By Jim Brace-Thompson
Beachcombers and fossil hunters alike love sand dollars! They belong to the phylum Echinodermata, which includes marine animals like starfish, sea cucumbers, crinoids, and sea urchins. One thing all hold in common is five-fold symmetry, as illustrated by the five-rayed star atop a sand dollar.
What beachcombers find is the test (skeleton), which is made up of interlocked plates. In life, these tests are studded with short spines that, in turn, are covered with small, hairlike structures called cilia, which make living sand dollars look fuzzy. They use the spines to move across the ocean floor, burrow within it, and to direct food particles to their mouths, located at the bottom center of the test. Their five-rayed stars on top are called petals and are used for gas exchange or respiration.
In the history of life, sand dollars are young. Their close relatives, the sea urchins, have a long fossil record. For instance, my collection includes 300 million-year-old spiny sea urchins. During the Age of Dinosaurs (Mesozoic Era), sea urchins became especially common, and [sand dollar ancestors] began diverging from other echinoids during this time. However, it wasn’t until 65 million years ago that true sand dollars appeared, during the Paleocene, or the first epoch of the “Age of Mammals” (Cenozoic Era).
Sand dollars have done well, with 250 living species. Their rigid tests, their tendency to live in large colonies and to burrow into sandy or muddy sea floors are all perfect for fossilization. Where you find one fossil sand dollar, you tend to find hundreds!