Sharks and rays have been declining for 45 million years

Sharks and rays may be ancient survivors, but new research shows their long history has been anything but steady.

An international team has reconstructed 100 million years of their evolution and uncovered a surprising trend. Instead of holding stable or rising through recent geologic time, their diversity has been falling for nearly 45 million years.

Led by researchers at the University of Vienna, the analysis overturns the common assumption that cartilaginous fish (neoselachians) held steady or even expanded in diversity through recent geologic time.

Instead, their peak came in a warmer world, and their decline since then helps explain why so many species are vulnerable today.

Ancient climate shaped evolution

Manuel Staggl and colleagues compiled extensive fossil records and paired them with reconstructions of past environments – ocean temperatures, atmospheric CO2, and the extent of shallow marine habitats.

That pairing allowed the team to test which factors best tracked speciation booms and busts across the past 66 million years.

The result is a fresh evolutionary narrative for some of the ocean’s most iconic predators, and a set of warning signs for the present.

“Cartilaginous fish, which include today’s sharks and rays, have existed on our planet for over 400 million years,” said Staggl.

“They have survived several mass extinction events during this time, yet today, over a third of neoselachians (i.e. modern sharks and rays) are at risk of extinction.”

Peak diversity of sharks and rays

The end-Cretaceous mass extinction 66 million years ago, which eliminated non-avian dinosaurs, had only a limited and short-lived effect on sharks and rays.

“These animals proved to be amazingly resilient and recovered quickly from the catastrophe,” said Staggl.

Their true zenith arrived later, during the Eocene’s greenhouse warmth about 45 million years ago. After that peak, their diversity began a long descent.

Coastlines shaped evolution

If there was a single stage on which shark and ray diversification played out, it was the continental shelf.

The team’s models point to habitat availability, especially shallow, structurally diverse coastal zones, as the strongest driver of new species.

“The more diverse the shallow marine habitats, the more species emerged,” said Jürgen Kriwet, who leads the Evolutionary Morphology Research Group at the University of Vienna.

Today, these same habitats are squeezed by coastal development, warming seas, pollution, and intense fishing pressure. The very nursery grounds that fostered past diversity are among the at risk.

Ocean chemistry and shark decline

The role of carbon dioxide wasn’t linear. Moderate atmospheric CO2 levels historically aligned with higher shark and ray diversity, likely by boosting primary production.

“In simple terms, CO2 promotes photosynthesis in algae and seagrass meadows. This has a positive effect on the entire food chain and ultimately on sharks and rays,” said Kriwet.

But push CO2 too high, and the benefits flip as acidification and heat stress begin to destabilize marine food webs.

“In a nutshell: moderate CO2 levels were beneficial for shark and ray biodiversity in the past, but excessive levels were harmful. This highlights the danger posed by current ocean acidification caused by human-induced climate change,” said Staggl.

Today’s oceans change faster

The study’s deep-time perspective also clarifies why the current biodiversity crisis is so acute. In the past, environmental changes unfolded slowly enough for species to shift ranges, adapt, or speciate. That buffer is now gone.

“In the past, sharks and rays had time to adapt to changes or migrate to other areas. However, the current changes are happening far too quickly. In this respect, the situation today is unprecedented,” Staggl warned.

Highly specialized species, such as deep-sea sharks adapted to cold, stable environments, are especially exposed to rapid warming and oxygen loss.

Habitat loss drives decline

If habitat diversity built shark and ray richness, protecting and rebuilding that diversity must be the cornerstone of conservation.

The authors argue for a two-pronged approach: safeguard and restore coastal ecosystems at scale and slash CO2 emissions to limit warming and acidification.

“Our study shows that marine conservation is not just about fishing quotas – we need to adopt a broader perspective that considers entire habitats and the climate system,” said Staggl.

Ongoing shark and ray decline

Seen through the lens of 100 million years, sharks and rays are resilient, but not invincible. They weathered the asteroid that ended the age of dinosaurs and flourished in warm, fertile seas when coastlines were plentiful and productive.

Their ongoing decline since the Eocene doesn’t stem from fate. Shrinking and degraded habitats, along with climate stressors, are driving it.

Reversing that trajectory today will require the same ingredients that fueled their past success. That means complex, well-connected coastal ecosystems and a climate within the bounds marine life can endure.

The study is published in the journal Scientific Reports.

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