Insights into Atlantic-Pacific divergence in Arbacia (Echinoidea) using revised systematics of Neogene fossils from the Caribbean and southeastern United States

The regular echinoid Arbacia Gray, 1835 is a key model in evolutionary, ecological, and biogeographic research, yet its fossil diversity and paleobiogeographic history in the western Atlantic remain understudied. We present a comprehensive revision of Plio-Pleistocene Arbacia from the southeastern United States and Caribbean based on examination of > 400 specimen lots from institutional collections, including preserved material retaining spines. Our results resolve longstanding taxonomic confusion by (1) recognizing and designating a neotype for Arbacia improcera (Conrad, 1843) as encompassing Arbacia sloani (W.B. Clark in W.B. Clark and Twitchell, 1915) and Arbacia crenulata Kier, 1963; (2) confirming the distinctiveness of Arbacia waccamaw Cooke, 1941 and Arbacia rivuli Cooke, 1941; and (3) describing Arbacia percaribbeana new species, from Venezuela, the Caribbean, and southern Florida. The revised taxonomy establishes a coherent regional succession: Arbacia improcera in the Late Pliocene, Arbacia waccamaw and Arbacia rivuli at the Pliocene-Pleistocene transition, and Arbacia punctulata persisting from the Late Pleistocene to the present. These species occur in well-dated formations and provide useful biostratigraphic markers. The absence of Arbacia from Miocene deposits of the eastern USA aligns with molecular estimates suggesting a Pliocene origin of modern Atlantic lineages. Morphological comparisons reveal a previously unrecognized biogeographic structure. Four western Atlantic species share a distinctive trait combination separating them from extant Pacific taxa, whereas Arbacia percaribbeana n. sp. exhibits stronger Pacific affinities. This pattern suggests a North Atlantic lineage and a lineage with Pacific relationship, possibly reflecting transisthmian exchanges during the progressive closure of the Central American Seaway. Together, these results establish Arbacia as a robust model for investigating macroevolutionary dynamics in Neogene western Atlantic echinoids. UUID: http://zoobank.org/4a9e9e9e-e101-49b0-aa34-bc247b10fe0a