Adaptive strategies of Lower Jurassic and Eocene multivincular bivalves

The adaptive strategies of three multivincular bivalves are described: two Liassic species of the Lithiotis Facies (Lithioperna scutata and Mytiloperna sp.) and an Eocene species (Pachyperna laverdana). These taxa are characterized by a high intraspecific variability and unusual shell form which are typical features of the secondary soft-bottom dwellers. P. laverdana shows a high ontogenetic variability with behavioural changes from pleurothetic, byssally attached, hard-bottom dweller to edgewise, soft-bottom recliner. Mytiloperna sp. shows a very high morphological and behavioural variability in the adult shells; its adaptive strategy ranges from an epifaunal, edgewise recliner to semi-infaunal mud sticker. The behaviour of L. scutata ranges from pleurothetic recliner to semi-infaunal mud sticker. P. laverdana probably inhabited higher energy environments with a minor spatial competition with respect to the Liassic species. A less static stability of Mytiloperna sp. with respect to the Eocene species has been proposed. Thus its bottom stabilization could also be guaranteed by other factors (e.g. dense package of the host bivalve community, semi-infaunal behaviour, anchorage with beaks). L. scutata has a shell with the lowest hydrodynamic stability. All three taxa have inner shell layers made up of an irregular simple prismatic microstructure. It becomes massive and fibrous in P. layerdana and Mytiloperna sp., where it mainly carried out a static function in the heavyweight strategy. The mechanical, environmental and taxonomical significance of the morphology and microstructure are discussed.