Giant Beaver

Castoroides ohioensis is the largest rodent ever to live in North America, reaching the size of a black bear. Its fossils have been found across the upper Midwest, including the glacial lake sediments and peat bogs of Minnesota. Despite its name, the giant beaver was not closely related to the modern North American beaver and did not share its wood-cutting habits.

Minnesota designated the giant beaver its state fossil in 2025. The state's thousands of lakes and wetlands are direct products of the same glacial retreat that ended the giant beaver's world roughly 10,000 years ago.

Castoroides ohioensis grew up to 8 feet (2.4 m) long and weighed up to 220 pounds (100 kg). Its incisors reached 6 inches (15 cm) in length and had a distinctive ridged outer surface, unlike the smooth teeth of modern beavers. The skull alone measured up to 14 inches long.

Despite its teeth and body shape, the giant beaver almost certainly did not cut trees or build dams. Its teeth were shaped for cutting aquatic plants such as sedges and cattails, and its limbs were better suited to swimming than to construction. It lived in shallow lakes, ponds, and wetlands throughout the Pleistocene and went extinct about 10,000 years ago at the end of the last Ice Age.

Minnesota designated the giant beaver its official state fossil in 2025. Castoroides ohioensis was chosen because its fossils occur statewide, the animal's preferred habitat of shallow glacial lakes and peat bogs mirrors the Minnesota landscape today, and the species is large and distinctive enough for students to connect with. The designation was championed by paleontologists, educators, and state legislators.

The species name ohioensis comes from Ohio, where the first scientific specimens were described in the early 19th century. Fossils have since been found across a wide range of the eastern and central United States and Canada, with Minnesota among the northern states where remains turn up in glacial lake deposits.

Giant beaver fossils in Minnesota come from the glacial lake sediments, peat bogs, and stream deposits left behind when Ice Age glaciers retreated roughly 12,000 years ago. Remains have been found in scattered counties across the state, typically as isolated bones or teeth rather than complete skeletons. The wet, low-oxygen conditions of Minnesota's ancient lake beds slow decay and preserve organic material better than most environments.

The Science Museum of Minnesota in St. Paul holds fossil specimens and has exhibits on the state's Pleistocene fauna, making it the main public resource for learning about Ice Age mammals in Minnesota.