NEW EOCRINOIDS FROM THE LOWER ORDOVICIAN GARDEN CITY FORMATION, NORTHEASTERN UTAH AND SOUTHEASTERN IDAHO

Eocrinoids are a paraphyletic class of blastozoan echinoderms that dominated the Middle and Late Cambrian echinoderm record and then continued through the Ordovician into the Silurian. During the past four years, 5-7 eocrinoid genera based on 11 specimens have been collected from the lower half of the Garden City Formation (Lower Ordovician) in northeastern Utah and southeastern Idaho. Most eocrinoids appear to be new although a few belong to genera known or described from the coeval Fillmore and Wah Wah Formations in western Utah.

A wide range of thecal shapes and appendage types occur among Garden City eocrinoids. These include: 1) ellipsoidal thecae having numerous epispire-bearing plates, recumbent ambulacra bearing erect brachioles, and a long stem; 2) small ellipsoidal thecae with fewer plates lacking pores, few long brachioles, and a medium-length stem; 3) medium to long, thick, biserial arms with a V-shaped array of biserial brachioles belonging to an otherwise unknown eocrinoid(?); 4) possible asteroblastids with wide ambulacra and short erect brachioles but no diplopores (similar to a taxon known from the Fillmore Formation); and 5) a specimen of Haimacystis (originally described from the Wah Wah Formation) having an elongate flattened theca, two short ambulacra with small erect brachioles, and a medium-length stem.

Nearly all Garden City eocrinoids with visible recumbent ambulacra have a single, thick, floor plate supporting each brachiole. This is the same ambulacral arrangement as in the Early Ordovician eocrinoid Rhopalocystis from Morocco, but differs from glyptocystitid rhombiferans and blastoids that have two paired floor (or side) plates supporting each brachiole. The long arms with densely branched brachioles represent a striking example of convergence on the pinnulate arms of crinoids, and indicate that eocrinoids evolved high density filters using brachioles before crinoids developed true pinnulate arms.