''Pachyaena'' (literally, "thick

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

") was a

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

of heavily built, relatively short-legged mesonychids, early Cenozoic

mammals A mammal () is a vertebrate animal of the class Mammalia (). Mammals are characterised by the presence of milk-producing mammary glands for feeding their young, a broad neocortex region of the brain, fur or hair, and three middle e ...

that

evolved Evolution is the change in the heritable Phenotypic trait, characteristics of biological populations over successive generations. It occurs when evolutionary processes such as natural selection and genetic drift act on genetic variation, re ...

before the origin of either modern hoofed

animals Animals are multicellular, eukaryotic organisms in the biological kingdom Animalia (). With few exceptions, animals consume organic material, breathe oxygen, have myocytes and are able to move, can reproduce sexually, and grow from a ...

carnivores A carnivore , or meat-eater (Latin, ''caro'', genitive ''carnis'', meaning meat or "flesh" and ''vorare'' meaning "to devour"), is an animal or plant whose nutrition and energy requirements are met by consumption of animal tissues (mainly mu ...

, and combined characteristics similar to both. The genus likely originated from

Asia Asia ( , ) is the largest continent in the world by both land area and population. It covers an area of more than 44 million square kilometres, about 30% of Earth's total land area and 8% of Earth's total surface area. The continent, which ...

and spread to

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

, and from there to

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

across a

land bridge In biogeography, a land bridge is an isthmus or wider land connection between otherwise separate areas, over which animals and plants are able to cross and colonize new lands. A land bridge can be created by marine regression, in which sea le ...

in what is now the

North Atlantic ocean The Atlantic Ocean is the second largest of the world's five oceanic divisions, with an area of about . It covers approximately 17% of Earth's surface and about 24% of its water surface area. During the Age of Discovery, it was known for se ...

. Various described species of ''Pachyaena'' ranged in size from a

coyote The coyote (''Canis latrans''), also known as the American jackal, prairie wolf, or brush wolf, is a species of canis, canine native to North America. It is smaller than its close relative, the Wolf, gray wolf, and slightly smaller than the c ...

to a

bear Bears are carnivoran mammals of the family (biology), 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 ...

. However, a 2007 paper suggests that ''Pachyaena'' may be

paraphyletic Paraphyly is a taxonomic term describing a grouping that consists of the grouping's last common ancestor and some but not all of its descendant lineages. The grouping is said to be paraphyletic ''with respect to'' the excluded subgroups. In co ...

, with ''P. ossifraga'' being closer to '' Synoplotherium'', '' Harpagolestes'' and '' Mesonyx'' than to ''P. gigantea''. Pachyaena gigantea24

Unlike many

mesonychids Mesonychidae (meaning "middle claws") is an extinct family of small to large-sized omnivorous-carnivorous mammals. They were endemic to North America and Eurasia during the Early Paleocene to the Early Oligocene, and were the earliest group of l ...

, ''Pachyaena'' is known from

skeletal A skeleton is the structural frame that supports the body of most animals. There are several types of skeletons, including the exoskeleton, which is a rigid outer shell that holds up an organism's shape; the endoskeleton, a rigid internal fram ...

material in addition to skulls and jaws. Analysis of three wolf- to bear-sized species from the early Eocene of

Wyoming Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...

( Willwood formation) indicates they all had many

adaptations In biology, adaptation has three related meanings. Firstly, it is the dynamic evolutionary process of natural selection that fits organisms to their environment, enhancing their evolutionary fitness. Secondly, it is a state reached by the p ...

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 Ungulates ( ) are members of the diverse clade Euungulata ("true ungulates"), which primarily consists of large mammals with hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined to b ...

that run to escape

predators Predation is a biological interaction in which one organism, the predator, kills and eats another organism, its prey. It is one of a family of common feeding behaviours that includes parasitism and micropredation (which usually do not kill ...

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 Suidae, pig, with a short, prehensile nose trunk (proboscis). Tapirs inhabit jungle and forest regions of South America, South and Centr ...

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

femur The femur (; : femurs or femora ), or thigh bone is the only long bone, bone in the thigh — the region of the lower limb between the hip and the knee. In many quadrupeds, four-legged animals the femur is the upper bone of the hindleg. The Femo ...

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''.

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), ...

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

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

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

Mongolia Mongolia is a landlocked country in East Asia, bordered by Russia to the north and China to the south and southeast. It covers an area of , with a population of 3.5 million, making it the world's List of countries and dependencies by po ...

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