Out of Africa: Caviomorph Origins

South America was isolated from Africa and the North American continent during most of the Cenozoic. As a result, early mammalian immigrants evolved in isolation resulting in a unique assemblage of terrestrial mammals. Approximately 34 million years ago (Eocene-Oligocene) global cooling and drying periods are believed to have spurred major faunal changes in South America terrestrial fauna,
including the arrival of
caviomorph rodents.

One question that has long puzzled mammalogists is how did caviomorphs get to South America? Did they arrive via dispersal from Africa? Did they enter across the Central American isthmus during the Miocene? The exact mode and timing of the caviomorph invasion remained hotly debated because early (pre-Oligocene) rodent fossils from South America were non-existent.

All that changed with the recent description of several new Middle Eocene (approximately 41 million year old) rodent fossils from Amazonian Peru (Figure 1). A group of European and South American researchers describe five small stem caviomorph rodents that may help clarify the debate.

teeth
Figure 1. Scanning electron microscope images of fossil caviomorph teeth belonging to a new genus and species, Cachiyacuy contamanensis. (From Antoine et al. 2011).

The newly described caviomorphs are remarkably small compared to later caviomorphs. However, their small size and dental morphology are similar to African
phiomorph rodents. In addition, the new fossils are 41 million years old, which demonstrates that proto-caviomorphs entered South America much earlier than previously thought at a time when the climate was more tropical (Figure 2). The new fossil evidence strongly supports the Middle Eocene caviomorph/phiomorph split, which is also consistent with molecular data.

rodentphylo
Figure 2. A phylogeny and dispersal scenario of Palaeogene caviomorph rodents including the newly discovered fossils and related taxa. (From Antoine et al. 2011)


Thus, it appears that Africa is the “homeland for the last common ancestor of caviomorphs.” Furthermore, the evidence from reconstructions of South Atlantic “palaeowinds and palaeocurrents during the Middle Eocene” suggest that Eocene African hystricognaths entered South America via sweepstakes dispersal across the Atlantic from Africa to South America.


References

Antoine, P., Marivaux, L., Croft, D., Billet, G., Ganerod, M., Jaramillo, C., Martin, T., Orliac, M., Tejada, J., Altamirano, A., Duranthon, F., Fanjat, G., Rousse, S., & Gismondi, R. (2011). Middle Eocene rodents from Peruvian Amazonia reveal the pattern and timing of caviomorph origins and biogeography Proceedings of the Royal Society B: Biological Sciences DOI: 10.1098/rspb.2011.1732