The evolutionary history of bivalves is represented by an extensive fossil record. It begins in the Cambrian period (544-505 million years ago [mya]) with a laterally compressed stenothecid monoplacophoran (primitive single-shelled marine mollusk) as the most likely immediate ancestor. By the Middle Ordovician period (about 460 mya), recognizable members of all modern subclasses had appeared. Bivalves formed important components of marine communities since they first diversified, especially in shallow-water marine sediments, but also in the intertidal zone, the deep sea, and freshwater habitats. In the Cretaceous period (145-65 mya), epibenthic rudist bivalves formed tropical reef-like structures similar to modern coral reefs. Rudists were massive extinct cemented bivalves that had two different-sized shell valves.
Most authors think that scaphopods and bivalves are closely related taxa, based on the configuration of the nervous system, lateral expansion of the mantle, and elongation of the foot for burrowing. Within the class of bivalves itself, the widely variable shell characteristics (shape, sculpture, color, hinge teeth) have been historically and consistently used in identification and classification at all levels. Various other evolutionary schemes have been based primarily on single organ systems, especially the ligament, stomach, digestive tract, and gills. Elements of each of these continue to be important in modern comprehensive phylogenetic analyses.
Current classification schemes recognize five subclasses of bivalves. The Protobranchia (e.g., families Nuculidae, Sole-
myidae) are presumably the most primitive, using simple gills solely for respiration and palp proboscides (enlarged labial palps, normally used for sorting food particles) for collecting food from the sediment surface. The Pteriomorphia (e.g., Mytilidae, Pteriidae) include many of the most familiar bivalves, all of which share an epibenthic habitat (byssate or cemented), an unfused mantle edge, and a reduced foot. Byssate bivalves are attached to their substrate by a byssus (bundle of elastic collagen-rich threads) secreted by the foot. The Pale-oheterodonta (e.g., Unionidae, Margaritiferidae) include the freshwater mussels, with their specialized glochidia larvae. Heterodonta (e.g., Veneridae, Donacidae) is the most species-rich and most widely distributed subclass, containing the classic burrowing clams with well-developed hinges, siphons, and active feet. The Anomalodesmata (e.g., Pandoridae, Clavag-ellidae) include the most unusual and most specialized bivalves, featuring modified ctenidia (gills), an edentulous hinge, fused mantle margins, and hermaphroditism. The evolutionary relationships, and thus the accepted classification, among and within bivalve subgroups continue to be revised through the application of phylogenetic analysis. These analyses makes use of a wide range of morphological (especially anatomical) and molecular characters.
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