Constructional morphology of the shell/ligament system in opisthogyrate rostrate bivalves.

ECHEVARRÍA, JAVIER; DAMBORENEA, SUSANA E.; MANCEÑIDO, MIGUEL O.

EARTH AND ENVIRONMENTAL SCIENCE TRANSACTIONS OF THE ROYAL SOCIETY OF EDINBURGH

ROYAL SOC EDINBURGH

Lugar: Edinburgo; Año: 2017 vol. 106 p. 221 - 227

1755-6910

The bivalve ligamentprovides the thrust for shell opening, acting as the resistance in a leversystem against which adductor muscles effort applies. Usually its outerlamellar layer is subjected to tensile stress, while the inner fibrous layer iscompressed, with the pivotal axis located between them. However, opisthogyraterostrate bivalves display a concave dorsal margin, and both the umbo and the postero-dorsalangle of the shell project dorsally to the ligament, which then fails to act aspivotal axis. Three opisthogyrate rostrate genera of unrelated lineages showsomewhat different solutions to this morpho-functional challenge. In Cuspidaria (Anomalodesmata) the ligamentis internal, subjected only to compression and ventral to the pivotal axis, a thickenedperiostracum develops forcing the dorsal margins of the valves to act aspivotal axis, and the posterior parts of shell dorsal margins gape dorsally. InNuculana (Palaeotaxodonta) the innerlayer of the ligament is internal, the outer layer is external but reduced;some species develop a dorsal ridge parallel to the commissural plane, levelledwith the rostrum and acting as pivotal axis. In Pterotrigonia (Palaeoheterodonta) and other rostrate trigoniidesthe ligament is external opisthodetic, but it is allometrically reduced. Trigoniidesmay also develop a dorsal ridge.