Premaxillae of the Extinct Megalonychid Sloths Acratocnus, Neocnus, and Megalonyx, and their Phylogenetic Implications (Mammalia, Xenarthra)

Abstract

In most folivorans, the premaxilla is loosely attached to the maxilla, so that it is often missing in otherwise very well-preserved fossil skulls. Despite its infrequent preservation in sloths, the premaxilla has been shown to have phylogenetically significant variation among the taxa that do preserve the element. In the family Megalonychidae, the premaxilla is known only in the early taxon Eucholoeops (Santacrucian South American Land Mammal Age [SALMA]), the extant two-toed sloth Choloepus, and the North American Neogene taxon Megalonyx, the last described only in an unpublished Master’s thesis. We report here the discovery of the premaxilla in two genera of extinct megalonychids, Neocnus and Acratocnus. These small bodied, semiarboreal megalonychid sloths are endemic to the islands of the Greater Antilles. Though the presence of sloths in the Caribbean dates at least to the late Oligocene, the best known taxa derive from Pleistocene to Holocene cave deposits in Puerto Rico, Hispaniola, and Cuba. We also describe the premaxilla in two species of Megalonyx from North America, the Blancan North American Land Mammal Age (NALMA) M. leptostomus and Rancholabrean NALMA M. jeffersonii. These species show a progressive reorientation of the premaxilla within Megalonyx from a primitive horizontal element to a nearly vertical element, and some significant changes in the anatomy of the incisive foramen. Morphological evidence suggests that a broadened, plate-like premaxilla constitutes a synapomorphy for the entire clade Megalonychidae. Furthermore, although Eucholoeops retains a short anterior process of the premaxilla like that of megatherioid sloths, this process is lacking in the other megalonychids, suggesting that the loss of this process may unite late Miocene to Recent megalonychids.