Functional morphology and palaeontological significance of the conchiolin layers in corbulid pelecypods

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Abstract
The Corbulidae, which today are slow, cumbersome and very shallow burrowers, developed special morphological features by which they obtained an outstanding capability to withstand the physical and biological stresses characteristic of their preferred habitat. These features are: an inequivalve, globose shape, thick shells and conchiolin layers (at least 1) embedded within their valves in a unique way. These features enable the corbulids to close their valves tightly during the unfavorable environmental conditions (e.g., low salinity, low O2 content) which may prevail in the marginal marine regions inhabited by several corbulid species. The conchiolin layers act as a barrier preventing all chemically boring organisms from penetrating into the bivalve shell, or shell dissolution by sea water undersaturated with respect to calcium carbonate. The layered conchiolin weakens the shell mechanically especially during fossilization, when the conchiolin is decomposed. The valve splits apart into 2 shells so completely different in appearance that they may be attributed to different taxa. The nature of these conchiolin layers in Corbula (Varicorbula) gibba (Olivi) is described and illustrated and their functional significance discussed in relation to other living and fossil corbulid species.