Plax et al.—New stylonurid eurypterid from Upper Devonian of Belarus


crab mortalities have been shown to float intact for up to two days after death before settling on the sediment surface (Babcock et al., 2000), facilitating the transport potential of recent carcasses (see also Allison, 1986). Alternatively, living Soligorskopterus n. gen. might have been swept into the hypersaline lagoon during storm events before expiring in the toxic environment—a similar scenario to that suggested for occurrences of the horseshoe crab Mesolimulus Størmer, 1952 in the Upper Jurassic Solnhofen Limestone in Germany (Lomax and Racay, 2012). However, a large influx of meteoric water would likely have lowered the salinity of the lagoon at the same time that the eurypterids were swept in, and hence it seems more likely that the specimens were transported into the lagoon soon after death. Once in this setting, burial of the arthropod carcasses in fine-grained clays, and the onset of hypersaline conditions in the water column or subsurface, would have been inimical to growth of decay bacteria and conducive to long-term preserva- tion (Seki and Taga, 1963; McMahon et al., 2016). The occur- rence of Soligorskopterus n. gen. in what appear to be semiregular intervals in the Soligorsk Formation could also reflect a broader sequence stratigraphic control on preservation (see Vrazo et al., 2017), and it is clear that further sedimento- logical and stratigraphic work is needed to fully understand the environmental factors that facilitated eurypterid preservation in this evaporitic lithofacies. There is no direct evidence for the diet of Soligorskopterus n.


gen.; however, analysis of the visual system of Rhenopterus diensti Størmer, 1936, a stylonurine eurypterid from the Early Devonian of Germany, indicates that at least some stylonurines had a visual acuity comparable to that of modern horseshoe crabs (Poschmann et al., 2016). It is unlikely that Soligorskopterus n. gen. would have been an active predator like certain swimming eurypterids (Anderson et al., 2014; McCoy et al., 2015). Soligorskopterus n. gen. most likely had a diet like that of modern horseshoe crabs, consisting of small invertebrates (ostracods, gastropods, bivalves, polychaetes, etc.; Botton and Haskin, 1984). The morphology of appendages II–IV, with their rows of multiple spines per podo- mere, indicates that Soligorskopterus n. gen. was a dragnet-style sweep-feeder (Lamsdell et al., 2010b) and would have raked the spines of these appendages through the sediment to entangle and excavate partially buried prey.


Eurypterid paleogeography and patterns of extinction


Stylonuroidea was until now known with confidence from the lower Silurian (Llandovery) to the Lower Devonian (Pragian), with Ctenopterus (?) beecheri (Hall, 1884), a species of ques- tionable taxonomy, potentially extending the range of the group into the Famennian (Lamsdell and Braddy, 2010). Soli- gorskopterus tchepeliensis n. gen. n. sp. is the first confirmed occurrence of Stylonuroidea in the Famennian and as such extends the age range of the clade some 50 million years. The extension of Stylonuroidea into the Famennian means


that four clades (Mycteropoidea, Kokomopteroidea, Stylo- nuroidea, and Adelophthalmoidea) persisted beyond the major drop in eurypterid diversity in the Early Devonian and into the Late Devonian biotic crisis (Lamsdell and Selden, 2017). However, each of these clades are represented by only a few species (in the case of adelopthalmoids and mycteropoids) or a


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single species (for kokomopteroids and stylonuroids), and the discovery of Soligorskopterus n. gen. does not suggest that Late Devonian eurypterid diversity was significantly higher than previously suggested. It is noteworthy that three of these clades are members of Stylonurina, which are consistently lower in diversity than the swimming Eurypterina and tend to have a more cryptic fossil record (Lamsdell et al., 2010b). The fact that the majority of Late Devonian eurypterids are stylo- nurines—including the new species described here—supports existing hypotheses regarding the global collapse of eurypterid diversity during the Devonian (Lamsdell and Selden, 2017). The nektonic and nektobenthic Eurypterina were particularly hard hit by the biotic crisis, dwindling exponentially in diver- sity from the latest Silurian through to the Middle Devonian. The survivors were relegated to the margins of eurypterid niche space and invaded continental freshwater ecosystems (Lamsdell and Braddy, 2010; Lamsdell and Selden, 2017), although the exact timing of this transition is still uncertain (Vrazo et al., 2017). The morphological disparity of eurypterids has also been


demonstrated to decrease over the course of the Devonian, with an associated reduction of morphospace occupation (Lamsdell and Selden, 2017). The discovery of Soligorskopterus n. gen. does not alter these patterns in disparity and morphospace occupation. Sty- lonuroid morphospace almost wholly overlaps with the koko- mopteroid and mycteropoid morphospace, and as such does not expand the occupied morphospace of Famennian eurypterids. Soligorskopterus n. gen. does, however, expand the geographic occurrence of eurypterids during the Famennian. Previous records of Famennian eurypterids are known from western and central Laurussia and largely comprise stylonurines from the continental United States (Hall, 1884; Ehlers, 1935; Tetlie, 2008), with a single stylonurine known from southwestern England (Lamsdell et al., 2009) and another from Belgium (Fraipoint, 1889). The eurypterine genus Adelophthalmus Jordan in Jordan and von Meyer, 1854 is also known from the Famennian of the USA (Hall and Clarke, 1888) and Belgium (Fraipoint, 1889). Soligorskopterus n. gen. therefore extends the occurrence of Famennian eurypterids into eastern Laurussia. Despite Famennian eurypterids only being known from Laurussia, however, it is likely that they had a global distribution. In the Frasnian, eurypterids are known from both Laurussia (Ruedemann, 1921) and Gondwana (Tetlie et al., 2004), and the same is true for the Tournasian (Waterston, 1985; Tetlie and Poschmann, 2008). Whatever the cause of the eurypterid shift from marine to


freshwater habitats during the Devonian, it seems likely to have been the result of widespread, global pressure rather than an inherited trait through the chance survival of a few eurypterid groups within a single biogeographic region. As supported by phylogenetic information (Lamsdell and Selden, 2017), the concerted, independent habitat shift among eurypterids during this time period represents a case of macroevolutionary con- vergence driven by a selective trend (Lamsdell et al., 2017; Congreve et al., 2018). Such data are integral to expanding our understanding of biotic responses to environmental processes in the Late Devonian; further studies into other contemporaneous clades will undoubtedly refine our understanding of the events underpinning the biotic crisis that characterizes the end of the period (Stigall, 2010, 2012).


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