page_title: Castoridae
section_title: Evolution
hierarchical_section_title: Castoridae / Evolution
caption_attribution_description: 日本語: カストロイデス（Castoroides ohioensis）の骨格標本。国立科学博物館の展示。 English: Skeleton of Giant beaver (Castoroides ohioensis). Exhibit in the National Museum of Nature and Science, Tokyo, Japan.
context_page_description: The family Castoridae contains the two living species of beavers and their fossil relatives. This was once a highly diverse group of rodents, but is now restricted to a single genus, Castor.
context_section_description: The earliest castorids belong to the genus Agnotocastor, known from the late Eocene and Oligocene of North America and Asia. Other early castorids included genera such as Steneofiber, from the Oligocene and Miocene of Europe, the earliest member of the subfamily Castorinae, which contains castorids closely related to living beavers. Their teeth were not well suited to gnawing wood, suggesting this habit evolved at a later point, but they do appear adapted to semiaquatic living. Later, such early species evolved into forms such as Palaeocastor from the Miocene of Nebraska. Palaeocastor was about the size of a muskrat, and dug corkscrew-shaped burrows up to 2.5 m (8.2 ft) deep.
   
  Giant forms evolved in the Pleistocene, including Trogontherium in Europe, and Castoroides in North America. The latter animal was as large as a black bear, yet had a brain only marginally larger than that of modern beavers. Its shape suggests it would have been a good swimmer, and it probably lived in swampy habitats.
caption: Mounted skeleton of Castoroides ohioensis
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