Diagnosis: Pseudancistrus sidereus is diagnosed by a unique modification of the ventral plates on the caudal peduncle. In loricariids, the plates of the ventral row on the caudal
peduncle are typically bent at an approximately 90° angle to follow the contour of the
body. The bend is often the site of a slight keel formed from one or more rows of slightly
longer odontodes. In P. sidereus, the keel is accentuated by having the dorsal laminae of
the plates strongly concave. Although some loricariids may have the dorsal laminae
slightly concave, it is much more pronounced in P. sidereus.