''Hyopsodus'' is 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 ...

extinct Extinction is the termination of an organism by the death of its Endling, last member. A taxon may become Functional extinction, functionally extinct before the death of its last member if it loses the capacity to Reproduction, reproduce and ...

early

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

mammal A mammal () is a vertebrate animal of the Class (biology), 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 ...

of the family

Hyopsodontidae Hyopsodontidae is an extinct family of primitive mammals, initially assigned to the order Condylarthra, living from the Paleocene to the Eocene in North America and Eurasia. Condylarthra is now thought to be a wastebasket taxon; hyopsodontids h ...

, a group associated with or basal to the

Perissodactyla Perissodactyla (, ), or odd-toed ungulates, is an order of ungulates. The order includes about 17 living species divided into three families: Equidae (horses, asses, and zebras), Rhinocerotidae (rhinoceroses), and Tapiridae (tapirs). They t ...

. It was a small mammal with skull of about 6 cm in length.

Fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

s of this genus have been found in the Eocene of

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 ...

, especially the Bighorn Basin region of the United States. It has also been found in Eurasia.

Taxonomy

Eighteen species of ''Hyopsodus'' have been described from North America, four from Asia, and two from Europe. The exact number and identity of species has been contested, as is common when taxa are erected based on fragmentary materials. However, there is broad agreement that multiple species in the genus lived in the Wasatchian, Uintan, and Bridgerian North American Land Mammal Ages, and they have been used both to reconstruct paleoenvironments and to study evolutionary change.

Appearance and habits

Members of the genus had a long spinal column with short limbs and tail, making them one of the few genera of Paleogene mammal with a common name: "tube-sheep" Reconstructing their appearance in life was originally difficult, as remains are common but often consist only of fragmentary jaws or skulls and teeth. (In ''H. paulus'', this preservation has been observed in predator coprolites, along with lizard scutes.) However, several whole individuals in the genus have been found beautifully preserved in life positions in langerstatten in fossil lakes. They show ''Hyopsodus'' individuals had an arched back with a posture similar to a

hyrax Hyraxes (), also called dassies, are small, stout, thickset, herbivorous mammals in the family Procaviidae within the order Hyracoidea. Hyraxes are well-furred, rotund animals with short tails. Modern hyraxes are typically between in length a ...

, rather than a weasel-like tube-shaped body. Study of the limbs in ''H. lepidus'' has shown these animals were not adapted either for life as underground diggers like moles and

groundhogs The groundhog (''Marmota monax''), also known as the woodchuck, is a rodent of the family Sciuridae, belonging to the group of large ground squirrels known as marmots. A lowland creature of North America, it is found through much of the Easte ...

, or for life in the trees. However, they would have moved quickly on the ground, and could have dug for food or shelter as

pigs The pig (''Sus domesticus''), also called swine (: swine) or hog, is an omnivorous, domesticated, even-toed, hoofed mammal. It is named the domestic pig when distinguishing it from other members of the genus '' Sus''. Some authorities cons ...

and

bandicoots Bandicoots are a group of more than 20 species of small to medium-sized, terrestrial, largely nocturnal marsupial omnivores in the order Peramelemorphia. They are endemic to the Australia–New Guinea region, including the Bismarck Archipelago ...

do. An endocast of the brain of ''H. lepidus'' has shown it had an excellent sense of smell and an expanded ability to process sounds. Though they did not have a sophisticated echolocation system like

bats Bats are flying mammals of the order Chiroptera (). With their forelimbs adapted as wings, they are the only mammals capable of true and sustained flight. Bats are more agile in flight than most birds, flying with their very long spread-out ...

, they may have had basic terrestrial echolocation to help them avoid or find objects in the dark, like modern

shrews Shrews (family Soricidae) are small mole-like mammals classified in the order Eulipotyphla. True shrews are not to be confused with treeshrews, otter shrews, elephant shrews, West Indies shrews, or marsupial shrews, which belong to diffe ...

and

tenrecs A tenrec () is a mammal belonging to any species within the afrotherian family Tenrecidae, which is endemic to Madagascar. Tenrecs are a very diverse group, as a result of adaptive radiation, and exhibit convergent evolution, some resemble hedge ...

. It is likely ''H. lepidus'' was a nocturnal ground-dweller in damp closed forests and by the edges of lakes. ''H. wortmani'' had strong chest muscles and could have dug burrows or swum. The teeth of all species show they were

omnivores An omnivore () is an animal that regularly consumes significant quantities of both plant and animal matter. Obtaining energy and nutrients from plant and animal matter, omnivores digest carbohydrates, protein, fat, and fiber, and metabolize t ...

; at least one species, ''H. lovei,'' had a battery of procumbent incisors worn by chewing, and likely had a specialized diet. In their heyday, ''Hyopsodus'' species may have occupied a variety of niches later taken by other flexible forest feeders such as

rats Rats are various medium-sized, long-tailed rodents. Species of rats are found throughout the order Rodentia, but stereotypical rats are found in the genus ''Rattus''. Other rat genera include ''Neotoma'' (pack rats), '' Bandicota'' (bandicoot ...

, small early perissodactyls, and hypocarnivores like

raccoons The raccoon ( or , ''Procyon lotor''), sometimes called the North American, northern or common raccoon (also spelled racoon) to distinguish it from Procyonina, other species of raccoon, is a mammal native to North America. It is the largest ...

References

Hyopsodontidae Eocene mammals of North America Eocene mammals of Asia Eocene mammals of Europe Fossil taxa described in 1870 Prehistoric placental genera {{paleo-oddtoedungulate-stub