Bivalves (clams, oysters, mussels and their relatives) have thrived for more than 500 million years but became particularly common from about 250 million years ago and are still abundant today. Although the earliest types lived in the sea, they also colonised lakes and rivers several hundred million years ago and are still common there today. The huge variety of fossil and living bivalve show a diverse range of lifestyles. Some burrow into mud or sand or lie on its surface; others are firmly attached to rocks or other shells; while some (the scallops) even swim! The smallest are no more than a few millimetres across; the largest more than a metre.
In Northern Ireland bivalves are particularly
common fossils in Jurassic and Cretaceous rocks, but they have
also been found in rocks as old as the Ordovician in the Pomeroy
area of Co. Tyrone, and they are not uncommon in some of the Carboniferous
rocks of Co. Fermanagh. However, in these older Ordovician and
Carboniferous rocks the superficially similar brachiopods generally
are more common and conspicuous.
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