Anatomy of Arctodictis sinclairi (Mammalia, Metatheria) and the phylogenetic relationships of Sparassodonta, large carnivores of the Cenozoic of South America

Sparassodonta is a group of extinct carnivorous metatherians (Mammalia, Metatheria, Sparassodonta) that evolved in South America during the temporal interval of Paleocene to Pliocene (Peligran-Tiupampan to Chapadmalalan SALMAs). This group of mammals includes different morph types and sizes, ranging from opossum-like to bear-like taxa. The knowledge about the group dates back to the end of the Nineteenth century, nevertheless it has included many controversies, especially related to their phylogenetic position among other metatherians.; A new specimen of Arctodictis sinclairi (Sparassodonta, Borhyaenidae) is studied here and is the core of the present dissertation. The specimen consists of a nearly complete skeleton exquisitely preserved, collected at Gran Barranca (Chubut Province, Argentina) in outcrops of' Sarmiento Formation (Colhuehuapian SALMA; early Miocene). The detailed description provided new information about the morphology of the group in general, which served as material basis for comparisons with other sparassodonts and more distant metatherians. A phylogenetic analysis was conducted based on morphological characters taken from the skull, dentition, and postcranial skeleton. The features were analyzed among taxa of different lineages from the Mesozoic and Cenozoic of Asia, North America, and South America by means of cladistic methodology. The results obtained suggest that Sparassodonta is a monophyletic group closely related to living marsupials and other fossil extinct metatherians (but not related to didelphids, nor to the Australian carnivorous marsupials as previously considered).; The sparassodonts are mainly formed by two groups: Hathliacynidae and Borhyaenidae and some basal stem taxa. The pattern of relationships obtained led to review the traditional systematics of the group and to propose a new modified taxonomic list. Based on the cladogram and the known fossil record, ghost lineages for some groups (e.g., Thylacosmilinae, Borhyaeninae) were recognized that are probably revealing deficiency in the fossil record of the Paleogene of South America. A general discussion about Sparassodonta was lastly performed, which includes a review of the morphological changes in the basicranial anatomy of the group, a proposition of a new hypothesis of homologies of the lower molar cusps, comments about the paleobiology of some sparassodonts focused on Arctodictis, and comments about the extinction of the last sparassodonts.