The first dendrasterid sand dollars appear in the Upper Miocene and continued to the present day. They first appeared in California, ranged south to Baja California during the Pliocene, and as far north as Alaska during the Quaternary (Beadle 1991). Two genera, Dendraster and Merriamaster (the latter from the Pliocene only), are recognized. Three Dendraster species are living today (Mooi 1997).
Dendrasterids are known as 'eccentric' sand dollars because their petaloid morphology is not centered on the aboral side of the test (and petaloid growth itself is uneven). As Beadle (1991) notes, "Even the oldest species are highly eccentric." (As you can see in the specimen of Dendraster gibbsii above, it is even more eccentric that the modern specimen below.) There are no 'transitional' eccentric forms; instead, the morphological distinction may have been rapidly introduced and spread, as it relates directly to the timing in the sand dollar's regulatory mechanism for growth (Beadle 1989). The oral and aboral areas in a sand dollar usually grow at different rates, but even out as they develop. In dendrasterids, a timing quirk results in a neotenic growth pattern, and thus eccentricity.
The living Dendraster excentricus can bottom-feed flat on the substrate as do other sand dollars, but will also tilt itself upright as the tide comes in, enhancing its filter-feeding capabilities (Mooi 1997). Eccentricity may aid this feeding adaptation, and may have driven their rapid radiation (Beadle 1991).
