Phylum Mollusca (2)
The first bivalves appeared in the early Cambrian. They became more abundant by middle Ordovician. They probably lived in shallow water on top of the sediment or buried at shallow depths, filter feeding. After the Permian, many families of brachiopods became extinct and were replaced ecologically by bivalves.
During the Mesozoic and Cenozoic, burrowers were more abundant, and scallops made their appearance. Following is a discussion of some of the life habits of members of the class Bivalvia.
Deep burrowers are clams with long siphons to reach the sediment-water interface from the deep burrow. The valves gape to accommodate the long siphons and the siphons cannot be retracted into the shell.
Attached bivalves such as mussels range from late Paleozoic to recent times.
They attach by byssus fibers to sediment or rock. Rudists were unique attached bivalves that ranged from late Jurassic to Cretaceous. The lower valve is conical (like a horn coral) and the upper valve caps the top of the cone. They were reef-builders during the late Mesozoic. The largest rudists were 6 feet in length.
Swimming scallops range from Carboniferous to Quaternary. They lie on one valve and can move by snapping the valves together with a single muscle and jetting water from the mantle. Scallops possess light-sensing "eyes" around the outer edge of the mantle.
Cemented oysters range from late Paleozoic to recent Quaternary. The lower valve grows to conform to the shape of the object to which it is cemented, and oysters can form banks or reefs by cementing to each other.
Borers are clams with rough front edges on their shells for boring into rock, wood, or other shells. An example is the shipworm, Teredo. To help in boring into hard substances, some of these bivalves secrete corrosive chemicals.