Pachyaena

 ( Paleogene Mammals Of North America ) 

Pachyaena (literally, "thick hyena") was a genus of heavily built, relatively short-legged Mesonychidae, early Cenozoic Mammal that Evolution before the origin of either modern hoofed Animal or Carnivore, and combined characteristics similar to both. The genus likely originated from Asia and spread to Europe, and from there to North America across a land bridge in what is now the North Atlantic ocean. Various described species of Pachyaena ranged in size from a coyote to a bear. However, a 2007 paper suggests that Pachyaena may be Paraphyly, with P. ossifraga being closer to Synoplotherium, Harpagolestes and Mesonyx than to P. gigantea.

Unlike many Mesonychidae, Pachyaena is known from Skeleton material in addition to Skull and Jaw. Analysis of three wolf- to bear-sized species from the early Eocene of Wyoming (Willwood formation) indicates they all had many Adaptation for running, including paraxonic compressed feet with a vestigial first digit, lower sections of the limbs elongated compared with the upper sections, and limb joints with movement mostly restricted to a sagittal plane (back-and-forth movement). All these characteristics are common to both Ungulate that run to escape Predation and carnivores that run to pursue prey, though they probably evolved independently in mesonychids. Some adaptations are more typical of the grade of cursorial carnivores; others are more specialized, as in ungulates. Pachyaena was likely built for endurance rather than speed; the overall body shape of the genus would have resembled a modern tapir.

Size varied depending on the species: For P. gracilis, the body mass estimated based in femur length is 24.0 kg, for P. ossifraga is about 56.9 kg, and for the large P. gigantea is 81.7 kg. However, weight estimations of P. gigantea vary from 129 to 396 kg.J. Sargis, Eric (2009) Mammalian Evolutionary Morphology: A Tribute to Frederick S. Szalay p.130 Using Zhou's equation (Unpublished thesis, 1995) of ln(body mass) = 1.327 * ln(m2 length * width) -3.355, a body mass of 130 kg is calculated for P. gigantea.

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Species The genus' greatest diversity is seen in North America, with P. ossifraga known from Latest Paleocene strata of Wyoming, and P. gigantea, P. intermedia, and P. gracilis known from Early Eocene strata of Wyoming. P. nemegetica is known from Late Paleocene strata of Mongolia.

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Palaeoecology The dental microwear patterns of Pachyaena show that it processed large, irregular objects. They resemble those of the extant genus Crocuta. This suggests that like Crocuta, Pachyaena had an osteophagy diet.

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