Plio-Pleistocene Megalonychid Sloths of North America

Abstract

Although ground sloths appear in South America in Mustersan (late Eocene) time, they are not found in North America until the Hemphillian (middle Pliocene) when three genera appear, two of them megalonychids. It now appears that the genus Megalonyx was widely distributed through North America by that time. The most primitive is M. mathisi, new species, from the upper Mehrtens formation, Merced Co., California. Additional material of M. curvidens Matthew from the Upper Snake Creek channels of Sioux Co., Nebraska, differs in its shorter cheek teeth and narrower caniniforms from later species. Likewise a fine collection of late Blancan (early Pleistocene) M. leptostomus Cope from Cita Canyon, Randall Co., Texas, exhibits small cranial, dental and podial differences from typical late Pleistocene species of Megalonyx. M. leptonyx (Marsh) is a nomen dubium. The general features of Megalonyx are reviewed and the great variability of its dentition is analyzed. The earliest megalonychid from North America is a new genus and species, Pliometanastes protistus, from early Hemphillian deposits at McGehee Farm, Alachua Co., Florida. Relatively complete material of this new genus is described. Another new species from New Mexico, ?P. galushai, and other records from western North America are presented. The genus bears no special relationship to Megalonyx nor to any of the West Indian megalonychids, but stems from some Mio-Pliocene stock that gave rise to all of these.