Thecideide brachiopods are an anomalous group of invertebrates. In this study, we discuss the evolution of thecideide brachiopods from the Triassic to the Holocene and base our results and conclusions on microstructure and texture measurements gained from electron backscatter diffraction (EBSD). In fossil and Recent thecideide shells, we observe the following mineral units: (1) nanometric to small granules; (2) acicles; (3) fibres; (4) polygonal crystals; and (5) large roundish crystals. We trace for thecideide shells the change of mineral unit characteristics such as morphology, size, orientation, arrangement and distribution pattern. Triassic thecideide shells contain extensive sections formed of fibres interspersed with large, roundish crystals. Upper Cretaceous to Pleistocene thecideide hard tissues consist of a matrix of minute to small grains reinforced by acicles and small polygonal crystals. Recent thecideide species form their shell of mineral units that show a wide range of shapes, sizes and arrangements. We find from Late Triassic to Recent a gradual decrease in mineral unit size, regularity of mineral unit morphology and orientation and the degree of calcite co-orientation. While crystallite co-orientation is the highest for fibrous microstructures, it is strikingly low for taxa that form their shell out of nanogranular to acicular mineral units. Our results indicate that Upper Jurassic species represent transitional forms between ancient taxa with fibrous shells and Recent forms that construct their shells of acicles and granules. We attribute the observed changes in microstructure and texture to be an adaptation to a different habitat and lifestyle associated with cementation to hard substrates.

中文翻译：

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杀虫剂微观结构和质地的演变：从三叠纪到全新世

Thecide 腕足动物是一组异常的无脊椎动物。在这项研究中，我们讨论了从三叠纪到全新世的杀虫类腕足动物的演变，并将我们的结果和结论基于从电子背散射衍射 (EBSD) 获得的微观结构和纹理测量。在化石和最近的杀虫剂壳中，我们观察到以下矿物单位：（1）纳米到小颗粒；(2) 针头；(3) 纤维；(4)多边形晶体；(5)大的圆形晶体。我们追踪杀虫壳矿物单元特征的变化，如形态、大小、取向、排列和分布模式。三叠纪杀虫剂壳包含由散布着大的圆形晶体的纤维形成的广泛部分。上白垩纪至更新世杀虫剂硬组织由微小到小颗粒的基质组成，这些颗粒由针状体和小多边形晶体增强。最近的杀虫剂物种形成了它们的矿物单元外壳，显示出各种形状、大小和排列。我们发现从晚三叠世到近代，矿物单元尺寸、矿物单元形态和取向的规律性以及方解石的共取向程度逐渐减小。虽然微晶共取向对于纤维微结构来说是最高的，但对于从纳米颗粒到针状矿物单元形成壳的分类群来说，它是非常低的。我们的研究结果表明，上侏罗纪物种代表了具有纤维壳的古代分类群与构造其针状和颗粒壳的最近形式之间的过渡形式。