QTA of Prismatic Calcite Layers of some Bivalves, a Link to Trichite Ancestrals

Abstract. Eight calcite layers of bivalves are examined in the view of QTA. We use the QTAinformation to compare calcite prisms of a fossil species, Trichite , with other outer prismatic calcitelayers. The results indicate that the fossil species exhibit textures close to the ones of Pteriomorphia,but considerably different from the ones of mytilids, as expected from molecular analysis. Textureanalysis results point out that inside the Pterioids, the Pectinoidea family is probably more distantthan Pterioidea and Pinnoidea ones, coherently with other studies, and perhaps closer to Mytiloidea.However, texture analysis seems to favour a closer relationship of Trichites with Pinnoids than withPteriids. Introduction Modern bivalved molluscs make their calcareous shells out of layers of two or more of thefollowing microstructures (aragonitic prisms, nacre, crossed-lamellar, complex crossed-lamellar andhomogeneous, and, or calcitic prisms and foliae). There is good evidence that the earliest bivalves,recorded from the Middle Cambrian, which were wholly aragonitic, comprising a thin outer layer ofprisms and a substantially thicker layer of nacre [1,2], and that the diversity of microstructures aroserelatively rapidly, most by the Ordovician [3]. Quite why this diversity of structures evolved isunclear but raises extremely interesting questions as to the potential benefits of the more derivedmicrostructures. What is well established is that the microstructural arrangements shown bydifferent bivalve taxa is of considerable phylogenetic value [4,5,6].Of particular interest is the evolution of calcitic microstructures from the originally whollyaragonitic primitive shell. Calcitic microstructures are found in only two of the subclasses of theBivalvia: the Pteriomorphia and the Heteroconchia. In each of these subclasses calcite prisms arefound in two apparently separate clades and it is thus entirely reasonable to suggest this mineralogyhas evolved on at least four separate occasions [7,8]. Within the Heteroconchia, outer layers ofcalcite prisms characterise the rudists, large, bizarre extinct bivalves which dominated shallowmarine carbonate platforms for much of the Mesozoic, and also occur in some members of theChamidae, an extant family of tropical bivalves [8]. Within the Pteriomorphia, a large clade ofcalcite secreters, the pterioids, includes major groups such as the oysters, scallops, pinnoids andpteriids, many of which have expanded their calcitic layers such that they may account for most ofthe shell thickness. This expansion of the calcitic shell layers is most extreme in the oysters wherethe only aragonite present is restricted to the muscle attachment sites and ligament [9]. The otherpteriomorphian group to have some members to secrete calcite is the mytiloids (mussels). In boththe chamids and mytiloids, however, there is clear evidence that not all members secrete calcite,implying that even within these taxa the ability to secrete calcite has evolved more than once.The study of crystallographic textures in gastropods has revealed that this may be a method ofrevealing phylogenetic relationships [10]. It may be instructive to study bivalve microstructures inthe same way. This paper focuses on a QTA study of the calcite prisms of a number of modernpteriomorph bivalves. Specifically, we examined and compared calcite prisms from two mytilids,