Castoroides (Latin: "beaver" (castor), "like" (oides)), or giant beaver, is an extinct genus of enormous, bear-sized beavers that lived in North America during the Pleistocene. Two species are currently recognized, C. dilophidus in the Southeastern US and C. ohioensis in the rest of its range. C. leiseyorum was previously described from the Irvingtonian of Florida, but is now regarded as an invalid name. All specimens previously described as C. leiseyorum are considered to belong to C. dilophidus.
Description
Species of Castoroides were much larger than modern beavers. Their average length was approximately , and they could grow as large as . The weight of the giant beaver could vary from to . This makes it the largest known rodent in North America during the Pleistocene and the largest known beaver. Recent analyses suggest that they weighed less, closer to , but this is disputable.
The hind feet of the giant beaver were much larger than in modern beavers, while the hind legs were shorter. The tail was longer and may not have been paddle-shaped as in modern beavers. It can only be assumed that its feet were webbed as in modern species. The skull structure of the giant beaver suggests that it participated in extended underwater activity, thanks to the ability to take more oxygen into its lungs.
One of the defining characteristics of the giant beaver was their incisors, which differed in size and shape from those of modern beavers. Modern beavers have incisor teeth with smooth enamel, while the teeth of the giant beaver had a striated, textured enamel surface. Their teeth were also much larger, up to 15 cm (6 in) long.
One other major difference between the giant beaver and the modern beaver is that the size of its brain was proportionally smaller. As a result, the giant beaver may have had inferior interactions in its environment, as well as less complex patterns of thoughts and behavior.
Classification
There are two known species: Castoroides dilophidus (found in Florida and the southeastern states only) Castoroides ohioensis, synonym Castoroides nebrascensis (found throughout continental United States and Canada)
These two species of giant beaver (genus Castoroides) are not close relatives to modern beavers (genus Castor).
This genus typifies the extinct subfamily Castoroidinae, which forms a North American lineage beginning with the Hemingfordian genus Monosaulax, followed by Eucastor, Dipoides, and Procastoroides, to finally culminate and go extinct with Castoroides.
Discovery and species
Castoroides fossils were first discovered in 1837 in a peat bog in Ohio, hence the species epithet ohioensis. Catalogue no.1195, Mus. North. Ind. Hist. Soc. Well-preserved skull of Castoroides ohioensis but with the mandibles lost, both zygomatic arches missing, and the facial portions of the maxillae broken away; dental series complete and in good condition. Castoroides had cutting teeth up to 15 cm-long with prominently-ridged outer surfaces. These strong enamel ridges would have acted as girders to support such long teeth. Further, the deep masseteric fossa of the lower jaw suggests a very powerful bite. Remains of the giant beaver, along with Paleo Indian artifacts and the remains of the flat-headed peccary, giant short-faced bear, and the stag moose were found in the Sheriden Cave in Wyandot County, Ohio.
Fossils of Castoroides are concentrated around the midwestern United States in states near the Great Lakes, particularly Illinois and Indiana, but specimens are recorded from Alaska and Canada to Florida. In Canada, fossils of this species are commonly found in the Old Crow Basin, Yukon, and single specimens are known from Toronto, Ontario and Indian Island, New Brunswick. A hitherto overlooked 1891 record of a Castoroides skull from near Highgate, Ontario is the earliest for Canada. In Old Crow region, Castoroides fossils occur in deposits of the Sangamonian interglacial.