The development and shell microstructure of the pseudodeltidium and interarea in thecideide brachiopods

Abstract

This study examines the development of the delthyrium, pseudodeltidium and interarea, their growth during the early juvenile stages of ontogeny and the extrapolation of morphology from adult shells to the probable juvenile state. Examination of the development and shell microstructure of the cardinalia of early juvenile thecideide brachiopod ventral valves from Jurassic, Cretaceous and Holocene specimens suggests that the delthyrium develops early in ontogeny and that the initial development of the pseudodeltidium precedes that of the interarea. Also, until the interarea is formed, the postero-lateral flanks of the ventral umbo are palintropic. The development of the interarea can be seen to be a consequence of the lateral extension of the early juvenile hinge line. Initially, the pseudodeltidium consists solely of a thin plate of primary shell material. Comparison of the morphology of the pseudodeltidium of early juveniles with that of adults suggests that the initially curved lateral profile of the pseudodeltidium is retained, or even accentuated, in the ontogeny of lacazellines, but in all thecidellinines, with the exception of Pachymoorellina and Minutella, following the appearance of the interarea, the pseudodeltidium becomes flattened and often appears continuous with the interarea. However, we do not support any proposal that suggests that, in thecideides, only those forms in which the delthyrium is closed by a dorsally convex plate should be considered to possess a pseudodeltidium sensu stricto, mainly because of physiological differences and the prospect of possible taxonomic confusion in the future. Instead, we propose the term planodeltidium for a flat pseudodeltidium, typically developed in the thecidellinines, and rugideltidium for a convex pseudodeltidium, typically developed in the lacazellines, but also in Pachymoorellina and Minutella. Despite the presence of a rugideltidium, we believe the affinities of Minutella are more strongly with the thecidellinines and have included it in the new subfamily Minutellinae of the family Thecidellinidae.

Description and figures of new lectotype and paralectotype material of Recent brachiopod Thecidellina maxilla (Hedley, 1899)

“…The purpose of this paper is: to outline the published misconceptions of the morphology of T. maxilla from the type locality; to describe (using modern terminology) and figure the newly designated lectotype and paralectotypes to show (at least part of) the range of morphological variation of T. maxilla from the type locality; and to provide a new diagnosis of the species T. maxilla based on the lectotype and paralectotypes examined. The terminology follows Hoffmann & Lüter (2009, fig. 2), , and Logan & Baker (2013).…”

Recent thecideide brachiopods from a submarine cave in the Department of Mayotte (France), northern Mozambique Channel

“…The interarea is a planodeltidium (Logan & Baker 2013) with an isosceles triangular outline and a flat (Pl. 1, Figs.…”

Late Eocene (Priabonian) brachiopod fauna from Dnipropetrovsk, eastern Ukraine

“…Its shell is small (maximum observed length 5.2 mm), subtriangular in outline, with a strongly convex ventral valve and almost flat dorsal one. The beak is high, without foramen; the delthyrium is closed by a convex, triangular pseudodeltidium for which Logan & Baker (2013) propose the name rugideltidium. The hemispondylium, supported by a small median septum, projects as two prominent prongs (Fig.…”