| Summary: | Background: Radiodonta, large Palaeozoic nektonic predators, occupy a pivotal evolutionary position as stem-euarthropods and filled important ecological niches in early animal ecosystems. Analyses of the anatomy andphylogenetic affinity of these large nektonic animals have revealed the origins of the euarthropod compound eye andbiramous limb, and interpretations of their diverse feeding styles have placed various radiodont taxa as primaryconsumers and apex predators. Critical to our understanding of both radiodont evolution and ecology are the pairedfrontal appendages; however, the vast differences in frontal appendage morphology between and within differentradiodont families have made it difficult to identify the relative timings of character acquisitions for this body part.Results: Here we describe a new genus of hurdiid, Ursulinacaris, from the middle Cambrian (Miaolingian, Wuliuan) MountCap Formation (Northwest Territories, Canada) and Jangle Limestone (Nevada, USA). Ursulinacaris has the sameorganisation as other hurdiid frontal appendages, with elongate endites on the first five podomeres in the distalarticulated region and auxiliary spines on the distal margin of endites only. Unlike all other hurdiid genera, whichpossess a single row of elongated and blade-like ventral endites, this taxon uniquely bears paired slender endites.Conclusion: The blade-like endite morphology is shown to be a hurdiid autapomorphy. Two other frontal appendagecharacters known only in hurdiids, namely auxiliary spines on the distal margin of endites only, and elongate enditeson the first five podomeres in the distal articulated region only, predate this innovation.
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