Crystallographic orientations of structural elements in skeletons of Syringoporicae (tabulate corals, Carboniferous): implications for biomineralization processes in Palaeozoic corals

Abstract

The crystallographic orientation of structural elements in skeletons of representatives of Carboniferous Syringoporicae (Auloporida) has been analysed by scanning electron microscopy (SEM), petrographic microscopy and electron backscatter diffraction (EBSD) on specimens from the Iberian Peninsula. The skeletons of the tabulate corals of the Syringoporicae consist of biogenic calcite crystals, and their microstructure is composed of lamellae, fibres and granules, or of a combination of these. Independent of the microstructure, the c-axis is oriented towards the lumen, quasi-perpendicular to the growth direction of the skeleton (perpendicular to the morphological axis lamellae, parallel to fibres). Most phaceloid taxa have a turbostratic distribution, as a biogenic response to prevent the cleavage of crystals. Cerioid and some phaceloid corals, whose microstructure is conditioned by wall elements, do not exhibit turbostratic distribution. Wall elements are determined by the biology of each taxon. Holacanth septal spines are composed of fibres arranged in a cone-shape structure, sometimes clamped to the external part of the corallite and show a complex crystallography. Monacanth septal spines are spindle shaped and composed of bundles of fibres. Tabulae are composed of lamellae. Their development and crystallographic orientation depends on the position of the epithelium in each case. Shared walls are formed by a combination of the walls of two independent corallites with a median lamina, composed of granules; these have a crystallographic orientation between that of the two corallites. The growth of the microstructure is derived by a coordinated stepping mode of growth, similar to other groups of organisms such as molluscs and scleractinians. The nucleation and formation of packages of cooriented microcrystals suggest a growth mode similar to mineral bridges with a competitive growth mode between each crystal. The growth pattern of corallites suggests that the growth direction is divided into two main components: a horizontal growth direction towards the lumen and a vertical direction towards the top.

Reconstructing a Carboniferous inferred coral–alcyonarian association using a biomineralogical approach

“…D). In the transverse section, the c‐axes of the lamellae point to the lumen and are slightly tilted to the growth direction, whereas in longitudinal section, the c‐axis is even more perpendicular to the coral wall, and the tilting is lower (Coronado et al ., , , ). Coronado et al .…”

“…Coronado et al . () describe the growth mechanisms of this superfamily of tabulate corals from the crystallographic study of the generation of calices; the growth direction is divided into two main components, a horizontal growth direction towards the lumen and a vertical direction towards the outside. In contrast, the scales have a variable c‐ axis orientation and its direction is from the footstep shape insertion area towards the apex, a pattern that has never been described before in Syringoporicae.…”

“…In the transverse section, the c-axes of the lamellae point to the lumen and are slightly tilted to the growth direction, whereas in longitudinal section, the c-axis is even more perpendicular to the coral wall, and the tilting is lower (Coronado et al, 2013(Coronado et al, , 2014(Coronado et al, , 2015. Coronado et al (2015) describe the growth mechanisms of this superfamily of tabulate corals from the crystallographic study of the generation of calices; the growth direction is divided into two main components, a horizontal growth direction towards the lumen and a vertical direction towards the outside. In contrast, the scales have a variable c-axis orientation and its direction is from the footstep shape insertion area towards the apex, a pattern that has never been described before in Syringoporicae.…”

“…Diagenetic alteration can be mapped with CIP because during the process of dissolution/coprecipitation, the original crystallographic orientations are lost, even if the microstructure is preserved (P erez-Huerta et al, 2007(P erez-Huerta et al, , 2011(P erez-Huerta et al, , 2012Coronado et al, 2013Coronado et al, , 2015Moreno-Azanza et al, 2013). In polycrystalline and well-structured skeletons, as is the case of those with carbonate-controlled biominerals (e.g., corals, molluscs, brachiopods, eggshells, which have a great crystallographic control), the crystallographic orientations are random after diagenesis.…”

Biomineral electron backscatter diffraction for palaeontology

“…), fossil and modern corals (Floquet and Vielzeuf ; Coronado et al . ) and conodonts (Pérez‐Huerta et al . ).…”