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.