What it is
Thylacoleo carnifex, the marsupial lion, was a leopard- to lion-sized predator with enormous slicing premolars. It is extinct. It is a wombat relative, not a cat.
Range
Pleistocene Australia, in woodland and open country. Cave deposits record it across much of the continent.
What happened
Australian megafauna: genera of large mammals, reptiles, and birds: disappeared around 50,000-40,000 years ago, after people reached Sahul (~65,000-50,000). A continent-wide dating study put the pulse near 46,000 years ago (Roberts et al. 2001). Climate is discussed; the Last Glacial Maximum itself is too late to be the sole cause. The human role is central in most recent syntheses and still debated in detail.
When / status
Extinct about 46,000-40,000 years ago, with Australian megafauna.
Human role
Arrival of people is the leading cited cause, with climate as a possible co-factor: not a proven single mechanism.
Sources
Roberts et al. 2001 (Science) on ~46 ka extinction; later Australian megafauna reviews.