April 2023

Around 66 million years ago, a massive meteorite struck Earth, triggering a series of ecological disasters that wiped out T. rex along with all the other non-bird dinosaurs. For most of us, that’s old news – and might be as much as we know about this dramatic time. But scientists, of course, dig much deeper into the past, investigating exactly how mass extinctions played out and how they affected different groups of organisms. Now, new research explores this same extinction event from the view of another ancient and famously toothy group: sharks, skates, and rays.

Where's the evolution?

The dino-killing extinction event is known as the K-Pg (for Cretaceous-Paleogene) mass extinction and is sometimes called the K-T (Cretaceous-Tertiary) event.  Much of what we know about the size of this mass extinction comes from some of Earth’s tinier and more unassuming inhabitants: shelly, ocean-dwelling invertebrates, like clams, which fossilize readily, as shown in the photo below. Their dense fossil record allows scientists to track the ups and downs of different groups, and importantly, figure out if groups with certain characteristics were hit harder than others by mass extinctions.

While fossils from the cartilaginous skeletons of sharks, skates, and rays (which are called elasmobranchs) are rare, these animals do boast a mouthful of potential fossils. Elasmobranchs shed and regrow teeth throughout their lives and these fossilize readily. Of course, a tooth can’t tell you what the whole animal looked like, but differently shaped teeth can indicate that the teeth belonged to different species and what their owners ate. Furthermore, the geologic and geographic setting in which the tooth fossil was found can yield important information about where that animal lived. The scientists behind the new research used fossilized teeth to investigate how elasmobranchs fared in the ancient oceans at the same time that non-bird dinosaurs were dying out on land. How bad was this mass extinction for sharks, skates, and rays?

The short answer to this question is: not dinosaur-bad, but still pretty bad. Around 62% of elasmobranchs went extinct during the K-Pg mass extinction. The diversity disaster is shown in these graphs by the plunge in species numbers around 66 million years ago. It was worse for skates and rays, which narrowly avoided complete annihilation.

Scientists have long thought that having a large geographic range protects against extinction. Even if one part of an organism’s range is affected by a disaster, it can survive in another area and recolonize a habitat once it has recovered. The research team found that sharks, skates, and rays show the same pattern. The reconstructed geographic ranges of groups that survived the K-Pg mass extinction were larger than the ranges of groups that went extinct.

The scientists also found that diet strongly affected extinction risk. Species with teeth adapted for crunching through hard prey (think bottom-dwelling clams and lobsters) were much more likely to go extinct than species that fed on softer prey. The graphs below show this difference. The pattern fits with other evidence that suggests that, during the K-Pg event, changes in ocean cycles decimated the small invertebrates at the bottom of the marine food web, and that species relying directly on this part of the food web faced greater chances of extinction.

In fact, those abundant, fossilizable, crunchy, shelled critters that make easy prey for animals, as well as scientists trying to understand mass extinction, seem to be much more than just data points. The researchers found that extinction among clams and their relatives is the best predictor of extinction among the sharks, rays, and skates that eat them. They may also be a key to understanding elasmobranch’s recovery after the extinction. The extinction of many species specializing on hard prey left open niches. As ecosystems and their invertebrates recovered, some of the surviving lineages of sharks, skates, and rays diversified in ways that took advantage of these opportunities: new lineages of hard-prey specialists evolved.

Investigating past mass extinctions is not just a way to satisfy our curiosity about the world that the dinosaurs inhabited. An understanding of extinction risk can also help us protect the diversity that remains today as we enter Earth’s sixth mass extinction. Around 2/3 of shark and ray species that depend on coral reefs are threatened with extinction today. These threats come from fishing, habitat degradation, and climate change, as well as geopolitical factors that leave some species more at risk than others. Overlaying these threats on internal risk factors, like geographic range and specialized feeding strategies, could help identify the species that can’t survive without more protection.