Megistotherium (from Greek, megistos "greatest" + therion "beast" and osteon 'bone' + thlaston 'crushed, bruised' with -es being an agent noun: 'bone-crusher') is an extinct genus of hyaenodontid, the only known species of which is Megistotherium osteothlastes. It is most likely a junior synonym of Hyainailouros sulzeri.
Megistotherium osteothlastes was a large hyaenodontid that lived during the early Miocene epoch some . Its remains have been found in the Ngorora and Muruyur Formations of Kenya, Egypt, Namibia, Uganda and Libya. Named by Robert Savage in 1973, Megistotherium is one of the largest hyaenodontids known. Like the other hyaenodontids, it had an enormous skull relative to its body; up to 66 cm (2 ft 2 in) in length and a body mass estimated at 500 kg (1,100 lb).
The carnassial teeth of Megistotherium (like those of other hyaenodontids) were the upper first molars, and overlapped with their lower molar counterparts like scissors to form a formidable and powerful shearing action. The land that is now the Sahara desert was much more fertile in the Miocene. A considerable amount of it was grassland and rainfall was plentiful. Lakes and ponds provided water for large fauna, which provided Megistotherium and other predators with an ample supply of prey. Large hyaenodontids like this one could have originally evolved as specialized predators or scavengers of large African herbivores. Gomphothere bones have been found with its fossils, indicating that Megistotherium may have hunted them for food.
The clade Hyaenodontidae comprised a diverse group of creodont predators that were most successful during the Eocene before being possibly ecologically displaced by the order Carnivora during the late Oligocene. Megistotherium emerged in the Miocene towards the end of the hyaenodontids' flourishing; it was a part of a radiation of African hyaenodontids that occurred at that time. Hyainailouros sulzeri is very closely related to Megistotherium, extremely similar in size, structure and ratios - with a long tail, short limbs and robust body. Morales & Pickford (2005), Morlo, Miller & El-Barkooky (2007) and Morales, Pickford & Salesa (2008) suggested that Megistotherium is actually a junior synonym of Hyainailouros sulzeri, which is known by an almost complete skeleton, among other remains, and has been found in Europe, Asia and Namibia, and therefore comes from the same localities.
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