Castorocauda lutrasimilis

Castorocauda Temporal range: Middle or Late Jurassic, 164 Ma PreЄ Є O S D C P T J K Pg N ↓
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Order:	Therapsida
Suborder:	Cynodontia
Family:	†Docodontidae
Genus:	†Castorocauda Ji et al., 2006
Type species
†Castorocauda lutrasimilis Ji et al., 2006

Castorocauda is a genus of small, semi-aquatic mammal relatives living in the Jurassic period, around 164 million years ago, found in lakebed sediments of the Daohugou Beds of Inner Mongolia. It contains the single species Castorocauda lutrasimilis. They were highly specialized, with adaptations evolved convergently with those of modern semi-aquatic mammals such as beavers, otters, and the platypus.

Castorocauda lutrasimilis is a member of the order Docodonta, which is a wholly extinct group of Mammaliaformes. It is not considered to be a mammal by the crown group definition, which takes the mammals to be the group containing the most recent common ancestor of all living mammals (the monotremes, placentals, and marsupials) and its descendants. Many writers, however, do not define Mammalia as a crown group; Kielan-Jaworowska et al. (2004), for example, defines Mammalia as the group originating with the last common ancestor of Sinoconodon and living mammals, a definition that includes Docodonta.

A Castorocauda fossil was discovered in 2004 in the fossil-rich beds of Liaoning province, China; it was reported in the journal Science by an international team led by Qiang Ji of Nanjing University. The fossil was so well preserved that an important feature of its soft anatomy — hair — was preserved. Hair is present in all modern mammals and is therefore assumed, under principles of maximum parsimony, to have been present in all descendants of the last common ancestor of Castorocauda and today's mammals, including crown mammals and other docodonts. The hair appears to have been a very advanced dense pelage including guard hairs and underfur.