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.