Castoridae is a family of rodents that contains the two living species of and their fossil relatives. A formerly diverse group, only a single genus is extant today, beaver. Two other genera of "giant beavers", Castoroides and Trogontherium, became extinct in the Late Pleistocene.
Characteristics
Castorids are medium-sized mammals, although large compared with most other rodents. They are
semiaquatic, with sleek bodies and webbed hind feet, and are more agile in the water than on land. Their tails are flattened and scaly, adaptations that help them manoeuvre in the water.
Castorids live in small family groups that each occupy a specific territory, based around a lodge and
dam constructed from sticks and mud. They are herbivores, feeding on leaves and grasses in the summer, and woody plants such as willow in the winter.
They have powerful
and the typical rodent
dentition:
Evolution
The earliest castorids belong to the genus
Agnotocastor, known from the late
Eocene and
Oligocene of
North America and
Asia.
Other early castorids included genera such as
Steneofiber, from the Oligocene and
Miocene of Europe, the earliest member of the subfamily Castorinae, which contains castorids closely related to living beavers.
Their teeth were not well suited to gnawing wood, suggesting this habit evolved at a later point, but they do appear adapted to semiaquatic living.
Later, such early species evolved into forms such as
Palaeocastor from the
Miocene of
Nebraska.
Palaeocastor was about the size of a muskrat, and dug
corkscrew-shaped burrows up to deep.
Giant forms evolved in the Pleistocene, including Trogontherium in Europe, and Castoroides in North America. The latter animal was as large as a black bear, yet had a brain only marginally larger than that of modern beavers. Its shape suggests it would have been a good swimmer, and it probably lived in habitats.[Savage, R.J.G., and Long, M.R. 1986. Mammal Evolution: an Illustrated Guide. Facts on File, New York, pp. 120–121 .]
Taxonomy
McKenna and Bell
[McKenna, Malcolm C., and Bell, Susan K. 1997. Classification of Mammals Above the Species Level. Columbia University Press, New York, 631 pp. .] divided Castoridae into two subfamilies, Castoroidinae and Castorinae. More recent studies
have recognized two additional subfamilies of basal castorids, Agnotocastorinae and Palaeocastorinae, which is followed here. Within the family, Castorinae and Castoroidinae are sister taxa; they share a more recent common ancestor with each other than with members of the other two subfamilies. Both subfamilies include semiaquatic species capable of constructing dams.
The Palaeocastorinae include beavers that are interpreted as
fossorial (burrowing),
as are nothodipoidins and
Migmacastor.
[Korth W.W., 2007b. The skull of Nothodipoides (Castoridae, Rodentia) and the occurrence of fossorial adaptations in beavers Journal of Paleontology 81(6):1533-1537.] The following taxonomy is based on Korth
[Korth W.W., 2007a. A new genus of beaver (Rodentia, Castoridae) from the Miocene (Clarendonian) of North America and systematics of the Castoroidinae based on comparative cranial anatomy Annals of Carnegie Museum 76(2):117-134.] and Rybczynski,
with preference given to the latter where these differ.
