Could the asymmetrical commissure in rhynchonellide brachiopods be an adaptive trait?

Abstract

This paper presents new arguments that contribute to support the hypothesis about the functional meaning of the commissural asymmetry in the Coniacian (Upper Cretaceous) rhynchonellide Cyclothyris cardiatelia Berrocal-Casero. According to this hypothesis, commissural asymmetry is interpreted as an adaptation to life on soft substrates, which leads to a life position that is oblique and partially sunk in relation to the substrate. The taphonomic compression of one of the shell lobes observed in asymmetrical C. cardiatelia has been compared to the compression found in a symmetrical rhynchonellide in which the compression is located at the frontal part of the shell, supporting the idea of a different life position than in C. cardiatelia. Additionally, the fossil record shows that brachiopods exhibiting external asymmetry usually display the corresponding asymmetry in the brachidium, related to dysfunction or atrophy of one of the lophophore arms. Observations in extant rhynchonellides show they can live with one arm of the lophophore atrophied or misfunctioning, and even amputated. The applicability of this hypothesis to other cases of commissural asymmetry in rhynchonellides, such as those living in closely packed clusters, reef environments or affected by unidirectional currents, is discussed, considering that the common physiological response to these different palaeoenvironmental scenarios would be a differential use of the arms of the lophophore, in turn affecting commissure plication. This physiological response could explain the origin of obligate asymmetry in other rhynchonellide lineages besides Cyclothyris M´Coy, for instance in Torquirhynchia Childs. Continuous records of both facultative and obligate asymmetrical rhynchonellides along single phyletic lines will be necessary, in order to establish whether this interpretation is a generalized explanation for asymmetrical shells in rhynchonellides.