Evolutionary History and Fossil Record
The Chelonoidis genus has two main sub-categories based on appearance and habitat- the carbonaria and the chilensis groups. The carbonaria group has the closely related red- and yellow-footed tortoises that clearly share a common ancestor. The chilensis group features the Argentine tortoise (C. chilensis), Chaco tortoise (C. petersi), and Galápagos tortoise (C. niger) which share similar habitats and a basic appearance but otherwise do not seem to be closely related. The relationship between the groups is unclear.
There are several theories to explain the relatively small number of tortoise species in South America and the relationship between them, but the fossil record is not very complete. One long-held theory is that they came from Asia using the land bridge, then spread down through North America and shared ancestors with the gopher tortoises (Gopherus species). Another holds Geochelone ancestors floated over from central Africa, taking advantage of their ability to float, resist salt water, and go without food for extended periods.
DNA studies suggest that the carbonaria group may be related to the African hingeback tortoises (Kinixys species). This suggests that they might have come from Gondwana before it separated into Africa and South America some 130 million years ago. One ancestral form from about 5mya, Chelonoidis hesterna (Auffenberg 1971), is thought to have lived in wet forests and split into two species in the Miocene with the yellow-footed tortoioses remaining in the deep forest and the red-footeds colonizing the edges of the forests and the emerging savannahs. As the climate and topography changed, groups of red-footed tortoises became physically separated and genetically isolated.
Read more about this topic: Red-footed Tortoise
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