Ptychodus

Due to a global distribution Ptychodus is well represented in the fossil record; 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 and Germany in the first half of the 18th century. Ptychodus teeth have long been identified as palates of diodon, or porcupinefish (Osteichthyes, Diodontidae), well-known for their ability to inflate their bodies in defense. At the beginning of the 19th century, several authors including Swiss paleontologist Louis Agassiz eventually demonstrated the affinities of Ptychodus teeth with those of elasmobranchs (rays and sharks). 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. == Taxonomy ==

While the affinity of Ptychodus as some kind of cartilaginous fish has long been accepted, the exact position in the group was long uncertain. A 2016 publication found that Ptychodus are likely true sharks belonging to Selachimorpha, rather than hybodonts or batoids as previously thought. A 2024 study of a complete skeleton concluded that they belong to the Lamniformes (mackerel sharks). ==Description==

Size Ptychodus was a large shark, previously estimated at long based on extrapolation from teeth. The subadult specimen with the largest vertebra showed that it could reach lengths of , so a length is possible, but more analysis is required for verification. Dentition 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 of crushing plate teeth. A Ptychodus jaw contains many teeth, up to 550 teeth, 220 of which are on the lower jaw and 260 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 plates 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 was in the Late Santonian-Early Campanian deposits. Body The only known complete body remains are from the Agua Nueva Formation in Mexico, which suggest that Ptychodus had a fusiform body, with a large and elongate head, the primarily dorsal fin was large and placed forward on the body while the second posterior dorsal fin was much smaller, with a small anal fin being present. The overall body form closely resembles that of the great white shark and Lamna (which includes the porbeagle and salmon shark). ==Paleobiology==

Based on analysis of vertebrae from Spain, it is suggested that species of Ptychodus lived relatively long lives and were slow growing and produced large offspring and small litters. Extinction It is suspected that Ptychodus went extinct during the Campanian due to increased competition with shell crushing mosasaurs in the groups Globidensini and Prognathodontini. ==References==