Protocercal is a coordinate term of heterocercal.

As adjectives the difference between protocercal and heterocercal

is that protocercal is (zoology) having a caudal fin extending around the end of the vertebral column, like that which is first formed in the embryo of fishes; diphycercal while heterocercal is (ichthyology) of a fish's tail, such that the vertebral column bends and extends upwards into the upper lobe of the tail, making it larger than the lower lobe; as in sharks, rays, and skates.