Dimock, Ronald V., Jr. Department of Biology, Wake Forest University, Winston-Salem, North Carolina.
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A subclass of bivalve mollusks (class Bivalvia, phylum Mollusca), whose shells often have mother-of-pearl (nacre), and possessing no or prominent teeth. The Paleoheterodonta (same as Palaeoheterodonta) date from the Cambrian Period, with two extant orders—the marine Trigonioida, with six species in the genus Neotrigonia (family Trigoniidae) from Australia; and the Unionoida, with six families and approximately 845 species that are exclusively freshwater and have a larva that must become parasitic on a host fish to complete its metamorphosis. The Unionoida are colloquially called freshwater or pearly mussels, or naiads, and often have colorful common names such as spectacle case, pig toe, washboard, heel splitter, or monkey face. Paleoheterodonts are essentially synonymous with freshwater mussels, which occur on all continents except Antarctica, with the greatest diversity, roughly 300 species, in North America. The largest family, the Unionidae, has approximately 675 species in the New World (from North America south into Mesoamerica), the northern Old World, eastern Asia, and Ethiopia. Although global in distribution, freshwater mussels are among the most endangered of animal groups, with at least 70% of the species in North America being of special conservation concern. See also: Bivalvia; Endangered species; Mollusca
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