Pleolobites (Decapoda: Brachyura) from the Paleogene of Africa revisited, with implications on taxonomy of fossil portunoid crabs

Abstract

Since its first description in 1960, the brachyuran crab genus Pleolobites was restricted to the Eocene (Ypresian) strata of Ivory Coast. Here, we report newly collected specimens that indicate its presence in the Paleocene (Thanetian) of Togo. This monotypic genus has been classified within various higher taxa, including Portunidae, Macropipidae, and Carcinidae, depending on the concurrent classification of portunoid crabs. Through the re-examination of the type material of Pleolobites erinaceus, the most recent assignment of Pleolobites within the subfamily Polybiinae of the family Carcinidae is disputed herein. Nevertheless, we refrain from assigning Pleolobites to any family and opine for its placement as Portunoidea incertae sedis. Additionally, the crab genus Rhachiosoma, morphologically similar to Pleolobites, is also removed from Polybiinae and placed in Portunoidea incertae sedis. It is further argued that Palaeoxanthopsis, Paraverrucoides, Parazanthopsis, and Verrucoides currently classified within the family Palaeoxanthopsidae of the superfamily Carpilioidea show striking similarities with Pleolobites and Rhachiosoma, including the morphology of dorsal carapace, thoracic sternum and pleon. Additionally, chelipeds with a spiny propodal margin and multi-lobed serial finger-teeth, otherwise characteristic for Portunoidea are found in Parazanthopsis and Palaeoxanthopsis. It remains to be investigated whether these resemblances are the result of evolutionary convergence or close phylogenetic relationships. add a blank line