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Paucituberculata is an order of marsupials which has a long evolutionary history, developed exclusively in South America. The few species that represent this group nowadays are relatively generalized, small sized, and similar to placental insectivores of the Family Soricidae. They are grouped in the Family Caenolestidae and are distributed in three disjunctive areas along the Cordillera de los Andes, between Venezuela and southern Chile and Argentina. On the contrary, according to the paleontological record, the Paucituberculata were notably diverse and had a wide distribution in this continent. The families Palaeothentidae and Abderitidae include the most derived species of paucituberculatans, characterized by the reduction of the anterior dentition, the presence of incipient to well-developed lophs (crests), and in the case of abderitids, the transformation of the first lower molar, or last premolar and first molar, in a very compressed and cutting structure, known as “plagiaulacoid” morphology. This type of dentition appeared independently in some multituberculate groups and plesiadapiform primates, as well as in other marsupial groups as the burramids. The Paucituberculata have been long time considered more closely related to other South American marsupials, the Polydolopimorphia, with which they would have formed the monophyletic group “Pseudiprotodontia”. Today this hypothesis is slightly supported. “Pseudiprotodonty” or “pseudodiprotodonty” refers to the hypertrophy and procumbency of the first lower incisor, which appeared as a convergence in kangaroos and other related groups from Australasia. Recent phylogenetic analyses suggest that Didelphimorphia is the most closely related group to the Paucituberculata among South American marsupials. Application of new prospecting and recovery techniques of very small fossils (e.g. sieving, picking) yielded materials suggesting much higher diversity of paucituberculates than previously known. In this thesis, 2800 specimens were studied, among which seven new species and two genera of Paucituberculata were recognized, all of them recovered from Early Miocene sediments of Patagonia. These collections allowed the current understanding of the dental morphology of the members of this order. The new specimens, most of which are represented by teeth with very scarce wear, permitted the recognition of structures so far unknown for some members of the group, as well as the reconsideration of the serial homology of incisors and cheek teeth. The analysis of all the lineages of Paucituberculata resulted in new hypotheses of primary homology. In this sense, the study of the molar pattern of basal Paucituberculata, all of them recently published, was essential. Among the proposals of primary homology are the first incisor of the Paucituberculata with the I2/i2, and the recognition in the upper molars of the following elements of the tribosphenic pattern: anterolabial cusp= StB, posterolabial cusp= StC+D, anterolingual cusp= protocone, posterolingual cusp= metaconule, cusp lingual to StB = paracone, cusp lingual to StC+D and labial to metaconule= metacone. Establishing primary homologies was the key to elucidate the relationships among the different studied species.

Sistemática y bioestratigrafía de los <i>Paucituberculata</i> (Mammalia: marsupialia) del Cenozoico de América del Sur