[B-PT02] Biotic History

[BPT02-05] Shell microstructure of extinct protobranch bivalves

*Kei Sato¹ (1.Faculty of Education and Integrated Arts and Sciences, Waseda University)

Keywords:Shell microstructure, nacreous structure, bivalve, Protobranchia

The shell microstructure is the micro-scale morphological trait of molluscan shells. It can be potentially preserved in fossils, and therefore is considered as a clue for inference on evolution and phylogeny of the past life. The presenter has described the shell microstructures of Recent protobranchs, the ancestral taxon of bivalve, using scanning electron microscopy. Furthermore, microstructural characters have been evaluated phylogenetically through molecular phylogenetic analysis using DNA sequences. Mapping of shell-microstructure characters on the phylogenetic tree confirmed that the composition of shell microstructures is conservative at the superfamily level and can be divided into three major groups; RESP (radially elongate simple prismatic structure), homogeneous, and nacreous groups. To assess the evolutional history of protobranch bivalves, the presenter newly described shell microstructures of fossil protobranchs mainly from the Mesozoic and revealed that several taxa changed their shell microstructural composition from homogeneous to nacreous group. This microstructural evolution can be one of the driving forces of their diversity modulation according to the paleobiological evidence.