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Journal of Paleontology 92(5):838–849
the clade. The prosomal carapace in Parastylonurus and Soligorskopterus n. gen. has a more rounded outline, being horseshoe-shaped rather than quadrate as in species of Pagea (Waterston, 1962), Stylonurus Page, 1856 (Woodward, 1872), and Stylonurella Kjellesvig-Waering, 1966 (Waterston, 1979), spatulate as in Laurieipterus (Waterston, 1962), or triangular as in Ctenopterus Clarke and Ruedemann, 1912. The prosomal ventral plates are of the Eurypterus-type as in Parastylonurus and Stylonurella rather than possessing an epistoma as in Laurieipterus. The metastoma is generally narrow and straight- sided as in Parastylonurus and Pagea, quite unlike the broad metastoma of
Laurieipterus.As in Pagea and Parastylonurus, Soligorskopterus n. gen. has a slight median ridge on the metastoma; however, the anterior notch is more reminiscent of Parastylonurus rather than the flattened anterior margin present in Pagea.
Soligorskopterus n. gen. differs from Parastylonurus
primarily in the armature of prosomal appendages II–IV, which bear multiple spines per podomere and are of the Ctenopterus- type. This appendage morphology is shared with Laurieipterus, Pagea, and Ctenopterus, the appendages of Parastylonurus and Stylonurella only having a single pair of fully-developed spines per podomere. The spines of Soligorskopterus n. gen. are all equal in length along each podomere and so more closely resemble the armature of Lauriepterus and Ctenopterus than that of Pagea. The spines bear longitudinal striations, as do those of Laurieipterus and Stylonurella. The structure of prosomal appendages V and VI in Soligorskopterus n. gen. closely resemble those of Parastylonurus, with a row of enlarged scales present on the anterior third of each podomere and with each podomere possessing a margin of thickened overlapping scales. This type of appendage ornament is shared with Stylonurus, Laurieipterus, and Stylonurella. However, Parastylonurus and Soligorskopterus n. gen. are distinct in having the anterodistal margin of podomere VI-8 developed into a long spur. Soligorskopterus n. gen. differs from Parastylo- nurus in lacking the crenulate distal podomere margins. The available material of the genital appendages in
Soligorskopterus n. gen. reveals them to be almost identical to those in Parastylonurus,with the typeAappendage being large, composed of at least two segments, and possessing a median ridge running along its length. The type B appendage is short, narrow, and also has a median ridge. The only other genital appendage known from Stylonuroidea is the type B genital appendage of Stylonurella, which agrees in overall morphology with those of Parastylonurus and Soligorskopterus n. gen. The postgenital opercula of both Soligorskopterus n. gen. and Parastylonurus also have small median keels running along their lengths. The overall shape of the opisthosoma is broad as in Parastylonurus and Stylonurus, although never expanding beyond the width of the prosomal carapace. The metasomal segments of Soligorskopterus n. gen. are extended into epimera, although they are short projections as in Pagea and not elongated like those of Parastylonurus. The epimera do, however, exhibit the same terraced ornamentation as observed in Parastylonurus. An elongate pretelson is common between Soligorskopterus n. gen., Parastylonurus, and Pagea, with Ctenopterus and Stylonurus having the regular, short pretelson morph. The general cuticular ornamentation, consisting of a
mixture of semilunate and acicular scales, is also consistent between Soligorskopterus n. gen. and Parastylonurus. The scale ridges on the pretelson of Soligorskopterus n. gen. are not known from any other stylonuroid eurypterid and appear to be an autapomorphy for the genus; however, they could be an indication of trilobation as seen in Pagea. Soligorskopterus n. gen. appears to be an intermediate
between the traditionally considered parastylonurid (Parastylo- nurus and Stylonurella) and stylonurid (Stylonurus, Pagea, Ctenopterus,and Laurieipterus) groups. Parastylonurids are paraphyletic, with Stylonurella resolving closer to the stylonur- ids and Parastylonurus forming the sister taxon to all other stylonuroids (Lamsdell et al., 2010a, 2010b). Soligorskopterus n. gen. likely resolves intermediately between Stylonurella and the Stylonurus/Pagea + Laurieipterus/Ctenopterus clade due to the armature of appendages II–IV comprising multiple spines per podomere (as in the Stylonuridae) and the morphology of appendage VI and the horseshoe-shaped carapace (which are parastylonurid in form).
Paleoecology of Soligorskopterus n. gen
The evaporitic nature of the eurypterid-bearing halopelite and the close proximity of the eurypterid-bearing beds to thick evaporite layers suggests deposition in a hypersaline environ- ment. Moreover, potassium salts (e.g., carnallite, sylvite) are some of the last evaporite minerals to precipitate out of solution in marine settings (Warren, 1997), and therefore the occurrence of one specimen (BKM 1052; Fig. 3) on a thin lamina of halo- pelite that is immediately adjacent to a sylvite layer would suggest deposition in or near a supersaline seawater brine. Yet despite the occurrence of Soligorskopterus tchepeliensis n. gen. n. sp. in this extreme environment, it is unlikely that this setting is representative of its preferred life habitat. Eurypterids, including stylonurids, in the Late Devonian were almost entirely constrained to freshwater habitats (Lamsdell and Braddy, 2010). In addition, a previous study of eurypterids preserved in eva- poritic settings has demonstrated that the onset of hypersaline conditions (and any subsequent formation of associated intra- sedimentary evaporites) occurred after burial of the eurypterid specimens, and thus are not reflective of life habitat (Vrazo et al., 2016). Eurypterids have been suggested to congregate in sheltered
regions to molt (Braddy, 2001; Vrazo and Braddy, 2011), and it is possible that the specimens of Soligorskopterus n. gen. are shed exuvia of individuals molting within a nearshore lagoon or tidal flat prior to the onset of hypersaline conditions. However, the majority of specimens are well articulated and do not exhibit any of the characteristic patterns of disarticulation or contortion characteristic of exuvia (Tetlie et al., 2008; McCoy and Brandt, 2009), and thus it seems more likely that the specimens are allochthonous carcasses transported into the environment of preservation following death. The co-occurrence of terrestrial plant material with the eurypterids indicates some terrestrial influence on the paleoenvironment and it is probable that the carcasses of Soligorskopterus n. gen. were transported into the Soligorsk environment via freshwater streams. Additionally, the general lack of disarticulation suggests that the individuals were only recently dead when transport occurred. Modern horseshoe
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