''Pachyaena'' (literally, "thick

hyena Hyenas, or hyaenas (from Ancient Greek , ), are feliform carnivoran mammals of the family Hyaenidae . With only four extant species (each in its own genus), it is the fifth-smallest family in the Carnivora and one of the smallest in the clas ...

") was a

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In the hierarchy of biological classification, genus com ...

of heavily built, relatively short-legged

mesonychid Mesonychia ("middle claws") is an extinct taxon of small- to large-sized carnivorous ungulates related to artiodactyls. Mesonychids first appeared in the early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely ...

s, early Cenozoic mammals that evolved before the origin of either modern hoofed animals or carnivores, and combined characteristics similar to both. The genus likely originated from

Asia Asia (, ) is one of the world's most notable geographical regions, which is either considered a continent in its own right or a subcontinent of Eurasia, which shares the continental landmass of Afro-Eurasia with Africa. Asia covers an area ...

and spread to Europe, and from there to

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

across a land bridge in what is now the North Atlantic ocean. Various described species of ''Pachyaena'' ranged in size from a

coyote The coyote (''Canis latrans'') is a species of canis, canine native to North America. It is smaller than its close relative, the wolf, and slightly smaller than the closely related eastern wolf and red wolf. It fills much of the same ecologica ...

to a

bear Bears are carnivoran mammals of the family Ursidae. They are classified as caniforms, or doglike carnivorans. Although only eight species of bears are extant, they are widespread, appearing in a wide variety of habitats throughout the Nor ...

. However, recent work indicates that ''Pachyaena'' is paraphyletic, with ''P. ossifraga'' being closer to '' Synoplotherium'', '' Harpagolestes'' and ''

Mesonyx ''Mesonyx'' ("middle claw") is a genus of extinct mesonychid mesonychian mammal: fossils of the various species are found in Early to Late Eocene-age strata in the United States and Early Eocene-aged strata in China, 51.8—51.7 Ma ( AEO). Wh ...

'' than to ''P. gigantea''. Hapalodectes serus

Unlike many mesonychids, ''Pachyaena'' is known from skeletal material in addition to skulls and jaws. Analysis of three wolf- to bear-sized species from the early Eocene of Wyoming (Willwood formation) indicates they all had many adaptations 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 ungulates that run to escape predators 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 Tapirs ( ) are large, herbivorous mammals belonging to the family Tapiridae. They are similar in shape to a pig, with a short, prehensile nose trunk. Tapirs inhabit jungle and forest regions of South and Central America, with one species inhabit ...

. Size varied depending on the species: For ''P. gracilis'', the body mass estimated based in

femur The femur (; ), or thigh bone, is the proximal bone of the hindlimb in tetrapod vertebrates. The head of the femur articulates with the acetabulum in the pelvic bone forming the hip joint, while the distal part of the femur articulates with ...

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

Species

The genus' greatest diversity is seen in North America, with ''P. ossifraga'' known from Latest

Paleocene The Paleocene, ( ) or Palaeocene, is a geological epoch (geology), epoch that lasted from about 66 to 56 mya (unit), million years ago (mya). It is the first epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), E ...

strata of Wyoming, and ''P. gigantea'', ''P. intermedia'', and ''P. gracilis'' known from Early

Eocene The Eocene ( ) Epoch is a geological epoch (geology), epoch that lasted from about 56 to 33.9 million years ago (mya). It is the second epoch of the Paleogene Period (geology), Period in the modern Cenozoic Era (geology), Era. The name ''Eocene' ...

strata of Wyoming. ''P. nemegetica'' is known from Late Paleocene strata of

Mongolia Mongolia; Mongolian script: , , ; lit. "Mongol Nation" or "State of Mongolia" () is a landlocked country in East Asia, bordered by Russia to the north and China to the south. It covers an area of , with a population of just 3.3 million, ...

References

{{Taxonbar, from=Q254141 Paleogene mammals of North America Mesonychids Paleocene mammals Eocene mammals Eocene genus extinctions Paleogene mammals of Asia Fossil taxa described in 1874 Prehistoric placental genera