The secretion and structural evolution of the shell of thecideidine brachiopods

Abstract

Secretion of the exoskeleton of Thecidellina barretti proceeds in the same way as in other living brachiopods, but the structure of the mantle edge is different. Mucin cells occupy the core of the outer mantle lobe, and periostracal secretion begins within a slot separating lobate cells charged with secretion droplets from long vesicular cells. The former exude an impersistent mucopolysaccharide film and may be regarded as an integral part of the generative zone; the latter secrete most of the periostracum and, thereafter, the carbonate succession as they become part of the outer epithelial layer of the mantle. The periostracum consists of an outer coat initially differentiated into a triple-unit structure and a polysaccharide layer. It is rarely more than 200 nm thick except at the hinge-line where it forms a narrow fold secreted by columnar cells as a cover to the interareas of both valves. Papillose outgrowths of the mantle (caeca), rich in secietion droplets, arise at the mantle edges at regular intervals and persist throughout life. The distal cells are highly microvillous at first, and as they withdraw from the periostracum they secrete numerous proteinous sheaths (the brush) containing mucopolysaccharides, which become encased within the carbonate succession. As the mineral layer thickens, the caeca retreat with the mantle and periodically secrete proteinous partitions within vacated distal parts of the canals (puncta). The carbonate skeleton mainly consists of acicular and granular calcite forming a primary layer with an internal surface ornamented by tubercles, rhombic bodies, and regularly arranged pits accommodating epithelial cells underlying muscle tissue. The teeth and socket ridges, however, are composed of fibres sheathed in anastomosing proteinous sheets and stacked in alternating rows as in fibrous secondary layers found in most other articulate brachiopods. Superficial traces of fibres also occur as patches and on tubercles in both valves. A similar distribution of secondary fibres is found in T. australis ; but fibres are limited to spinose outgrowths in the pedicle valve of T. hedleyi , while incipient fibres develop only on the surfaces of mature teeth in Lacazella mediterranea . Study of the shell fabric of nine fossil genera shows that a continuous fibrous secondary layer occurs in all of the earliest thecideidines. During subsequent evolution the layer became neotenously reduced in the three post-Jurassic Subfamilies of the Suborder; and, although the loss was accelerated in the Lacazelhnae, the main change to a sporadic secretion of secondary shell took place during late Jurassic or early Cretaceous times in all three groups. A review of the principal morphological features of thecideidines, on balance, corroborates the evidence of the ultrastructure of the shell that the Suborder was caenogenetically derived from spiriferide rather than strophomenide brachiopods.