The holotype specimen, JZMP04117, was discovered in the Daohugou Beds of the Tiaojishan Formation in the Inner Mongolia region of China, which dates to about 159–164 million years ago (mya) in the Middle to Late Jurassic.
It comprises a partial skeleton including an incomplete skull but well-preserved lower jaws, most of the ribs, the limbs (save for the right hind leg), the pelvis and the tail.
The remains are so well preserved that there are elements of its soft anatomy and hair.

Castorocauda is a member of the order Docodonta, an extinct group of mammaliaforms.
Mammaliaformes includes mammal-like creatures and the crown mammals (all descendants, living or extinct, of the last common ancestor of all living mammals).
Docodonts are not crown mammals.
When Castorocauda was first described in 2006, it was thought to be most closely related to the European Krusatodon and Simpsonodon.
In a 2010 review of docodonts, Docodonta was split into Docodontidae, Simpsonodontidae and Tegotheriidae, with Castorocauda considered incertae sedis with indeterminate affinities.
Simpsonodontidae is now considered to be paraphyletic and thus invalid, and Castorocauda appears to have been most closely related to Dsungarodon, which came from the Junggar Basin of China and probably ate plants and soft invertebrates.

Castorocauda is part of a Middle Jurassic mammaliaform diversification event, wherein mammaliaforms radiated into a wide array of niches and evolved several modern traits, such as more modern mammalian teeth and middle ear bones.
It was previously thought that mammals were small and ground-dwelling until the Cretaceous–Paleogene boundary (K–Pg boundary) when dinosaurs went extinct.
The discovery of Castrocauda, and evidence for an explosive diversification in the Middle Jurassic – such as the appearance of eutriconodontans, multituberculates, australosphenidans, metatherians and eutherians, among others – disproves this notion.
This may have been caused by the breakup of Pangaea, which started in the Early to Middle Jurassic and diversified habitats and niches, or modern traits that had been slowly accumulating since mammaliaforms evolved until reaching a critical point which allowed for a massive expansion into different habitats.
The holotype specimen, JZMP04117, was discovered in the Daohugou Beds of the Tiaojishan Formation in the Inner Mongolia region of China, which dates to about 159–164 million years ago (mya) in the Middle to Late Jurassic. It comprises a partial skeleton including an incomplete skull but well-preserved lower jaws, most of the ribs, the limbs (save for the right hind leg), the pelvis and the tail. The remains are so well preserved that there are elements of its soft anatomy and hair.
Castorocauda is a member of the order Docodonta, an extinct group of mammaliaforms. Mammaliaformes includes mammal-like creatures and the crown mammals (all descendants, living or extinct, of the last common ancestor of all living mammals). Docodonts are not crown mammals. When Castorocauda was first described in 2006, it was thought to be most closely related to the European Krusatodon and Simpsonodon. In a 2010 review of docodonts, Docodonta was split into Docodontidae, Simpsonodontidae and Tegotheriidae, with Castorocauda considered incertae sedis with indeterminate affinities. Simpsonodontidae is now considered to be paraphyletic and thus invalid, and Castorocauda appears to have been most closely related to Dsungarodon, which came from the Junggar Basin of China and probably ate plants and soft invertebrates.
Castorocauda is part of a Middle Jurassic mammaliaform diversification event, wherein mammaliaforms radiated into a wide array of niches and evolved several modern traits, such as more modern mammalian teeth and middle ear bones. It was previously thought that mammals were small and ground-dwelling until the Cretaceous–Paleogene boundary (K–Pg boundary) when dinosaurs went extinct. The discovery of Castrocauda, and evidence for an explosive diversification in the Middle Jurassic – such as the appearance of eutriconodontans, multituberculates, australosphenidans, metatherians and eutherians, among others – disproves this notion. This may have been caused by the breakup of Pangaea, which started in the Early to Middle Jurassic and diversified habitats and niches, or modern traits that had been slowly accumulating since mammaliaforms evolved until reaching a critical point which allowed for a massive expansion into different habitats.
Castorocauda