An analysis of data from more than 3,200 fossils of sharks and rays has yielded a better understanding of the reasons for those species’ extinction or survival during Earth’s fifth mass extinction event, 66 million years ago. The findings by Guillaume Guinot and Fabien Condamine, a paleontologist and evolutionary biologist, respectively, at the Institute of Evolutionary Sciences of Montpellier (ISEM, “Institute of Evolution Sciences of Montpellier”), were published on February 24 in the newspaper Science.
The details of the cataclysmic events that caused this extinction are still being debated, but one of the most commonly accepted hypotheses involves a meteorite impact. The extinction affected many terrestrial and marine species, of which only a few survived. The reasons for their survival are not entirely understood, but this study has helped to provide some answers. Scientists from Montpellier studied the evolution of shark and ray diversity from between 93 million and 56 million years ago. The appearance of new species and the disappearance of others from the fossil data were evaluated.
Nearly 63% of the species disappeared, the majority during an interval of 800,000 years occurring around 66 million years ago. “But above all, we were able to demonstrate that not all groups were equal when faced with this extinction: The rays showed higher extinction rates – with 72% of species going extinct – than those of the sharks, with 59% of species becoming extinct We can say that in this case, extinction was selective,” said Condamine.
One of the factors that influenced the ability to survive such a crisis was diet. Scientists sorted species by their diet, determining the latter by the shape of their teeth. Nearly 74% of species that had teeth that were adapted to grinding shells went extinct, a potential indicator of significant depletion among the shellfish that were their primary food source. “This is an interesting point because we know that this extinction heavily impacted the first links of the marine food chain and the organisms directly dependent on them,” said Condamine.
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On the other hand, those with teeth that were adapted to more diverse diets fared better, with roughly 60% of species disappearing. “In general, living beings located in the [part of the] food chain that links on decomposing matter survived better,” said Lionel Cavin, curator at the Museum of Natural History in Geneva.
The effect of the geographic distribution of species on their likelihood of surviving the crisis was then assessed. The researchers observed that species with a wide geographic distribution had a higher survival rate than others. “We discovered that species living in lower latitudes were more strongly affected, suggesting geographic selectivity this time. What is striking is that there seems to have been the same geographic selectivity in other groups of marine invertebrates, such as bivalves,” noted Guinot. The specific causes of this geographic selectivity have yet to be established.