and showing mainly ventral features of the exoskeleton, is deemed the part (Figures 2 and 3); the slab with the specimen in concave relief, preserving mostly dorsal features, is the counterpart (Figure 4). The larger slab, bearing the part (Figure 2) is some 32 cm at its widest, by 21 cm. Numerous other faunal elements are present on the slab, including eurypterid fragments, two tergites of Pasternakevia, and an orthicon nautiloid (Figure 2).
Smotrychaspis kurtopleurae gen. et sp. nov. holotype G.2025.2.1. (a) Photograph of entire slab bearing the part. (b) Interpretive drawing of the slab, showing associated fauna.
Smotrychaspis kurtopleurae gen. et sp. nov. holotype G.2025.2.1. (a) Photograph of part. (b) Interpretive drawing of part. All scale bars = 5 mm. Tergites numbered 1–11.
Smotrychaspis kurtopleurae gen. et sp. nov. holotype G.2025.2.1. (a) Photograph of counterpart. (b) Interpretive drawing of counterpart. All scale bars = 5 mm. Tergites numbered 1–11.
The external surface of the carapace (the counterpart) is smooth. Both parts and counterparts bear fragments of darker material interpreted as cuticle remnants (shown in grey on Figures 3 and 4). On the counterpart (Figure 4), these remnants lie mainly along the anterior edges of the tergites (where the tergites overlap one another), along the edges of the epimera, and around the edges of the carapace. On the part (Figure 3), cuticle remnants mainly occur on lateral areas of the axial parts of the tergites, parts of the epimera, around the edges of the carapace, at the base of the telson and in radiating lines on the carapace. In some studies (e.g. Lustri et al. Reference Lustri, Antcliffe, Gueriau and Daley2024 a, b), these radiating cuticle traces have been interpreted as appendages. However, no podomeres can be discerned, and it is likely that the traces mark ridges between concavities on the ventral surface of the carapace, which correspond to the prosomal appendages. As such, spaces can be seen for the chelicerae (prosomal appendage I), followed by five corresponding to post-cheliceral appendages (II–VI).
The anterior and lateral margins of the carapace form a continuous, smooth curve, similar to part of an ellipse, which terminates posterolaterally in an acute renal angle. The anterior border of the carapace shows a narrow strip which may be interpreted as the doublure beneath the carapace rim. The posterior border of the carapace is preserved laterally, but the median portion is broken in the fossil, revealing the first opisthosomal tergite beneath. Hence, the border is reconstructed as a smooth recurved shape (Figure 5). Eleven opisthosomal tergites are present, 1–10 slightly overlapping the more posterior on their posterior median border. Laterally, the termites have epimera developed, which are more prominent and backwardly curbed on more posterior tergites, except 11, which is a narrower pretelsonic tergites lacking large epimera. The posterior border of the pretelsonic tergite is procurved around a depressed area, which bears brown cuticle on the (ventral) part (Figure 3). This is interpreted as the area where a muscle for operating the telson would be attached. The telson shows a ridge with small nodes on the part (Figure 3), which is presumed to be dorsal because it is in positive relief. The length of the telson is not determinable, but more is preserved on the counterpart.
Smotrychaspis kurtopleurae gen. et sp. nov. Reconstruction.
3.b. Study methods
The specimen was studied using a Leica MZ16 stereomicroscope and photographed with a Canon EOS 5DSR digital camera on the microscope. Photographs were captured using DSLR Assistant (www.dslrassistant.com) running on Mac OS 15, and manipulated in Affinity Photo 2.5 (affinity.serif.com), in dry conditions with low-angle incident light. Several shots at different focus levels were stacked using Focus Merge in Affinity, and several stacks taken across the specimen were merged into a composite using the Panorama tool (see Selden, Reference Selden2014). Drawings and the reconstruction were made from the photographs using Affinity Designer 2.5 (affinity.serif.com). Measurements were made using Graphic 3.1 (www.graphic.com).
4. Systematic palaeontology
Chelicerata Heymons, Reference Heymons1901
Euchelicerata Weygoldt & Paulus, Reference Weygoldt and Paulus1979
Smotrychaspis gen. nov.
Etymology. The genus name comes from the type locality, the Smotrych river, and -aspis, a common suffix for synziphosurines, from the Greek ᾰ̓σπίς, a shield.
Type and only species. Smotrychaspis kurtopleurae gen. et sp. nov.
Diagnosis. Euchelicerate with 11 visible tergites, lacking anterior median carapace projection. Carapace horseshoe-shaped in dorsal outline; smooth dorsally, lacking cardiac or ocular ridges; with recurved posterior margin; small, acute genal angles; approximately as wide as long; length ∼⅓ of total (exc. telson), ∼2× that of opisthosoma. Opisthosoma with subparallel sides from tergites 1 to 9; increasing gape between progressively more falcate epimera posteriorly. Styliform telson ∼½ length of opisthosoma; stout.
Smotrychaspis kurtopleurae sp. nov.
Diagnosis. As for the genus.
Etymology. The species epithet kurtopleurae is derived from the Greek ᾰ̓σπίς, meaning hooked, and πλϵυρόν, a rib, with reference to the hooked lateral edges of the posterior pleural segments.
Material, locality, horizon. Specimen G.2025.2.1, part and counterpart, held in the National Museums of Scotland, Edinburgh. Collected from the banks of the Smotrych river, near Velikozalissya, 48°47′6.71″N 26°32′6.84″E, Podolia, Ukraine. Horizon: uppermost 5–10 m of the Ustya Formation (Silurian: topmost Wenlock: Homerian to Ludlow: Gorstian; ∼427 Ma).
Description. Total length inc. telson 36.2, exc. telson 28.1. Carapace horseshoe-shaped in outline, slightly convex dorsally, with somewhat more extended genal angles (∼50°); anterior margin with narrow doublure, posterior margin recurved, straighter in axial region, lateral parts slightly recurved; total length (inc. genae) 11.0, sagittal length 9.2, maximum width 14.4, posterior width across genae 13.9; L/W ratio 0.66; no visible anterior median projection. Carapace length ∼⅓× total length of animal (exc. telson). Carapace ventral (interior) surface bearing trace of segment in posterior, axial part, length 1.6, width 5.8. Central part of carapace, immediately anterior to segment trace, bearing trace of subcircular structure, ∼3.0 × 3.0; lines radiate from this structure at angles of (from posteriormost forward) ∼190°, ∼180°, ∼110°, ∼75°, ∼30°. Anteriormost two lines each joined by a curve at the anterior of the carapace, forming a bilobed structure on either side of the midline.
Opisthosoma parallel parallel-sided over anterior ∼80% of length, with axial region and lateral epimera. Total length (exc. telson) 18.8, maximum width 13.2, L/W ratio 1.4, minimum width 12.0, width of axial region 7.4. Eleven visible tergites (I–XI), measurements (sagittal length, axial width, total width): I 1.1, 5.9, 14.0; II L 1.6, 7.1, 13.4; III 1.8, 6.9, 12.5; IV 1.8, 6.7, 12.3; V 1.6, 6.9, 11.9; VI 1.5, 7.0, 12.3; VII 1.6, 7.2, 12.5; VIII 1.9, ∼7.4, 12.3; IX 2.2, ∼6.9, 12.0; X ∼1.7, 6.2, 10.3; XI 2.0, ∼4.5, 7.1. Division line between axial and pleural regions of tergites is parallel for anterior 5 tergites, then diverges on tergites 6–7, where it ends. More posterior tergites are longer and lack clear division between axial and pleural regions. Epimera become more posteriorly curved and pointed posteriorly; those on tergites IV–IX bear a groove along their lengths. Telson length >9.4, width at base 2.7.
4. Discussion
4.a. Identity of the new taxon
Synziphosurines, previously considered to be the stem group of the Xiphosurida (Anderson & Selden, Reference Anderson and Selden1997), have since been shown to form a polyphyletic group of chelicerate arthropods composed of various basal euchelicerates (i.e. Chelicerata excluding Pycnogonida) (Lamsdell, Reference Lamsdell2013). In Lamsdell’s (Reference Lamsdell2013) analysis, the Herefordshire Lagerstätte euchelicerates Offacolus Orr et al. Reference Orr, Briggs, Siveter and Siveter2000 and Dibasterium (= family Offacolidae Sutton et al. Reference Sutton, Briggs, Siveter and Orr2002), which show biramous appendages, are resolved as sister to all other euchelicerates, the Prosomapoda Lamsdell, Reference Lamsdell2013. Within Prosomapoda, Xiphosura sensu stricto forms a sister group to Planaterga Lamsdell, Reference Lamsdell2013, consisting of the bunodid and pseudoniscid synziphosurines, as well as a group called Dekatriata Lamsdell, Reference Lamsdell2013, which includes chasmataspidids, eurypterids and arachnids. Also in Prosomapoda is a stem group of other synziphosurines, e.g. Camanchia Moore et al. Reference Moore, Briggs, Braddy and Shultz2011, Legrandella Eldredge, Reference Eldredge1974, Venustulus Moore, in Moore et al. Reference Moore, Briggs, Braddy, Anderson, Mikulic and Kluessendorf2005, and Weinbergina Richter and Richter, Reference Richter and Richter1929. The relationships proposed by Lamsdell (Reference Lamsdell2013) are well expressed in a stratigraphic context in Figure 3 of Garwood and Dunlop (Reference Garwood and Dunlop2023). More recently, Lustri et al. (Reference Lustri, Gueriau and Daley2024 b) described a new synziphosurine, Setapedites Lustri et al. Reference Lustri, Gueriau and Daley2024 b, from the Ordovician Fezouata biota, as a member of the basal euchelicerate Offacolidae, and Lustri et al. (Reference Lustri, Antcliffe, Gueriau and Daley2024 a) redescribed the Silurian synziphosurine Bunaia woodwardi Clarke, Reference Clarke1919 from new material, also placing it in Offacolidae. This family is diagnosed mainly on appendage morphology (e.g. biramous prosomal appendages) and a bifurcate tip to the telson (Sutton et al. Reference Sutton, Briggs, Siveter and Orr2002; Lustri et al. Reference Lustri, Gueriau and Daley2024 b). Thus, at present, synziphosurines occur as stem taxa in three places in the euchelicerate tree: sister to all Prosomapoda (Offacolidae), stem to Xiphosura + Planaterga and within Planaterga as stem to Dekatriata (see Garwood & Dunlop, Reference Garwood and Dunlop2023: Figure 3).
Smotrychaspis lacks the buckler of chasmataspidids and the distinct division of the opisthosoma into mesosoma and metasoma of eurypterids; hence, it can be excluded from Dekatriata Lamsdell, Reference Lamsdell2013. Xiphosura sensu stricto are characterised by cardiac lobes and ridges on the proximal shield, which are lacking in the new genus. Among the polyphyletic synziphosurines, few show the smooth carapace, lack of opisthosomal differentiation and lack of dorsal tuberculation as shown by the new taxon. Smotrychaspis lacks the axial nodes which characterise the more basal synziphosurines, such as Weinbergina and Legrandella (Lamsdell, Reference Lamsdell2013). The new genus mainly resembles the pseudoniscines such as Cyamocephalus and Pseudoniscus. These taxa show a smooth carapace, approximately equal in length and width, with small genal angles, an opisthosoma not narrowing rapidly posteriorly. and with increasing lateral gape between the epimera posteriorly (Lamsdell, Reference Lamsdell2013).
4.a.1. Pseudoniscidae
Two genera, Cyamocephalus and Pseudoniscus, are included in the family Pseudoniscidae (Anderson, Reference Anderson1999). Note that, in their horseshoe crab atlas, Bicknell and Pates (Reference Bicknell and Pates2020: Figure 8) listed Cyamocephalus and Pseudoniscus as lacking a family assignment, and that, in his most recent phylogenetic analysis, Lamsdell (2025, supplementary figure S1) included pseudoniscids within Bunodidae. The type genus Pseudoniscus was described by Nieszkoswki (Reference Nieszkowski1859) from the famous quarry on Viita Farm, Rootsiküla, Saaremaa, Estonia (in beds of upper Wenlock age), which has yielded abundant eurypterids (Eurypterus tetragonophthalmus Fischer, Reference Fischer de Waldheim1839), other synziphosurines (Bunodes lunula Eichwald, Reference Eichwald1854), fish, etc. (e.g. Märss et al. Reference Märss, Afanassieva and Blom2014). The type specimen of Pseudoniscus aculeatus shows a portion of a smooth carapace, followed by nine segments which become increasingly curved and hastate posteriorly, and terminates in a tail spine (Nieszkowski, Reference Nieszkowski1859: 39–40, pl. II, figure 15). Thus, the type resembles Smotrychaspis except that the anterior part of the opisthosoma (where presumably additional tergites may be present) and much of the rest of the carapace is missing. Since the first description of the genus, other species have been added to Pseudoniscus: P. clarkei Ruedemann, Reference Ruedemann1916 from Pittsford, New York; P. falcatus (Woodward, Reference Woodward1868) from Lesmahagow, Scotland; and P. roosevelti Clarke, Reference Clarke1902 from the Bertie Waterlime of New York. Recent photographs of these specimens were published by Bicknell et al. (Reference Bicknell, Amati and Ortega Hernández2019: Figure 3–4), in their study of the presence of lateral compound eyes in Palaeozoic horseshoe crabs, and in the horseshoe crab atlas of Bicknell and Pates (Reference Bicknell and Pates2020: Figure 9). The carapace of Pseudoniscus is smooth in most species, but P. falcatus shows strong cephalic ridges. This species was transferred to Pseudoniscus from Neolimulus Woodward, Reference Woodward1868 by Bergström (Reference Bergström1975), but differs from other members of the genus by this strong relief. Bicknell et al. (Reference Bicknell, Amati and Ortega Hernández2019) confirmed the presence of lateral eyes in P. clarkei, P. falcatus and P. roosevelti, as well as in Cyamocephalus loganensis Currie, Reference Currie1927, but not in P. roosevelti. Pseudoniscus species have an undifferentiated opisthosoma of 10 visible tergites, followed by a short telson (Lamsdell, Reference Lamsdell2013). Cyamocephalus was first described by Currie (Reference Currie1927) from the Silurian (Llandovery) of Lesmahagow, Scotland. The type specimen (Bicknell et al. Reference Bicknell, Amati and Ortega Hernández2019: Figure 2C) is preserved as carbonaceous film on the rock matrix, but additional examples (Eldredge & Plotnick, Reference Eldredge and Plotnick1974: Figure 2; Anderson, Reference Anderson1999: Figure 1) are preserved in relief. A characteristic feature of Cyamocephalus is the apparent fusion of opisthosomal tergites 6 and 7 (Anderson, Reference Anderson1999).
Smotrychaspis would fit into Pseudoniscidae except that it lacks a median anterior projection on the carapace (present in most Pseudoniscus spp.), there is no fusion of opisthosomal tergites (seen in Cyamocephalus) but, more importantly, it bears 11 visible tergites while pseudoniscids show only 10.
4.a.2. Offacolidae
Offacolus, from the Herefordshire Lagerstätte, was described as a euchelicerate and placed in the new family Offacolidae by Sutton et al. (Reference Sutton, Briggs, Siveter and Orr2002). Their analysis placed the family within Chelicerata in a more derived position than Weinbergina (one of the few synziphosurines with appendages preserved). The analysis of Lamsdell (Reference Lamsdell2013) placed Offacolus (along with Dibasterium, described in the same year) as basal euchelicerates, sister to all others (Prosomapoda). Members of Offacolidae are diagnosed on their elongate chelicerae; at least second to fifth prosomal appendage pairs being biramous; seventh pair of appendages uniramous and lobate, paddle-like, fringed by lateral spines; and tip of the telson bifurcate (Lustri et al. Reference Lustri, Gueriau and Daley2024 b). Lustri et al. (Reference Lustri, Gueriau and Daley2024 b) added the new genus Setapedites to the Offacolidae, and Lustri et al. (Reference Lustri, Antcliffe, Gueriau and Daley2024 a) transferred Bunaia woodwardi to the family. Note that another Bunaia species, B. heintzi Størmer, Reference Størmer1934, consists of a single carapace quite unlike that of the type species, so it was excluded informally from the genus by Eldredge (Reference Eldredge1974). Both Setapedites and Bunaia woodwardi were placed in Offacolidae on the basis of their biramous prosomal appendages and bifurcate telson tip.
Smotrychaspis differs from members of Offacolidae in that its opisthosoma is rather longer, has a narrower axial region, and remains quite wide towards the posterior than in offacolids. See, for example, Sutton et al. (Reference Sutton, Briggs, Siveter and Orr2002, Figure 1: Offacolus), Briggs et al. (Reference Briggs, Siveter, Siveter, Sutton, Garwood and Legg2012, Figure 1: Dibasterium), Lustri et al. (Reference Lustri, Gueriau and Daley2024 b, Figure 1: Setapedites) and Lustri et al. (Reference Lustri, Antcliffe, Gueriau and Daley2024 a, Figure 1: Bunaia woodwardi).
It is interesting to mention here that Bunaia woodwardi was suggested as a possible synonym of Pseudoniscus clarkei (both are from the Bertie Waterlime) by Eldredge (Reference Eldredge1974: 31). Furthermore, Eldredge (Reference Eldredge1974) also suggested that P. clarkei might be a junior synonym of P. roosevelti (also Bertie Waterlime). Thus, all of these animals were considered somewhat similar in morphology until Lustri et al. (Reference Lustri, Antcliffe, Gueriau and Daley2024 a) redescribed Bunaia woodwardi, based on new material, and discovered that it possessed not only biramous appendages but also 11 opisthosomal tergites. Hence, these authors moved B. woodwardi from the pseudoniscines to the basal Offacolidae.
Smotrychaspis is in a rather similar situation: it resembles pseudoniscids but possesses 11 visible opisthosomal tergites. The material to hand does not preserve appendages, so we cannot know whether its prosomal appendages are biramous. Hence, it is not possible to place Smotrychaspis into a lower taxon than Euchelicerata, though it is clearly a synziphosurine of some kind. Two features of Smotrychaspis make it stand out from other pseudoniscid/bunodid synziphosurines. (1) The axis of Smotrychaspis does not taper posteriorly until very close to the base of the telson. Indeed, the large, falcate epimera on the posterior opisthosomal tergites (8 and 9 in particular) increase the opisthosomal width in this region (Figure 3–4). Moreover, tergites 8 and 9 are somewhat longer than the more anterior ones. In this respect, Smotrychaspis somewhat resembles Cyamocephalus (see, e.g. Anderson, Reference Anderson1999), which shows curved epimera on the posterior three tergites, following fused tergites 6 and 7. However, the opisthosomal width decreases normally in this animal, and the fused tergites are different from the larger ones in Smotrychaspis. (2) The base of the telson of Smotrychaspis is relatively stout compared with other synziphosurines. The large muscle-attachment area on the pretelsonic tergite probably reflects this more powerful telson.
5. Conclusions
The new synziphosurine described here increases the known diversity of arthropods from the Silurian eurypterid-bearing strata of Podolia. Several other new forms have been collected (Polypenko pers. comm) and are yet to be described. Once this fauna is more fully documented, comparisons with other Silurian eurypterid-bearing EPBs, in Europe and North America, will be fruitful.
Acknowledgements
We thank Dmytro Polypenko and his field collectors who found the specimen and made it available for purchase and subsequent donation by RDAS. We thank Andrew Ross (National Museums Scotland) for arranging the accession of this specimen to their collections, and Lorenzo Lustri and an anonymous reviewer for very helpful comments on the manuscript.
Open access
