Sphenodiscus

Sphenodiscus is an extinct genus of acanthoceratacean ammonite. The genus has been found from many continents and is thought to have had a large global distribution during the Maastrichtian stage of the Late Cretaceous. It was one of the last ammonoids to have evolved before the entire subclass became extinct at the end of the period during the Cretaceous–Paleogene extinction event.

Opalized specimen
Suture pattern of Sphenodiscus

Sphenodiscus
Temporal range: early Campanian-late Maastrichtian
~83.5–66 Ma
Sphenodiscus lenticularis
Scientific classification
Kingdom:
Animalia
Phylum:
Mollusca
Class:
Cephalopoda
Subclass:
Ammonoidea
Superfamily:
Acanthocerataceae
Family:
Sphenodiscidae
Genus:
Sphenodiscus

Meek 1871
Species
  • S. lobatus (Tuomey, 1854 [originally Ammonites lobatus])
  • S. lenticularis (Owen, 1852 [originally Ammonites lenticularis])
  • S. pleurisepta (Conrad, 1857 [originally Ammonites pleurisepta])
  • S. brasiliensis Maury, 1930
  • S. binkhorsti Böhm, 1898
  • S. intermedius Böse, 1927
  • S. aberrans Böse, 1927
  • S. parahybensis Maury, 1930
  • S. prepleurisepta Böse, 1927
  • S. ubaghsi de Groussouvre, 1894
  • S. siva

Fossils have been found throughout North America from localities in South Carolina,[1] North Carolina,[2] South Dakota,[3] Maryland,[4] New Jersey[5] and Mexico.[6] There is also evidence of the genus being present from the island of Trinidad, although the material found from here cannot be classified at the species level.[7] Common species found in North America include S. lobatus, S. lenticularis, and S. pleurisepta. New species have been found from localities outside of North America such as S. binkhorsti from the Maastricht Formation in the Netherlands, S. siva from the Valudavur Formation in India and S. brasiliensis from the beds along the banks of the Rio Gramame in Brazil.[8][9][10] Many specimens of S. lobatus have also been found from the Nkporo Shale in Nigeria.

The shell of Sphenodiscus was streamlined and lateromedially compressed with overlapping whorls and a small umbilicus. The ventral edge of the shell tends to be sharply angled. The outer surface is generally smooth in fossil specimens, although certain species at different stages of ontogenic development may possess many small tubercles along their surfaces.[11] Sphenodiscus had a complex suture pattern with many small branching lobes and saddles.

References
  1. Allmon, W. D.; Knight, J. L. (1993). "Paleoecological significance of a turritelline gastropod-dominated assemblage in the Cretaceous of South Carolina". Journal of Paleontology. 67 (3): 355–360. doi:10.1017/S0022336000036830.
  2. "Online Collections | North Carolina Museum of Natural Sciences". collections.naturalsciences.org. Archived from the original on 2016-03-16. Retrieved 2016-05-16.
  3. Turekian, K. K.; Armstrong, R. L. (1961). "Chemical and mineralogical composition of fossil molluscan shells from the Fox Hills Formation, South Dakota". GSA Bulletin. 72 (12): 1817–1828. Bibcode:1961GSAB...72.1817T. doi:10.1130/0016-7606(1961)72[1817:CAMCOF]2.0.CO;2.
  4. Landman, N. H.; Johnson, R. O.; Edwards, L. E. (2004). "Cephalopods from the Cretaceous/Tertiary boundary interval on the Atlantic Coastal Plain, with a description of the highest ammonite zones in North America. Part 1, Maryland and North Carolina". American Museum Novitates. 3454 (3454): 1. doi:10.1206/0003-0082(2004)454<0001:CFTTBI>2.0.CO;2. hdl:2246/2819.
  5. Kennedy, W. J.; Cobban, W. A. (1996). "Maastrichtian Ammonites from the Hornerstown Formation in New Jersey". Journal of Paleontology. 70 (5): 798–804. doi:10.1017/S0022336000023842. JSTOR 1306482.
  6. Böse, E. (1927). "Cretaceous ammonites from Texas and northern Mexico". University of Texas Bulletin. 2748 (2): 143–357.
  7. Rutsch, R. F. (1939). "Upper Cretaceous Fossils from Trinidad, B. W. I". Journal of Paleontology. 13 (5): 521–523.
  8. Kennedy, W. J. (1986). "The Campanian-Maastrichtian ammonite sequence in the environs of Maastricht (Limburg, the Netherlands), Limburg and Liège provinces (Belgium)". Newsletters on Stratigraphy. 16 (3): 149–168. doi:10.1127/nos/16/1986/149.
  9. Hancock, J. M. (1967). "Some Cretaceous-Tertiary marine faunal changes". Geological Society, London, Special Publications. 2 (1): 91–104. Bibcode:1967GSLSP...2...91H. doi:10.1144/GSL.SP.1967.002.01.10.
  10. Maury, C. J. (1930). "O Cretaceo da Parahyba do Norte". Serviço Geologico e Mineralogico do Brasil Monographia. 8: 1–305.
  11. Bandel, K.; Landman, N. H.; Waage, K. M. (1982). "Micro-Ornament on Early Whorls of Mesozoic Ammonites: Implications for Early Ontogeny". Journal of Paleontology. 56 (2): 386–391. JSTOR 1304464.
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