Calveriosoma gracile

Sea urchins are a group of marine invertebrates that usually possess a rigid skeleton composed of thick calcitic plates. However, a small group of deep sea forms known as echinothurioids have a leathery skin with thin, embedded plates. Calveriosoma gracile is a representative of this group. 

Echinothurioids were discovered first in the fossil record in the 1860s, when they were described from chalk sediments in England. Only later, when deep sea exploration began in earnest, were living examples found.

Species detail

Calveriosoma gracile

A close-up of part of the test of Calveriosoma gracile

The test of C. gracile is up to 240mm in diameter and has a cushion-like appearance. The plate boundaries show up clearly as white regions because of the presence of exposed ligament. 

The animal has a sparse covering of short, hoofed spines.

There are 2 species of Calveriosoma:

  • C. hystrix
  • C. gracile

They can be distinguished by the extent of tuberculation developed on their upper side.


Calveriosoma gracile

The body of Calveriosoma gracile, like other sea urchins, has a 5-fold symmetry

Calveriosoma gracile

A close up of Calveriosoma gracile showing the apical disc - a ring of 10 plates around the anal opening.

Close-up of Calveriosoma gracile

A close-up of part of the test of Calveriosoma gracile, showing the pores where the animal's tube-feet extend from

About the author

A Smith

Merit Researcher, Palaeontology Department.

Share this


The solid skeleton of an echinoid, formed of plates.

A knob-like projection that a spine attaches to on the surface of the test.