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, with possible earliest late Miocene records. 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.
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). Some experts have argued that 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, as well as being outcompeted by the larger-brained and allegedly more social carnivorans. However, this is questionable as studies have found little evidence of brain size and sociality among carnivorans and as most carnivorans are not social animals, with habitats being unfavorable for pack hunting in Africa during the Early and Middle Miocene.
Taxonomy
Early history The Gebel Zelten locality in Libya was first discovered in 1931 by the Italian scientific mission to Kufra. Starting in 1962, a series of geologic expeditions were carried out at the site, with some, such as R. C. Selley's 1963 expedition, being aimed at studying stratigraphy and sedimentology, and others, such as the expeditions of Robert J. G. Savage, being aimed toward the discovery of vertebrate fossils. The first remains of an unknown large predator were discovered in 1962 and 1963 by prospectors. In 1964, during his first expedition, Savage would himself come upon scattered postcranial fragments from such an animal. Only two years later, he discovered the complete skull of a large predatory mammal, specifically a hyaenodont, at a site designated Site 6412, in the southeast corner of the locality. The specimen was catalogued as M.26173 and found its way into the collections of the British Museum of Natural History (now the Natural History Museum, London). In 1973, described both the skull and various other cranial and skull elements, assigning them to a new genus and species of hyaenodont which he named Megistotherium osteothlates; M.26173 was designated as the holotype. Savage believed that several other specimens, such as the very large mandible M.12049 from the Bugti Hills of Pakistan (originally attributed to Anthracotherium), might have belonged to Megistotherium. The postcranial remains assigned to Megistotherium have since been reinterpreted as belonging to an amphicyonid. Remains assigned to Megistotherium have also been described from the Ngorora and Muruyur Formations of Kenya, Egypt,Morlo, M., Miller, E.R., and El-Barkooky, A.N. 2007. Creodonta and Carnivora from Wadi Moghra, Egypt. Journal of Vertebrate Paleontology 27: 145–159. Namibia, and Uganda.Morales, J. and M. Pickford. (2008). "Creodonts and carnivores from the Middle Miocene Muruyur Formation at Kipsaraman and Cheparawa, Baringo District, Kenya." Comptes Rendus Palevol 7 (8): 487-497 A partial right mandible (lower jaw) assigned to the genus, catalogued as DPC 6611 and discovered in strata from the Moghara Formation of Egypt, was described in 1989.
The generic name of Megistotherium comes and . The species name, osteothlastes, meanwhile, comes the and (with -es being an agent noun: 'bone-crusher').
Taxonomy Megistotherium belongs to the subfamily Hyainailourinae, part of the hyainailourid lineage of hyaenodonts. The family Hyainailouridae comprised a diverse group of hyaenodonts that were most successful during the Eocene before being possibly ecologically displaced by the order Carnivora during the late Oligocene. A relationship between M. osteothlastes and Hyainailouros was noted by Savage, who regarded them as "closely allied" with one another. Savage believed that the radiation of large mid-Tertiary hyaenodonts could be encompassed within three genera: Hyainailouros, Megistotherium, and Pterodon. The cladogram below is based on the results recovered by Matthew R. Borths and Nancy J. Stevens (2019):The validity of Megistotherium has been questioned by several authors, who suggest that it is actually a junior synonym of Hyainailouros.J. Morales, M. Pickford, S. Fraile, M. J. Salesa and D. Soria (2003.) "Creodonta and Carnivora from Arrisdrift, early Middle Miocene of southern Namibia" Mem. Geol. Surv. Namibia 19 177–194. However, other studies have maintained M. osteothlastes as a separate taxon based on dental characteristics and body size, and others still have done so as a conservative measure;Morales, J. and Pickford, M. (2005.) "Carnivores from the Middle Miocene Ngorora Formation (13-12 Ma) Kenya." Estudios Geol., 61, 271-284 David Tab Rasmussen and colleagues suggested that they may have shared a common ancestor, perhaps a species of Pterodon like P. africanus. The matter is complicated by the paucity of dentition in M. osteothlastes.
Description Megistotherium is one of the largest hyaenodonts, though the lack of postcranial elements means that its body size is difficult to determine. Body mass estimates for the genus have thus varied. In his 1973 description of M. osteothlastes, Savage used the distal width of an assigned humerus and that of brown bears to provide a lower estimate of ratio of brain weight/body weight to provide an upper estimate of . However, as hyaenodonts have particularly large heads compared to other large carnivorous mammals, he believed that the lower estimate was too low and the upper estimate was too high, thus settling on an intermediate of . In 2008, B. Sorkin provided a body mass estimate of for M. osteothlastes. In 2019, Borth and Stevens used three different methods to determine body masses for various hyaenodonts. The first, using an equation derived from the length of the third lower premolar in various mid-sized and large carnivorans, recovered a mass of . The second and third used comparisons between the third molar lengths in hyaenodonts and felids, and yielded mass estimates of . However, the paucity of remains for M. osteothlastes and the necessary use of anatomical proxies poses inherent issues for body mass estimates, and given the abnormal proportions of hyaenodonts, upper estimates may be very unreliable. Furthermore, body size would have been constrainted by energetic demands; in 2007, Chris Carbone, Amber Teacher, and J. Marcus Rowcliffe suggested a maximum size limit for carnivorous mammals of about .
Skull
The skull of Megistotherium was very large and heavily built. It had a condylobasal length (measured from the anterior, or front, portion of the premaxilla to the posterior, or rear, portion of the occipital condyles) of , and a width when measured from the tip of each zygomatic arch of . The fact that few tooth crowns are preserved, and the presence of areas deformed by bone disease, suggest that the holotype was very old when it died. The anterior portion of M. osteothlastes' maxilla is expanded laterally (outward) to accommodate a very large upper canine. The palate was V-shaped, very narrow at the level of the first upper premolar, yet very wide posterior to the third upper molar. M. osteothlestes' nasals appear to have extended as far posteriorly as the postorbital process of the frontal bone. Its nasal opening was fairly high and wide. The jugal bones are well-preserved, with large infraorbital tubercles. They contributed anteriorly to a very large zygomatic arch. The sagittal crest of M. osteothlastes was the largest known from any taxon at the time, around long and high, partly due to the reduced size of the braincase. The wide zygomatic arches and high sagittal crest of M. osteothlastes suggest that its temporalis muscles were very large, to the point where Savage suggested they may have weighed around apiece. The zygomatic arches are not particularly robust, suggesting that the temporalis was the primary jaw-closing muscle, as in many carnivorous taxa, as opposed to the masseter. The occipital region had a trefoil shape, with a circular foramen magnum with a diameter of . The basisphenoid was extraordinarily robust, forming a solid wedge where the sphenoids and squamosals articulated, preventing them and the brain itself being damaged by the powerful jaw muscles. Like with Hyaenodon, M. osteothlastes had no alisphenoid canal, and the carotid artery would have lay in a shallow groove running along the alar process. As in other hyaenodonts, a tympanic bulla appears to have been absent. The mandible (lower jaw) of Megistotherium is known from a fragment of the right mandiblular ramus. The mandible was very deep and bowed.
Brain anatomy The brain anatomy of Megistotherium is known from an endocast of the holotype. Its overall structure was similar to that of Hyaenodon and Pterodon, though far larger and "rather more complex". The volume of what is known of the brain was about 280 cubic centimetres (cc), although since the olfactory lobes and the anterior part of the cerebellum are missing, it may have exceeded 400 cc in its totality. The cerebrum bore a series of four well-developed sulci, traversed by blood vessels which gave it an appearance likened to a checkerboard. The cerebellum is very large, occupying around a quarter of the total volume of the brain, and is divided into vermis and two distinct lateral lobes. A 2019 study found estimated, assuming Megistotherium weighed , it had a low EQ score of 0.24-0.29. A 2021 study found a higher EQ score of 0.49-0.61, assuming it weighed . However, the low EQ scores of Megistotherium may have been due to the failure of Jerison's EQ regression taking into account brain sizes for high body mass.
Dentition