MOOI, R.*; BALL, J.E.; California Academy of Sciences, San Francisco; California Academy of Sciences, San Francisco: Size, shape, and phylogeny in northern Pacific sand dollars from the Miocene to the present
Most sand dollars (Clypeasteroida, Echinoidea) are associated with extant tropical or subtropical habitats. However, in the northern Pacific Ocean there is a rich diversity of over 130 nominal clypeasteroid taxa arranged in about 20 genera that extend back to the Miocene. Although a wide range of morphotypes suggests at least 5 disparate lineages, the majority of these taxa belongs to a single, taxonomically problematic group, the Echinarachniidae. Echinarachniids are represented today by only a single extant species, but over-splitting and outdated concepts of major fossil clades have made it difficult to formulate reliable statements about biodiversity patterns and the origins of these important groups. We present some results from a long-term effort to enhance our knowledge of these otherwise enigmatic northern Pacific clades, focusing on the echinarachniid genus, Vaquerosella Durham, 1955. The type species, V. andersoni, is a small form that, along with two other species of similar size, V. merriami, and V. durhami, makes up the �button-beds� of parts of the Californian Miocene. Size is all too often used as a classificatory criterion, leading to substantial confusion among these forms, and raising questions concerning the roles that changes in both shape and size have in the evolution of all northern Pacific taxa. Using some relatively simple morphometrics and plate pattern maps of the test, we have been able to clarify the differences among Vaquerosella species and other key taxa. Phylogenetic analysis of a spectrum of northern Pacific forms results in a more precise placement of these miniaturized clypeasteroids. Our results demonstrate that character analysis of test shape and plate architecture can contribute to a greater understanding of the polyphyletic origins and diversification of the entire northern Pacific sand dollar fauna.
