"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.