''Ischyromys'' is an extinct genus of

rodent Rodents (from Latin , 'to gnaw') are mammals of the Order (biology), order Rodentia (), which are characterized by a single pair of continuously growing incisors in each of the upper and lower jaws. About 40% of all mammal species are roden ...

from North America. Ischyromys 1

The 60 cm (2 ft) long creature is one of the oldest rodents known. It resembled a

mouse A mouse ( : mice) is a small rodent. Characteristically, mice are known to have a pointed snout, small rounded ears, a body-length scaly tail, and a high breeding rate. The best known mouse species is the common house mouse (''Mus musculus' ...

and already had characteristic rodent

incisor Incisors (from Latin ''incidere'', "to cut") are the front teeth present in most mammals. They are located in the premaxilla above and on the mandible below. Humans have a total of eight (two on each side, top and bottom). Opossums have 18, w ...

s. ''Ischyromyss hind legs were longer than the forelegs, which could be used for other means than walking. Unlike most other mammals of its time, ''Ischyromys'' was probably

arboreal Arboreal locomotion is the locomotion of animals in trees. In habitats in which trees are present, animals have evolved to move in them. Some animals may scale trees only occasionally, but others are exclusively arboreal. The habitats pose num ...

(along with its relative ''

Paramys ''Paramys'' is an extinct genus of rodents from North America, Europe, and Asia. It is one of the oldest genera of rodents known and probably lived in trees. While the genus name literally means "near a mouse", it coexisted with ''Thisbemys'', ...

''). It was a well-adapted climber that gradually beat out competition from rodent-like arboreal

plesiadapiform Plesiadapiformes ("Adapid-like" or "near Adapiformes") is a group of Primates, a sister of the Dermoptera. While none of the groups normally directly assigned to this group survived, the group appears actually not to be literally extinct (in t ...

primate Primates are a diverse order (biology), order of mammals. They are divided into the Strepsirrhini, strepsirrhines, which include the lemurs, galagos, and lorisids, and the Haplorhini, haplorhines, which include the Tarsiiformes, tarsiers and ...

References

Eocene rodents Oligocene rodents Eocene animals of North America White River Fauna Prehistoric rodent genera Fossil taxa described in 1856 Oligocene mammals of North America {{paleo-rodent-stub