Functional significance of the shell/ligament system in opisthogyrate rostrate bivalves

ECHEVARRÍA, J.; DAMBORENEA, S.E.; MANCEÑIDO, M.O.

Mendoza

Congreso; 4th International Palaeontological Congress; 2014

International Palaeontological Association

In bivalve molluscs the ligament joins both valves dorsally, and being flexible, provides the thrust for opening the shell, resulting in the resistance in/of a lever system against which adductor muscles act.  Usually the outer lamellar layer of the ligament is subjected to tensile stress, while the inner fibrous layer is exposed to compression, with the pivotal axis located between them, being a combination of class 1 and class 2 levers. However, some shells display a concave dorsal margin , as often happens in rostrate opisthogyrate bivalves, probably because such growth favours the upward orientation of the posterior margin which aids in reaching the water-sediment interface. When this happens, the umbo and the dorsal-posterior angle of the shell project dorsally relative to the ligament, which then fails to act as a pivotal axis and so, the adductor muscles-ligament system results in a class 2 lever. In this contribution three opisthogyrate rostrate genera of unrelated lineages are analyzed, each showing a somewhat different solution to this morpho-functional challenge: Nuculana (Paleotaxodonta), Pterotrigonia (Paleoheterodonta) and Cuspidaria (Anomalodesmata). In Cuspidaria the ligament becomes completely internal, hence it is subjected only to compression and can act ventrally to the pivotal axis. Nevertheless, a secondary ligament develops joining the straight anterior part of the dorsal margins of the valves, forcing them to act as pivotal axis despite the dorsal projection of the rostrum; consequently, the posterior parts of dorsal margins of the calcified valves tend to diverge from the sagittal plane as they direct dorsally. In Nuculana the inner layer of the ligament is well developed and becomes internal, while the outer layer is external but strongly reduced, so the whole shell can effectively act as a class 2 lever; nevertheless, some species develop a dorsal ridge parallel to the commissural plane, thus resulting in a straight dorsal margin levelled with the rostrum and acting as pivotal axis. In Pterotrigonia (as well as in other rostrate trigoniides) the ligament is external opisthodetic, and so tensile stress on the outer layer would prevent the opening of the valves. Although most of these species develop a dorsal ridge, usually the rostrum projects dorsally to it, without divergent dorsal margins. When compared with other trigoniides lacking a rostrum, the ligament seems to be shorter in rostrate species; this may point to a reduction in the opening action of the ligament, a function possibly taken over by the strongly muscular foot.