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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The Evolutionary Biology of the Bivalvia
Bivalves are key components of Recent marine and freshwater ecosystems and have been so for most of the Phanerozoic. Their rich and long fossil record, combined with their abundance and diversity in modern seas, has made bivalves the ideal subject of palaeobiological and evolutionary studies. Despite this, however, topics such as the early evolution of the class, relationships between various taxa and the life habits of some key extinct forms have remained remarkably unclear.
In the last few years there has been enormous expansion in the range of techniques available to both palaeontologists and zoologists and key discoveries of new faunas which shed new light on the evolutionary biology of this important class.
This volume integrates palaeontological and zoological approaches and sheds new light on the course of bivalve evolution. This series of 32 original papers tackles key issues including: up to date molecular phylogenies of major groups; new hard and soft tissue morphological cladistic analyses; reassessments of the early Palaeozoic radiation; important new observations on form and functional morphology; analyses of biogeography and biodiversity; novel (palaeo)ecological studies