Fur was preserved on the holotype, and it is the earliest known pelt; this showed that fur, with its many uses including heat retention and as a tactile sense, was an ancestral trait of mammals. Mammals preserved with fur from the Chinese Yixian Formation show little hair on the tail, whereas the fur outline preserved on the "Castorocauda" tail was 50% wider than the pelvis. The first quarter is covered by guard hairs, the middle half by scales and little hair cover and the last quarter by scales with some guard hair. Beavers have a very similar tail. 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.