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

Mandibles represent a key evolutionary innovation that have 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 repaid morphological diversification
early in their evolution, likely due to adaptive radiation.