''Agnotocastor'' is an extinct member of the beaver family

Castoridae The family Castoridae contains the two living species of beavers and their fossil relatives. A highly diverse group of rodents within this family once roamed the earth, but only a single genus is extant today, '' Castor''. Characteristics ...

. Unlike its modern relative, this species took the place of

muskrats The muskrat (''Ondatra zibethicus'') is a medium-sized semiaquatic rodent native to North America and an introduced species in parts of Europe, Asia, and South America. The muskrat is found in wetlands over a wide range of climates and habitat ...

in the rivers of North America during the

Oligocene The Oligocene ( ) is a geologic epoch of the Paleogene Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define the epoch are well identified but ...

epoch. The earliest species is ''A. galushai'', which is also the first reliable member of the Castoridae.Emry, R. J. 1972
A new species of ''Agnotocastor'' (Rodentia, Castoridae) from the early Oligocene of Wyoming.
American Museum Novitates, no. 2485.

References

Prehistoric beavers Eocene rodents Oligocene rodents Prehistoric rodent genera Eocene mammals of Asia Oligocene mammals of Asia Eocene mammals of North America Oligocene mammals of North America {{paleo-rodent-stub