Adaptive radiation of pelagiarian fishes at the K/Pg boundary led to rapid diversification of mandible morphology

Abstract Mandibles represent a key evolutionary innovation that has enabled jawed vertebrates to adapt and diversify in response to a range of food sources, and changes in mandible shape are often linked with adaptive radiations into new ecological niches. Using a phylogenetic comparative approach, we explore the phenotypic disparity and mechanical properties of the lower jaw in Pelagiaria, a morphologically diverse but relatively small clade of open-ocean fishes which arose near the Cretaceous/Paleogene (K/Pg) mass extinction event. Using three-dimensional (3D) geometric morphometrics, we find that the high phenotypic disparity of the pelagiarian mandible was established very early in their evolutionary history, and high levels of disparity have been maintained over tens of millions of years; this is consistent with the hypothesis that Pelagiaria represents an ancient adaptive radiation. We test the correlation of mandible shape with potential drivers of shape evolution and find that mandible shape is correlated with habitat depth and tooth type but not with body elongation or diet. Moreover, mandible shape is significantly correlated with mechanical advantage (MA), with closing MA being most strongly correlated. Pelagiarian jaw shapes fall broadly into six morphotypes, of which two show significantly higher closing MA than other groups, despite differing substantially in shape. Our results demonstrate both the mechanical and morphological diversity of the pelagiarian mandible and support the hypothesis that Pelagiaria underwent rapid morphological diversification early in their evolution, likely due to adaptive radiation.