Bivalvia
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The class Bivalvia are known as the bivalves because they typically have two-part shellss, with the parts 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 life but include both marine and freshwater forms.
Bivalves lack a radula and feed by siphoning and filtering small particles from water. Some forms attach to surfaces (epifaunal e.g. Pteriomorpha) in the water (by means of a byssus) and others bury themselves in sand or other sediments (infaunal e.g. Heterodonta, Palaeotaxadonta). 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 extinct, such as the Jurassic Gryphaea, also known as "devil's toenails".
Bivalves appeared late in the Cambrian explosion and came to dominate over Brachiopods during the Palaeozoic; indeed, by the End Permian extinction, bivalves were undergoing a huge radiation in numbers whilst brachiopods took a devastating hit, along with almost all other life (~95% species become extinct).
So how and why did bivalvia come to challenge the brachiopoda niche before the extinction event, and why didn't the bivalves suffer a similar fate to most other life? Although inevitable biases exist in the fossil record and our documentation thereof, bivalves essentially appear to be better adapted to aquatic life. Far more sophisticated than the brachiopods, bivalves are 1. seen to operate by means of an energetically-efficient ligament-muscle system for opening valves, and thus require less food to subsist; 2. burrowing allows evasion of predators - the extension of a siphon to the surface permits feeding (indicated by the presence of a palial sinus, the size of which is proportional to the burrowing depth, and represented by their dentition); and 3. becoming mobile, developing spines for buoyancy or by sucking in and ejecting water to enable propulsion, means bivalves themselves became predators.
By far the best website on all Palaeontology is from the University of California, Berkeley:
