The largest seems to be the freshwater turtle Stupendemys, with an estimated total carapace length of more than 3.3 m (11 ft) and weight of up to 1,814–2,268 kg (3,999–5,000 lb
Stupendemys is a prehistoric genus of freshwater turtle. Its fossils have been found in northern South America, in rocks dating from the late Miocene to the very start of the Pliocene, about 6 to 5 million years ago.
Stupendemys's carapace measured over 1.80 m (5.9 ft) in length and was also very wide. With an estimated total carapace length of more than 3.3 m (11 ft), it was the largest turtle that ever existed, surpassing even Archelon.The largest freshwater turtle living today is the Arrau turtle (Podocnemis expansa), a pleurodire closely related to Stupendemys, but the Arrau turtle measures only 75 centimetres (30 in).
Two species have been described to date. Stupendemys geographicus was more robust; its remains have been found in the Urumaco Formation of Venezuela. Stupendemys souzai, marginally smaller and more slender, was recovered from the Solimões Formation in Acre State, Brazil.
Its weight helped Stupendemys stay under water for extended periods of time, grazing on aquatic plants. On the other hand, it was probably a very weak swimmer, unable to move its bulk against a swift current, and thus probably avoided smaller streams.
Since S. souzai fossils have been found in sites which yield a rich fossil fauna, even though little is known with certainty, much can be inferred about the ecology of these animals. Among the aquatic animals that shared the habitat with S. souzai were fish, including catfish such as Phractocephalus and Callichthyidae, characids such as Acregoliath rancii and the tambaqui (Colossoma macropomum), the South American lungfish (Lepidosiren paradoxa), trahiras (e.g. Paleohoplias assisbrasiliensis) and freshwater rays and sharks. Crocodilians and other Crurotarsi were diverse and abundant, among them such taxa as Charactosuchus fisheri, Gryposuchus, Mourasuchus, Nettosuchidae and the giant Purussaurus brasiliensis. Other turtles and tortoises found in the same deposits are Chelus columbiana (a prehistoric relative of the mata mata) and Chelonoidis. Further aquatic vertebrates included river dolphins and the large darter "Anhinga" fraileyi.
Terrestrial mammals were plentiful, and the fauna included many megaherbivores, like the ground sloth Acremylodon campbelli, Toxodontidae (e.g. Gyrinodon and Trigodon), Proterotheriidae, and caviomorph rodents, some of them of immense size also (e.g. Kiyutherium, Neoepiblema, Phoberomys burmeisteri, Potamarchus murinus, Telicomys amazonensis and Tetrastylus). Smaller mammals living in that time and place were the ateline monkey Stirtonia and the bulldog bat Noctilio lacrimaelunaris.
Altogether, this fauna is massively dominated by large herbivores and generally lacks terrestrial carnivores. It can thus be assumed that the habitat was mostly low-lying rainforest that was seasonally flooded, as well as floodplains and swampland. The rivers must have been wide and slow-moving, as the fossil-rich rocks are alluvial deposits and do not show evidence of fast-flowing riverbeds that would have dug into the sediment deeply while depositing little of their own.
Carbonemys cofrinii has a shell that measures about 1.72 m (5.6 ft) and was estimated to weigh 916 kg (2,019 lb).
Carbonemys is an extinct genus of pelomedusoid turtle known from the early Paleocene Cerrejón Formation of Colombia. It contains a single species, Carbonemys cofrinii.
In 2005, a 60-million-year-old fossil specimen was discovered in a Colombian coal mine by a North Carolina State doctoral student named Edwin Cadena. It had a shell that measured about 1.72 metres (5 ft 8 in), making it one of the world's largest turtles.
They lived 5 million years after the mass extinction event of many species of dinosaurs. Their jaws were massive and would be powerful enough to eat a crocodile.
The Carbonemys holotype remains were first discovered in a coal mine, and since coal is essentially fossilised carbon, the name Carbonemys was chosen for the genus. The form of the shell is very interesting as it indicates that Carbonemys would have been a member of the Pleurodira, a group better known as the ‘side-necked turtles’. Side-necked turtles tend to have proportionately longer necks than other turtles, which means that they are too long to retract inwards under the shell. Because of this problem, these turtles instead bend their necks to one side so that they lay against the body and under the ridge of the carapace (upper shell), hence the name ‘side-necked turtle’.
Of course, being of a physically large size means that not many predators would pose a threat to you. However, even though the dinosaurs seem to have gone extinct from South America just a few millions of years before the first known of appearance of Carbonemys, giant super predators had already appeared on the landscape. For example, the giant snake Titanoboa, which at the time of writing may be the largest snake so far discovered, is also known from the same formation as Carbonemys. The presence of a giant turtle and a giant snake at the same time as one another is a good indication as to the kinds of fauna that rose up to replace the ecological gaps that were left behind by the extinction of the dinosaurs, and would continue to be quite dominant until the combined rise of mammals and birds.
Skull remains of Carbonemys are also known, and the form of the skull indicates that Carbonemys had a very strong bite. One idea is that these jaws may have been used for crunching through the armoured bodies of crocodiles such as the genus Cerrejonisuchus, which as the name suggests is also from the Cerrejуn Formation. However it must be remembered that the mouth of Carbonemys could have been used to kill either animals or crop vegetation, or perhaps even both. We also can’t look to other modern turtles of the Pleurodira because some genera are carnivorous while others are herbivorous, which can only further confusion as to the speculated diet of Carbonemys. Only the stomach contents of multiple individuals of Carbonemys could give us a clearer idea as to the exact diet of this genus.
Two tortoises share the title of largest ever tortoise: Meiolania at 8 ft.long and well over a ton,and Colossochelys atlas at 8 to 9 ft. and weighing over half a ton.
Meiolania had an unusually shaped skull that sported many knob-like and horn-like protrusions. Two large horns faced sideways, giving the skull a total width of 60 centimetres (2.0 ft), and would have prevented the animal fully withdrawing its head into its shell. The tail was protected by armored 'rings', and sported thorn-like spikes at the end.The body form of Meiolania may be viewed as having converged towards those of dinosaurian ankylosaurids and xenarthran glyptodonts.
The animal was rather large, measuring 2.5 metres (8.2 ft) in length, making it the second-largest known nonmarine turtle or tortoise, surpassed only by Megalochelys atlas from Asia, which lived in the Pleistocene. It lived in Australia, Lord Howe Island, New Caledonia and Vanuatu.
Meiolania turtles fed on plants. Because the remains known by 1925 were found close to beaches, it was thought to be aquatic. It is now known to have been terrestrial.
The genus was erected in 1886 based on remains found on Lord Howe Island, which Richard Owen assigned to the two species M. platyceps and M. minor (now a synonym of the latter).These were the first good meiolaniid remains, and were used to show that the first known remains of a related animal, a species from Queensland now known as Ninjemys oweni (which was assigned to Meiolania until 1992), did not belong to lizards as initially thought, but to turtles.Woodward sank Niolamia argentina into Meiolania, but this was not accepted by later authors.
In New Caledonia, M. mackayi was described from Walpole Island in 1925. It was smaller and less robust than M. platyceps.Meiolania remains are also known from the Pindai Caves, Grande Terre, and from Tiga Island.
M. brevicollis was described in 1992 from the Camfield Beds of northern Australia, and differed from M. platyceps in having a flatter skull and other horn proportions.
Remains of M. damelipi have been found on the island of Efate in Vanuatu, associated with settlements from the Lapita culture.
It is thought that M. damelipi was hunted to extinction by the Lapita people about 3,000 years ago, based on the presence of bones at the bottom of a garbage midden at an archeological site.The remains were primarily leg bones, indicating the turtles were butchered elsewhere.The bones were not present in younger layers of the mound, suggesting the turtles disappeared within 300 years of first human contact.
Colossochelys atlas at 8 to 9 ft. and weighing over half a ton.
Other members of the family Testudinidae are generally small (7–35 cm (2.8–13.8 in) long). M. atlas is the largest known member of the family, with a shell length of about 2.1 m (6.9 ft), an estimated total length of 2.5 to 2.7 m (8.2 to 8.9 ft), and an approximate total height of 1.8 m (5.9 ft). Weight estimates vary greatly: some go as high as 3 to 4 metric tonnes (3000 to 4000 kg), but a weight of around 1 mt (1000 kg) is probably more realistic.The only larger turtles were the oceanic Archelon and Protostega from the Cretaceous period, and the freshwater Stupendemys of the South American late Miocene. In life, M. atlas would have resembled a giant Galápagos tortoise.
Like the modern Galápagos tortoise, M. atlas's weight was supported by four elephantine feet. Since most members of the related genus Testudo are herbivores, paleontologists presume M. atlas had the same diet. When a predator threatened it, M. atlas could probably retract its limbs and head into its shell, like its modern relatives.