Ptychodus

***Ptychodus*** is an extinct genus of hybodont shark in the Ptychodontidae. *Ptychodus* primarily ate shellfish. They had well-adapted crushing plates allowing it to crush through the shells of its prey. Ptychodus went extinct approximately 85 million years ago. A recent publication also found that *Ptychodus* are classified as neoselachian versus hybodont or batoid. The genus name *Ptychodus* comes from the Greek words *ptychos* (fold/layer) and *odon* (tooth), so "fold teeth" describing the shape of their crushing and grinding teeth that were recovered in deposits around the Niobrara Formation.

## Discovery

*Ptychodus* is well represented in the fossil history due to its global distribution; many fossils have been uncovered such as isolated teeth, fragments of dentition, calcified vertebral centra, denticles, and associated fragments of calcified cartilage. The very first remains of *Ptychodus* were found in England in the early 19th century and was characterized as "palates of fish". The first discovery of Ptychodus teeth in Kansas came in 1868 when Leidy reported and described a damaged tooth near Fort Hays, Kansas. After, many more teeth were uncovered in almost perfect conditions and other species within the genus were identified. Fossils of species within this genus have been found in the marine strata of countries such as the United States of America, Brazil, Canada, the Czech Republic, France, Germany, India, Japan, Jordan, Mexico, Sweden and the United Kingdom. The fact that so many fossils of *Ptychodus* have been found in different regions of the world provides evidence of a cosmopolitan distribution of species during the Albian and Turonian stages of the cretaceous.

## Description

The largest Ptychodus species grew to about 10 meters (33 feet) long. It was covered in placoid scales like other members of Hybodontoidea, reinforced with a large cartilaginous skeleton, and was a bearer of large serrated spines along the dorsal fin. Unlike the colossal nektonic planktivores *Rhincodon* (whale sharks) and *Cetorhinus* (basking sharks) which relied upon gill rakers to acquire their food, the *Ptychodus* had a massive arrangement crushing plate teeth. A *Ptychodus'* jaws contained many teeth, up to 550 teeth, 220 of which were set in the lower jaw and 260 set in the upper jaw. These teeth were very large as well. Paleontologists believe that the largest tooth plate measured 55 centimeters in length and 45 centimeters in width. There are two distinct formations of tooth plate between the genus; one being juxtaposed, non-overlapping tooth rows and another being imbricated tooth rows. It is believed that the shape coincides with the diet of the species and their geographic locations, but the time it lived has a big part as well. *Ptychodus marginalis* teeth differ from *Ptychodus polygyrus*. *P marginalis* was in the Middle Cenomanian to Middle Turonian deposits in the English Chalk, while *P. polygyrus* were in the Late Santonian-Early Campanian deposits.

## Paleobiology

It is believed that Ptychodus species generally lived in upper water layers, which were also inhabited by mosasaurs, pliosaurs, and other large sharks such as *Cretoxyrhina*. While there is no solid evidence of members of the *Ptychodus* species living among other durophagous sharks like members of Heterodontidae (bullhead sharks), it is believed that this Cretaceous macropredator was the precursor to the crushing plate teeth seen in many similar sharks and rays. *Ptychodus* would likely have been a benthic predator, straying from the upper layers of the oceans that would have been inhabited by Mosasaurs, Pliosaurs, other sharks such as the *Cretoxyrhina*, which it was ill-equipped to tackle or compete with. It was capable of growing to enormous size because of this, decreasing the contact it had with macropredatory organisms, and securing a vast food source with little to no competition. Its biological range was linked to the Western Interior Seaway, where it was restricted to the middle and southern end, away from the highly concentrated remains of *Cretoxyrhina* and *Squalicorax* in the same period. It is believed that *Ptychodus* species not only preferred this area because of the subtropical environment, but due to the higher concentration of their prey source; *Cremnoceramus*, *Volviceramus* and other members of the inoceramids.

*Ptychodus* was a molluscivore (shellfish hunter) that dined upon the extremely large bivalves and crustaceans inhabiting the Western Interior Seaway. The *Ptychodus* diet was probably restricted to slow-moving or sessile shellfish, mollusks, invertebrates, larvae, and the occasional sunken carrion of Cretaceous megafauna that it could manipulate into its mouth. *P. decurrens* (found in southern India) seems to have eaten animals with hard shells. One of the largest bivalves at the time was the 9-foot *Platyceramus*, a shelled mollusk that would have provided a difficult meal for any other creature, but with its crushing palate *Ptychodus* could have broken through this durable mollusk with ease. Giant ammonites such as the *Parapuzosia seppenradensis*, members of the Belemnite family, squid, and a variety of Cretaceous crustaceans would also make up the majority of the shark's food.