It lived in a wet, seasonal, cool temperate environment – which possibly had an average temperature not exceeding 15 °C (59 °F) – alongside salamanders, pterosaurs, birdlike dinosaurs, and other mammaliaforms.

The holotype specimen, JZMP04117, was discovered in the Daohugou Beds of the Jiulongshan Formation in the Inner Mongolia region of China, which dates to about 159–164 million years ago (mya) in the Middle to Late Jurassic.

[1][2] 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.

[1] Evidence of fur and assumed heightened tactile senses indicate it had a well-developed neocortex, a portion of the brain unique to mammals which, among other things, controls sensory perception.

[7] Adelobasileus Sinoconodon Morganucodon Megazostrodon Haramiyavia Thomasia Megaconus Eleutherodon Sineleutherus Sibirotherium Tegotherium Hutegotherium Itatodon Krusatodon Agilodocodon Simpsonodon Tashkumyrodon Castorocauda Dsungarodon Borealestes Docofossor Docodon Haldanodon Hadrocodium Crown mammals Castorocauda is a member of the order Docodonta, an extinct group of mammaliaforms.

[1] In a 2010 review of docodonts, Docodonta was split into Docodontidae, Simpsonodontidae and Tegotheriidae, with Castorocauda considered incertae sedis with indeterminate affinities.

[10] Simpsonodontidae is now considered to be paraphyletic and thus invalid, and Castorocauda appears to have been most closely related to Dsungarodon,[8][6] which came from the Junggar Basin of China and probably ate plants and soft invertebrates.

The discovery of Castorocauda,[5] 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.

Based on these, its adaptations to swimming and digging and its large size, Castorocauda was probably comparable to the modern day platypus, river otters and similar semi-aquatic mammals in ecology and fed primarily on fish (piscivory).

[1] The Daohugou Beds also include several salamanders, numerous pterosaur species (of which many likely were piscivorous),[2] several insects, the clam shrimp Euestheria[1] and some birdlike dinosaurs.