Megistotherium

Megistotherium ("greatest beast") is an extinct genus of very large hyaenodont belonging to the family Hyainailouridae that lived in Africa and possibly Asia during the early-middle Miocene epoch. The first specimen, a near-complete skull, was discovered in 1963 by Robert J. G. Savage in Libya, and was kept at the Natural History Museum in London alongside postcrania discovered in the prior few years. In 1973, Savage described Megistotherium based on these elements, with the skull serving as the type specimen. It was later determined that the assigned postcrania actually came from an amphicyonid, or "bear-dog". One species of Megistotherium, M. osteothlastes, has been described. There is some debate over whether Megistotherium is a genus of its own or a junior synonym of another, related genus, Hyainailouros.

Due to the paucity of material assigned to Megistotherium, size estimates for the genus have varied considerably over the years. The estimate put forward by Savage was 880 kg (1,940 lb). Later efforts have produced estimates ranging from 500–3,002 kg (1,102–6,618 lb), as varying estimates have yielded considerably different results. However, given an apparent upper size limit of 1,100 kg (2,400 lb) for mammalian carnivores due to energetic demands, and the issues with applying size estimates to such fragmentary material, the highest estimates appear unlikely. Regardless, Megistotherium was very large, with a skull measuring 66.4 cm (26.1 in) from front to back, and 47.1 cm (18.5 in) on either side. The number of upper incisors had been reduced to just one, and its upper canines were very large, to such a degree that the front of the snout had expanded outwards to accommodate them. Megistotherium had a very powerful bite force, as evidenced by the size of its zygomatic (cheek) arches and its large sagittal crest. At the same time, it had a very large gape.

Remains assigned to Megistotherium have been discovered in Namibia, Uganda, Kenya, Libya, Egypt, and Pakistan. There is some faunal overlap between some of these localities, such as the Gebel Zelten of Libya and the Moghara of Egypt, and it is likely that Megistotherium would have hunted similar prey across its range. Due to its large body size, strong bite force, and large gape, Megistotherium may have specialised in large prey, such as proboscideans (elephants and their relatives). Its extinction may correlate to declines in the populations of these animals, due to the amount of time it takes large mammalian herbivores to breed, and increased competition with the more social carnivorans.