The first known rodent fossils in South America are represented by the three taxa Cachiyacuy contamanensis, C. kummeli, and Canaanimys maquiensis, as well as teeth from Eobranisamys sp. and Eospina sp., the latter two found also in the Santa Rosa fauna from the late Eocene or early Oligocene. By the late Oligocene, all superfamilies and most families of caviomorphs are present in the fossil record. During this time, South America was isolated from all other continents. Several hypotheses have been proposed as to how hystricognath rodents colonized this island continent. Most require that a small group of these rodents traveled across ocean bodies atop a raft of mangroves or driftwood. The most common hypothesis suggests that the ancestor to all modern caviomorphs rafted across the Atlantic Ocean from Africa. This is supported by molecular results, which suggest that the Phiomorpha are sister taxa to the Caviomorpha. In fact, until the discovery of the Laotian rock rat, all modern hystricognath families were restricted to South America, Africa, or had a range that included Africa. The principal alternative hypothesis is that the caviomorph ancestor arose in Asia and migrated to South America through another continent. Most commonly, North America is cited as the most likely continent as connections between North America and Asia were common via Beringia, and North America appears to have been closer to South America than any other continent at this time. The "Franimorpha" were once proposed as a potential North American rodent group that may represent an ancestor to the Caviomorpha, but most modern researchers consider franimorphs to have been protrogomorphous instead of hystricomorphous. Fossil evidence suggests the Entodacrya may have originated in Asia and this is cited as potential evidence for an Asian origin for Caviomorpha as well. Likewise, Jenkins et al. argue that their discovery of a hystricognath rodent family exclusive to Asia may be further evidence for an Asian origin of caviomorphs. Alternatively, the caviomorphs may have originated in Asia, but traveled through Africa, Australia and Antarctica, or Africa and Antarctica. Alternatively they may have originated in Africa and traveled to South America via Antarctica. New World monkeys appear to have colonized South America from Africa at a similar time. Caviomorphs went on to colonize the West Indies as far as the Bahamas, reaching the Greater Antilles by the early Oligocene. This is commonly viewed as another example of oceanic dispersal, although a role for a possible land bridge has also been considered.
Diversity
Caviomorph rodents underwent an explosive diversification upon arrival into South America. They managed to outcompete other animals in rodent-like niches such as certain South American marsupials. Retaining predominantly herbivorous diets, they expanded their sizes to encompass a range from rat-sized echimyids to the bison-sized Phoberomys. Their ecologies included burrowing gopher-like forms such as tuco-tucos, arboreal forms such as porcupines and certain spiny rats, running forms such as maras, and aquatic forms such as the capybara and nutria. Habitats range from grasslands, high mountains, forest edges and dense tropical forests. Although many species of caviomorphs have migrated into Central America since the Great American Interchange, only a single living species, the North American porcupine, has naturally colonized North America north of Mexico. The nutria has been introduced into North America and has proven a highly successful invasive species there.
Note that some changes to this taxonomy have been suggested by molecular studies. The Dinomyidae may belong to the Chinchilloidea, the Abrocomidae may belong to the Octodontoidea, and the Hydrochaeridae may have evolved from within the Caviidae.
