A comparative study of the life style of two Jurassic irregular echinoids

Abstract

The irregular echinoids Plesiechinus ornatus (Buckman) (Pygasteroida) and Galeropygus agariciformis (Forbes) (Cassiduloida) occur together in beds of the murchisonoe Zone, Bajocian, outcropping in the Cheltenham region of Gloucestershire. These species were largely restricted to different lithofacies within the carbonate shelf environment. Both adopted a hidden mode of life but achieved this by different techniques. Plesiechinus had fairly short spines and strongly muscular podia over the whole corona and was able to cover itself with coarse substrate particles. The oral tubercles are bilaterally symmetrical and are radially arranged. The oral spines are thought to have pulled sediment out from beneath the test, excavating a small depression for it. Galeropygus bore a dense covering of very small spines and its tube feet were differentiated into aboral respiratory podia and oral suckered podia. It had a preferred anterior direction of locomotion and is thought to have buried itself completely by excavating and ploughing into the substrate as it moved forward. Plesiechinus fed using only its lantern and postulated peristomial tube feet, whereas Galeropygus was a continugus sediment swallower and used its phyllode tube feet and peristomal lip spines in transferring particles towards the mouth.