Palaeotaxodonta

bivalve subclass

Also known as: Protobranchia, protobranch

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Last updated

July 1, 2026

Quick Summary

The Palaeotaxodonta, also known as Protobranchia, are a subclass of bivalves. They are considered among the most primitive living bivalves and are characterized by numerous similar teeth along their hinge plate and a unique shell microstructure. These marine detritivores are typically found in near-surface waters and are infaunal, meaning they live buried in the sediment. The subclass includes the order Nuculoida, which features equal shell valves with taxodont hinge teeth and large labial palps used for feeding.12

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bivalve
Internal features
…the earliest mollusks—hence the name protobranch, or “first gills.” The paired gills, separated by a central axis, are suspended from the mantle roof. Individual short gill filaments extend outward from either side of the axis, and cilia on their surfaces create an upward respiratory water current that passes from the…
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…the earliest mollusks—hence the name protobranch, or “first gills.” The paired gills, separated by a central axis, are suspended from the mantle roof. Individual short gill filaments extend outward from either side of the axis, and cilia on their surfaces create an upward respiratory water current that passes from the…
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bivalve
Annotated classification
Subclass Palaeotaxodonta (Protobranchia) Numerous similar teeth along the hinge plate; isomyarian; unique shell microstructure of aragonitic composite prisms and internal nacre; posterior ctenidia comprising 2 divergent rows of flat, short, filaments; protobranch respiratory gill; food collected by labial palps; mostly near-surface-dwelling marine detritivores; considered to be…
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Subclass Palaeotaxodonta (Protobranchia) Numerous similar teeth along the hinge plate; isomyarian; unique shell microstructure of aragonitic composite prisms and internal nacre; posterior ctenidia comprising 2 divergent rows of flat, short, filaments; protobranch respiratory gill; food collected by labial palps; mostly near-surface-dwelling marine detritivores; considered to be…
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bivalve
Critical appraisal
…group of primitive detrivores with protobranch gills. Differences in shell structure, however, argue against this. Similarly, the order Arcoida is separated by some from the subclass Pteriomorphia; shell structure again supports this, but other anatomic features do not. The order Trigonioida traditionally has been located within the subclass Palaeoheterodonta, but…
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…group of primitive detrivores with protobranch gills. Differences in shell structure, however, argue against this. Similarly, the order Arcoida is separated by some from the subclass Pteriomorphia; shell structure again supports this, but other anatomic features do not. The order Trigonioida traditionally has been located within the subclass Palaeoheterodonta, but…
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