Sea Urchins: Fossil and Recent

Sea urchins (regular echinoids) are a major group within phylum Echinodermata, which also includes sand dollars (= irregular echinoids), crinoids, starfish, sea cucumbers, etc.). Sea urchins are characterized by having an inflated to globular shell (test) with prominent spines on the top and sides of the shell. Sea urchins can live in a wide range of temperature: from polar seas to the tropics. There are about 940 species, and they are confined to the strictly marine environment.

Sea urchins tests are relatively rare in the fossil record because these animals lived where turbulent waters broke their tests after death. Their spines, however, can be common fossils.

Strongylocentrotus purpuratus, shown above (5.5 cm diameter), is the common sea urchin found in coastal waters of California. The image on the left is the dorsal surface of the exterior of its test, with the spines intact.  The image on the right shows the interior of this specimen. Notice the five-ray symmetry in the interior. This symmetry is a diagnostic feature of sea urchins and most other echinoderms.

This sketch shows the internal anatomy of a typical sea urchin. The mouth is on the bottom of the shell, the gut is twisted, and the anus is at the top of the shell. Sea urchins have a jaw structure, called the "Aristotle's lantern," which consists of five strong jaws, each with a single calcareous (calcite) tooth.  These jaws can drill holes in rock, kelp holdfasts, or shells. Sea urchins feed primarily on algae, but they can feed on mussels, worms, sponges, and even other echinoderms.

Colobocentrotus atratus (7 cm diameter, including the spines; Hawaii). This sea urchin has wide, flattened spines that provide a "cage" for the mouth area. This species lives on wave-swept intertidal shores in the Indo-Pacific area and is especially common in Hawaii. Their flattened shell has a very wave-resistant shape.

Bottom side of same specimen shown above. In the mouth area, two of the pincher-like teeth of the Aristotle's lantern are visible. The small spines around the mouth are cylindrical, but farther out from the mouth such spines become more flattened.

These diagrams show the technical terms for morphologic structures visible on sea-urchin tests, once their spines are missing.

The white-colored ambulacra (plural) are easily seen on this dorsal view of a test of a sea urchin (4.5 cm diameter) from Mulegé, Gulf of California, Baja California Sur, Mexico. The hole at the top is where the periproct was located but is now missing because that particular plate has fallen out. 

The fossil sea urchin shown here has some of its tubercle base plates preserved. These plates
support the spines during life.  




This piece of Paleozoic (Pennsylvanian?) age limestone contains numerous sea-urchin spines, which show current or wave alignment. The longest spine on the right side of the image is 2.7 cm in length. Sea urchins have a geologic time range from the Ordovician (450 million years ago) to modern day.