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In Latin: castor “beaver”, oides “like”, are giant beaver
An extinct genus of an enormous, bear-sized beaver that lived in north America during the Pleistocene. They are kinda like giant rats with large flat tails.

Description:
Species of castoroides are much larger than modern beavers. Their average length was approximately 6.2 ft, and they could grow as large as 7.2 ft. The weight could vary from 198 lb to 276 lb. This makes it the largest known rodent in north America during the Pleistocene and the largest known beaver(1).

Recent analyses suggest that they could have weighed less, closer to 170 lb, but this is disputable(2)
The hind feet of the giant beaver were still much larger than in modern beavers, though the hind legs were shorter. The tail was longer and may not have been as paddle-shaped as in modern beavers. More like a musk rat.

It can only be assumed that its feet were webbed, as in modern species and allowing it maneuverability underwater
Its skull was different too, suggesting that it participated in greater underwater activity. They had a greater ability to take oxygen into its lungs.

One of the defining characteristics of the giant beavers, or any beaver for that matter, is their incisor teeth
The giant beavers teeth were much larger and the shape was different too. Modern beavers have incisor teeth with smooth enamel, while the teeth of the giant beaver had a striated, textured enamel surface. Perhaps for digesting plants? Their teeth were also much larger, up to 6 in long(3).

Larger and dumber?
One other major differences between the giant beaver and our modern day beaver is that the size of brains. The giant beavers brain was proportionally smaller. Given that less humans were around, they probably had less complex patterns of thoughts and behaviour.

There are two known species:
Castoroides dilophidus (found in Florida and the southeastern states only)
Castoroides ohioensis (found throughout continental United States and Canada)
These two genus castoroides are not close relatives to modern beavers, genus castor.

Discovery and species:
In 1837, castoroides fossils were first discovered in a peat bog in Ohio. Its why the species was named ohioensis. The north Indiana historical society found a decent preserved skull though a few pieces were missing. Eventually groups put an entire skeleton together in around 1900.

Giant beavers had cutting teeth up to 6″ long, and they were shaped with prominently ridged outer surfaces
These strong enamel ridges would have acted as girders to support such long teeth. Further, the ‘deepmasseteric fossa’ of the lower jaw suggests a very powerful bite. It is unknown weaither the giant beaver felled trees or built dams, however a possible lodge was discovered near New Knoxville, Ohio around 1912. Part of a giant beaver skull was found in the peaty loam, which could have been a part of the damn. Their teeth could have acted as wood-cutters and gouges.

Dams: four feet high, 8 feet diameter?
In Ohio, there have been claims of a possible giant beaver lodge, formed from small (aspen, cottonwood, birch, popular) saplings. And, it is significant size.

Sheridan cave:
Remains of the giant beaver, paleo indian artifacts and other pleistocene aged critters were found in Wyandot county, Ohio. In the Sheridan cave, remains of the flat-headed peccary, giant short-faced bear, the stag moose and artifacts were all found.

Where do you find giant beavers?
Fossils are concentrated around the midwestern USA near the Great Lakes, particularly Illinois and Indiana. Specimens have also been recorded in Alaska, Yukon, and Florida.

In Canada, fossils of this species are commonly found around Old Crow, and northern Yukon. Single specimens are known from Toronto and Indian Island, New Brunswick. The Toronto areas record of a giant beaver skull from near Highgate, Ontario is the earliest in Canada from 1891. In the Old Crow region, fossils occur in the sangamonian interglacial deposits, and are still being discovered.

The quaternary terrestrial mammal fauna of New Brunswick has not been significant. The discovery of giant beaver suggests this area may have had greater activity than previously indicated. It is debatable if more megafauna will be found here, and in the region.

Castoroides dilophidus has been placed in a separate species, because it is from the southeastern US, and has differences in premolar and molar features. Martin (1969)
More than 25 pleistocene localities in Florida have been observed, 23 of Rancholabrean age. 1 is of possible Irvingtonian age, and 1 of late Blancan(3).

North America ice age distribution:
During our most recent ice age, and the 30,000 to 11,700 years, giant beavers were restricted primarily to the central and eastern U.S. (McDonald and Bryson 2010). They were most abundant south of the Great Lakes in Illinois and Indiana.

Midwestern paleontological Finds: Illinois has had the greatest remains of giant beaver in the midwest
There are at least seven localities in central and northern Illinois: Alton, hopwood, Clear Lake Sand and Gravel, Polecat Creek, Bellflower, New Bedford and Phillips Park. In central Indiana, there are at least 3: Prairie creek, Shoals, and Christensen bog. It has also been documented in Michigan: I-96 site, Dowagiac river, and near the city of Ludington. In Ohio, giant beaver documented from Carter site and Sheriden pit. It has been recovered from the Witte Farm in southern Wisconsin, Boney springs central Missouri, and, as well, two localities near Minneapolis, Minnesota.

What was happening at the end of the Pleistocene?
Something happened to extinct the giant beaver at end of the Pleistocene. Most agree it probably was due to reduction and disappearance of there preferred habitat. The climate warmed and the glaciers retreated north. Clovis people eventually came. And, scientists are still research many of these areas.

McDonald and Bryson in 2010, claimed beavers liked the cooler annual temperatures, with strong growing seasons. However it may have been the spring rains, that wiped food supply, and the giant beavers out
Where glaciers were melting, it was cooler in the north. If the spring rains were increasing, it would have shortened growing seasons significantly.

Swinehart and Richards in 2001, also claimed that for a period during the late pleistocene, lakes, ponds marshes may have actually increased habitat

Extinction:
Following the last glacial maximum. Castoroides disappeared from northern populations, Alaska and the Yukon about 18,000 years ago. Castoroides went totally extinct during the pleistocene–holocene transition, around southern great lakes and south eastern us region, 11,700 years ago. This coincides with the arrival of the Clovis people in the region.

Climate change; humans; competition from other species/predators; a great event, or combination(s). Could have led to there extinction

Folklore: The Innu and Mississaugas do feature a giant beaver in their traditional mythology Nation members believe there is evidence of some human interaction with with giant beaver (5)
In 1972, sociologists claimed ‘giant beaver’ was the basis of an Algonquin myth. Gargantuan beavers created dams so big, on the Saint John River, that the lake behind it almost reached the sea. A popular figure, Glooscap. Struck down the dam with his axe, creating the Reversing Falls. Glooscap chased the monster beaver upstream, creating several islands in the river while attempting to strike him through the ice. The beaver constructed another dam, which created the Great Lakes, and fled through these to the land beyond.
Several versions of an Anishinaabe story tell of giant beavers who “walked upright and stood as tall as the tallest man” as well.

Did the giant beavers jam up creeks and rivers like they do today?
Many scholars believe that stories like these could be evidence of humans and giant beavers. North American indigenous people probably encountered giant beaver or, at the very least, their fossils in a cave. It could indicate evidence of beaver/human conflict similar to what we have today.

Chomp chomp.
Be safe.

Cite: (1) Swinehart, Anthony L.; Richards, Ronald L. (2001). “Paleoecology of Northeast
Indiana Wetland Harboring Remains of the Pleistocene Giant Beaver (Castoroides Ohioensis)” (https://journals.iupui.edu/index.php/ias/article/download/7219/7239).
Proceedings of the Indiana Academy of Science. 110: 151.

2) Grayson, Donald (2011-04-18). The Great Basin: A Natural Prehistory (https://books.google.com/books?id=HAGgwbjoeEwC). University of California Press. ISBN 9780520267473.

3) Harrington, C.R. (1996). “Yukon Beringia Interpretive Center – Giant Beaver” (https://web.archive.org/web/20070914223710/http://www.beringia.com/02/02maina6.html). Archived from the original (http://www.beringia.com/02/02maina6.html) on 2007-09-14.

4) Martin, R.A. (1969). “Taxonomy of the giant Pleistocene beaver Castoroides from Florida”. Journal of Paleontology.

5) Simpson, Leanne Betasamosake (2021). A Short History of the Blockade: Giant Beavers, Diplomacy, and Regeneration in Nishnaabewin. University of Alberta Press. ISBN 9781772125382.

Bibliography: Kurtén, B. and E. Anderson (1980). Pleistocene Mammals of North America. Columbia University Press. pp. 236–237. ISBN978-0-231-03733-4.

Swinehart, Anthony L.; Richards, Ronald L. (2001). “Paleoecology of Northeast Indiana Wetland Harboring Remains of the Pleistocene Giant Beaver (Castoroides Ohioensis)”. Proceedings of the Indiana Academy of Science. 110: 151. Retrieved May 2022