ROTATING RHYNCHONELLIDES REVISITED: FUNCTIONAL INSIGHTS INTO THE MORPHOLOGY OF BICONVEX BRACHIOPODS

The brachiopod order Rhynchonellida is one of the longest surviving metazoan clades (Cambrian – Recent). Rhynchonellides are also basal to most biconvex brachiopod clades; understanding rhynchonellide shape may provide insight into the functional morphology of biconvex brachiopods in general. Most rhynchonellides are of one of two morphological types: (a) weakly-biconvex, unisulcate (median deflection towards the pedicle valve); or (b) strongly globose, uniplicate (median deflection towards the brachial valve). Both forms normally live attached by a pedicle, with the brachial valve down. Ontogenetic study reveals that globose forms begin weakly-biconvex and subsequently inflate the brachial valve. Assuming that the pedicle remains attached and of fixed length, this ontogenetic change rotates the commissure from a position subparallel to the substrate to a position perpendicular to the substrate. This change may be essential – all brachiopod lophophores ontogenetically pass through a simple, flat-lying schizolophe stage with ventral-dorsal flow, but as the rhynchonellide lophophore grows into a 3-D spirolophe, flow changes to lateral-medial. If the brachiopod remains with its commissure subparallel to the substrate, then a spirolophe with a uniplicate commissure would have an exhalant (medial) flow lower in the water column than the inhalant (lateral) flow, increasing the likelihood of refiltering waste water. Rotation positions exhalant flow higher than inhalant flow. In contrast, unisulcate forms do not need to rotate because the exhalant flow already is higher; thus unisulcate forms remain weakly biconvex. These hypotheses were tested by (a) examining the association of globosity with uniplication; (b) correlation between degree of inflation and angle of orientation, using exceptionally-preserved surfaces of fossil rhynchonellides in various growth-stages; and (c) performing biomechanical experiments with gaping models to determine if passive flow is consistent with the proposed flow. All tests produced significant results corroborating the hypothesis that shape change in rhynchonellide ontogeny is a function of a changing lophophore. The results may apply to other biconvex taxa, and may help identify spirolophous fossil taxa even when calcareous lophophore supports are lacking.