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the extant two-toed sloth Choloepus, and the North American Neogene taxon Megalonyx, the last described only in an unpublished Master’s thesis.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.

The Turin Pit locality (Monona county, Iowa) has been known to paleontologists since 1908, yet the age of the fauna has been unclear. Early paleontologists considered Turin Pit to date to a pre-Illinoian interglacial (the “Aftonian).” Subsequent researchers suggested it dated to the last glaciation. This study provides a partial list of mammals in the Turin Pit fauna, and together with stratigraphic information, uses the known age ranges of taxa to estimate an age for the assemblage. The presence of Mammuthus, Aenocyon, and Castoroides combined with a magnetically-reversed till located stratigraphically above fossil-bearing deposits, suggest the Turin Pit assemblage dates between ~1.3 and 0.773 Ma. The fauna can be assigned to the Irvingtonian North American Land Mammal Age based on Mammuthus, Aenocyon, Castoroides, and Ondatra zibethicus annectens. This fossil assemblage provides a rare window into the Quaternary paleontology of Iowa that pre-dates the Illinoian glaciation.

Excavations of ACb-3 Cave have uncovered the remains of at least seven individual Megalonyx jeffersonii, providing the most complete ontogenetic sequence of the taxon from one locality. Four individuals representing four distinct age classifications (infant, juvenile, subadult, and adult) were described and examined for pathologies. Cranial and major forelimb elements depict a change in morphology (trending from robust and stout to gracile and elongate) through ontogeny. Pathologies on the scapula and radius of the adult ground sloth indicate a potential attack from fighting or mating. The presence of infant and juvenile sloths suggests this cave may have been used as a maternity den. The nearly complete adult (RMM 5353) was compared to Megalonyx from other North American localities. Bivariate plots, created using linear measurements, showed that RMM 5353 was within the size range of M. jeffersonii and further supports the concept of Megalonyx chronospecies.