Sharks across mass extinctions

Sharks and their relatives are often described as survivors of every ancient catastrophe. That phrasing hides an important fact: cartilaginous fishes have repeatedly lost lineages and ecological roles. The group includes sharks, rays and chimaeras, whose skeletons are made largely of cartilage. Teeth and tiny dermal denticles preserve well, but the rest of the body is much less likely to fossilise.

This uneven record makes survival difficult to measure. A tooth-bearing rock layer can document a community in detail, yet a gap may reflect missing rocks or collecting as well as extinction. To understand a crisis, researchers compare many well-dated sites and distinguish the disappearance of species from the persistence of broader lineages.

Deep history and repeated change

Cartilaginous fishes are an ancient branch of jawed vertebrates. Their fossil history includes major changes in body form and habitat, from early marine communities to later sharks and rays with specialised feeding styles. The term “shark” is often used casually for the whole group, but rays and chimaeras followed their own evolutionary paths.

Studies of Paleozoic fossils suggest that chondrichthyan diversity changed during crises such as the end-Devonian and end-Permian events. The completeness of the Paleozoic record is comparatively low, so estimates depend strongly on how isolated teeth and other fragmentary remains are classified. Researchers continue to refine which groups disappeared, which survived and how quickly communities recovered.

The end-Permian crisis

The Permian–Triassic extinction, about 252 million years ago, was the most severe known mass extinction in the marine record. Cartilaginous fishes did not simply pass through it unaffected. Fossil studies report substantial shifts in diversity across the boundary, followed by survival and later diversification among several lineages. The exact scale of losses is difficult to compare across groups because shark fossils are often isolated teeth and rock records differ between regions.

A conclusion that a whole group “survived” can therefore be true at the level of ancestry while concealing the loss of many species. The major mass extinctions differed in timing, environment and severity, and their effects need to be evaluated separately.

Sharks, rays and the K–Pg boundary

The end-Cretaceous event provides one of the better quantified examples. A global analysis of more than 3,200 fossil occurrences representing 675 shark, skate and ray species found that elasmobranch diversity declined by more than 62 percent across the Cretaceous–Paleogene boundary. The losses were selective: species with broad geographic ranges and those restricted to high latitudes had higher survival in that dataset.

This result challenges the idea that sharks were protected from the impact because they lived in water. Many marine food webs were disrupted, and species did not all face the same conditions. A separate study using fossil teeth and shark denticles from deep-sea sediments found a strong change in the balance between sharks and ray-finned fishes after the extinction. The surviving sharks remained important, but the broader marine community was reorganised.

Local records can look different from global averages. At some boundary sections, family-level diversity appears to recover earlier than species-level diversity. At others, faunal turnover is pronounced. Researchers therefore report the geographic scope, time resolution and taxonomic level of each result instead of treating one formation as the worldwide pattern.

Why teeth dominate the story

Sharks replace teeth throughout life, so a single animal can contribute many fossils. Tooth shape carries useful information about feeding and can help distinguish taxa, but isolated teeth may be difficult to identify to species. Changes in sea level and sediment supply also affect how many teeth are collected from a rock interval. A rise in fossil counts does not automatically mean a population became more abundant.

More complete skeletons, associated tooth sets and carefully sampled boundary sections improve the picture. Researchers combine them with stratigraphy and statistical methods that account for missing time and uneven rock volume. This approach has revised earlier claims that sharks experienced little extinction at the K–Pg boundary.

Persistence without immunity

Cartilaginous fishes have survived through deep-time crises because some lineages persisted and later diversified. That does not mean they were unchanged or universally resilient. Some species vanished, others shifted in abundance or habitat, and new groups occupied ecological roles left open by extinction.

The history of Megalodon offers a much later example of a successful shark lineage that eventually disappeared. Across all these episodes, fossils support a more precise conclusion than “sharks survived everything”: the group endured, but its diversity and ecological structure were repeatedly transformed.