MORPHOMETRIC ASSESSMENT OF THE CENOZOIC ECHINOID GENUS ASTRODAPSIS

The extinct echinarachniid sand dollar Astrodapsis Conrad, 1856, endemic to marine deposits from the late Miocene to earliest Pliocene of California (Hall 1962) and Baja California Sur (Squires & Demetrion 1993), is an important index fossil as it is relatively speciose, abundant, and easily identified at the genus level. Despite this, species-level identification can be challenging; Astrodapsis displays a high level of morphological plasticity, even within the same population at the same horizon (Hall 1962). Of the 55 nominal species in the published literature, 13 are currently considered valid (Hall 1962, Squires & Demetrion 1993) following a 1962 study that focused on quantitative analysis of the genus. However, this consolidation of Astrodapsis has not been corroborated since its publication, and morphometric methods have been greatly elaborated in the past 60 years. This study examines eight species of Astrodapsis, as well as a related extant species, Echinarachnius parma, from the collections of the California Academy of Sciences, using two-dimensional and three-dimensional morphometrics to better determine the range of features within each species. Our results suggest that individual variation within Astrodapsis is greater than that within Echinarachnius. The three stratigraphically earliest species in the study, Astrodapsis brewerianus, A. diabloensis, and A. cierboensis, are indistinguishable in two-dimensional principal components analysis despite Hall 1962 reporting little stratigraphic overlap between the three. Examinations of the collections resulted in the recovery of the Astrodapsis peltoides holotype which, according to documents in the collections was reported missing in 1921 and believed permanently lost (Hall 1962).