13 Mayıs 2013 Pazartesi

Fossil Representative of the U.S. Virgin Islands

Fossil Representative of the U.S. Virgin Islands

Rudists (common name)
Rudists are an extinct order of marine mollusks that lived from the late Jurassic to the late Cretaceous periods, approximately 65.5 to 160 million years ago. Rudist belong to the taxonomic class of bivalves, which means the organism had two hinged shells. Since they have been extinct as long as the dinosaurs, these mollusks are known today only through their fossils. Over one thousand fossil species of these prehistoric mollusks have been discovered and described in the Caribbean, Middle East, and Mediterranean regions.
Today, rudist fossils are found in limestone rock throughout the tropics. In the Caribbean, 214 species of rudists have been described. One notable genus of rudists can be found at Coki Point Cliffs National Natural Landmark on St. Thomas. These and other marine bivalves occur in the Lower Cretaceous rocks of this U.S. Virgin Islands park. Although the U.S. Virgin Islands have not designated an official fossil to represent the territory, rudists are excellent candidates because they represent the geology of the islands well.
Most bivalves’ shells are symmetrical, but rudists had two asymmetrical shells. Also unlike other bivalves, one or both rudist shells were uncoiled, allowing them to grow by accretion, or buildup. This mollusk lived with one shell attached to the seafloor. It inhabited shallow tropical seas, where it fed by filtration. Cretaceous seas were warmer and saltier than today’s oceans, and these conditions were favorable to rudists.
Rudists had muscles and ligaments to move their thick valves, as well as sockets to hold the valves together. Since soft tissues do not fossilize well, details about the organs of rudists are not well known. The shell that was not attached to the ocean bottom had two teeth, and the attached valve had one tooth. Like corals, rudists normally grew in clusters, quickly constructing walls up to the water’s surface.
Rudists first appeared late in the Jurassic period, almost 160 million years ago. They lived in the Tethys Ocean, the body of water between the major north and south continents of Laurasia and Gondwana before the Indian Ocean formed.
Rudists seem to have gone extinct at the mass extinction event that occurred at the end of the Late Cretaceous epoch and the beginning of the Paleocene epoch. The Cretaceous–Tertiary extinction event took place approximately 65.5 million years ago and is associated with the extinction of a number of species, including many plants, most mollusks, and all non-avian dinosaurs. 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 present-day Chicxulub, Mexico, triggered this mass extinction.
Another school of thought among paleontologists is that the extinction of rudists in the Caribbean was caused prior to the end of the Cretaceous period by changing sea levels. Rudists were especially sensitive to rising and falling sea levels, which would have also caused changes to the platforms they inhabited.
The Tethys Ocean of the Mesozoic era eventually became mainly the Mediterranean Sea, with some sections becoming the Caribbean, Black, Caspian, and Aral Seas, as well as the Indian Ocean. Rudists inhabited the Tethys Sea, which explains why today their fossils are found in these areas of the world.
Although the majority of rudist fossils are found in the previously mentioned locations near large, tropical seas and oceans at low latitudes, some have been found as far north and inland as Saskatchewan, west-central Russia, and southern Sweden.

Click to enlarge an image
State Prehistoric Creature
Rudist Bivalves

Author: World Trade Press

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