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.

Please see the Methods section in the submitted manuscript.

The archived data includes the primary dataset for phylogenetic analysis in both Excel Spreadsheet (Appendix S1) and Plain Text form, which is derived from the character matrix of Aria and Caron, 2017a (Nature). The DOI for this work is provided in the "Related Works" section. In the Excel Spreadsheet, several rows are added for Parioscorpio venator Wendruff et al., 2020, and the traits of the stem-group comparison taxa are colour-coded based on whether or not and how they match with the "Parioscorpio preferred interpretation" row in the matrix. The Plain Text files contain the Nexus code for the phylogenetic analyses as well as the character states for the included taxa and are ready to be run by PAUP*. Other than "Data_Preferred_Interpretation," which was used for the primary analysis of Figure 15 in the manuscript and "Data_FuxianhuiidHead_Interpretation," which was used for the alternative analysis of Figure 17, each text file is named for the alternative state and main trunk leg interpretation of P. venator contained within it.

Supporting information includes the following:

Table S1. Measurement information on the whole body and head of specimens of Parioscorpio venator on which the rediagnosis and redescription were primarily based.

Table S2. Measurement information for the trunk of the specimens of Parioscorpio venator on which the rediagnosis and redescription were primarily based. Body length and width are included from Table S1 for reference. 

Fig. S1. Phylogenetic analyses of the character matrix of Aria and Caron (2017a) including Parioscorpio venator. Presented trees are those where alternative states are consistently coded for each analysis. Each alternative state scheme is analyzed for two interpretations: one where the anteriormost (as preserved in the fossils) main trunk leg ramus is interpreted as an exopod and the subsequent ramus an endopodal exite (exopod/endopodal exite interpretation), and one where the anteriormost ramus is interpreted as an epipod and the subsequent ramus an exopod (epipod/exopod interpretation). All alternative state analysis trees are strict consensus trees; any pertinent information that can be gleaned from the 50% majority rule trees is included in the Main Text or Appendix S2. Parioscorpio venator is highlighted in yellow in all cladograms, and bold numbers to the left of nodes are Bremer support values. A–B, the “uncertain” analyses where alternative states are coded as ‘?’ when ‘?’ was an optional alternative state; A, exopod/endopodal exite interpretation; B, epipod/exopod interpretation; C–D, the “negative certainty” analyses where alternative states are coded as absent or the smallest possible number of a repeating morphology when such optional states were available; C, exopod/endopodal exite interpretation; D, epipod/exopod interpretation; E–F, the “positive certainty” analyses where alternative states are coded as present or the largest possible number of a repeating morphology when such optional states were available; E, exopod/endopodal exite interpretation; F, epipod/exopod interpretation.

Appendix S1. Character table which serves as the basis for the phylogenetic analyses in this study. Based on the table of Aria and Caron (2017a), it includes Parioscorpio venator and the various alternative interpretations of its morphological character states described in the Main Text and Appendix S2.

Appendix S2. Additional considerations of the phylogenetic affinities of Parioscorpio venator with the stem-group comparison taxa and their implications on arthropod phylogeny.

Note that a copy of the above table, figure, and appendix list is available for download as a separate document.