TF0904 Irregular echinoid

Cretaceous

Phylum: Echinodermata
Class: Echinoidea
Subclass: Irregularia
Order: Echinoneoida
Genus: Conulus

Bilateral symmetry
Five radial ambulacra
Interambulacra
Migrated periproct (opening for anus)
Peristome (opening for mouth)

This fossil was collected in Quaternary river gravel. Originally it was preserved in the Upper Chalk (Cretaceous). This was broken up into clasts by erosive processes, one of which was this irregular echinoid. The echinoid clast was carried by a river downstream and deposited in the river gravel unit during the Quaternary.

Note how the anus has migrated, but the peristome is still in the position you would expect from a regular echinoid (not migrated). Notice also how the ambulacra bearing the tube feet continue onto the underside, a feature not seen in more streamlined and derived irregular echinoids.