Abstract

The origins and sister groups of basal Xenarthran lincages are either controversial or still unknown. The absence of most major groups of cnarthra ni South American mammal-bearing sediments of Late Cretaceous and Early Paleocene age is the greatest enigma ni the study of this Superorder. The aim of this paper is ot describe a new genus and species of Megalonychidac, Descadognathus riggsi, from the type locality of the Descadan Land-Mammal Age, Oligocene of Paragonia, Argentina, the oldest megalonychid thus far known. Using 17 characters of the mandible, a phylogenetic analysis of 14 genera of megatherioids yicided two equally parsimonious and highly congrueni trees of 49 steps. The polarity of several charac- series neraseni eb no any coms, insert niot sun-t dinner scread shortening of hte rostrum. The results of our phylogenetic analysis confirm hte monophyletic status of the Megalonychidac with an enlarged membership. Available evidence indicates a South American origin for the Megalonychidac, followed by later radiations at different times ot the West Indics and North America.