The "clawed marsupial sentinel" refers to Thylacoleo carnifex, known as the Marsupial Lion. It was Australia's largest predatory mammal, living from the Pliocene to the late Pleistocene epoch (approx. 2 million to 40,000 years ago). It is known as a "sentinel" or "fearsome predator" for being the top carnivore of its time, occupying an ecological role similar to big cats today.