Macroevolutionary Biomechanics: Linking Form, Function and the Environment across Deep Time

The tempo and mode of evolution shifts over time as species adapt to new environmental challenges, ecological interactions, and balance internal constraints. The environment and geography thus influence the rates at which traits evolve, as well as rates of biogeographic dispersal. Understanding how and why these rates vary has long been a primary interest in paleontology and is now crucial for understanding the impacts of climate change. Locomotor morphology provides a good model system to study these interactions because they directly reflect the environment. Functional morphology is also well-understood in both extant and extinct vertebrates, and is linked to other physiological and functional systems. Researchers have long studied ecomorphological traits in a comparative framework and in extinct species. In contrast, in this project, we are interested in how these systems evolve from ancestor to descendent along lineages using recently developed variable rate models of evolution.

Our overall objective is to quantify the evolutionary dynamics of movement at two scales: 1) direct proxies for locomotion using lever arms, and 2) biogeographic dispersal. These dynamics will be investigated in Mesozoic mammals and dinosaurs. The rationale for our taxonomic focus is that dinosaur locomotor evolution has remarkable parallels with mammals and therefore offers a natural experiment for investigating general principles of how trait-environment interactions affect evolutionary dynamics. Dinosaurs and mammals both have upright limb posture and leverarmsvrmparasagittal limb movements – a locomotor strategy that is arguably associated with a derived homeothermic physiology. The rationale for focusing on ecomorphologic traits is that, unlike complex biomechanical models, the evolution of functional parameters (such as lever arms) can be analyzed across many species owing to their simplicity. We don’t expect to discover nuanced insights into how these animals moved, but by using functionally relevant parameters, measured across whole clades, we expect to discover shifts in the rate of locomotor evolution that cannot be detected by more complex taxon-specific models. The rationale for investigating biogeographic dispersal is that current methods assume the earth is flat (an obvious problem) and that map projection techniques used to address this contain significant amounts of distortion. We are working with colleagues Chris Venditti and Andrew Meade (University of Reading, UK) on new models to analyze the tempo and mode at which Mesozoic dinosaurs and mammals speciated and dispersed across the surface of the planet.

This figure shows one analysis from this project. The rate of gear ratio (log10 lever-out/lever-in) evolution of the primary forelimb retractors mapped onto a time-calibrated phylogeny (scale bar = 100 million years) of dinosaurs. Warm colors indicate higher rates of gear ratio evolution, while cool colors indicate lower rates of gear ratio evolution. Note that the rate of forelimb gear ratio evolution increases dramatically throughout the entire theropod clade, while the ornithischian clade primarily maintains low rates of forelimb gear ratio evolution.