New Paleogene caviomorphs (Rodentia, Hystricognathi) from Santa Rosa, Peru: biochronology, biogeographic and early evolutionary contributions

ARNAL M.; KRAMARZ, ALEJANDRO G.; VUCETICH, MARÍA GUIOMAR; FRAILEY C.D.; CAMPBELL KENNETH

Congreso; VI Congreso Latinoamericano de Paleontología de Vertebrados; 2018

The origin and early evolution of South American rodents (Hystricognathi, Caviomorpha)is an ongoing debate subject. There is uncertainty about the age and the way in whichthey arrived into the continent, but the idea that their first steps of evolution have takenplace at low latitudes is widely accepted. The caviomorph rodents of the Peruvianlocalities of Santa Rosa (SR) and Contamana are the oldest so far known. In SR they arevery abundant and diverse, but dating is based on biostratigraphy (late middle Eocene?late Eocene), while in Contamana the rodent diversity is somewhat lower but radiometric,vertebrate biochronology and palynostratigraphy dates indicated an age of 41.6?40.94 Ma(late middle Eocene). Previous faunal comparisons between rodents from both localitiesidentified few shared elements (i.e. Eobranisamys, cf. Eoespina). In this work we studiednew rodent fossils from SR and analyzed its biochronologic and biogeographicimplications in order to infer and discuss the possible age of the SR local fauna and thepossible early evolutionary scenarios. As result of the taxonomic study, we recognized fivenew species for SR, four of them previously endemic of Contamana (C. contamanensis, C.kummeli, Canaanimys maquiensis, and Pozomys ucayaliensis). Except for minor variants, thedental pattern of most of them hardly differs from the hypothetic ancestral caviomorphpattern. The occurrence of longlasting rodent lineages in SR for arguing a late Oligoceneage is not supported by the comparisons among the Paleogene Peruvian faunas: SR sharesonly one rodent genus (Eoincamys) and no species with the immediately younger Shapajafauna (Early Oligocene). The Shapaja fauna shares no taxa with the late Oligocene rodentassemblage from the Chambira Formation exposed at Contamana. Thus, evidencessuggest that faunal replacement would have been the ruling process among Paleogenetropical caviomorphs, instead of persistence of conservative long-lasting lineages.Additionally, all the taxa recognized in Contamana also occur in SR, and thus, thetaxonomic affinities between these assemblages are greater than previously though. Thelate middle Eocene?late Eocene age hypothesis of SR is also substantiated by othermammals which occur in the same levels (e.g. primates and marsupials). Thus, weconclude that SR and Contamana assemblages are chronologically closer than previouslythought, and even contemporaneity can be considered. Otherwise, Santa Rosa rodentsrepresent the oldest and species-rich South American Paleogene rodent assemblage. Thevery fast Eocene radiation that gave rise to this diversity was not associated with animmediate geographic dispersion to higher latitudes since different biogeogeographicbarriers (arid diagonal?) or competitions factors would have delayed their dispersal.