Pachyperna laverdana is a large Eocene bivalve characterized by an extremely thick shell wall. Rediscovery of the type locality (Pernabanke Auctt.) after more than a century has made it possible to collect abundant material which is used here to provide a better morphological definition of the taxon. In particular, indication is given of its broad intraspecific variability mostly due to the gregarious habit (ecomorphism) and by change of mode of life through ontogenesis. As regards the latter factor, functional analysis of the shell suggests that in its early juvenile stages the bivalve was an epibyssatc, pleurothetic form, attached to hard substrata. Then, it moved to soft-bottom substrates, where a “heavy weight” strategy was developed to compensate for a weak byssal attachment. In the adult stage, it may be considered a reclining, orthothetic form. The shell is made up of a thin outer layer formed by simple prismatic calcite and by thick, aragonitic, irregular fibrous-prismatic inner layers, both with a well marked periodicity of growth. Mechanical, functional and systematic significance of shell microstructures are discussed. The diagnosis of the genus Pachyperna is herewith emended.
Palaeobiology and taxonomy of Pachyperna laverdana Oppenheim, an Eocene bivalve of Mesozoic heritage
POSENATO, Renato
1995
Abstract
Pachyperna laverdana is a large Eocene bivalve characterized by an extremely thick shell wall. Rediscovery of the type locality (Pernabanke Auctt.) after more than a century has made it possible to collect abundant material which is used here to provide a better morphological definition of the taxon. In particular, indication is given of its broad intraspecific variability mostly due to the gregarious habit (ecomorphism) and by change of mode of life through ontogenesis. As regards the latter factor, functional analysis of the shell suggests that in its early juvenile stages the bivalve was an epibyssatc, pleurothetic form, attached to hard substrata. Then, it moved to soft-bottom substrates, where a “heavy weight” strategy was developed to compensate for a weak byssal attachment. In the adult stage, it may be considered a reclining, orthothetic form. The shell is made up of a thin outer layer formed by simple prismatic calcite and by thick, aragonitic, irregular fibrous-prismatic inner layers, both with a well marked periodicity of growth. Mechanical, functional and systematic significance of shell microstructures are discussed. The diagnosis of the genus Pachyperna is herewith emended.I documenti in SFERA sono protetti da copyright e tutti i diritti sono riservati, salvo diversa indicazione.
