The evolution of thecideide microstructures and textures: traced from Triassic to Holocene

Abstract

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.

The architecture of Recent brachiopod shells: diversity of biocrystal and biopolymer assemblages in rhynchonellide, terebratulide, thecideide and craniide shells

“…In summary, our review demonstrates that the microstructure and texture of Recent thecideides is a unique structure amongst extant taxa, specific to this brachiopod order (Simonet Roda et al 2021). It is dissimilar to the microstructure and texture of the primary layer of Recent rhynchonellide and terebratulide brachiopods; the latter consists of interlocked crystals with dendritic boundaries, being devoid of organic substance and having a strong texture.…”

“…In contrast to the rhynchonellides and terebratulides, thecideides are small-bodied, relatively thick-shelled animals that live in tropical and warm seas, in cryptic habitats and are cemented to the hard substrate (e.g. Rudwick 1970;Baker 1990;Richardson 1997a, b;Lüter 2005;Carlson 1995Carlson , 2007Nebelsick et al 2011;Simonet Roda et al 2021). Thecideide shell microstructure was proposed to be similar to the primary layer microstructure of rhynchonellides and terebratulides (Williams 1973(Williams , 1997Baker 2006;Carlson 2007).…”

Gigantoproductid shell spiral and microstructure of tertiary layer: evaluation as taxonomical characters

“…Nevertheless, brachiopod microstructure has been widely studied with different techniques such as petrological microscopy, scanning electron microscopy, electron backscatter diffraction and atomic force microscopy (Williams 1956(Williams , 1968Rush & Chafetz 1990;Motchurova-Dekova 2001;Garbelli 2017;Ye et al 2018;Simonet-Roda et al 2019, 2021. Three different layers have been identified in the brachiopod shell: a primary layer with microgranular appearance, usually absent in fossils; a secondary layer with laminar or fibrous microstructure; and a tertiary layer with a columnar microstructure (Williams 1968).…”

“…Radulovic ́et al (2007) noted size differences of fibres into the secondary layer and proposed a new genus of fossil brachiopod based on its microstructure. Recently, Simonet-Roda et al (2021) studied the microstructure (crystal morphology and crystallographic orientation) of thecideide brachiopods within the context of the group's phylogeny.…”

“…2018; Simonet-Roda et al . 2019, 2021). Three different layers have been identified in the brachiopod shell: a primary layer with microgranular appearance, usually absent in fossils; a secondary layer with laminar or fibrous microstructure; and a tertiary layer with a columnar microstructure (Williams 1968).…”