Among sharks and rays, species within the first four million years of existence are significantly more likely to go extinct than older species, according to a study led by researchers from the University of Zurich based on fossils from the last 145 million years. This shows that in addition to environmental stressors, the evolutionary age of species also plays a crucial role in their survival.

Whether a species just freshly emerged, or it has been around for millions of years does not dictate its vulnerability. This has been the assumption of an old debate on whether species’ age plays a role in extinction risk. Researchers at the University of Zurich (UZH) have now led an international study that challenges this idea, at least when it comes to sharks and rays. According to their study, young species are by far the most likely to go extinct.

The team examined data from over 20,000 fossil records worldwide dating back to the Cretaceous period, using innovative methods to reconstruct the age of origin and extinction for each of the about 1,500 species. “We were particularly interested in identifying when, over the past 145 million years, large numbers of new species emerged or disappeared, and how this can be explained,” says first author Kristína Kocáková of UZH’s Department of Paleontology.

Previously unknown extinction events
Shark and ray species (Neoselachii) experienced high rates of loss during the mass extinction of dinosaurs at the end of the Cretaceous period around 66 million years ago, but this didn’t come as a surprise to the researchers. “We discovered other, previously unknown extinction events. However, many of these events, including the one at the end of the Cretaceous, were followed by the origination of new species", explains Catalina Pimiento, UZH professor of paleobiology. “What’s remarkable is that the more recent extinctions were not followed by the emergence of new species, including one around 30 million years ago. This one was far the most impactful, because many species went extinct but hardly any new species emerged afterward,” says Pimiento, the last author of the study.

High extinction risk for young species
A remarkably consistent pattern emerged across the entire 145-million-year period. Evolutionarily young species went extinct far more often than older ones, regardless of whether their extinction was caused by an asteroid impact, or any other driver. “If a species had existed for only about four million years, it was more vulnerable than one that had been around for 20 million years. The older species remained remarkably stable,” says Kocáková.

Importance of conservation
The study shows that modern-day sharks and rays are the survivors of a long history of ups and downs, including several extinction events that were previously unknown. The data also demonstrate that over the past 40 to 50 million years, not enough new species have emerged to make up for previous losses. Importantly, the study reveals that species’ age is a is a persistent predictor of extinction risk over evolutionary time.“Modern sharks and rays have already lost much of their evolutionary potential and have now also come under pressure from humans. Understanding their past helps us recognize how important it is to protect the species that still exist today,” says Dr. Daniele Silvestro, a coauthor of this study and one of the developers of the methods used.