Sharks: Profiteers of mass extinction?

The basic sharks that dominate today are better at the end of the Cretaceous

Primeval sharks at the end of the Cretaceous. Many of them survived the mass extinction - but not all ... © Julius Csotonyi
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Winners and losers at the same time: The sharks owe their present biodiversity to a global catastrophe - the mass extinction at the end of the Cretaceous. Because comparisons of fossil shark teeth reveal that the previously dominant shark order was decimated at that time. For this, the ancestors of today's basal sharks spread strong and laid the foundation for their current dominance. However, it is still unclear why some Haigruppen profited and others disappeared.

Sharks were among the top predators of the oceans in the dinosaur era. At that time dominated especially species from the order of the mackerel shark-like (Lamniformes), among them ancestors of today's Great White Sharks, the Mako and sand sharks, but also of about 2.6 million years ago extinct giant shark Megalodon.

Enigmatic shift

Strange, however: Today, this once so dominant order makes up only a small part of the shark species. Far more widespread are now the basal sharks (Carcharhiniformes), which today include about half of all shark species, including tiger sharks, cat sharks, hammerhead sharks and many reef sharks. What triggered this shift within the sharks and when it took place remained a mystery.

One of the reasons: "Shark cartilage skeletons are rarely fossilized, and so our knowledge of these fish is largely limited to isolated shark teeth found by the thousands, " says lead author Mohamad Bazzi of Uppsala University in Sweden. Based on the teeth of 597 fossil sharks, he and his team have now reconstructed what happened to these predators during and after the mass extinction at the end of the Cretaceous.

"We already knew that sharks were one of the few big sea predators that survived this event, " say the researchers. Which Haigruppen nevertheless had to accept losses and which perhaps even benefited, was so far unclear. display

Mohamad Bazzi with the fossil tooth of a shark from the order of the mackerel-type Jordi Estefa

Selective losses

Surprisingly, the study revealed that the variety of sharks remained virtually unchanged during the mass extinction - at least at first sight. Apparently, the sharks as a whole weathered the disaster relatively well. However, further analysis revealed that there were very selective losses in which some forms of mackerel shark became extinct.

"Specific patterns indicate that this asymmetric extinction mainly affected lamniformes with triangular teeth and flat dental crowns, " the researchers report. "Sharks with narrow and high-crowned teeth were barely affected. The transition from the Cretaceous to the Pal ogen thus had a lasting, asymmetrical effect on this order. "

Was the prey spectrum to blame?

The interesting thing is that the basic sharks did not have these selective losses back then on the contrary. Especially the basal sharks with triangular and flat teeth spread after the mass extinction particularly strong. The reason for this is still unclear, but the researchers suspect that these sharks benefited from a shift in food supply after the disaster.

"This shift in the shark tribal lineages coincided on the one hand with the loss of many Kopff er and sea dinosaurs, on the other hand with an explosive increase of bony fishes of the middle trophic level, " explain the scientists. The latter might have benefited the smaller-prey sharks.

"The basal sharks today form the largest group of sharks and it seems that the first steps towards this dominance began 66 million years ago, " says Bazzi. How it went on and whether the prey spectrum was the deciding factor has now to be determined by further studies. (Current Biology, 2018; doi: 10.1016 / j.cub.2018.05.093)

(Uppsala University, 03.08.2018 - NPO)