A sting in the tale of Parioscorpio venator from the Silurian of Wisconsin: is it a cheloniellid arthropod?

Abstract

Parioscorpio venator Wendruff et al., 2020a from the early Silurian Waukesha biota of Wisconsin, USA, interpreted as the earliest scorpion, then a basal euarthropod, is reinterpreted here as a cheloniellid-like arthropod with large raptorial appendages. The diversity of Cheloniellida Broili, 1932 is reviewed. Drabovaspis complexa Chlupáč, 1963, from the Upper Ordovician Letná Formation of Czechia, interpreted as an aglaspidid, then a xiphosuran, is also reinterpreted here as a cheloniellid. □ cheloniellid, scorpion, Silurian, Wisconsin.

Redescription of the cheloniellid euarthropodTriopus draboviensisfrom the Upper Ordovician of Bohemia, with comments on the affinities ofParioscorpio venator

“…The only aberrant character for a cheloniellid is the pronounced ridges running from the eyes to the anterior margin of the cephalic shield. Therefore, we concur with Braddy and Dunlop (2021) that it is highly likely that Drabovaspis complexa represents an isolated cephalic shield of a cheloniellid euarthropod, of which to date no parts of the trunk have been identified. If this is indeed correct, this would make the Letná Formation the world's cheloniellid hotspot, in that case harbouring three different genera (Triopus, Duslia and Drabovaspis).…”

“…Lamsdell (2020) formally removed Drabovaspis from Xiphosura, considering it to represent a bradoriid. Braddy and Dunlop (2021) attributed the fossil to Cheloniellida, a view with which we agree.…”

“…Parioscorpio venator Wendruff, Babcock, Wirkner, Kluessendorf and Mikulic, 2020a is a problematic euarthropod (Anderson et al 2021) from the Telychian (Llandovery, Silurian) Waukesha exceptionally preserved fauna, Brandon Bridge Formation of Wisconsin, which has been the subject of considerable recent debate, with some authors (Wendruff et al 2018(Wendruff et al , 2020aBraddy & Dunlop, 2021) incorrectly asserting a cheloniellid affinity for some or all of the material assigned to this species. In view of the major confusion surrounding this fossil, including the conflation of a genuine cheloniellid specimen with entirely unrelated material, it seems prudent to review why Parioscorpio venator cannot possibly be accommodated among either Cheloniellida or any other vicissicaudate clade.…”

“…They base this attribution almost exclusively on purported similarities in the cephalic and posterior trunk areas of UWGM 2439 and material previously assigned to Parioscorpio venator by Wendruff et al (2020a) and Anderson et al (2021); the cheloniellid specimen UWGM 2345 is only mentioned briefly with respect to its poorly preserved anterior appendages, ignoring the fact that this fossil lacks the cephalic and posterior trunk characters used to align UWGM 2439 with Parioscorpio venator, and has a lower segment count. Braddy and Dunlop (2021) use the presence of a supposed procurved posterior margin of the cephalic shield as an argument for a cheloniellid affinity of Parioscorpio. While strongly procurved posterior cephalic margins are indeed characteristic of cheloniellids, this character is not exclusive to them and is to a lesser extent also shown by e.g.…”

“…Such a morphology precludes any interpretation of these structures as a cheloniellid furca, and Anderson et al (2021) considered them to represent the tergopleura of the last tergite. Nevertheless, these critical differences did not prevent Braddy and Dunlop (2021) from summarily accepting the posterior lateral spines of Parioscorpio as cheloniellid furcal appendages, while also noting the presence of a rounded, ventrally located valve similar to the anal plate of Sidneyia, which they argued to be homologous with the aglaspidid post-ventral platesin that case leaving open the question of homology of Sidneyia's uropods. Because this would mean that Parioscorpio (and, in effect, also Sidneyia) has two pairs of modified appendages associated with the termination of the body, whereas real vicissicaudates only have one pair, Braddy and Dunlop (2021) tried to get around this problem by arguing that the cheloniellid furca was not homologous with the aglaspidid post-ventral plates, invoking a scenario where the former evolved from the exopods of the terminal somite, while the latter was derived from the endopods.…”