|Castoroides ohioensis specimen at the Field Museum|
Castoroides, 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.
Species of Castoroides were much larger than modern beavers. Their average length was approximately 1.9 m (6.2 ft), and they could grow as large as 2.2 m (7.2 ft). The weight of the giant beaver could vary from 90 kg (198 lb) to 125 kg (276 lb). 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 77 kg (170 lb), 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.[failed verification]
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)
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. Perhaps their teeth could have acted as both wood-cutters and gouges. There is no clear evidence that the giant beaver felled trees or built dams, but a possible lodge was discovered near New Knoxville, Ohio around 1912. Part of a giant beaver skull and the lodge were located in a peaty layer surrounded by loam. In Ohio, there have been claims of a possible giant beaver lodge four feet high and eight feet in diameter, formed from small saplings. The recent discovery of clear evidence for lodge building in the related genus Dipoides indicates that the giant beaver probably also built lodges. 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.
The discovery of giant beaver remains in New Brunswick adds significantly to the Quaternary terrestrial mammal fauna of New Brunswick, and suggests that the terrestrial fauna was probably richer than earlier evidence indicated. The known North American distribution of giant beaver is not significantly changed by this occurrence. Specimens from the southeastern US have been placed in a separate species, Castoroides dilophidus, based on differences in premolar and molar features. Martin (1969) considered it a subspecies, but new research by Hulbert et al. place them in their own species, Castoroides dilophidus., It is recorded from more than 25 Pleistocene localities in Florida, 23 of Rancholabrean age, one possibly of Irvingtonian age, and one of late Blancan age.
Castoroides dilophidus specimens have been unearthed in Florida and South Carolina. The latter site (Cooper River) was dated at 1.8 million—11,000 years ago. The Castoroides leiseyorum was named by S. Morgan and J. A. White in 1995 for the Leisey shell pit .. Specimens were found in Leisey Shell Pit 1A and 3B, Hillsborough County, Florida, in paleontological sites about 2.1 Mya. These specimens are now considered to belong to C. dilophidus, C. leiseyorum is no longer a valid species name. Specimens were also found at the Strawberry Hill site, (Cooper River dredging) Charleston County, South Carolina from about 1.8 Mya to 11,000 years ago.
Castoroides went extinct during the Pleistocene–Holocene Transition 12,800–11,500 years ago, alongside several other iconic North American Pleistocene megafauna, including mammoths, mastodons, steppe bison, and so on. This roughly coincides with the arrival of the Clovis people in the region—who rapidly colonized the area by 12,800 years ago—as well the beginning of an aridity trend. It has been long debated if humans ("overkill hypothesis") or climate change had a bigger effect in the extinction event, but they took several thousands of years to completely die out. There is no conclusive evidence that humans hunted Castoroides. Castoroides disappeared from Alaska and the Yukon about 18,000 years ago following the Last Glacial Maximum.
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