UNCOVERING THE DIVERSITY AND BIOGEOGRAPHY OF SILURIAN DIPLOPORITANS: NEW DATA FROM MISSOURI, USA

Evolutionary relationships among echinoderms are still poorly understood, especially within the various stalked echinoderm groups. Many of the traditional groups, such as the diploporitans, do not represent natural groupings, but groups of unrelated or distantly related taxa united only by convergent features. These aphyletic groupings are problematic because they cannot be used to address evolutionary questions, such as biogeographic patterns, or diversity of life through time.

Part of the difficulty of assessing the relationships of these diploporitans is a lack of available material. The few known occurrences of Silurian diploporitans show limited diversity especially at the higher taxonomic levels. They belong almost exclusively to the Holocystites Fauna, comprising diploporitans characterized by reduced ambulacra bearing stout, but otherwise undocumented appendages, large thecae, and respiratory humatipores. The only other described diploporitans from Silurian Laurentia are gomphocystitids, bearing long, spiraling ambulacra and true diplopores.

Two specimens from the Bainbridge Formation near Cape Girardeau, Missouri provide new data increasing the high taxonomic level diploporitan diversity known from the Silurian of Laurentia. These specimens show strong morphological affinities with Ordovician Baltican and peri-Gondwanan faunas and closely resemble Eucystis. The theca is globular with a semi-protuberant oral summit. Five multi-branched ambulacra extend without underlying floor plates from an ovate peristome and each ends in large brachiole facets, ranging from two-four facets per ambulacrum. The peristome is surrounded by seven oral plates, with O1, O6, and O7 being contained within the CD interray. The periproct is much closer to the peristome than evidenced in Eucystis, but it is still separated from the peristome by O7. Diplopores are contained within shallow, elliptical peripores and are distributed randomly across the theca. The holdfast is large and flares slightly at the attachment surface.

This specimen is the first eucystitid-type taxon found in Laurentia, as well as one of the very few Silurian diploporitan occurrences. It provides additional data with which to understand the phylogenetic and biogeographic relationships of Silurian diploporitans.