Parioscorpio is an extinct genus of arthropod containing the species P. venator known from the Silurian-aged Waukesha Biota of the Brandon Bridge Formation near Waukesha, Wisconsin. This animal has gone through a confusing taxonomic history, being called an arachnid, crustacean, and an artiopodan arthropod at various points.[1][2][3] This animal is one of the more famous fossil finds from Wisconsin, due to the media coverage it received based on its original description in 2020 as a basal scorpion.[4][5][6]
Taxonomy
The fossils were originally discovered in 1985, tentatively identified as a branchiopod or remipedecrustacean[1][7] but were neglected for decades. [8] In 2016, some of the fossils now assigned to Parioscorpio were given the name Latromirus and were assigned to an extinct group of early Paleozoic arthropods known as cheloniellids in a Ph.D dissertation,[9] but the name was never published in a peer-reviewed journal and is therefore not valid in accordance with the International Code of Zoological Nomenclature. The fossils known as “Latromirus” were also mistakenly named “Xus yus” in a preprint of a separate paper. [10] Upon initial publication in 2020, Parioscorpio was considered the world's oldest and most primitive known scorpion, older than Dolichophonus from Scotland by several million years.[3] In 2021, the fossils were reanalysed, and Parioscorpio was found not to be a scorpion, but an arthropod of uncertain placement, outside of Mandibulata, Chelicerata and all other groups of extinct arthropods (e.g. Megacheira, Fuxianhuiida, Artiopoda and so on).[8]
In 2021 another paper stated that Parioscorpio venator, including the fossils previously called Latromirus, might be a cheloniellid.[2] If this is correct, it means that P. venator is related to trilobites, nektaspids, aglaspidids, xenopods, and xandarellids.[11] However in 2022, its affinity as cheloniellid is questioned, and firmly rejected from that clade.[12] Currently the most resolved tree in the paper considered P. venator as an enigmatic stem-group arthropod.[8][12]
In 2022 a study was published describing Acheronauta stimulapis, a new species of possible mandibulate arthropod from the biota.[13] While coding the phylogenetic trees for this arthropod, the authors of the paper also included Parioscorpio, and all of the trees preformed presented this creature as a basal taxon of arthropod that sat in between the groups Artiopoda and Mandibulata.[13] This discovery actually is consistent with the rejection of P. venator as a cheloniellid.[12] As of 2023, P. venator is regarded as a basal euarthropod.[14]
Morphology
Movement of the raptorial appendages of P. venator
The animal is around 1.6–4.5 cm (0.63–1.77 in) long.[8][2] It is characterized by a trapezoidal head with a pair of eyes located antero-medially, a pair of enlarged raptorial appendages (previously thought to be scorpion's clawedpedipalps[3]), as well as another pair of small appendages.[8] Central to the head was a mouth-covering hypostome and a pair of muscular blocks articulated to the raptorial appendages.[8] The trunk is composed of 14 segments, each associated with a pair of thin pleurae (lateral extension of tergite) and appendages.[8] The first segment is covered by the head while the posterior segments may have lateral spines.[8] The anterior 12 pairs of trunk appendages are multiramus (each composed of 4 bundles of setae and a segmented endopod) while the last two pairs are simple fan-like structures.[8] The trunk ends with 3 spines.[8]
^Wendruff, Andrew J.; Babcock, Loren E.; Mikulic, Donald G.; Kluessendorf, Joanne (7 September 2018). "New cheloniellid arthropod with large raptorial appendages from the Silurian of Wisconsin, USA". bioRxiv10.1101/407379.
^Ortega-Hernández, Javier; Legg, David A.; Braddy, Simon J. (February 2013). "The phylogeny of aglaspidid arthropods and the internal relationships within Artiopoda". Cladistics. 29 (1): 15–45. doi:10.1111/j.1096-0031.2012.00413.x. PMID34814371. S2CID85744103.
^ abPulsipher, M. A.; Anderson, E. P.; Wright, L. S.; Kluessendorf, J.; Mikulic, D. G.; Schiffbauer, J. D. (2022). "Description of Acheronauta gen. nov., a possible mandibulate from the Silurian Waukesha Lagerstätte, Wisconsin, USA". Journal of Systematic Palaeontology. 20 (1). 2109216. doi:10.1080/14772019.2022.2109216. S2CID252839113.
