ORIGIN AND DIVERGENCE OF HARD-SUBSTRATE ATTACHMENT STRUCTURES IN EARLY ECHINODERMS

Many early suspension-feeding echinoderms in the Cambrian attached to free skeletal fragments lying on muddy substrates or to archaeocyathid mounds. Late Cambrian and Early Ordovician echinoderms became more widespread and common on early diagenetically cemented carbonate hardgrounds that formed on flat-pebble conglomerates, grainstones, and sponge-algal buildups. The early edrioasteroid Totiglobus (Middle Cambrian) used a suctorial plated disk with internal radiating ridges for ligament attachment. Later derived blastozoans, crinoids, and many edrioasteroids converted this suctorial disk into a cemented disklike holdfast, with each group developing a distinctive type. Most early blastozoans had a 1-2 element conical cemented disk to attach a stem made of 1-piece (holomeric) columnals having a triangular or round lumen. Early crinoids, in contrast, had a multiplated, domal to conical, Disconia- or Lichenocrinus-type holdfast to attach a stem having 5-piece (pentameric) columnals with a pentagonal or round lumen.

Early and Middle Cambrian attached eocrinoids only reached 10-30 cm above the sea floor with their short stalks and relatively short brachioles. Trachelocrinid eocrinoids in the Late Cambrian reached 50-75 cm using relatively long stems and erect brachiole-bearing arms. Early Ordovician large-calyx protocrinoids probably reached 100 cm in height based on a 26 cm calyx and proximal stem, and on another 48 cm conical holdfast and distal-to-medial stem. This latter specimen shows irregular multiplating in the holdfast and distal stem, partly organized alternating pentameres above, and well-organized opposite pentameres medially, indicating that stem ontogeny was recapitulating the likely phylogenetic history. Other Early Ordovician hard-substrate edrioasteroids, blastozoans, and crinoids occupied lower tiers from 1-5 cm and 20-50 cm high. Large Late Ordovician crinoids reached 100-110 cm height based on stems at least 90 cm long. The first crinoids having rootlike holdfasts, distal stems with cirri, and distal stems coiled around other fossils also appeared at this time, indicating that crinoids had expanded their ability to attach in soft substrates.