Science

Catastrophic End-Cretaceous Extinction Was Not Dramatic for Sharks, Study Shows

Paleontologists have examined tooth morphologies in multiple lineages of sharks that lived during the 27.6-million-year interval around the end-Cretaceous mass extinction event.

Bazzi et al. found that shark-tooth diversity remained relatively constant across the mass extinction event at the end of the Cretaceous period. Image credit: Karen Carr / CC BY 3.0.

Bazzi et al. found that shark-tooth diversity remained relatively constant across the mass extinction event at the end of the Cretaceous period. Image credit: Karen Carr / CC BY 3.0.

Sharks are iconic marine predators that have survived multiple mass extinctions over geologic time.

Their prolific fossil record is represented mainly by isolated shed teeth, which provide the basis for reconstructing deep time diversity changes affecting different shark lineages.

Approximately 66 million years ago, the end-Cretaceous mass extinction eradicated roughly 75% of the animal and plant species on Earth, including whole groups such as non-avian dinosaurs, ammonites, and large marine reptiles like mosasaurs and plesiosaurs. But what happened to the sharks?

In the new research, a team of paleontologists from Uppsala University and the University of New England analyzed the morphology of 1,239 fossil shark teeth, including species in eight existing orders and one now-extinct order.

“These groups include the following: the Galeomorphii orders Carcharhiniformes, Heterodontiformes, Lamniformes, Orectolobiformes; the Squalomorphii orders Echinorhiniformes, Hexanchiformes, Squaliformes, Squatiniformes; and the extinct Synechodontiformes,” they said.

The shark teeth span a 27-million-year period from the Late Cretaceous epoch 83.6 million years ago to the early Paleogene epoch 56 million years ago.

The researchers found that shark dental diversity was already declining prior to the end-Cretaceous, but remained relatively constant during the mass-extinction event itself.

Some groups of apex predators, particularly those with triangular blade-like teeth, did suffer selective extinctions during the period studied, which may have been linked to the extinction of their prey species.

However, other shark lineages increased in dental diversity after the end-Cretaceous.

For example, sharks in the Odontaspididae family, which have narrow, cusped teeth adapted for feeding on fish, showed increases in diversity that coincided with the rapid diversification of finned fish in the early Paleogene.

“This pattern of selective extinctions may reflect an ecological shift from specialist tetrapod predators to more general bony fish diets,” the scientists said.

Their paper appears in the journal PLoS Biology.

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M. Bazzi et al. 2021. Tooth morphology elucidates shark evolution across the end-Cretaceous mass extinction. PLoS Biol 19 (8): e3001108; doi: 10.1371/journal.pbio.3001108

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