Paleontologists have discovered a baby tooth in the coprolite aka fossilised faeces, of a prehistoric shark called Orthacanthus (illustration above), and concluded that these sharks may have fed on juvenile sharks of the same species amongst other prey. These fossil were found on the coast of New Brunswick, Canada. 

Heteropolar spiral coprolites -with asymmetrical form - were and are produced by fishes with valvular intestines, same structure found in modern sharks and represent fully evacuated coprolites. 

Orthacanthus was a three-metre-long xenacanth shark with a dorsal spine, an eel-like body, and tricusped teeth.  About 260 million years ago, Orthacanthus was the apex predator of freshwater swamps and bayous in Europe and North America

Cannibalism in sharks occurs in the present, Sand tiger sharks have been known for devouring each other in the womb. And top predators as white sharks are know of predate on other as result of a territorial disputed, or in case of possibly extreme hunger.