Heterocercal tail
A tail with a longer upper lobe that holds the end of the backbone, as in sharks and sturgeons.
Also: homocercal tail
In a heterocercal tail the spine bends up into the top lobe, which is larger than the bottom one. Each beat pushes the fish forwards and slightly upwards or downwards, which helps sharks, which have no swim bladder, stay at depth. Sharks, sturgeons and paddlefish have this tail, as did many early fish in the fossil record. Most living bony fish have a homocercal tail, where the backbone ends at the tail's base and the two lobes look equal. Gars and the bowfin show an in-between form.