Tooth morphology elucidates shark evolution across the end-Cretaceous mass extinction

Sharks (Selachimorpha) are iconic marine predators that have survived
multiple mass extinctions over geologic time. Their prolific fossil record
is represented mainly by isolated shed teeth, which provide the basis for
reconstructing deep time diversity changes affecting different
selachimorph clades. By contrast, corresponding shifts in shark ecology,
as measured through morphological disparity, have received comparatively
limited analytical attention. Here, we use a geometric morphometric
approach to comprehensively examine tooth morphologies in multiple shark
lineages traversing the catastrophic end-Cretaceous mass extinction — this
event terminated the Mesozoic Era 66 million years ago. Our results show
that selachimorphs maintained virtually static levels of dental disparity
in most of their constituent clades across the Cretaceous–Paleogene
interval. Nevertheless, selective extinctions did impact apex predator
species characterized by triangular blade-like teeth. This is particularly
evident among lamniforms, which included the dominant Cretaceous
anacoracids. Conversely, other groups, such as carcharhiniforms and
orectolobiforms, experienced disparity modifications, while
heterodontiforms, hexanchiforms, squaliforms, squatiniforms, and
†synechodontiforms were not overtly affected. Finally, while some
lamniform lineages disappeared, others underwent post-extinction disparity
increases, especially odontaspidids, which are typified by narrow-cusped
teeth adapted for feeding on fishes. Notably, this increase coincides with
the early Paleogene radiation of teleosts as a possible prey source, and
the geographic relocation of disparity sampling ‘hotspots’, perhaps
indicating a regionally disjunct extinction recovery. Ultimately, our
study reveals a complex morphological response to the end-Cretaceous mass
extinction and highlights an event that influenced the evolution of modern
sharks.