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Creodonts
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  *This image is copyright of its original author Creodonts are an extinct group of carnivorous mammals that were long thought to be the ancestors of modern Carnivora. This is no longer thought to be the case. Creodonts were the dominant group of carnivorous mammals in the early Tertiaryand were quite diverse. They ranged from very large, wolf-like animals as Hyaenodon to small mongoose-like forms such asPrototomus vulpeculus. http://www.ucmp.berkeley.edu/mammal/euth...donta.html
http://animaliaenthusiasts.proboards.com...eothlastes
Description: Megistotherium osteothlastes was an enormous hyaenodontid creodont that lived during the early Miocene Epoch some 23 million years ago. It has been found in the Ngorora Formation and Muruyur Formation Kenya, Egypt, Namibia, Uganda and Libya. Named by Robert Savage in 1973, it is one of the largest terrestrial carnivorous mammals known to have existed. It stood about 1.5 meters at the shoulders, 3.5 meters in the head and body length, 1 meter long tail and its skull was 65 cm long and weighing 500 kg. The carnassial teeth of Megistotherium (like those of other Creodonta) 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. Mastodon bones have been found with its fossils, indicating that Megistotherium may have hunted them for food. Taxonomy: The order Creodonta comprised a diverse group of predators that were most successful during the Eocene before being ecologically displaced by the order Carnivora during the late Oligocene. Megistotherium emerged in the Miocene towards the end of the creodents 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.  *This image is copyright of its original author Read more: http://animaliaenthusiasts.proboards.com/thread/213/megistotherium-osteothlastes#ixzz47CsGk3Dh  *This image is copyright of its original author
Megistotherium could join the competition of the biggest terrestrial mammalian predator ever along with Daenodon, Andrewsarchus, and the giant short-faced bears. At 500 kg or 1100 pounds, I cannot visualize this big creodont hunting and killing mature mastodons.
These wonderful new threads piqued my interest. I did a little digging and a little number crunching and look what I've found.
This is where most of Megistotherium fossils are found: Savage (1973) So the two massive specimens are these, 664 mm x 471 mm skull:  *This image is copyright of its original author Distal humerus fragment of width 123 mm and articular width 107 mm.  *This image is copyright of its original author Savage (1973) used a bear humerus and guessed a weight attached to it come up with an estimate and also did a brain mass to body weight formula which is outdated at this point. Skull size is not a good measure as proportion differed in Megistotherium. Nonetheless some form of giant must've held that head up. In terms of the humerus, the diameter is an okay measure to compare between species because widths are better related to weight than lengths are. I packed up all of the cats of Christiansen and Harris (2005) and came with the following felid averages: Mass: 80.3 kg Humeral Articular width (HAW): 42 mm Applying isometry: Mass Megistotherium = (HAW Megistotherium / HAW felids)^3 x mass felids Mass Megistotherium = (107 mm / 42 mm) ^3 x 80.3 Mass Megistotherium = 1,328 kg Now isometry probably does not work in this case. Generally really big animals have bones wider than what you expect for their weight due to weight-bearing and gait issues. So instead of using isometry, I changed the scale factor to 2.7 (some negative allometry and a scale factor I've found that works well interspecifically) to account for this physiological trend. So the math as before except for the scale factor change: Mass Megistotherium = (HAW Megistotherium / HAW felids)^2.7 x mass felids Mass Megistotherium = (107 mm / 42 mm) ^2.7 x 80.3 Mass Megistotherium = 1,003 kg So I'd say around 1 metric ton would be a good range for at least specimen. Whether it is average or large, no one can tell.
1,003 kg = 2,211 pounds! Yes, this was indeed a mega-predator. Thank you Tigerluver.
I will edit and add: 664 mm skull equals 26 inches; the size of a coastal brown bear skull. 
Hyaenodon horridus skull
Measurements; 29,5 cm long - 18 cm wide length of upper fang: 4,5 cm length of lower fang: 4,2 cm Pictures comparing it with a big adult female Spotted Hyena (Crocuta crocuta) Hyena skull measures 29 cm long - 18 cm wide  *This image is copyright of its original author  *This image is copyright of its original author  *This image is copyright of its original author  *This image is copyright of its original author  *This image is copyright of its original author  *This image is copyright of its original author Hyaenodons is a group of carnivorous creodonts of the family Hyaenodontidae endemic to all continents except South America, Australia and Antarctica, living from 42-15.9 mya, existing for approximately 26.1 million years. Hyaenodon horridus was the largest North American species with an estimated weight of 40 to 60 kgs. 
Don't forget Sarkastodon, another creodont that can match Megistotherium in size.
 (05-29-2017, 06:24 AM)Polar Wrote: How large was H.gigas skull? In addition to this. Does anyone have any sources about the size of this animal. I've seen sources that say it weighed 500 kg, and some other sources claiming it was only a big as a big cat. My guess is skull size will help us determine that. (05-29-2017, 07:45 AM)Kingtheropod Wrote:@Kingtheropod(05-29-2017, 06:24 AM)Polar Wrote: How large was H.gigas skull? I am trying to find information of H gigas skull but I have not found anything  *This image is copyright of its original author Hyainailouros sulzeri was one of the largest members of the Creodonta, and the skull of the closely related (if not congeneric) Magistotherium from northern Africa measures over 60 cm in length, larger than that of any other creodont or any modern carnivore. Although complete skulls of Hyainailouros are not known, the size of the dentition is at par with that of magistotherium, but one must take into account that these giant creodonts, like all members of the family Hyaenodontidae, had very big skulls for their body size. The skeleton of H, sulzeri is well known thanks to a fine specimen from a site of Chevilly, France, which shows an animal with relatively short, robust legs and long body and tail. Compared with the contemporary amphicyonid Amphicyon giganteus, the giant creodont had a considerably larger head, but a smaller body. The modern wolf is the biggest extant carnivore whose dentition shows a combination of meat slicing and bone cracking adaptations comparable to that of Hyainailouros and Amphicyon, but it would, of course, be dwarfed by both Miocene meat predators.
Sarkastodon mongoliensis skull, Sarkastodon is only know by a partial skull with a reconstructed length of 46 centimeters long and a mandible. Credit to American Museum of Natural History
 *This image is copyright of its original author  *This image is copyright of its original author
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