A New Species of Fossil Homolid Crab (Decapoda: Brachyura) from the Río Foyel Formation (Paleogene), Río Negro Province, Argentina

Abstract

A previously undescribed fossil specimen of deep water crab belonging to the family Homolidae de Haan, 1839 was discovered in the collection of the Museo Paleontologica de Bariloche of Rio Negro Province, Argentina. The specimen is exceptional because one of the extra-lineal flanks is present. These lateral portions of the carapace are often absent in homolid crab specimens, due to disarticulation of the carapace along the lineae homolicae (Glaessner, 1969). The specimen was collected from the (middle?) Oligocene Rio Foyel Formation (Casadio et al., 2004), which crops out south of the town of San Carlos de Bariloche, in the foothills of the Andes Mountains (Fig. 1). Paleogene specimens of the Homolidae are exceedingly rare, due in large part to the lack of preserved rocks from deepwater environments (Feldmann et al., 1991), which they prefer in modern oceans. This specimen is of particular importance because it is not only the first reported fossil occurrence of the genus Paromola Wood-Mason, 1891 but also provides additional evidence as to the depositional environment of the Rio Foyel Formation. Figure 1 —Collection locality of Paromola vetula n. sp. specimen, near San Carlos de Bariloche, Rio Negro Province, Argentina. The Rio Foyel Formation was deposited within the Nirihuau Basin, Rio Negro Province, Argentina. The lithology of the Rio Foyel Formation is reported as being composed primarily of beds of massive shale with isolated fossiliferous calcareous concretions (Casadio et al., 2004). The concretions contain the remains of corals, brachiopods including Terebratula sp., several species of clams, gastropods, echinoids, and decapods (Casadio et al., 2004). The stratigraphy and exact age of the Rio Foyel Formation is currently a matter of debate. The region has been tectonically deformed, and outcrop exposures are limited. Various conflicting interpretations have been made regarding the depositional environment(s) and the age of the …