A revision of helicoplacoids and other early Cambrian echinoderms of North America

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Wilbur, Bryan Charles

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The Lower Cambrian rocks of North America contain the remains of several species of echinoderms, including helicoplacoids, the eocrinoids Gogia, Lepidocystis, and Kinzercystis, and the edrioasteroids Camptostroma and Stromatocystites. Camptostroma roddyi Ruedemann, 1933 is a large echinoderm from the Lower Cambrian Kinzers Formation of southeastern Pennsylvania. The animal has a unique dual-layered oral surface, and a large aboral suction pad used for attachment to skeletal debris. Thought by some authors to be ancestral to the edrioasteroids, it clearly belongs in a clade with the other early edrioasteroids. Helicoplacoids are from the Lower Cambrian Poleta Formation and equivalent units across the western edge of North America. The group was thought to be represented by nine species; this number is reduced here to three species, with the remainder of helicoplacoid disparity accounted for by vii ontogenetic variation. These triradiate spindle-shaped echinoderms were capable of expansion and contraction by means of cloacal pumping, and attached to skeletal debris or semi-lithified matgrounds by means of a suction pad. Rather than representing an ancestral triradiate form that served as the template for a “2- 1-2” ambulacral arrangement, these disparate echinoderms are derived from the ancestral pentameral edrioasteroid bauplan. Gogia is well represented in the Lower Cambrian rocks of California and Nevada by five species, four of which are new and are described here. This is now understood to be the most diverse echinoderm genus from the Early Cambrian. Members of this group attached to skeletal debris by using a suction pad, and fed by employing retractable spiraled brachioles. Stromatocystites walcotti Schuchert, 1919 is from Lower Cambrian rocks from Bonne Bay, Newfoundland. It is a flat edrioasteroid with a retractable suctorial aboral surface devoid of a pad. This morphologic feature delineates the group from other Middle Cambrian Stromatocystites, warranting renaming herein. A tree generated for these groups shows that helicoplacoids, Gogia, Imbricata, and the eocrinoids (Gogia + Imbricata) all constitute monophyletic groups, while edrioasteroids are polyphyletic. Early Cambrian echinoderms are found with only one or (rarely) two species occupying the same morphospace, a pattern repeated by Middle Cambrian groups.