Castoroides, or giant beaver, is an extinct genus of enormous beavers that lived in North America during the Pleistocene. C. leiseyorum and its northern sister species Castoroides ohioensis, were the largest beavers to ever exist.
The giant beaver looked similar to modern beavers but, as the name implies, was considerably larger: it grew over 8 ft (2.4 m) in length — making it the largest rodent in North America during the last ice age and the largest known beaver. It weighed roughly 60 to 100 kg (130 to 220 lb), the size of a modern black bear.
Its hind feet were much larger than in modern beavers, but because soft tissues decay, it is not known whether its tail resembled the tails in modern beavers, and it can only be assumed that its feet were webbed like in modern species.
The incisors were 15 cm (5.9 in) long, and had blunt, rounded tips, in contrast to the chisel-like tips found in modern beaver cutting teeth. The molars were well adapted to grinding and resembled those of capybaras with an S-shaped pattern on the grinding surfaces.
One of the important anatomical differences between the giant beaver and modern beaver species, besides size, is the structure of their teeth. Modern beavers have chisel-like incisor teeth for gnawing on wood. The teeth of the giant beaver are bigger and broader, and grew to about 15 centimeters (6 inches) long. In addition, the tail of the giant beaver must have been longer but narrower and its hind legs shorter.
Castoroides ohioensis had a length of up to 2.5 m (8.2 ft) and an estimated weight of 60-100 kg (130-220 lbs); past estimates went up to 220 kg (485 lbs). It lived in North America during the Pleistocene epoch and went extinct at the end of the last Ice Age, 12,000 years ago.
There are two known species:
- Castoroides leiseyorum (found in Florida 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 
The first giant beaver fossils were discovered in 1837 in a peat bog in Ohio, hence its species epithet ohioensis. Nothing is known on whether the giant beaver built lodges like modern beavers. 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.
Fossils of the giant beaver 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. Specimens from Florida have been placed in a subspecies, Castoroides ohioensis dilophidus, based on differences in premolar and molar features.
Castorides leiseyorum specimens were unearthed in Florida and South Carolina. Mark D. Uhen, Ph.D., George Mason University dated the latter site (Cooper River) at 1.8 million—11,000 years ago. The Florida specimens were dated by John Alroy, Ph.D. using Appearance Event Ordination for an age of 2.1 million years ago. Castoroides leiseyorum was named by S. Morgan and J. A. White in 1995 for the Leisey family, phosphate quarry owners who found the first skull.  Specimens were found in Leisey Shell Pit 1A and 3B, Hillsborough County, Florida paleontological sites ~2.1 Ma. Age is based on J. Alroy's evaluation on November 11, 2007. Strawberry Hill site, (Cooper River dredging) Charleston County, South Carolina. ~1.8 Ma—11,000 years ago.
Fossils of the older species, C. leiseyorum, from Florida are , while fossils of the younger species, C. ohioensis, from Toronto, Ontario, and the Old Crow Basin, Yukon Territory, are 130,000 years old, but it is believed that the giant beaver may have died out about 10,000 years ago along with several other American species such as mammoths, mastodons, and ice-age horses. Giant beavers were most abundant south of the Great Lakes in present day Indiana and Illinois.
The extinction of the giant beaver may have been caused by ecological restructuring at the end of the Pleistocene. The arrival of humans in the Americas could have been a factor, but there is no evidence that humans hunted the giant beaver. It was one of the abundant Pleistocene megafauna—a wide variety of very large mammals that lived during the Pleistocene.
Both the native Mi'kmaq people of Canada and the native Pocumtuck people of the Connecticut River Valley of Massachusetts have related significant myths about giant beavers. The Cree people also have myths about giant beavers.
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