The preserved length from head to tail is 425 mm (16.7 in), but in life it was much larger.
Based on the dimensions of the platypus, the lower weight limit was estimated to be 518 g (1.1 lb) in life, and the upper 700 to 800 g (1.5 to 1.8 lb), making it the largest known Jurassic mammaliaform, surpassing the previous record of 500 g (1.1 lb) for Sinoconodon.

The recurved teeth of Castorocauda are more are similar in shape to those of mesonychians, Eocene whales, and seals than other docodonts.
The first two molars have cusps in a straight row, and interlocked during biting.
This feature is similar to the ancestral condition in Mammaliaformes (such as in triconodonts) but is a derived character (it was specially evolved instead of inherited) in Castorocauda.
The lower jaw contained 4 incisors, 1 canine, 5 premolars and 6 molars.

The forelimbs of Castorocauda are very similar to those of the modern platypus: the humerus widens towards the elbow; the forearm bones have hypertrophied (large) epicondyles (where the joint attaches); the radial and ulnal joints are widely separated; the ulna has a massive olecranon (where it attaches to the elbow); the wrist bones are block-like; and the finger bones are robust.
Docodontans were likely burrowing creatures and had a sprawling gait, and Castorocauda may have also used its arms for rowing, similar to the platypus.
There are traces of soft tissue between the toes, suggesting webbed hind feet.
It likely also had claws, and the holotype shows a spur on the hind ankle, which, in male platypuses, is venomous.

Castorocauda likely had 14 thoracic, 7 lumbar, 3 sacral and 25 tail vertebrae.
Like some mammals, it had plated ribs, and the ribs extended into the lumbar vertebrae.
Plating occurred on the proximal margins (the part of the rib closest to the vertebra), and, in Castorocauda, they may have served to increase the insertion area (the part of a muscle which moves while contracting) of the iliocostalis muscle on the back, which would interlock nearby ribs and better support the torso of the animal.
Plated ribs are present in arboreal (tree-dwelling) and fossorial (burrowing) xenarthrans (sloths, anteaters, armadillos and relatives).
The tail vertebrae are flattened dorsoventrally (shortened vertically and widened more horizontally); and each centrum has two pairs of transverse processes (which jut out diagonally from the centrum) on the headward side and another on the tailward side, making the centrum appear somewhat like the letter H from the top-view looking down.
This tail anatomy is similar to beavers and otters, which use their tails for paddling and propulsion.

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.
The preserved length from head to tail is 425 mm (16.7 in), but in life it was much larger. Based on the dimensions of the platypus, the lower weight limit was estimated to be 518 g (1.1 lb) in life, and the upper 700 to 800 g (1.5 to 1.8 lb), making it the largest known Jurassic mammaliaform, surpassing the previous record of 500 g (1.1 lb) for Sinoconodon.
The recurved teeth of Castorocauda are more are similar in shape to those of mesonychians, Eocene whales, and seals than other docodonts. The first two molars have cusps in a straight row, and interlocked during biting. This feature is similar to the ancestral condition in Mammaliaformes (such as in triconodonts) but is a derived character (it was specially evolved instead of inherited) in Castorocauda. The lower jaw contained 4 incisors, 1 canine, 5 premolars and 6 molars.
The forelimbs of Castorocauda are very similar to those of the modern platypus: the humerus widens towards the elbow; the forearm bones have hypertrophied (large) epicondyles (where the joint attaches); the radial and ulnal joints are widely separated; the ulna has a massive olecranon (where it attaches to the elbow); the wrist bones are block-like; and the finger bones are robust. Docodontans were likely burrowing creatures and had a sprawling gait, and Castorocauda may have also used its arms for rowing, similar to the platypus. There are traces of soft tissue between the toes, suggesting webbed hind feet. It likely also had claws, and the holotype shows a spur on the hind ankle, which, in male platypuses, is venomous.
Castorocauda likely had 14 thoracic, 7 lumbar, 3 sacral and 25 tail vertebrae. Like some mammals, it had plated ribs, and the ribs extended into the lumbar vertebrae. Plating occurred on the proximal margins (the part of the rib closest to the vertebra), and, in Castorocauda, they may have served to increase the insertion area (the part of a muscle which moves while contracting) of the iliocostalis muscle on the back, which would interlock nearby ribs and better support the torso of the animal. Plated ribs are present in arboreal (tree-dwelling) and fossorial (burrowing) xenarthrans (sloths, anteaters, armadillos and relatives). The tail vertebrae are flattened dorsoventrally (shortened vertically and widened more horizontally); and each centrum has two pairs of transverse processes (which jut out diagonally from the centrum) on the headward side and another on the tailward side, making the centrum appear somewhat like the letter H from the top-view looking down. This tail anatomy is similar to beavers and otters, which use their tails for paddling and propulsion.
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.
Castorocauda