A HETEROCHRONIC LINE OF LEAST EVOLUTIONARY RESISTANCE MEDIATES FUNCTIONAL TRADE-OFFS BETWEEN HYDRODYNAMIC AND FEEDING PERFORMANCE IN THE CINCINNATIAN BRACHIOPOD VINLANDOSTROPHIA SPP.

Selection for functional efficiency typically pushes a morphological trait toward an adaptive optimum, but when a structure performs more than one function, the optimal solution may require a compromise in functional efficiency. This optimal solution is necessarily also a compromise with ontogenetic change and other processes that canalize morphological variation into lines of least evolutionary resistance (LLER). The shape of brachiopod valves changes substantially during ontogeny and plays a role both in minimizing the likelihood of being swept away by currents and in directing nutrient filled water to the lophophore. Using 60 micro-CT scanned brachiopod shells of the Late Ordovician Vinlandostorphia, we synthesized computation fluid dynamics simulations with 3D surface geometric morphometrics to test if shape variation is constrained by tradeoffs between feeding efficiency, hydrodynamic efficiency, and ontogeny. A negative relationship between feeding and hydrodynamic efficiency was found (p-value=0.001, R²=0.18), indicating that there is a functional trade-off between the two. The axis of shape variation associated with this trade-off was estimated using a two-block partial least squares regression on the two performance variables and geometric morphometric shape. The functional trade-off axis was significantly correlated with the ontogenetic axis (using size as age proxy), both of which are aligned with the first principal component of shape variation, suggesting that these axes define a heterochronic LLER and that their interaction is important in defining shape variation within this taxon. The heterochronic LLER was projected onto phenotypic adaptive landscapes, and, using a randomization test, we found that species are distributed in morphospace closer to this heterochronic LLER than would be expected by chance alone (p-value = 0.001). Taken together, these findings suggest that in Vinlandostrophia, selection for feeding and hydrodynamic performance can be accomplished via changes in developmental timing. The LLER of this taxon may be associated with onshore-offshore transitions, as these niches face divergent selective pressures that are likely to involve trade-offs between flow regimes and food availability. This work also provides a case study in synthesizing computational fluid models with 3D morphometrics to elucidate patterns of selection and constraint in the fossil record.