Pound for pound, Thylacoleo carnifex had the strongest bite of any mammal species living or extinct; a T. carnifex weighing 101 kg (223 lb) had a bite comparable to that of a 250-kg African lion, and research suggests that Thylacoleo could hunt and take prey much larger than itself. Larger animals it may have hunted include Diprotodon spp. and giant kangaroos. It also had extremely strong fore limbs, with retractable, cat-like claws, a trait previously unseen in marsupials. Thylacoleo also possessed enormous hooded claws set on large semiopposable thumbs, which were used to capture and disembowel prey. The long muscular tail was similar to that of a kangaroo. Specialized tail bones called chevrons allowed the animal to tripod itself, and freed the front legs for slashing and grasping.
Its strong forelimbs, retracting claws, and incredibly powerful jaws mean it may have been possible for Thylacoleo to climb trees and perhaps to carry carcasses to keep the kill for itself (similar to the leopard today). Due to its unique predatory morphology, scientists repeatedly claim Thylacoleo to be the most specialized mammalian carnivore of all time. Thylacoleo had vertical shearing ‘carnassial’ cheek–teeth that are relatively larger than in any other mammalian carnivore. Thylacoleo was clearly derived from the diprotodontian ancestry due to the pronounced development of upper and lower third pre-molars which functioned extreme carnassials with complementary reduction in the molar teeth row. They also had canines but they served little purpose as they were stubby and not very sharp.
Thylacoleo was 71 cm (28 in) at the shoulder and about 114 cm (45 in) long from head to tail. The species T. carnifex is the largest, and skulls indicate they averaged 101 to 130 kg (223 to 287 lb), and individuals reaching 124 to 160 kg (273 to 353 lb) were common.
Thylacoleo was first described by Sir Richard Owen in 1859.
In 2002, eight remarkably complete skeletons of T. carnifex were discovered in a limestone cave under Nullarbor Plain, where the animals fell through a narrow opening in the plain above. Based on the placement of their skeletons, at least some survived the fall, only to die of thirst and starvation.
In 2008, rock art depicting what is thought to be a Thylacoleo was discovered on the north-western coast of the Kimberley. This represented only the second example of megafauna depicted by the indigenous inhabitants of Australia. The image contains details that would otherwise have remained only conjecture; the tail is depicted with a tufted tip, it has pointed ears rather than rounded, and the coat is striped. The prominence of the eye, a feature rarely shown in other animal images of the region, raises the possibility that the creature may have been a nocturnal hunter. In 2009, a second image was found that depicts a Thylacoleo interacting with a hunter who is in the act of spearing or fending the animal off with a multiple-barbed spear. Much smaller and less detailed than the 2008 find, it may depict a thylacine, but the comparative size indicates a Thylacoleo is more likely.