a to z the usa: State Fossil of Tennessee

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State Fossil of Tennessee

Bivalve (common name of class)
Pterotrigonia thoracica (scientific name of species)

STATUS

Official

DESCRIPTION

Pterotrigonia thoracica is an extinct species of bivalve that lived about 65 to 70 million years ago in the Maastrichtian age of the late Cretaceous period. A bivalve is a mollusk that has two symmetrical shells, such as modern oysters, clams, and mussels. In the Cretaceous period, present-day western Tennessee and portions of three neighboring states were covered by or bordering a shallow sea. A number of mollusks, crustaceans, fish, foraminifera, worms, and even the huge aquatic reptiles called plesiosaurs are known from fossilized material from this region.

STATE SYMBOL

The world-famous Coon Creek formation in western Tennessee is important to paleontology and geology because of the number and diversity of fossils that occur there, as well as the perfect preservation of these specimens. One fossil mollusk that occurs at this formation is Pterotrigonia thoracica, and in fact, some of the best fossil specimens of this bivalve are found at the Coon Creek formation and other locations in western Tennessee.

In 1996, a geological and geographical club at the University of Tennessee at Martin began a campaign to nominate a suitable fossil to represent the state’s geology and paleontology. In 1998, the club’s choice, Pterotrigonia thoracica, was named the official state fossil by the Tennessee legislature.

NAME ORIGIN

The genus Pterotrigonia was named by E.C.N. van Hoepen in 1929. Van Hoepen was a South African paleontologist and director of that country’s national museum. The word is derived from the Greek words öôåñü (ptero), which means "wing" or "feather," and ôñßãùíï (trigono), which means "triangle."

BIOLOGY/ANATOMY

Pterotrigonia thoracica has a distinctive curved shell with deep ribs. It was a small mollusk that inhabited the soft clay sands of shallow prehistoric seas. It lived with its smaller end burrowed into the sand and its posterior up. The shell interior was sometimes lined with mother-of-pearl.

EVOLUTION/EXTINCTION

Mollusks first appear in the fossil record about 545 million years ago at the very beginning of the Cambrian period, but these fossils do not represent the origin and early evolution of this mollusk. Later in the Cambrian period, approximately 505 to 520 million years ago, most of the modern groups of mollusks can be found in some primitive form as fossils.

Pterotrigonia thoracica went extinct at the end of the Cretaceous period, approximately 65 million years ago. Fossils of this bivalve have been found in masses, which may indicate that these mollusks died all at once. Reasons for this are not known, but this date does coincide with the Cretaceous-Tertiary mass extinction event that wiped out most of the large vertebrates of the time, including dinosaurs, as well as many plankton and tropical invertebrate species. Plants were also largely affected. In March 2010, a panel of paleontologists and geologists from leading universities and research institutes from around the globe concluded that an asteroid that hit Chicxulub, Mexico, triggered this mass extinction.

MODERN DISCOVERY

Coon Creek is a geological formation located in western Tennessee and extreme northeastern Mississippi, approximately 90 miles east of Memphis. The formation is a deposit of sedimentary marl that formed about 70 million years ago. At that time, the climate was semi-tropical. The waters of the region have produced fossil specimens of over 600 animal species. These include plesiosaurs, crocodiles, crustaceans, sea turtles, sharks, and mollusks such as Tennessee’s state fossil, Pterotrigonia thoracica.

INTERESTING FACTS

Although Pterotrigonia thoracica is extinct, one relative of this mollusk, Neotrigonia, survives in the Pacific Ocean off the coast of New Zealand and Australia.

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