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 . Later efforts have produced estimates ranging from , as varying estimates have yielded considerably different results. However, given an apparent upper size limit of 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 from front to back, and 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.