Exogyra Classification Essay

Left and right valves of Exogyra

A lesson we can learn from oysters is that even though they have no heart to feel and no brain to reason, many of their species build massive reef communities which provide protection for one another; and not only for their own kind, but for many other ocean organisms. Very fittingly,  they’ve been referred to as the “unshellfish”.

This extinct oyster was a large species that lived in the soft sediment of ancient shallow marine waters. It possessed a thick shell with a distinct pattern of ribbing and pitting representing growth lines. Many of its kind thrived during the Upper Cretaceous Period around 65 to 100 million years ago. Their shells opened to expose a foot and a siphon to filter food and take in oxygen from the ocean water.

 

CLASSIFICATION

Scientific Name: Exogyra, ponderosa

Common Name: Oyster

Phylum: Mollusk (large group of marine and fresh water invertebrates having soft bodies enclosed in a shell)

Class: Pelecypod or Bivalve (means hinged shell)

Order: Ostreoida (means true oyster with irregular shell and adductor muscle; pearl oysters are not true oysters)

Family: Gryphaeidae (includes honeycomb oyster or foam oyster characterized under magnification distinct shell structure)

Genus: Exogyra (extinct group of large shallow marine oysters possessing thick shells with distinctive spiraled peak and ribbing on left valve; right valve was smaller and flattened)

Species: Ponderosa

Exogyra, ponderosa (extinct oyster)

People and Oysters

In Greek mythology, the Greek goddess of love, Aphrodite, was said to have sprang up out of the ocean on an oyster shell. The term “Aphrodisiac”, meaning to heighten love, has been related to oysters ever since.  Also, the charismatic Casanova was known to have eaten twelve oysters a day, believing it would  enrich his love life.

 

Oysters have been a part of the human diet since the Greeks and Romans. Today, two-billion pounds are eaten every year around the world. Oysters are prepared in a variety of ways, but raw on the half shell is the most nutritious. Oysters contain rich sources of B vitamins, and scarce minerals such as calcium, iron, zinc, selenium and magnesium.

 

Hand crafted oyster jewelry making is a popular past time. Only one in 10,000 oysters produce a pearl so human intervention has found a way to culture them artificially, but it still takes about six years for the oyster to complete the process. Many artists also craft beautiful jewelry using the shells of various oysters.

 

Today many of these small oyster fossils (shown below) are found in abundance within shell banks along North American coast lines. They were originally likely washed ashore during storms and deposited on the beaches. Eventually, layers and layers of sand  buried them deep where they fossilized into limestone. I’ve made my best guesstimates to identify them.

 

Graphea, navia (from Triasic 210mya – Jurassic 150mya

Exogyra, Graphea and Texigryphaea Extinct Oyster Fossil Rendering

 

 

Texigryphaea oyster fossil (Cretaceous) 135mya – (Miocene) 40mya

Texigryphaea fossil (Cretaceous) 135mya – (Miocene) 40mya

Graphea, navia oyster fossil (Triasic) 210mya – (Jurassic) 150mya

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Exogyra is an extinct genus of fossilmarineoysters in the family Gryphaeidae, the foam oysters or honeycomb oysters.[1] These bivalves grew cemented by the more cupped left valve. The right valve is flatter, and the beak is curved to one side. Exogyra lived on solid substrates in warm seas during the Jurassic and Cretaceous periods.

The subgenus Aetostreon is sometimes considered a separate genus, due to a lack of the fine set of parallel ribs (chomata) separated by pits, on the inner surface of the valves (which is present in the nominate subgenus).[2]

Species[edit]

Exogyra (subgenus Exogyra) (Say, 1820)
  • Exogyra africana Say, 1820[3]
  • Exogyra aquillana Stephenson, 1953
  • Exogyra arietina[4][5]
  • Exogyra callophyla Ihering, 1903
  • Exogyra cancellata Stephenson, 1914
  • Exogyra clarki Shattuck, 1903
  • Exogyra columbella Meek, 1876
  • Exogyra contorta Eichwald, 1868
  • Exogyra costata
  • Exogyra davidsoni[6]
  • Exogyra columba
  • Exogyra erraticostata Stephenson[7]
  • Exogyra fimbriata Conrad, 1855
  • Exogyra flabellata
  • Exogyra ganhamoroba Maury, 1936
  • Exogyra guadalupae Whitney, 1937 (thesis)
  • Exogyra haliotoidea Maury, 1936
  • Exogyra laevigata[8]
  • Exogyra laeviuscula Roemer, 1849
  • Exogyra lancha Stoyanow, 1949
  • Exogyra levis Stephenson, 1952
  • Exogyra mutatoria White, 1887
  • Exogyra obliquata Pulteney[9]
  • Exogyra paupercula Cragin, 1893
  • Exogyra plexa Cragin, 1893
  • Exogyra potosina Castillo and Aguilera, 1895
  • Exogyra ponderosa Roemer, 1852
  • Exogyra praevirgula Douville & Jourdy, 1924[10][11]
  • Exogyra quitmanensis Cragin, 1893
  • Exogyra sergipensis Maury, 1936
  • Exogyra sigmoidea Reuss, 1844[12]
  • Exogyra solea Muller, 1910
  • Exogyra upatoiensis Stephenson, 1914
  • Exogyra whitneyi Bose, 1910
  • Exogyra woolmani Richards, 1947

Exogyra (subgenus Aetostreon) (Bayle, 1878)[13]

  • Exogyra aquila Brongniart, 1871
  • Exogyra bale
  • Exogyra couloni Say, 1820
  • Exogyra imbricatum (possibly a morphotype of E. couloni) Kraus, 1843[2]
  • Exogyra latissimum[14]
  • Exogyra miotaurinensis Sacco, 1897[15] (Type species of subgenus)
  • Exogyra neocomiensis[16]
  • Exogyra pilmatuegrossum[17]
  • Exogyra rectangularis[16]

Distribution[edit]

Fossils of Exogyra have been found in:[18]

Jurassic

Afghanistan, Chile, China, Eritrea, Ethiopia, France, Germany, India, Kenya, Poland, Portugal, Somalia, Spain, Tanzania, the United Kingdom, and Yemen.

Cretaceous

Afghanistan, Algeria, Argentina, Brazil, Canada (British Columbia), Chile, Colombia (Hiló Formation, Tolima, Macanal and Chipaque Formations, Eastern Ranges),[19][20] Cuba, the Czech Republic, Egypt, Ethiopia, France, Germany, Greenland, Hungary, India, Iran, Italy, Jordan, Lebanon, Libya, Madagascar, Mexico, Morocco, Mozambique, New Zealand, Nigeria, Oman, Pakistan, Peru, Poland, Portugal, Serbia and Montenegro, Slovakia, South Africa, Spain, Sweden, Switzerland, Tanzania, Trinidad and Tobago, Tunisia, Turkey, USSR, Ukraine, the United Kingdom, United States (Alabama, Arizona, Arkansas, California, Colorado, Delaware, Georgia, Maryland, Minnesota, Mississippi, Missouri, Montana, New Jersey, New Mexico, North Carolina, Oklahoma, South Carolina, Tennessee, Texas, Utah, Wyoming), Venezuela, and Yemen.

References[edit]

  1. ^Evolution of Exogyra plexa
  2. ^ abPewgaczewska.H.
  3. ^Exogyra africana
  4. ^Bivalve Fossil Record curriculum
  5. ^Catalog number 528130
  6. ^Exogyra davidsoni at Fossilworks.org
  7. ^Exogyra
  8. ^Lake.P., and Rastall.R.H., (1913), A Text Book of Geology, 2nd edition, London: Edward Arnold's Geological series Page 426 and 436
  9. ^Castell. C.P., and Cox. L.R., (1975), British Mesozoic Fossils, 5th edition, London: Trustees of the British Museum (Natural History), page 86
  10. ^Exogyra praevirgula
  11. ^Global Names Index
  12. ^Ivanov. M., Hrdlickova. S., and Gregorova. R., (2005), The Complete Encyclopedia of Fossils, 3rd. ed., Lisse: Rebo International, page 133
  13. ^Paleobiology Database, Exogyra (Aetostreon), accessed 7 May 2013
  14. ^Anon, Aetostreon latissium As a Derived Fossil, accessed 7 May 2013
  15. ^Paleobiology Database, Exogyra (Aetostreon miotaurinensis), accessed 7 May 2013
  16. ^ abGlobal Names Index, (2000), Global Names Index, accessed 7 May 2013
  17. ^Rubillar, A.E; Lazo, E.B. (2008). "Description of Aetostreon pilmatuegrossum sp. Nov. from the Lower Cretaceous of Argentina (Neuquén Basin), and significance of the conservative left valve morphology in oysters of the genus Aetostreon Bayle". Cretaceous Research. 30: 727–748. doi:10.1016/j.cretres.2008.12.013. 
  18. ^Exogyra at Fossilworks.org
  19. ^Piraquive et al., 2011, p.204
  20. ^Acosta & Ulloa, 2002, p.54

Bibliography[edit]

Further reading[edit]

  • National Audubon Society Field Guide to North American Fossils

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