Article: Ornamentation and shell structure of acrotretoid brachiopods
Alwyn Williams and Lars E. Holmer
The surface ornamentation of acrotretoid brachiopods includes three kinds of microstructural features which appear to reflect the nature of the periostracum and the outer mantle lobe responsible for secreting the first-formed layers of the phosphatic shell. Radial folds, less than 1 um in wavelength, must have been casts of wrinkles in a thick, inner sealing membrane of a vesicular periostracum. Coarser folds are discrete segments of concentric ridges (fila), disposed as outwardly convex arcs (drapes) with chords up to 40 um in length. The drapes could have been formed by spasmodic stresses within the outer mantle lobe induced by muscles controlling setae. Sporadically developed, microscopic networks of superficial grooves simulate the outlines of radially elongated vesicular cells forming the outer mantle lobe of living brachiopods, and are interpreted as casts of intercellular spaces. The grooves are continuous internally with the partitions of camerate laminae forming the secondary shell of some acrotretoids. In other acrotretoids, secondary as well as tertiary laminae contain columns and domes. The relatively smooth surfaces, bounding the laminae and defining slots within the partitions and canals within the columns, are believed to represent interfaces between organic membranes and seeded phosphatic crystallites so that the uniquely camerate and columnar nature of the acrotretoid laminae are original features of shell successions.