Named By: Louis Agassiz - 1835.
Classification: Chordata, Chondrichthyes, Elasmobranchii, Selachimorpha, Ptychodontidae.
Species: P. mortoni (type), P. anonymus, P. janewayii, P. polygurus, P. rugosus, P. whippleyi.
Diet: Durophagus (shellfish/crustaceans).
Size: Estimated at 10 meters long.
Known locations: Known mostly from Europe, Canada - Saskatchewan, and the USA - Alabama, Colorado, Georgia, Kansas, New Mexico and South Dakota. Similar teeth are also known from other parts of the world hinting at a possibly cosmopolitan distribution.
Time period: Coniacian through to the Maastrichtian of the Cretaceous.
Fossil representation: Mostly teeth, but scales, vertebra and a jawbone fragment are also known.
was one of the most specialist sharks of the late Cretaceous oceans,
as the teeth are adapted for crushing shells rather than tearing
through flesh. As such the teeth are rounded rather than being
triangular and pointed, and have a series of ridges that run across
the surface of the crown. These ridges would have increased the bite
pressure along the length of the ridges, something that would have
helped to hold onto smooth shelled shellfish while also focusing the
bite pressure onto key areas of the shell, making it easier for
Ptychodus to break it. It is actually these
ridges that give a
folded appearance to the teeth that were the inspiration for the name
of the shark which when broken down becomes a combination of ‘ptych’
which means ‘fold’ or ‘layer’, and ‘dus’ which means
When first discovered the teeth of Ptychodus were thought to have belonged to a ray, a bottom dwelling type of cartilaginous fish that are related to the sharks. Later it was thought to be hybodont, a shark similar to Hybodus that had crushing teeth at the back of its mouth but more regular pointed teeth at the back. Even later analysis combined with new fossil discoveries now suggests that Ptychodus was a neoselachian, the group of sharks that appeared during the Cretaceous which are thought to be the direct ancestors to today’s modern sharks.
Analysis of Ptychodus fossils has indicated that it was possibly around ten meters long, something that would actually make Ptychodus one of the largest sharks of the Cretaceous, even bigger than the hyper carnivorous Cretoxyrhina that grew up to seven meters long. However Cretoxyrhina was a pelagic (open ocean) predator of other marine creatures whereas Ptychodus was probably more of a benthic (bottom) predator of shellfish and crustaceans which it could easily eat with its specialised teeth. It might seem strange that a shark could grow so large upon a diet of shellfish, but back in the Cretaceous waters there were giant inoceramid bivalves such as the almost two meter wide Inoceramus. Large prey animals like this combined with a more sedentary lifestyle compared to the pelagic sharks meant that Ptychodus could easily afford to grow large.
Additionally by being a benthic creature Ptychodus may not have come into much contact with the apex predators of the Cretaceous seas, namely the mosasaurs. As marine reptiles the mosasaurs had to remain within reach of the surface so that they could breathe in air, similar in fashion to whales. Sharks do not need to do this however because they have gills that can extract oxygen from the water, which meant that if a Ptychodus was ever harassed by a mosasaur it would just have to stay down until the mosasaur couldn’t hold its breath any longer. Despite the reduction in the numbers of predators that a full grown Ptychodus faced, a small number of teeth from another shark called Squalicorax have been found in association with Ptychodus remains, something that raises the possibility that a Squalicorax fed upon a Ptychodus. This does not reveal the cause of death for the Ptychodus however, as it may not have been killed by the Squalicorax, but just scavenged by it. Nonetheless this does help piece together a part of the marine ecosystems of the Cretaceous where sharks like Ptychodus kept the numbers of large bivalves in check while other sharks ‘cleaned up’ the remains of dead animals by eating them.