Sanctacaris uncata: the oldest chelicerate (Arthropoda).

The morphology of the arthropod Sanctacaris uncata, from the Middle Cambrian Burgess Shale of Canada, is reinterpreted based on a restudy of previously described material. Although originally considered a chelicerate-like arthropod, these affinities were dismissed based primarily on interpretations of the anterior appendages and hypotheses which considered the megacheirans ('great-appendage' arthropods) as putative ancestors of chelicerates. The similarities between megacheirans and chelicerates appear to be overstated however, and this study instead reaffirms the identity of putative chelicerate feature in S. uncata and similar arthropods such as Sidneyia and Emeraldella, both also from the Middle Cambrian Burgess Shale. Newly interpreted features, including the presence of pediform exites, multi-partite trunk exopods, and a trunk differentiated into an anterior limb-bearing area and a differentiated posterior limbless abdomen, were coded into an extensive phylogenetic data set of fossil and recent arthropods. In all analyses, Sanctacaris resolved as the basal-most member of total-group Euchelicerata (the least inclusive group including horseshoe crabs and arachnids but not pycnogonids), thus making it the oldest chelicerate in the fossil record. The vicissicaudates (including Sidneyia, Emeraldella, aglaspidids, and cheloniellids--all of which have previously been allied to chelicerates) resolved as sister-taxon to crown-group Chelicerata. This topology indicates that many purported chelicerate features, such as lamellar gills, and a differentiated posterior abdomen evolved sequentially in the chelicerate stem-lineage.