SPECTACULAR SUMMIT STRUCTURES IN THE MIDDLE CAMBRIAN EOCRINOID GOGIA FROM NORTHERN UTAH
Nearly 500 complete specimens of a new Gogia species have been collected from several localities of the lower Middle Cambrian Spence Shale in northern Utah, and are now being described. Most specimens are preserved as flattened, etched molds on shale slabs, studied by making latex casts showing one or both sides of the flattened theca with its attachment stalk and long, feeding brachioles. About 10% of these new Gogia specimens from one northern Utah locality have beautifully preserved summits draped over one side of the theca. The summit plates are small, concave, and surrounded by numerous, deep epispires (respiratory sutural pores) covered by tiny platelets in a few specimens. The ambulacral system shows a ‘2-1-2’ branching pattern around the central mouth in adults with 3-5 brachioles at the end of each ambulacrum. Two sets of small, closed, cover plates protect the ambulacral food grooves, but these unfortunately hide the underlying floor plates. However, a few specimens have the cover plates stripped off, revealing a radial mouth frame and thick, uniserial, floor plates extending out each ambulacrum. A small pyramid with an apical pore (possibly the hydropore) is located just below the mouth near the C-ray ambulacrum, and the large anal pyramid is located laterally in an area of small plates just off the summit in the CD-interray.
The ambulacral arrangement in this new Gogia resembles that found in many Early Ordovician eocrinoids that have a single uniserial floor plate supporting each biserial brachiole. Other blastozoan groups (such as rhombiferans and blastoids) have 2 floor plates together supporting each biserial brachiole. The branching pattern of brachioles at the end of each ambulacrum is still uncertain, along with the rate at which new brachioles are added.