Evolution of chondrichthyan jaw morphology, from ecological generalists to specialists

Living chondrichthyans comprise only a fraction of their historical diversity represented in the fossil record, but together they provide insights into the evolution of this ancient clade. Using a theoretical morphology approach, we sought the evolutionary drivers of mandible morphology, a key factor in the feeding ecology of the clade, across their more than 400 million year evolutionary history. Using an empirical sample of 122 extant and 95 extinct species across 35 orders, we created a theoretical morphospace that encompasses, and expands beyond, sampled variation. We sampled morphologies from this theoretical morphospace and subjected them to biomechanical analysis of speed and strength, deriving landscapes of functional performance and optimality into which we projected a phylomorphospace. We examined how the optimality landscape has been navigated by chondrichthyan evolution and how it has been occupied by taxa characterized according to habitat and trophic level. The empirical chondrichthyan morphospace occupation was dispersed from the trade-off optimality peaks. Early chondrichthyans occupy morphospace characterized by narrow, curved jaws before expanding to more robust morphologies through time. This move toward robust morphologies does not follow the most optimal trade-off morphologies, instead avoiding areas that are least optimal. Deep-water species occupy the largest morphospace area, while higher trophic level species stay closer to the trade-off optimality peaks. Our study shows that chondrichthyans, rather than being living fossils, have explored increasingly specialized jaw morphologies, likely related to shifts in ecology such as increased numbers of durophagous taxa, as opposed to a more generalist optimization of component biomechanical constraints.