Functional meaning of asymmetrical commissures in Coniacian (Upper Cretaceous) rhynchonellide brachiopods from Northern Spain

Abstract

The functional meaning of asymmetry in brachiopods has been a matter of discussion for over a century. Several hypotheses have been proposed to explain why these morphologies appear, the predominant idea in the literature being that it might be a non-adaptive characteristic. Here we present an in-depth study of the asymmetry present in the commissure of the rhynchonellide brachiopod Cyclothyris aff. globata from three Upper Cretaceous sections of northern Spain, with the aim to establish whether it is correlated to some kind of palaeoenvironmental factor. A new hypothesis interpreting this asymmetry as a functional feature is proposed after the results obtained from the palaeoecological and taphonomical study of the specimens. A noteworthy aspect of the asymmetry displayed by this species is that juvenile forms are fully symmetric while adult specimens are asymmetrical, but internal structures remain symmetric throughout ontogeny. In adult forms, asymmetry is expressed as the division of the shell into two vertically displaced lobes, one of which is invariably shorter than the other. Characteristically, the beak is twisted towards the shorter lobe, and in some individuals the larger lobe is seen to be completely flattened due to burial and compaction. When the occurrence of these asymmetrical forms was analyzed, it became evident that they appeared wherever the lithology was mainly marly, thus seeming to be related to the predominance of fine detrital components in the sediments (soft bottom). These features enable us to suggest that C. aff. globata lived partially buried, with one side of the shell placed inside the soft sediment. Asymmetry developed gradually as the shell grew, being evident once it attained a certain size. This hypothesis fits well with the morphological and structural features displayed by this species and, particularly with its relationship with the consistency of the substrate in which these asymmetric brachiopods lived, corresponding to an adaptation of the growing to soft and unstable bottom conditions. Although this paper does not aim to conduct a systematic review of the species, the geometric morphometric analysis performed in order to describe the asymmetry provided results that give a basis for the discussion of the taxonomic relationships between certain Upper Cretaceous asymmetric Cyclothyris species.