Ptychodus

Ptychodus Temporal range: 99.6–61.7Ma PreꞒ Ꞓ O S D C P T J K Pg N Cretaceous to Paleogene
Ptychodus mortoni
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Subclass:	Elasmobranchii
Superorder:	Selachimorpha
Order:	†Hybodontiformes
Superfamily:	Hybodontoidea
Family:	Ptychodontidae
Genus:	Ptychodus Agassiz
Species
P. anonymus P. janewayii P. mortoni P. polygurus P. rugosus P. whippleyi

Ptychodus is a genus of extinct^[1] hybodontiform shark which lived from the Cretaceous to the Paleogene.^[2] Ptychodus was about 32 feet long (10 meters) and was unearthed in Kansas, United States.^[3] 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.^[4] It was a molluscivore predator that dined upon the extremely large bivalves and crustaceans inhabiting the Western Interior Seaway. 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 crushing plate teeth seen in many similar sharks and rays.^[5]

Paleobiology

Ptychodus would 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.^[6]

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. 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. 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 though this durable mollusk with ease.^[4] 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. The Genus name Ptychodus comes from the Greek words ptychos (fold/layer) and odon (tooth), describing the shape of their crushing and grinding teeth that were recovered in deposits around the Niobrara Formation.^[7]

References

• Williston, Samuel (1900) University Geological Survey of Kansas, Volume VI: Paleontology part II, (Carboniferous invertebrates and Cretaceous fish)

Ptychodus mammillaris teeth

External links

• Media related to Ptychodus at Wikimedia Commons
• BBC page on Ptychodus mortoni: "Giant predatory shark fossil unearthed in Kansas"