THE TIE THAT BINDS: ATTACHMENT STRUCTURE HOMOLOGIES IN EARLY CAMBRIAN ECHINODERMS

Lack of apparent homology coupled with autapomorphy has hindered systematic insight into Early Cambrian echinoderms. Recently discovered structures derived from the imperforate extraxial system (IER) in three groups for which these structures were poorly known provide a new suite of characters with which to trace the earliest diversification of echinoderms. The imperforate extraxial system is the portion of the skeleton set aside for the lower theca and attachment stalk. IER structures have been recognized in Middle Cambrian groups and the Early Cambrian lepidocystids, whereas other Early Cambrian groups were presumed to have lost it. Camptostroma roddyi has a multiplated aboral pad, which is encircled by a deep furrow. This structure has been interpreted as the bell of a hydrozoan, both the dorsal and ventral surfaces of an edrioasteroid-like echinoderm, or ignored altogether. The pad is flexible, with an internal raised central process, which served for muscular attachment. By contracting these muscles, the center of the pad would be raised while the edge remained in contact with the substrate, inducing a tight seal. Helicoplacoids have been interpreted by most workers as sediment stickers, which remained upright by embedding a tapered lower pole into soft substrates. Recently studied specimens show a multiplated pad at the tip of the lower pole, similar to that found in Camptostroma. Furthermore, a new specimen attached to a Nevadella trilobite cephalon indicates the group was capable of attachment to skeletal debris. Adherence through suction rather than cementation is assumed through the similarity to other echinodermal suction pads and a lack of disembodied helicoplacoid bases cemented to skeletal remains. Some Middle Cambrian Gogia specimens are affixed to trilobite debris, but it is unclear whether these were attached by cementation or suction. A newly studied specimen from the Early Cambrian Latham Shale of California suggests suction as the lower surface of the holdfast has been removed through taphonomic processes, revealing internal struts suggestive of those found in Stromatocystites and Totiglobus. These structures appear to have been the site of muscular attachment, functioning in the same manner as in Camptostroma.