Crinoid

Clidochirus? sp.

• Silurian
• Hughley Shale
• Hughley Brook, Shropshire, England, UK

Size: 2 cm crown

Fossils that don't quite conform to our established categories and definitions are always interesting. Case in point, here is an unusual crinoid that shows characteristics of both Flexibles and Cladids. It is technically classified as a Flexible due to its three unequal infrabasal plates and uniserial nonpinnulate, stout and somewhat incurving arms. On the other hand, it lacks the classic patelloid processes in the arms that are typical of flexibles, and has a tall, conical cup without interradial plates much like a cladid. In fact, Flexibles are thought to have been derived from Cladids, and while this Silurian-aged specimen is too young to be the transitional form, it is a fascinating crinoid with some primitive holdovers that clearly point to the Cladid origin of Flexibles. The uncertainty in the genus assignment is from the original description of this type of crinoid (Donovan et al. 2008), which was based on a single specimen of uncertain orientation and anal plate arrangement. On this specimen there is evidently one anal plate in the posterior side of the calyx.