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Bivalves Fossils

Belonging to the Nesosilicate group, zircon including Yellow Zircon has a chemical name, “zirconium silicate.”   Zircon is born out of melting silicate and is characterized by a good proportion of incompatible elements of good field strength, including Hafnium that is present 1 - 4 percent. Zircon has a tetragonal crystal system and its colorless samples are used as gem stones to substitute diamond.  They are also referred to as "Matura diamond".      

History

The Bivalves were seen in their fossilized form near the rocky areas after the Cambrian Explosion took place around 520 to 540 million years ago. The oldest Bivalves fossils are usually found in small sizes because the bivalve species present in the Cambrian region were too small. But, as the time passed, the bigger sized species started to evolve. Ancient people used many kinds of bivalves as a source of food. The bivalve mollusk was the most popular and about 1,000,000 + tons was traded worldwide, a fact that was revealed in 1950 by the Food and Agriculture Organization (FAO).

Anatomy
Similar to the living Bivalves, the fossils too are seen in different sizes and shapes. The two sides of the bivalves, the left and the right parts are usually found symmetrical except in oysters. The shells are made of Calcite or Aragonite and sometimes both; mostly the inner part is formed by Aragonite mineral. The outer part of the shell consists of an organic substance which is known as Periostracum. Sometimes, the Perostracum forms a skin layer on the outer part of the shell which peels out when dried for long hours.

Features Of Bivalves
In a bivalve body, a ventral foot and a visceral mass are formed inside the thin mantle. This mantle creates a two sided shell, secreted from its outer part. The mantle cavity is formed between the body and the mantle in which the right and left gills hang. The hinge, teeth and the ligaments are secreted by the mantle crest. The mantle retractor muscle helps it (the mantle) to get enclosed inside the shell valves and the point at which the two shells are attached is called the pallial line. The musculature consists of two equal muscles called posterior and anterior. Usually the anteriors get reduced, whereas the posteriors remain intact.

Reproduction
There are two types of reproduction in Bivalve species. Some species have either female or male organ; they are known as gonochoristic and the others possess both the male and female organs and are known as hermaphrodites.  In the hermaphroditism process, the follicles that produce eggs mingle with tubules that produce sperms in the gonads,  or sometimes the gonads form into a new testis and ovary to carry out the reproduction process. In the gonochoristic organisms, usually the sex is equally divided.

Habitats
The typically two shelled organisms can be found in numerous habitats due to more than 15000 species that are spread all over the globe. Usually it is  found in the coastal seas, Islands and continental landmasses like the rivers that provide the deltaic habitat suitable for the Bivalve organisms. It can be found in deep as well as shallow water growing on the sea grass, sand, mud, mangrove roots, rock and sea wall areas. With its easily adaptive nature, it is well distributed in all the aquatic habitats on the Earth.

Used As Food
Since the age of the Romans, bivalves have been very popular and used for consumption. In those days, it was an essential source of food, a fact that emerged from the huge quantity of empty shells excavated from the archeological sites. Some of the commonly used bivalves are Scallops, Clams, Oyster, Cockles, Mussels and Ark Clams, that were consumed raw or in cooked form.

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