Early Stage Biomineralization in the Periostracum of the ‘Living Fossil’ Bivalve Neotrigonia

摘要: A detailed investigation of the shell formation palaeoheterodont ‘living fossil’ Neotrigonia concentrated on timing and manufacture calcified ‘bosses’ which stud outside all trigonioid bivalves (extant fossil) has been conducted. Electron microscopy optical revealed that spp. have a spiral-shaped periostracal groove. The periostracum itself is secreted by basal cell, as thin dark pellicle, becoming progressively transformed into layer additions secretions from internal outer mantle fold. Later, intense secretion surface fold forms translucent layer, becomes tanning layer. initiation bosses occurred at very early stage formation, deep within groove immediately below initialmost At this stage, they consist series polycyclically twinned crystals. grow traverse through groove, in coordination with thickening until, upon reaching edge, impinge each other become large prisms separated walls. In conclusion, initial external part prismatic are fully intraperiostracal. With later growth, transform fibrous aggregates, although details process unknown. This reinforces relationships groups ability to form intraperiostracal calcifications, for example unionoids trigonioids clade Paleoheterodonta. presence similar structures anomalodesmatans euheterodonts raises question whether indicates relationship or represents convergence. identification calcification an organic sheet interesting implications our understanding how shells may evolved.