Giant Beaver Femur

(Not for sale)

Castoroides nebrascensis
was a prehistoric giant beaver that lived in North America during the last Ice Age. Modern beavers can grow over three feet long, whereas the extinct giant beaver was known to get as long as seven feet. It was the largest rodent to exist and probably lived life quite similarly to the beavers we have with us today.

Fossils of this giant rodent can be found around the Midwestern United States and Canada near the Great Lakes. This Giant Beaver Femur is roughly 15 million years old and was personally collected from the eastern Nodaway River in Iowa in June 2019. 

(Left Giant Beaver Femur)

(Right Modern Beaver Femur)


I was walking along the Nodaway River just after a heavy rainstorm moved through the area and flooded the river. We were looking for Pleistocene-aged fossils like mastodon, mammoth, sloth, etc. that might’ve washed out along the riverbank and after a good while searching, I spotted the virtually perfect Giant Beaver Femur completely exposed on the shore. Just a few days later I discovered the modern beaver femur in the same part of the river where different materials and stones were washed out. Finding these two exquisitely preserved bones provided an ideal way to better understand the size and anatomy of the Giant Beaver.

The giant beaver closely resembled its present-day relative, however, its hind legs were shorter and its back feet were longer; it had longer striated incisor teeth; and it most likely had a smaller brain. This large, left femur measures almost 8-inches long whereas the modern right femur measures roughly half that length at 4.5-inches. 

Castoroides went extinct roughly 12,000 years ago when much of the other megafauna like mammoths and mastodons did. Although it’s unclear why they went extinct—whether from human intervention or other natural reasons—it took several thousand years for them to completely die out. Its fossils can sometimes be found near and around Paleolithic sites, but whether or not it had much contact with man is unknown.


Giant Beaver Femur
Castoroides nebrascensis
Nodaway, SW Iowa
Personally collected 6/28/2019

Beaver Femur
Castoroides ohioensis 
Eastern Nodaway River, Iowa
Personally collected 6/30/2019

(The Giant Beaver Femur measures 7.88 inches long)

(The Modern Beaver Femur measures 4.5 inches long)

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