Conulus

A conulus is a small, cone-shaped structure found in certain echinoids (sea urchins), particularly in the Clypeasteroida (sand dollars and sea biscuits). It is an internal skeletal element located within the apical disc, above the periproct (anus) and posterior to the madreporite (a porous plate for water intake).

The conulus is typically composed of calcite, the same material that makes up the test (shell) of the sea urchin. Its precise function is not fully understood, but it is believed to be involved in the muscular attachments that control the position and movement of the periproct and potentially other aspects of the apical disc. The shape and size of the conulus can vary among different species and genera of Clypeasteroida, and can be useful in taxonomic identification.

The presence or absence, and the morphology of the conulus, are important characteristics used by paleontologists and zoologists in classifying and studying the evolution of sand dollars and related echinoids. Fossil conuli, although small, can sometimes be found in sedimentary rocks, providing evidence of the past presence of these organisms.