A new study conducted by researchers from Uppsala University in Sweden and the University of New England in Australia, and published this week in Current Biology, suggests that modern shark biodiversity was triggered by the end-Cretaceous mass extinction event about 66 million years ago.
As part of the study, researchers explored how certain groups of sharks responded to the mass extinction that killed-off non-bird dinosaurs and marked the end of the Cretaceous period and the Mesozoic era. Sharks are one of the major groups that survived the Cretaceous-Palaeogene mass extinction.
“Our study found that the shift from lamniform- to carcharhiniform-dominated assemblages may well have been the result of the end-Cretaceous mass extinction,” said project leader and Uppsala doctoral student Mohamad Bazzi.
The team used “cutting-edge” analytical techniques to explore the variation of tooth shape in carcharhiniforms and lamniforms and measured diversity by calculating the range of morphological variation, also called disparity.
“Unlike other vertebrates, the cartilaginous skeletons of sharks do not easily fossilize and so our knowledge of these fishes is largely limited to the thousands of isolated teeth they shed throughout their lives,” Bazzi said. “Fortunately, shark teeth can tell us a lot about their biology, including information about diet, which can shed light on the mechanisms behind their extinction and survival.”
“Going into this study, we knew that sharks underwent important losses in species richness across the extinction.” said Dr. Nicolás Campione at the University of New England, who co-devised the project. “But to our surprise, we found virtually no change in disparity across this major transition. This suggests to us that species richness and disparity may have been decoupled across this interval.”
The study found that extinction and survival patterns were substantially more complex. Morphologically, there were differential responses to extinction between lamniform and carcharhiniform sharks, with evidence for a selective extinction of lamniforms and a subsequent proliferation of carcharhiniforms (the largest order of living sharks today) in the immediate aftermath of the extinction.
“Carcharhiniforms are the most common shark group today and it would seem that the initial steps towards this dominance started approximately 66 million years ago,” Bazzi said.