Gigantofossor

Gigantofossor, or the Cavedigger Sloth, is a monospecific genus of mylodontid sloth that lived between 4 million and 11 thousand years ago. It was a massive mammal, one of the largest known to science, weighing nearly 40 tons and measuring well over 30 feet in length. It was native to all of Eldham at one point, but disappeared only a few thousand years ago. The creature's full scientific name, Gigantofossor paneldhami, means "Giant digger of all of Eldham".

Gigantofossor is most well known for possibly creating the river caves that are present throughout Eldham. Its large size made it necessary to remain somewhat aquatic to support its weight (unlike dinosaurs, Gigantofossor and other large mammals lack air sacs and have denser bones), and being large made it necessary to move across a large range to forage for food. It is hypothesized that the aquatic sloths dug into mountains to connect Eldham's rivers in underground tunnels, explaining why so many rivers there go underground frequently.

While other mylodontid ground sloths are thought to have burrowed, they are generally land-dwellers; only Gigantofossor has shown adaptations for swimming, and studies of oxygen isotopes in its bones show that it fed extensively on aquatic vegetation more so than any other extinct mylodontid, and only the unrelated megatheriid sloth Thalassocnus seems to have been equally adapted to living in water.

Being so huge, an adult Gigantofossor had no known natural predators. The massive sloth probably lived most of its days alone, only coming together to breed. They probably had very long gestation periods due to their massive size, and as such reproduced slowly. It is hypothesized that overhunting of the sloths drove them to extinction when humans first migrated to Eldham, as they could not replace their population as quickly as humans killed them. However, with their massive claws, a live Gigantofossor was almost certainly a difficult creature to hunt.