Definition of protocercal in US English:

protocercal

adjective

Zoology
  • (In fishes) of, relating to, or designating a tail of a primitive (or larval) type, in which the end of the vertebral column is straight and the caudal fin is symmetrical and often continuous with the dorsal and anal fins.

Origin

Mid 19th century; earliest use found in Philosophical Transactions of the Royal Society of London.