REEVALUATION OF EARLY MIDDLE CAMBRIAN EOCRINOIDS PERIDIONITES AND CYMBIONITES AND TWO NEW ECHINODERMS FROM THE THORNTONIA LIMESTONE, NORTHWEST QUEENSLAND, AUSTRALIA

Peridionites and Cymbionites are two of the most poorly known echinoderms from the Middle Cambrian, based on fused basal cups described by Whitehouse in 1941, and a few epispire-bearing thecal plates. We recollected slabs from the Thorntonia Limestone type locality in 2002. Between later Sprinkle visits, Jell spent five years etching the limestone slabs for silicified cups and thousands of separate plates, and many additional months picking out and sorting the best-preserved plates, before we tried to position them into reasonable thecal arrangements.

Peridionites, which has a fused, elongate cup of five, thick, basal plates, has five higher circlets of epispire-bearing plates, including a circlet of thick end radials, thin lateral radials, and bulged interradials, three circlets of supraradials with food grooves and brachiole facets that slightly zig-zag, and a circlet of small orals around the also elliptical summit, where four(?) food grooves enter the mouth. Cymbionites has a fused, rounded cup of five, thick basal plates, possibly has six additional circlets of epispire-bearing plates, with four slightly bulged radials and four bulged interradials, as many as four circlets of supraradials with food grooves and brachiole facets that zig-zag, and a circlet of five small orals where one short and four long food grooves enter the mouth on a round summit. Both genera seem most closely related to the late Middle Cambrian eocrinoid Lichenoides from the Czech Republic, which has reduced the thecal plating to five bulged basals, five radials, and five small orals. All plates have epispires, and the top two circlets have food grooves and scattered brachiole facets.

Two new echinoderm genera have also been recovered from these acid residues. Both differ from the genera above by having meric stalks and distal attachment holdfasts. The first appears to be another eocrinoid with 4-5 plate circlets, including basals, laterals, radials and supraradials (both with small interradials), and orals. This genus has thin epispires, food grooves that zig-zag, and about 20 brachiole facets with two shallow depressions. The second new genus appears to be an early edrioblastoid(?) with 3-4 plate circlets (basals, radials, interradials, and long orals), pustular plates and spiny stalks and holdfasts, short but wide epispires, and 4-5(?) long, straight, ambulacral grooves down the theca lacking pores to the interior and brachiole facets.