Stranger than a scorpion: a reassessment of Parioscorpio venator, a problematic arthropod from the Llandoverian Waukesha Lagerstätte

Abstract

A relatively uncommon arthropod of the Waukesha lagerstätte, Parioscorpio venator, is redescribed as an arthropod bearing a combination of characters that defy ready classification. Diagnostic features include sub‐chelate ‘great appendages’, a lack of antennae, multiramous anterior trunk appendages, filamentous fan‐like rear trunk appendages, and apparently thin and poorly preserved pleural fields. Phylogenetic analysis resolves this organism as basal to crown‐group Mandibulata and Chelicerata, but its exact placement is inconclusive. Thus, we compare its morphology to several stem groups of arthropods in a discussion of its plausible taxonomic affinities. The examined specimens are probably carcasses and preserve a variety of soft‐tissue details, including muscle blocks in the head, eyes and eye facets, likely ventral nerve cords, a central gut tract and trunk legs with multiple filamentous elements organized into stiff bundles. The preservation habits of P. venator are characterized and compared to previous assessments of Waukesha lagerstätte taxa. Four preservation habits are observed: a phosphatized habit showing flattened to partly three‐dimensional mineralization in francolite; a mouldic habit largely left behind by removed francolite that shows no carbon enrichment despite a darkened colour; sheet‐like or speckled carbonaceous compressions; and scattered pyrite crystals. This redescription highlights both the palaeobiological value of ‘small’ lagerstätten typical of the middle Palaeozoic and the caution that must be taken when interpreting their more enigmatic constituents.

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

“…In Parioscorpio, the cephalic shield widely overlaps the trunk, covering the anterior first and possibly also second tergites (Anderson et al 2021;Braddy & Dunlop, 2021). Such a large overlap of the shield with the trunk is not encountered in any true cheloniellid, including specimen UWGM 2345, and is uncharacteristic for vicissicaudates in general.…”

“…In a detailed restudy, Anderson et al (2021) conclusively demonstrated that the reinterpretation of some of the material from Wendruff et al (2018) as belonging to a scorpion was as misguided as its original cheloniellid attribution, and further showed that most of the remaining specimens from Wendruff et al (2018) also belong to Parioscorpio. While Anderson et al (2021) excluded UWGM 2439 and UWGM 2345 from their restudy, they considered that these two specimens were the only fossils among the material of Wendruff et al (2018Wendruff et al ( , 2020a that exhibited possible cheloniellid affinities. Parioscorpio venator itself was revealed as a problematic euarthropod exhibiting a highly intriguing organization, which likely places it in a basal position within Deuteropoda (Anderson et al 2021).…”

“…While Anderson et al (2021) excluded UWGM 2439 and UWGM 2345 from their restudy, they considered that these two specimens were the only fossils among the material of Wendruff et al (2018Wendruff et al ( , 2020a that exhibited possible cheloniellid affinities. Parioscorpio venator itself was revealed as a problematic euarthropod exhibiting a highly intriguing organization, which likely places it in a basal position within Deuteropoda (Anderson et al 2021). Braddy and Dunlop (2021) then recycled part of the defunct draft manuscript posted to bioarXiv by Wendruff et al (2018) and resurrected some of its misinterpretations in a formal publication which appeared online just before the current paper was about to be submitted.…”

“…In a throwback to Wendruff et al (2018), Braddy and Dunlop (2021) consider specimens UWGM 2439 and UWGM 2345 to be similar to the material attributed to Parioscopio venator by Anderson et al (2021) and use this supposed similarity to reassign Parioscorpio to Cheloniellida. 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.…”

“…The only exception to this is the genus Sidneyia Walcott, 1912, which has two cylindrical sclerites without pleura preceding the telson (Bruton, 1981); in this respect, it is interesting to note that Lerosey-Aubril et al (2017b) recovered Sidneyia as the earliest offshoot of Vicissicaudata. Conversely, none of the specimens attributed to Parioscorpio by Wendruff et al (2020a) or Anderson et al (2021) shows any evidence for the presence of a terminal cylindrical sclerite without pleura, which is a prime apomorphy uniting Vicissicaudata (Van Roy 2006a, b;Lerosey-Aubril et al 2017a, b). While a differentiation in preabdominal and abdominal regions may also be present in UWGM 2439, it is clearly absent in UWGM 2345, in common with other cheloniellids.…”