Fossil Brachiopods
Brachiopods are solitary, exclusively marine animals of the epifauna. Although superficially brachiopods may resemble bivalve molluscs, they are indeed different creatures.
For example, brachiopods are symmetrical about a plane down the middle of the valve (above, center), whereas bivalves are symmetrical about a plane separating the two valves.
The shell chamber is divided into two compartments by a membrane (above, right).
The animal itself resides in the posterior chamber, while the anterior chamber is occupied by the lophophore, a fleshy or calcified, ciliated extension of the animal designed to bring food into the mouth. Brachiopods are common as fossils from the Lower Cambrian throughout the Paleozoic. They occur as fossils less frequently in the Mesozoic and Cenozoic.
The three groups of brachiopods with modern representatives are: the Rynchonellata, the Craniata, and the Lingulata. The lingulate brachiopods are inarticulate, in that the two valves do not share a hinge line.
Most other groups have valves that articulate along a hinge line of some sort. It is worth noting that few differences separate the fossil forms from the living ones.
For example, modern lingulid brachiopods, such as the genus Lingula, shown above, are essentially identical with those from Lower Cambrian, with one species ranging from Devonian to Recent.
|
|
Extinct orders of brachiopods include the spiriferids (left), a group ranging from Ordovician to Jurassic. These brachiopods are often characterized by a wide hinge line and an internal spired brachidium.
Often the shells of spiriferids have the appearance of "wings". Other extinct groups are the orthids, pentamerids and strophomenids (right). The two valves of the shells of this latter group are concavo-convex, fitting one valve inside the other, and leading one to wonder where the animal lived.
