COSTACOPLUMA N. SP. (DECAPODA: RETROPLUMIDAE) FROM THE EOCENE TALLAHATTA FORMATION, ALABAMA: RANGE EXTENSION AND A REEVALUATION OF THE RETROPLUMIDAE

Ten species arrayed within the genus Costacopluma Collins and Morris, 1975, are known from rocks ranging in age from Late Cretaceous to Paleocene in Africa, South America, Mexico, and Greenland. The discovery of nine specimens of small crabs in the Eocene Tallahatta Formation of Alabama provides the first documentation of the genus in the United States and extends the geologic range into the middle Eocene. An earlier, questionable reference to a late Eocene species from Hungary by Müller and Collins (1991) cannot be confirmed as the specimen is fragmentary. This range extension makes Costacopluma contemporaneous with the earliest Retropluma Gill, 1894, a genus ranging from early Eocene to Recent, as well as the extinct Retrocypoda Via Boada, 1959. Costacopluma is one of seven genera within the Retroplumidae, as envisioned herein. The genus is the focal point of controversy regarding familial placement. Evaluation of the taxa then considered allied with Retropluma prompted de Saint Laurent (1989) to elevate the family to superfamily level and to propose the new family Costacoplumidae to embrace all the extinct genera then known, including Costacopluma, Retrocypoda, and other Western Hemisphere genera whereas the Retroplumidae sensu stricto was confined to the extant genera, Retropluma and Bathypluma de Saint Laurent, 1989. Examination of representatives of all fossil genera, including nine species of Costacopluma, as well as a specimen of Retropluma, preserved in the spirit collection of the United States Museum of Natural History, documents distinct morphologic characters of the dorsal and ventral carapace as well as the abdomen, uniting all genera within a single family. Furthermore, generic-level characters are shown to be subtle gradations from one taxon to another, suggesting that no clear familial distinctions can be made. Thus, the Retroplumidae is retained as a discrete taxon, embracing all seven genera, ranging from Late Cretaceous to Recent.