Infaunalization of Post-Paleozoic Echinoderms

The title of this post sounds like a complicated subject, but the concept is easy to understand. During most of Paleozoic time, animals without backbones (invertebrates) lived mostly on the surface of the ocean floor and are referred to as epifaunal ("epi," meaning on). The sea-urchin echinoderms (see my last post) are epifaunal animals. 

With the arrival of strong-jawed, bony predatory fish, marine reptiles, and other animals during Mesozoic time, many invertebrates, in order to survive, began to live partially buried or completely buried in sand or mud. In other words, they adapted by becoming burrowers, and they are referred to as infaunal ("in," meaning inside). Some echinoderms (the partially buried sand dollars and the completely buried heart urchins) are prime examples of this infaunalization event.

Today, epifaunal sea urchins, are still around, but many of them have retreated to rocky shorelines or very deep water, where they are safer from predators.

The above-mentioned three groups (non-burrowing, partially burrowing, or completely buried) of echinoderms, which, technically speaking, are referred to as "echinoid echinoderms," are illustrated below, with comments about each group in vertical alignment with its sketched shape. 

EPIFAUNAL SEA URCHIN
Strongylocentrotus purpuratus (6 cm diameter), from southern California. Dorsal (top) view, followed by the side view. The globular and spiny sea urchin shell (test) is not adapted for burrowing. Starting in Mesozoic time, living on the sea floor was no longer as safe as before, and many have since sought refuge by living on wave-swept rocky shorelines where many predatory fish cannot invade. 

PARTIALLY INFAUNAL SAND DOLLAR
Encope micropora (7 cm in height and width), from beach drift on the Pacific side of Baja California Sur, Mexico. Dorsal view, followed by the side view. Sand dollars (clypeasteroids; the prefix "clyp" rhymes with clip) are adapted for partial burrowing because they have a flattened shell with tiny spines. The upper surface of their shell (called a "test") is exposed, in order for the "star-shaped" ciliated structure on the top of their shell to move food particles found toward the edge of the shell. From there, other cilia move the particles to the underside of the shell, where the mouth is located. The holes in the shell allow sand to "flow through" the shell, thereby facilitating the process of shallow burrowing. The geologic record of sand dollars is Cretaceous to Recent, with most of the species occurring in the Cenozoic.

INFAUNAL HEART URCHIN
Cassidulus gouldii (4.5 cm height and width), a fossil of Oligocene age, Florida. Dorsal view, followed by the side view. Heart urchins (spatangoids) resemble sand dollars but are more streamlined and can burrow as deep as 18 cm. Heart urchins are totally infaunal.

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.