TEMPORAL AND GEOGRAPHIC BIAS IN THE SAMPLING OF NORTH AMERICAN GLYPTOCYSTITOID RHOMBIFERA (ECHINODERMATA)

The present distribution of glyptocystitoid rhombiferans records a pattern radically different from that which derived our current interpretations of both temporal diversity and disparity, and geographical diversity. Changes in these patterns are most acute regionally, but global patterns are also affected. We examined North American glyptocystitoid biodiversity using data from the Treatise on Invertebrate Paleontology (-1968), recent published accounts (1968-2002), and new, yet unpublished accounts. Pre-1968 data show glyptocystitoid genera exclusively in the Midwest and St. Lawrence region. The Treatise argued that this pattern reflected a true North Atlantic distribution of "Cystoid" taxa. However, more recent sampling, particularly in the Great Basin and southern Rockies, has uncovered a wealth of new taxa. These faunas have species diversity as great as those of their eastern counterparts and affect our interpretations in three fundamental ways. 1) Paleogeography. Glyptocystitoids are now known, albeit not necessarily described, from much of North America. Echinoderm specialists have not actively examined regions where glyptocystitoids are generally absent (Canadian Rockies, arctic islands, and Meso-America). 2) Diversity through time. All known Early Ordovician glyptocystitoids from North America were discovered after the publication of the Treatise. These now comprise most species known from the Western US, including nine new genera and two new higher-level clades. 3) Total morphological disparity. Taxa found in the Western US include examples with corrugated thecal plates akin to those seen in Macrocystella, species with covered epispires, and species with either extra or missing thecal plates; features previously unknown in North America. These discoveries should come as no surprise, given that Late Cambrian and Early Ordovician counterparts to these taxa are known from central and western Europe, and suggest that more complete, global scale sampling is necessary if we are to reconstruct an accurate account of glyptocystitoid evolution.