Sharks are a group of exceptional longevity. They have existed for hundreds of millions of years (420 million years according to some authors), that is to say long before there were trees on Earth! They thus escaped all the massive extinctions that our planet has known, but an ecological accident never yet identified almost caused their disappearance.
Collapse of populations
This was 19 million years ago at the start of the Miocene. And the sharks never really recovered, that’s in any case the conclusion of a study published in the journal Science by Elizabeth Sibert who was at Harvard University during this research. His work indicates that deep-sea shark populations have declined by 90% in abundance and 70% in morphological diversity. To reach this conclusion, the scientist analyzed microfossils called ichthyolites which are made up of fragments of scales and teeth of sharks and other fish. They line the seabed and the sampling of sedimentary cores makes it possible to reconstruct entire sections of underwater history.
For this study, she worked on extractions from two sites, one in the North Pacific and the other in the South Pacific, which allowed her to reconstruct a history of the abundance and diversity of sharks on the last 40 million years. While we find 1 shark fossil for every 5 other fish before this extinction event, there is only 1 in 100, 100,000 years later. Such a collapse was only observed after the crisis that caused the demise of the dinosaurs and yet it seems to have been less violent.
Conclusions to be confirmed
No apparent cause explains this virtual disappearance from the fossil record of sharks. In any case, it cannot be associated with a global climatic event. And this extinction in the oceans does not seem to have a counterpart on Earth either. Moreover, it is even impossible to say that all the oceans have been affected in the same way since the data only concern the Pacific. In addition, fossil records for this period concerning the oceanic environment are very rare. Other studies will therefore have to confirm these conclusions which also affirm that after this extinction their diversity was never fully restored. And that it is the large migratory sharks that then thrive best.
In another article “Perspective” also published in the journal Science, Catalina Pimiento of the University of Zurich and Nicholas Pyenson of the Smithsonian Institute in Washington draw a parallel between this 19-million-year-old story and what is happening in our oceans today. They observe that the pressure induced by mankind, and particularly overfishing, is causing a crisis similar to that of the Miocene with a 71% decrease in shark numbers between 1970 and 2018. The only difference being that the decline current life of sharks is faster than at any time in their history. “Pelagic shark communities have never recovered from a mysterious extinction event 19 million years ago; the ecological fate of what remains is now in our hands”, They write.