Ptychodus is a ptychodontid cartilaginous fish from the Early-Late Cretaceous of the entire world. It was named in 1837 by Louis Agassiz. It was one of the most specialized sharks to have evolved in the Cretaceous period, as well as one of the most widespread.
Not much is known about how Ptychodus looked, but based on its vertebrae and the shape of its scales, we can assume that it had a torpedo-shaped body and a mid-sized head with a conical nose; as well as this, it had triangular pectoral (arm) and dorsal (back) fins, a crescent-shaped caudal (tail) fin, large jaws, and 5 gills located near the head on each side of its body. Its body would have been covered in scaly skin.
Unlike many of the other sharks it lived with, such as Cretoxyrhina and Squalicorax, Ptychodus was a molluscivore, using its flattened, plate-like teeth to crush the hardened shells of ammonites swimming through the water. Due to the shape of its scales, it can be assumed that it was an active-swimming, epipelagic animal with a torpedo-shaped body (similar to its more typical counterparts), so this durophagous (able to eat armored prey) lifestyle would have benefitted it, as it would avoid competition with other predators; despite being adapted for living near the surface, it may have occasionally dove deep down to the seafloor to eat shellfish. Analyses of vertebrae on different specimens indicates that Ptychodus grew at a slow rate, entered sexual maturity at a late age, and was relatively long-lived; along with this, the fact that the juveniles were closer in size to live-birthing sharks like tiger sharks, great white sharks, and whale sharks rather than egg-laying sharks, indicating that it may have also given live birth to its young. All of these adaptations point to it having K-selected traits, such as slower growth, later sexual maturity, larger bodies, smaller litter size, and greater parental care; combined with a specialized lifestyle, these may have contributed to its extinction at the hands of adverse conditions.