Ptychodus

Ptychodus
Temporal range: 99.6–66 Ma

late Cretaceous

Ptychodus mortoni
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Chondrichthyes
Subclass: Elasmobranchii
Superorder: Selachimorpha
Order: †Hybodontiformes
Superfamily: Hybodontoidea
Family: Ptychodontidae
Genus: Ptychodus
Agassiz, 1837

Ptychodus is a genus of extinct hybodontiform sharks.[1]

Etymology

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.[2]

Fossil records

These sharks lived during the late Cretaceous (age range: 94.3 to 66 million years ago). Fossils of species within this genus have been found in the marine strata of United States, Brazil, Canada, the Czech Republic, France, Germany, India, Japan, Jordan, Mexico, Sweden and the United Kingdom.[3]

Species

Species within this genus include:[3]

  • 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 spectabilis Agassiz 1837
  • Ptychodus whipplei Marcou 1858

Description

Ptychodus was about 10 meters (33 feet) long and was unearthed in Kansas, United States.[4] 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.[5] 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.

Paleobiology

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. 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.[5] 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.

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.[6] 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.[7]

References

  1. ↑ Fossils (Smithsonian Handbooks) by David Ward (Page 200)
  2. ↑ David, Michelle A historical and mechanical description of Ptychodus (Chondrichthyes) dententions with notes on the distribution and systematics of the genus
  3. 1 2 The paleobioloy Database Ptychodus entry accessed on 8/23/09
  4. ↑ "BBC - Earth News - Giant predatory shark fossil unearthed in Kansas"
  5. 1 2 A Field Guide to Fossils of the Smoky Hill Chalk
  6. ↑ 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
  7. ↑ Shawn Hamm Ptychodus and species 2011 - SYSTEMATIC, STRATIGRAPHIC, GEOGRAPHIC AND PALEOECOLOGICAL DISTRIBUTION OF THE LATE CRETACEOUS SHARK GENUS PTYCHODUS WITHIN THE WESTERN INTERIOR SEAWAY


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