This sand dollar, Astrodapsis coalingaensis, is a representative of the echinoidea, a diverse and successful group of echinoderms. Echinoids first appear in the fossil record in the Ordovician, but their numbers and diversity don’t really begin to increase until late in the Triassic. Sand dollars make their appearance in the Paleocene, though this particular specimen is Miocene in age; it’s from the Santa Margarita Formation, Fresno County, CA. To learn more about echinoids in general, look here.