New insight from the Paleogene record of Amazonia into the early diversification of Caviomorpha (Hystricognathi, Rodentia): phylogenetic, macroevolutionary, and paleobiogeographic implications

Caviomorph rodents represent one of the most successful groups of placental mammals from South America. Despite their modern and Neogene high taxonomic diversity, the early evolutionary history of caviomorphs has long remained obscure. Until recently, the majority of Paleogene caviomorph assemblages were limited to South American localities at middle and high latitudes, whereas only a few were known at lower latitudes. Recent field expeditions in Peruvian Amazonia (Contamana and Tarapoto areas) have led to the discovery of about twenty Eocene and Oligocene localities yielding hundreds of fossils of caviomorphs (mainly isolated teeth). The systematic studies of these fossils have revealed the presence of numerous new species. A large-scale cladistic analysis (107 terminal taxa; 513 morphological characters), including these new Amazonian species, was undertaken here. For the first time, the four modern superfamilies (i.e., Erethizontoidea, Cavioidea, Octodontoidea, and Chinchilloidea) and all extant families were considered and included through modern and fossil terminal taxa. As a result, several stem Caviomorpha were recognized outside all four superfamilies. Close relationships between Erethizontoidea and Cavioidea on one hand and Octodontoidea and Chinchilloidea on the other hand, as hypothesized in most recent molecular phylogenies, are supported morphologically, thus leading to the formal definition of the clades Erethicavioi Boivin, n. taxon (Erethizontoidea + Cavioidea + extinct related taxa) and Octochinchilloi Boivin, n. taxon (Octodontoidea + Chinchilloidea + extinct related taxa). A synthetic classification of Caviomorpha is proposed. Three main diversification phases would have occurred during the Paleogene and early Miocene (early radiation of caviomorphs; emergence of modern superfamilies; diversification of superfamilies with emergence of some modern families/subfamilies). These three phases seemingly coincide with global climatic events and intense Andean uplift phases. Based on these recent fossil discoveries and on the phylogenetic results presented here, low latitudes of South America are viewed as the first diversity hotspot of early caviomorphs. Geographic origin of modern superfamilies remains somewhat ambiguous, except for chinchilloids which would have emerged in low-latitude regions.