The Echinoid Directory

Biology of Clypeasteroida


The Clypeasteroida includes a number of familiar sea urchin groups, including the sand dollars, sea biscuits and cake urchins. They are mainly shallow-water forms inhabiting sandy substrata. The sand dollars are mostly infaunal, living immediately underneath the sediment-water interface often in beach-face or shallow shoal settings. Clypeasteroids are unique in having large numbers of tiny tube feet and this has allowed them to harvest the small organic particles found amongst sand-sized grains. No other echinoid group has tube-feet of such minute proportions. Although all have a functioning lantern, the teeth are internal and are used solely for crushing and biting particles internally. A general review of clypeasteroid evolutionary biology was given by Ghiold (1984).

Ghiold, J. 1984. Adaptive shifts in clypeasteroid evolution - feeding strategies in the soft-bottom realm. Neues Jahrbuch fur Geologie und Palaontologie Abhandlungen 169, 41-73.

Geological history
The fossil record of the clypeasteroids is extremely good and they are found at all latitudes and in all continents. Clypeasteroids arose from amongst the paraphyletic cassiduloids in the late Cretaceous or early Tertiary and rapidly diversified during the Eocene. During this period they came to replace cassiduloids as the dominant echinoid group in inshore clastic facies. Today they are probably more diverse than at any time in the past.