The shell fabric of Palaeozoic brachiopods: patterns and trends

Abstract

The varied microstructures of brachiopod biominerals represent a robust archive to understand the evolution and adaptations of marine calcifiers in time. Therefore, a detailed study of the shell microstructure of Cambrian to Devonian brachiopods from Iran is here presented. The shell of 38 brachiopod species, representatives of 22 families and nine orders, has been analysed using scanning electron microscope (SEM), and a database has been built, including macro‐ and micro‐morphological features used to characterize the two‐ or three‐layered brachiopod shells. Two main microstructural variants of the secondary layer have been analysed: fibrous and laminar fabrics. The fibrous layer has a fabric comparable to that of recent brachiopods, whereas the laminar fabric is more complex in its structural organization and has no recent analogue. In cross‐section, the laminae are thinner than the fibres, and much less variable in size. There is evidence that taxa with laminar microstructure have diverged from the Billingsellida and then followed a trend implying a decrease in thickness of the laminae. Our linear discriminant analysis (LDA) shows that shell fabric and shell thickness are powerful predictors of shell shapes, which in turn approximate the brachiopod lifestyles and ecological strategies. Taxa with a fibrous fabric are mostly biconvex, whereas the groups with a laminar secondary layer are associated to a variety of shell shapes and lifestyles. Even if the relations between shell fabric and shell thickness remain enigmatic, as well as the metabolic cost they imply, shell fabrics, and the possible structural and mechanical advantages conferred, could have played a role in the evolutionary success of the Strophomenata during the Palaeozoic.

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

“…2014; Ye et al . in press). Accordingly, as outlined above, we wish to emphasize with this study that, on the basis of shell fabric and microstructure, it is very difficult to envisage a link between thecideides and strophomenates, as was previously suggested by Williams (1973), Baker (2006) and Carlson (2016).…”

“…2000; Ye et al . in press). Fibrous assemblies appear to be less suitable for a cemented lifestyle; (2) the Craniiformea also lived and live cemented to hard substrates and form shells with a distinct organocarbonate tabular laminar fabric (Williams 1997); (3) the granular–acicular fabric can possibly be secreted more easily and rapidly relative to the formation of fibres and columns; and (4) secretion of small mineral units, acicles and granules, might make it easier to attach to uneven substrate surfaces.…”

Fibrous or Prismatic? A Comparison of the Lamello-Fibrillar Nacre in Early Cambrian and Modern Lophotrochozoans

“…They share a generally fibrous morphology but exhibit high variability in crystal shape and size, crystallographic orientation, and hierarchical organization among different biocalcifying systems and their morphological features are usually not closely related to mineralogical compositions. Common fibrous microstructures are the calcitic fibrous prismatic form of mytilid bivalve shells [23], the primary and fibrous shell layer of calcitic brachiopods [24,25], aragonitic fibers of corals [26], and even the calcite-reinforced chitin fibers of arthropod cuticles [27].…”

Evolution and diversity of biomineralized columnar architecture in early Cambrian phosphatic-shelled brachiopods

“…However, the biologically controlled process of brachiopod shell secretion at the cellular level is still unclear, although organic substrates are observed to be available for biomineral deposition during mantle activity ( Williams et al, 1997b ). Extensive studies have been conducted on living and fossil shells, but most of them are focused on articulated or carbonate-shelled representatives ( Cusack et al, 2010 ; Griesshaber et al, 2007 ; Simonet Roda et al, 2022 ; Roda and Mar, 2021 ; Ye et al, 2021 ). By contrast, the linguliform brachiopods with shells composed of an organic matrix and apatite minerals that show extremely intricate architectures and permit exquisite preservation are less studied.…”