Sharks got smaller after mass extinction event

8 June 2017

Shorfin mako shark upper jaw

Scientists studied fossil and modern shark teeth to compare the size, diet and feeding habits of the animals before and after the end-Cretaceous mass extinction event 66 million years ago. The long, thin teeth of the modern shortfin mako shark, Isurus oxyrinchus, can be seen in this close-up of the shark's upper jaw.

Fossil teeth show that sharks shrank in size and changed their diet after a major extinction event 66 million years ago.

Research led by Museum palaeontologists found that sharks and other large predators were severely affected by environmental upheaval at the end of the Cretaceous Period - the same extinction that wiped out the dinosaurs.

In a paper published in PLOS ONE, the team studied the tooth shapes of modern and extinct shark species to compare sizes, diets and feeding habits.

The research was led by Museum palaeontologists Rachel Belben, Prof Richard Twitchett and Dr Zerina Johanson, as well as Dr Charlie Underwood of Birkbeck University.

Prof Twitchett says, 'Shark teeth are common fossils in marine rocks all over the world, because they are hard and because an individual shark produces hundreds in its lifetime.

'Tooth shapes reveal important details about the shark's diet and environment, so by studying these fossils we are able to show how these important marine predators responded to past episodes of environmental change.'

Such studies help us understand the wider ecological consequences of the extinction and the dramatic decline that shark species are currently experiencing.

Fossil tooth of the extinct shark Serratolamna maroccana

Fossil tooth of the extinct shark Serratolamna maroccana, which was the most common shark living in the seas of Morocco before the end-Cretaceous extinction © Rachel Belben
 

Shrinking seafood

Perfectly preserved fossil teeth reveal that seas teemed with a wide variety of sharks in the Cretaceous Period, 145 to 66 million years ago. This included many large predators with serrated teeth that would have fed on other large fish and marine reptiles.

The largest sharks vanished after the extinction. The teeth of the survivors were much smaller and less varied in shape than they had been before.

The fossil teeth the researchers examined were collected from rocks in the deserts of Morocco. It is one of the best places in the world to study the effects of the end-Cretaceous mass extinction on marine ecosystems.

Researcher carrying out fieldwork in Morocco

Sieving for fossil shark teeth in Morocco
 

Dr Underwood says, '66 million years ago, the deserts of Morocco were under a warm, shallow sea that was full of sharks, rays, other fish and marine reptiles - a fantastic mix of predators and prey.

'Thousands of fossil bones and teeth were carefully collected and brought back to the lab for identification and analysis, which revealed exciting new details of how the end-Cretaceous extinction affected marine food webs.'

Chart showing the relative sizes of extinct and living sharks

This chart shows the relative sizes of the sharks included in the study. It illustrates how there was a dramatic drop in the size of sharks after the end-Cretaceous mass extinction, before sizes eventually increased again in modern sharks, such as the great white Carcharodon carcharias.

Goodbye to apex predators

The team also measured the sizes and shapes of teeth from the Museum's collection of modern sharks, both young and adult, including the great white, thresher and sand tiger.

This confirmed that the shapes of shark teeth are closely linked to diet and feeding habits.

By comparing modern teeth with the fossil teeth, the scientists were able to find out the diet of the extinct sharks and show how feeding was affected by extinction. 

Large predators at the top of the food chain, with similar feeding habits to great whites, disappeared after the extinction.

The warmer seas of the post-extinction period were dominated by much smaller sharks, with diets similar to those of young sand tiger sharks living today, which eat small fish and squid.

Fossil tooth of Striatolamia whitei

Fossil tooth of the now-extinct shark Striatolamia whitei. It was the most common shark living in the seas of Morocco in the Danian (the geological stage directly after the mass extinction event) and had a diet similar to today's juvenile sand tiger sharks. © Rachel Belben
 

Belben says, 'It was exciting - and sometimes a little tricky - to measure the teeth of great white sharks from specimens in the Museum collections.

'Taking the same measurements of hundreds of beautiful fossil teeth was very time-consuming, but the results really brought the past to life by allowing me to recreate the diet of these long extinct species.'

Sharks remain a crucial part of the marine ecosystem and are presently under threat from overfishing and global warming.

This paper is one of few studies on the impact of past climate change and extinction on shark species, but may hold clues to the future.

'One of the most striking things about the fossil teeth is the really small size of the ones from right after the extinction,' says Belben.

'Marine scientists are predicting that as present oceans get warmer all fish, including sharks, will become smaller, and this is exactly what we see in the fossils.'

  • By Katie Pavid

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