Cambrian – Recent
Bivalvia is an extant class of facultatively mobile infaunal suspension feeders.
Characteristics of the Class
- Characterized by paired lateral shelly valves of conchiolin and calcium carbonate joined at hinge by flexible ligament
- Mantle connected with shell submarginally by pallial muscles not present in other mollusks
- Several small shell muscles activate the foot which is variously adapted for creeping, burrowing, and secretion of anchorage devices
- Feeding and respiration accomplished pair of lateral ctenidia bearing ciliary and mucous systems.
- Fertilization of eggs external, sexes commonly hermaphroditic and protandrous
Treatise on Invertebrate Paleontology, Part N, Vol. 1 of 3 (1969):
- Aquatic, acephalous, passive, and microphagous mollusks characterized by paired lateral shelly valves of conchiolin and calcium carbonate joined at hinge by flexible ligament; shell and ligament secreted by mantle which envelops organs and tissues within bivalved shell; mantle connected with shell submarginally by pallial muscles not present in other mollusks; other shell muscles generally consist of one anterior and one posterior adductor that close valves against opposing stresses within ligament; several small shell muscles activate the foot which is variously adapted for creeping, burrowing and secretion of anchorage devices; feeding and respiration are accomplished by more or less symmetrical pair of lateral ctenidia bearing ciliary and mucous systems; fertilization of eggs external, sexes commonly hermaphroditic and protandrous; gut convolute, nephridia paired.
- The origin of the class is shrouded in mystery and the oldest bivalves (Tremadocian and older) give no hint of their derivation. Speculation that they were derived from a monoplacophoran-like ancestor is reasonable but not at all conclusive. Great gaps in the fossil record of many groups probably reflect ineffectual competition with other benthonic organisms, restricted geographic distribution and small population numbers rather than diagenetic destruction of the shells. Gastropods, also with conchiolin-calcareous shells, have a good Cambro-Ordovician record in rocks that rarely show a trace of bivalves.
- Extinction of many major groups of competing brachiopods at the end of the Paleozoic Era was followed after a considerable time lapse by a steady evolutionary deployment of bivalves from Late Triassic time until the present. Many living superfamilies are virtually immortal, with known record and very little morphological change in more than 300 or 400 million years. A fascinating exception were the rudists which underwent rapid radiation and were distributed abundantly and widely at low latitudes during the Cretaceous Period. Certain other groups show evolutionary trends but a more prevalent pattern is for a group to appear and vanish abruptly with very little morphologic change.
- The class has been singularly successful and apparently, is now near the acme of its diversity, suggesting inherent advantages of conservatism, generalized modes of feeding and reproduction, and life in stable habitats.