Harper Elizabeth M, Checa Antonio
Biol Bull. 2017 Feb;232(1):19-29. doi: 10.1086/691382. Epub 2017 Mar 27.
Multiple groups of bivalve molluscs produce calcitic shell layers, many of these broadly classified as "prismatic." Various pteriomorphian bivalves (such as oysters, pterioids, and mussels) secrete prismatic microstructures with high organic content and clear, strong biological control. However, we present the results of a detailed analysis by scanning electron microscopy (SEM), thermogravimetric analysis, and electron backscatter diffraction to characterize the calcitic prisms in two different clades within the euheterodont bivalves: the extant Chama arcana and the extinct rudists. These results show that the form of prisms constructed is both closely similar between the two taxa and significantly different from those of the pteriomorph bivalves. Most notably, C. arcana and the extinct rudists lack the clear organic outer envelopes and uniform polygonal, cross-sectional appearance. Instead, they form interdigitating crystals of very varied diameters, with some crystals encapsulating others. We advocate retaining the term "fibrillar prisms" to classify these euheterodont microstructures. These fibrillar prisms are more closely similar to abiotic speleothem deposits than to the calcitic prisms of pteriomorph bivalves. We argue that calcite prism growth in euheterodonts is dominated by abiotic constraints whereas, in pteriomorphs (such as oysters, pterioids, and mussels), it is under strong biological control.
多组双壳贝类会产生方解石质的壳层，其中许多大致归类为“棱柱形”。各种翼形亚纲双壳贝类（如牡蛎、翼形贝类和贻贝）会分泌具有高有机物含量且有清晰、强大生物控制作用的棱柱形微结构。然而，我们通过扫描电子显微镜（SEM）、热重分析和电子背散射衍射进行了详细分析，以表征真异齿亚纲双壳贝类两个不同分支中的方解石棱柱：现存的神秘锉蛤和已灭绝的厚壳蛤。这些结果表明，所构建的棱柱形态在这两个分类单元之间非常相似，但与翼形双壳贝类的棱柱形态有显著差异。最值得注意的是，神秘锉蛤和已灭绝的厚壳蛤缺乏清晰的有机外包膜以及均匀的多边形横截面外观。相反，它们形成了直径差异很大的相互交错的晶体，有些晶体还包裹着其他晶体。我们主张保留“纤维状棱柱”这一术语来对这些真异齿亚纲的微结构进行分类。这些纤维状棱柱与非生物洞穴沉积物的相似性比与翼形双壳贝类的方解石棱柱更高。我们认为，真异齿亚纲中方解石棱柱的生长受非生物因素限制，而在翼形亚纲（如牡蛎、翼形贝类和贻贝）中，其生长受到强大的生物控制。
