WHERE IS THE MISSING CAMBRIAN ECHINODERM DIVERSITY - EVIDENCE FROM ARCTIC SIBERIA

Phylogenetic studies of the approximately 30 clades of Ordovician echinoderms indicate that the primary diversification occurred by Late Cambrian. However, only a few Middle and Late Cambrian species are known. Recent studies from the Great Basin of western North America indicate that much of the missing diversity is an artifact of global facies distribution and paleontological sampling. Here we expanded these studies to the uppermost Middle Cambrian section of the Siberian Platform in Yukutia, Russia, 66 N, 117 E. Fossils were collected in the Upper Cambrian (Mayan) section in limestone exposures along the Tyung River. The section is a cyclic package of thinly laminated micritic limestones interlaminated with mm-thick shale partings, overlain by thick-bedded stromatolitic limestones with occasional stromatolitic bioherms and local skeletal grainstones and stromatolite-derived rip-up beds. Traditionally, thinly laminated limestones have been interpreted as deep slope-to-basin sedimentation. However, shallow water sedimentary structures, such as interference ripples, suggest much shallower conditions.

Although only three complete echinoderm skeletons were recovered, numerous holdfasts, stems and plates were found. All taxa were hard-substrate dwelling. In the apparent absence of hardgrounds, stromatolitic buildups were the dominant attachment surfaces. Specimens were washed from these buildups and disarticulated prior to burial making species identification extremely difficult. The eocrinoid Gogia was the most common faunal element represented by numerous stem segments. Holomeric pelmatozoan columnals, almost unknown from coeval rocks elsewhere, were common and diverse and suggest a minimum of six yet-unknown taxa. No solid evidence was found for edrioasteroids or homalozoans, although both are known from the Upper Cambrian section along the nearby Lena River. Together, these data indicate a minimum echinoderm diversity of seven taxa, making it quite diverse by Middle Cambrian standards. This study suggests additional echinoderm sampling in traditionally overlooked geographic regions can produce a better understanding of echinoderm diversity at this critical interval.