Ptychodus

Ptychodus is a genus of extinct hybodontiform sharks.[1] As well as a genus of durophagous (shell-crushing) sharks from the Late Cretaceous.[2] Fossils of Ptychodus teeth are found in many Late Cretaceous marine sediments.[3] There are many species among the Ptychodus that have been uncovered on all the continents around the globe.[3] Such species are Ptychodus mortoni, P. decurrens, P. marginalis, P. mammillaris, P. rugosus and P. latissimus to name a few. They died out approximately 85 million years ago in the Western Interior Sea, where a majority of them were found.[2] A recent publication found that Ptychodus are classified as neoselachian versus hybodont or batoid. [4] [5]

Ptychodus Temporal range: late Cretaceous, 112–70 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Ptychodus mortoni
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Chondrichthyes
Order:	†Hybodontiformes
Family:	†Ptychodontidae
Genus:	†Ptychodus Agassiz, 1837
Species
Ptychodus altior Agassiz 1839 Ptychodus anonymus Williston 1900 Ptychodus arcuatus Agassiz 1837 Ptychodus articulatus Agassiz 1837 Ptychodus belluccii Bonarelli 1899 Ptychodus concentricus Agassiz 1839 Ptychodus decurrens Agassiz 1839 Ptychodus elevatus Leriche 1929 Ptychodus gibberulus Agassiz 1837 Ptychodus janewayii Cope 1874 Ptychodus latissimus Agassiz 1843 Ptychodus mahakalensis Chiplonkar and Ghare 1977 Ptychodus mammillaris Agassiz 1839 Ptychodus marginalis Agassiz 1839 Ptychodus mortoni Agassiz 1843 Ptychodus multistriatus Woodward 1889 Ptychodus oweni Dixon 1850 Ptychodus paucisulcatus Dixon 1850 Ptychodus polygyrus Agassiz 1839 Ptychodus rugosus Dixon 1850 Ptychodus spectabili Agassiz 1837 Ptychodus whipplei Marcou 1858

Etymology

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.[6]

Discovery

1822 illustration of the first Ptychodus teeth.

Due to a well global distribution the Ptychodus is well represented in the fossil history; many fossils have been uncovered such as isolated teeth, fragments of dentition, calcified vertebral centra, denticles, and associated fragments of calcified cartilage.[7] The very first remains of Ptychodus were found in England in the early 19th century and was characterized as "palates of fish".[8] The first discovery of Ptychodus teeth in Kansas came in 1868 when Leidy reported and described a damaged tooth near Fort Hays, Kansas.[4] After, many more teeth were uncovered in almost perfect conditions and other species within the genus were identified.[4] Fossils of species within this genus have been found in the marine strata of United States, Brazil, Canada, the Czech Republic, France, Germany, India, Israel, Japan, Jordan, Mexico, Sweden and the United Kingdom.[3] The fact that so many fossils of Ptychodus have been found in different regions of the world provides evidence of a distribution of species during the Albian-Turonian time.[7]

Description

Size of Ptychodus vs an average sized human

Ptychodus was about 10 meters (33 feet) long and was unearthed in Kansas, United States.[9] 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.[10] 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 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 plate between the genus; one being juxtaposed, non-overlapping tooth rows and another being imbricated tooth rows.[11] 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.[12]

Paleobiology

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.[13] Ptychodus would have been a benthic predator, straying from the upper layers of the oceans that would have been inhabited by mosasaurs, pliosaurs, and other sharks such as 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.[14]

Diet

Ptychodus was a molluscivore predator 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) ate animals with hard shells.[7] 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.[10] 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.

Species

There have been 23 species within this genus that have been discovered so far and they include:[3]

• Ptychodus altior Agassiz 1839
• Ptychodus anonymus Williston 1900
• Ptychodus arcuatus Agassiz 1837
• Ptychodus articulatus Agassiz 1837
• Ptychodus atcoensis Carrillo-Briceno 2013
• Ptychodus belluccii Bonarelli 1899
• Ptychodus concentricus Agassiz 1839
• Ptychodus decurrens Agassiz 1839
• Ptychodus elevatus Leriche 1929
• Ptychodus gibberulus Agassiz 1837
• Ptychodus janewayii Cope 1874
• Ptychodus latissimus Agassiz 1843
• Ptychodus mahakalensis Chiplonkar and Ghare 1977
• Ptychodus mammillaris Agassiz 1839
• Ptychodus marginalis Agassiz 1839
• Ptychodus mortoni Agassiz 1843
• Ptychodus multistriatus Woodward 1889
• Ptychodus oweni Dixon 1850
• Ptychodus paucisulcatus Dixon 1850
• Ptychodus polygyrus Agassiz 1839
• Ptychodus rugosus Dixon 1850
• Ptychodus spectabili Agassiz 1837
• Ptychodus whipplei Marcou 1858

Gallery

• 
  Ptychodus mammillaris teeth
• 
  Teeth of Ptychodus decurrens from Cretaceous of United States
• 
  Teeth of Ptychodus anonymous from Cretaceous of United States
• 
  Teeth of Ptychodus species
• 
  The Lower Jaw of the Ptychodus

References

1. Fossils (Smithsonian Handbooks) by David Ward (Page 200)
2. Everhart, Mike. "Ptychodus mortoni". Ocean of Kansas.
3. The paleobioloy Database Ptychodus entry accessed on 8/23/09
4. Everhart, Michael; Caggiano, Tom. "An associated dentition and calcified vertebral centra of the Late Cretaceous elasmobranch, Ptychodus anonymus Williston 1900". 4 (4): 125–136. Cite journal requires |journal= (help)
5. Hoffman, Brian (July 2016). "et. al". Journal of Paleontology. 90 (4): 741–762. doi:10.1017/jpa.2016.64.
6. David, Michelle A historical and mechanical description of Ptychodus (Chondrichthyes) dententions with notes on the distribution and systematics of the genus
7. Verma, Omkar; et al. (February 1, 2012). "Ptychodus decurrens Agassiz (Elasmobranchii: Ptychodontidae) from the Upper Cretaceous of India". Cretaceous Research. 33 (1): 183–188. doi:10.1016/j.cretres.2011.09.014.
8. Everhard, Mike. "Ptychodontid Sharks: Late Cretaceous Shell Crushers". Ocean of Kansas.
9. "BBC - Earth News - Giant predatory shark fossil unearthed in Kansas"
10. A Field Guide to Fossils of the Smoky Hill Chalk
11. Shimada, Kenshu (October 31, 2012). "Dentition of Late Cretaceous shark, Ptychodus mortoni (Elasmobranchii, Ptychodontidae)". Journal of Vertebrate Paleontology. 32 (6): 1271–1284. doi:10.1080/02724634.2012.707997.
12. Hamm, Shawn (May 2010). "The Late Cretaceous shark Ptychodus marginalis in the Western Interior Seaway, USA". Journal of Paleontology. 84 (3): 538–548. doi:10.1666/09-154.1.
13. Shawn A. Hamm The Late Cretaceous shark, Ptychodus rugosus, (Ptychodontidae) in the Western Interior Sea Transactions of the Kansas Academy of Science (1903) - Vol. 113, No. 1/2 (Spring 2010), pp. 44-55
14. Shawn Hamm Ptychodus and species 2011 - SYSTEMATIC, STRATIGRAPHIC, GEOGRAPHIC AND PALEOECOLOGICAL DISTRIBUTION OF THE LATE CRETACEOUS SHARK GENUS PTYCHODUS WITHIN THE WESTERN INTERIOR SEAWAY

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

External links

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