Secretion and ultrastructure of the periostracum of some terebratulide brachiopods

Abstract

A survey of the periostracum of representative species of living terebratulide brachiopods reveals that it originates within a slot separating the lobate and vesicular cells of the outer mantle lobe and that three fundamentally different types are synthesized beneath a mucopolysaccharide film exuded by the lobate cells. The simplest type, which is found among the Terebratulacea, consist of a uniformly textured, electron-dense layer, either splaying externally at irregular intervals into acutely disposed subsidiary sheets up to 4 μm long as in Terebratulina , or bearing externally scattered secretion droplets and rare vesicles as in Liothyrella . This simple type of periostracum is a correlative of an undifferentiated basal layer which underlies the periostracal succession of all known terebratellaceans, not only in its structure, but also in its secretion by the vesicular cell region of the outer mantle lobe. The basal layer of terebratellaceans, however, supports a complex superstructure. In Waltonia and other terebratellids as well as dallinids, kraussinids, laqueids and possibly platitiids, the superstructure is an elaborate proteinous succession composed of vesicles and secretion droplets amalgamated by accretionary growth and fashioned by partial resorption into a labyrinthine structure covered by fibrillar proteinous rods arranged in undulating interconnected rows. In the megathyridid Gwynia , the superstructure consists of proteinous sheets secreted as a series of isoclinal folds normal to the basal layer. Both types of superstructure as well as the secretion droplets and vesicles of Liothyrella are exuded mainly by distal lobate cells forming the outer boundary of the periostracal slot. This difference in origin between the basal layer and the superstructure precludes consideration of periostraca as chronologically stratified successions. Concordance of the various types with the phylogenetic history of the Order based mainly on the evolution of the lophophore supports, suggests that the simplest periostracum is the most primitive type and that it may have been widespread among Palaeozoic brachiopods.