THE MIDDLE CAMBRIAN EOCRINOID GOGIA LONGIDACTYLUS AS A SHALLOW SEDIMENT STICKER: IMPLICATIONS FOR CHISHOLM SHALE FAUNA PALEOECOLOGY

Specimens of the eocrinoid Gogia longidactylus, preserved within the middle Cambrian Chisholm Shale of Pioche, Nevada, USA, have been found associated with numerous small structures that resemble the distal ends of eocrinoid stems. A petrographic and paleoecological analysis was conducted to better understand the relationship between the preserved remains and interpret the origin of the stem-like structures. Eleven new specimens of G. longidactylus as well as nine additional bulk samples of the Chisholm Shale, preserving sixty separate stem-like structures, were examined for this study. Petrographic comparisons between G. longidactylus fossils and the stem structures suggest a common origin as organic remains subsequently recrystallized to glauconite. The mineralogical and morphological similarities of the known stems and stem-like structures leads to the interpretation that the latter are indeed the preserved remains of eocrinoid stems inserted sub-vertically into the substrate. Rock samples from seven Chisholm Shale localities were also analyzed for levels of bioturbation using the ichnofabric index (ii) method. The results of this analysis, combined with paleoecological and petrographic characteristics, indicate that G. longidactylus lived on firm unlithified substrates with minimal mixed layer development (avg. ii~1) that were present during the Ediacaran-Cambrian transition. The sub-vertical orientation and depth of sediment penetration of the observed stem-like structures, together with a lack of preserved holdfasts, makes it likely G. longidactylus attached to the substrate via shallow sediment sticking, perhaps aided by the use of a bioglue. The paleoecology of both G. longidactylus and the associated sediment-attaching edrioasteroid echinoderm Totiglobus indicate that the benthic paleoecology of the Chisholm Shale fauna was strongly influenced by the presence of firm substrates with minimal mixed layer development; substrates more typical of the Proterozoic than the Phanerozoic. The results of this study provide further evidence for the Proterozoic-style attachment affinities of many early epifaunal echinoderms and emphasizes the important role the Cambrian Substrate Revolution played in the evolution of Cambrian benthic metazoans.