Aragonitic dendritic prismatic microstructure of the anomalodesmatan bivalve Thracia

Checa, A. G. and Harper, E. M. and Willinger, M. (2012) Aragonitic dendritic prismatic microstructure of the anomalodesmatan bivalve Thracia. Invertebrate biology. DOI https://doi.org/10.1111/j.1744-7410.2011.00254.x

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Abstract

The shells of most anomalodesmatan bivalves are composed of an outer aragonitic prismatic granular or columnar layer and inner nacreous layers. The family Thraciidae is one of the very few which lack nacreous layers. In particular, the genus Thracia is exceptional in its possession of a very distinctive, but previously unreported microstructure, which we term ‘dendritic prisms’. Dendritic prisms, consist of slender fibres of aragonite which radiate perpendicular to, and which stack along, the axis of the prism. SEM and TEM study of the periostracum and its intervening mantle, as well as of the shell, in three species of Thracia, have been used to reconstruct the mode of shell calcification and to unravel the crystallography of the dendritic units. The periostracum is composed of an outer dark layer and an inner translucent layer. During the free periostracum phase the dark layer grows at the expense of the translucent layer but at the position of the shell edge, the thickness of the translucent layer which has not previously transformed into the dark layer mineralizes and produces the units typical to the dendritic prismatic layer. Within each unit, the caxis is oriented along the prismatic axis, whereas the a-axis of aragonite runs parallel to the long axis of the fibres. The six-rayed alignment of the latter implies that prisms are formed by {110} polycyclically twinned crystals. We conclude that, despite its distinctive appearance, the dendritic prismatic layer of Thracia is homologous to the outer granular prismatic or prismatic layer of other anomalodesmatans, while the nacreous layer present in most anomalodesmatans has been suppressed.

Item Type:	Article
Uncontrolled Keywords:	2011AREP; IA63;
Subjects:	04 - Palaeobiology
Divisions:	04 - Palaeobiology
Journal or Publication Title:	Invertebrate biology
Identification Number:	https://doi.org/10.1111/j.1744-7410.2011.00254.x
Depositing User:	Sarah Humbert
Date Deposited:	18 Nov 2011 18:09
Last Modified:	23 Jul 2013 10:02
URI:	http://eprints.esc.cam.ac.uk/id/eprint/2244

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