Some Thylacoleo species were the largest mammalian predators in Australia of that time, with Thylacoleo carnifex approaching the weight of a small lion.
Pound for pound, Thylacoleo carnifex had the strongest bite of any mammal species living or extinct; a 100 kg (220 lb) T. carnifex had a bite comparable to that of a 250 kg (550 lb) African Lion and is thought to have hunted large animals such as Diprotodon and giant kangaroos. It also had extremely strong forelimbs, with retractable catlike claws, a trait previously unseen in marsupials. Thylacoleo also possessed enormous hooded claws set on large semi-opposable 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 that 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 was 71 cm (28 in) at the shoulder and about 114 cm (45 in) long from head to tail. The T. carnifex species is the largest, and skulls indicate they averaged 101 to 130 kg (220 to 290 lb), and individuals reaching 124 to 160 kg (270 to 350 lb) were common.