|M. atlas skeleton at the American Museum of Natural History|
Falconer & Cautley, 1837
Megalochelys ("great turtle") is an extinct genus of tortoises that lived from the Miocene to Pleistocene. They are noted for their giant size, which is among the largest of any known testudine, with a maximum carapace length over 2 m (6.5 ft) in M. atlas. During the dry glacial periods it ranged from western India and Pakistan (possibly even as far west as southern and eastern Europe) to as far east as Sulawesi and Timor in Indonesia, though the island specimens likely represent distinct species.
One species of Megalochelys, M. atlas, is the largest known tortoise, with a shell length of 2 m (6.6 ft) and even 2.7 m (8.9 ft), and an approximate total height of 1.8 m (5.9 ft). Popular weight estimates for this taxon have varied greatly with the highest estimates reaching up to 4,000 kg (8,800 lb) in some instances. However, weights based on volumetric displacement of the skeleton, or inferences based on two-dimensional skeletal drawings, indicate that M. atlas was probably closer to 1,000 to 2,000 kg (2,200 to 4,400 lb) in mass. M. atlas is thus the largest known tortoise. The only larger turtles were the marine Archelon and Protostega from the Cretaceous Period, and the aquatic, freshwater Stupendemys of the South American Late Miocene. A similarly gigantic tortoise, Titanochelon, is known from the Miocene to Pleistocene of Europe, which had shell lengths of up to 2 m (6.6 ft).
Like the modern Galápagos tortoise, M. atlas' weight was supported by four elephantine feet. Like other tortoises, it is thought to have been herbivorous.
Megalochelys is the original and valid name for what has been called Colossochelys. It contains three named species with several unnamed taxa.
Cladistic analysis has suggested that Megalochelys' closest living relative is Centrochelys (the African spurred tortoise), with both also being closely related to Geochelone (the star tortoises).
The genus is suspected to have gone extinct due to the arrival of Homo erectus, due to staggered extinctions on islands coinciding with the arrival of H. erectus in these regions, as well as evidence of exploitation by H. erectus. The genus was largely extinct by the end of the Early Pleistocene, but persisted on Timor into the Middle Pleistocene.