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Abstract

Typical clavagellids are either tube dwellers in soft sediments or facultative semiendolithic borers. Bryopa deviates from this pattern by being fully and obligatorily endolithic. The left valve in Bryopa is permanently attached to the wall of the borehole. In spite of this, the bivalve migrates forwards within the substrate throughout growth. These seemingly incompatible feats are achieved by continuously elongating the shell in the anterior direction and sliding forwards the soft parts, hinge and right valve within the shell. While the left valve becomes elongated, the posterior region of the right valve is continuously destroyed by resorption. The resulting strongly inequivalve condition is unique among clavagellids, as well as endolithic bivalves.

In the lack of direct evidence, the evolution of clavagellids from less specialized stocks remains open to alternative hypotheses. Probable parallel evolution within the clavagellid stock further complicates the problem. Evolution of tube-dwelling clavagellids directly from burrowing ancestors is as likely as their evolution from a hypothetical, endolithic protoclavagellid. In the latter case, however, B. lata is probably too specialized to reflect the adaptations of such an ancestor.

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