Parioscorpio

The fossils were originally discovered in 1985, tentatively identified as a branchiopod or remipede crustacean^[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]

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 clawed pedipalps),^[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]

Parioscorpio may had been a marine or brackish water predator, using an ambush prey-capture method similar to extant waterbugs (Nepomorpha).^[8] It would have lived alongside many other bizarre organisms like the Conodont Panderodus, the enigmatic Papiliomaris, the thylacocephalan Thylacares, early synziphosurans, and Trilobites.^[15]