 |
Business | Industries | Finance | Tax |
| Index: > A B C D E F G H I J K L M N O P Q R S T U V W X Y Z |
|
|||||
| First Prev [ 1 2 ] Next Last |
| Bivalves | ||||||||
|---|---|---|---|---|---|---|---|---|
| Scientific classification | ||||||||
| ||||||||
| Orders | ||||||||
| ||||||||
Animals of the Class Bivalvia are known as bivalves because they typically have two-part shells, with both parts being more or less symmetrical. The Class has 8,000 species, including scallops, clams, oysters and mussels. The name is also spelled Bivalva. An old name for the Class is Pelecypoda.
Bivalves are exclusively aquatic; they include both marine and freshwater forms.
Bivalves lack a radula and feed by siphoning and filtering small particles from water. Some bivalves are epifauna l: that is, they attach themselves to surfaces in the water, by means of a byssus. Others are infauna l: they bury themselves in sand or other sediments. These forms typically have a strong digging foot. Some bivalves can swim.
The oldest known fossil bivalves are 500 million years old. Some are now extinctIn biology and ecology, extinction is the disappearance of a species or group of species. The moment of extinction is generally considered to be the death of the last individual of that species. In species which reproduce sexually, extinction of a species, such as the JurassicThe Jurassic period is a major unit of the geologic timescale that extends from about 135 million years BP at the end of the Triassic to 195 million years BP at the beginning of the Cretaceous. As with other geologic periods, the rock beds that define the GryphaeaGryphaea also known as Devil's toenails are a type of extinct oyster. They are bivalve fossils from the Jurassic period. They lived on the sea bed in shallow waters, possibly in large colonies like oysters. They are common in many parts of Britain. Extern, also known as "devil's toenails".
Examples of fossil freshwater bivalves are the large mussel Archanodon, found in the Old Red SandstoneThe Old Red Sandstone is a rock formation of considerable importance to early paleontology. Found in Wales and Scotland, in England where it borders those two, and in the area around Omagh, Northern Ireland, the Old Red Sandstone is a huge set of sediment formation of the Devonian Period in the United KingdomThe United Kingdom of Great Britain and Northern Ireland is a state in Western Europe, usually known simply as the United Kingdom the UK Britain or less accurately as Great Britain . The UK was formed by a series of Acts of Union which united the formerly. The non-marine bands of CarboniferousThe Carboniferous is a major division of the geologic timescale that extends from the end of the Devonian period (about 280 million years before the present (BP)) to the beginning of the Permian period (about 340 million Years BP). As with most older geol Coal Measures contain 'mussel bands' which are crowded with specimens such as Carbonicola and Naiadites. These freshwater bivalves prove useful as zone or index fossils for subdividing the strata of the Coal Measures.
Most fossil bivalves are relatively small, typically being a few centimetres (about an inch) long. However, a few giant forms are known, such as Inoceramus from the Cretaceous-period Chalk Formation of Europe, which grew to well over a meter (3.3 feet) in length.
The shells of bivalves are composed of calcium carbonate and are quite strong in most species, and hence easily become fossilized. However, in some bivalves the shell is made up of a form of calcium carbonate known as aragonite, which easily dissolves away when the fossil is entombed in sediment, leaving only an impression of the shells' internal and external surfaces (known as internal and external moulds).
Remarkable bivalve fossils have been found in which the original colour banding of the shell has been preserved, such as in specimens of Aviculopecten planoradiatus from the Carboniferous Period of Derbyshire in England.
Bivalves seem to have evolved at a relatively slow pace, especially when compared to some of the other molluscs - such as the ammonites.
The bivalves which appeared in the early Ordovician Period were marine, and like many other groups of molluscs they radiated out to fill many of their niches (which they still occupy today) by the end of the Ordovician. Bivalves begin to become reasonably common fossils in the rocks of the Silurian and Devonian Periods. The shells of bivalves in rocks older than those of the Carboniferous are not generally preserved; these fossils largely consist of mere impressions in the rock. Freshwater species first appear at the boundary between the Silurian and Devonian Periods. All of the major groups of bivalves were in existence by the end of the Palaeozoic Era .
Bivalves are much more numerous and varied in rocks from the Mesozoic Era, and they may occur in great numbers at some localities. Many of the ancient bivalve forms became extinct during the Triassic Period and were replaced by many new forms, some of which have persisted to the present day.
Like the gastropods, bivalves are very abundant fossils in rocks of the Cenozoic Era. Many of these bivalves are related to living forms so it is possible to state with some confidence the conditions in which the rocks they are found were deposited, such as shallow marine water, brackish or freshwater, under sub-tropical or a cold climate, etc.
Some living forms of bivalve have a very long ancestry, such as the modern Nucula, which has relatives in rocks from the Ordovician that are not very different from those found living today.
| Politics | Business | Finance |
 |
 |
 |