Palaeoheterodonta

bivalve subclass

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July 1, 2026

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The subclass Palaeoheterodonta is a group of bivalves characterized by equal shell valves and a variable hinge. Their shells are aragonitic, featuring outer prismatic and inner nacreous layers. Most are approximately isomyarian, meaning they have two adductor muscles of similar size, and possess eulamellibranch ctenidia (gills). They lack mantle fusions, especially ventrally, and often have complex life cycles. Palaeoheterodonta are exclusively freshwater inhabitants, are non-byssate (do not produce byssal threads for attachment), and live buried in sediment (infaunal). The primary order within this subclass is Unionoida, which includes freshwater mussels.1

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bivalve
Annotated classification
Subclass Palaeoheterodonta Characterized by equal shell valves with a variable hinge dentition; aragonitic shell with outer prismatic and inner layers of nacre; most approximately isomyarian; ctenidia eulamellibranch; mantle fusions lacking, especially ventrally; complicated life cycles; wholly freshwater; nonbyssate; infaunal. About 1,200 species. Order Unionoida
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Subclass Palaeoheterodonta Characterized by equal shell valves with a variable hinge dentition; aragonitic shell with outer prismatic and inner layers of nacre; most approximately isomyarian; ctenidia eulamellibranch; mantle fusions lacking, especially ventrally; complicated life cycles; wholly freshwater; nonbyssate; infaunal. About 1,200 species. Order Unionoida
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bivalve
Ecology and habitats
The Palaeoheterodonta (a group that includes the unionids) are exclusively freshwater species, but all have significantly more complicated life cycles.
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The Palaeoheterodonta (a group that includes the unionids) are exclusively freshwater species, but all have significantly more complicated life cycles.
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