TORONTO -- Shark populations experienced a period of mass species extinction nearly 19 million years ago, and researchers who discovered it say they have no idea why.
, conducted by earth scientists from Yale University and the College of the Atlantic, found that shark populations experienced a level of extinction that wiped out more than 70 per cent of the total population, indicating a level of species decline not seen since the extinction of dinosaurs.
鈥淲hat I looked at in the original study that told us that something must be up here, was that I looked at the ratio of fish teeth to shark scales in rudimentary sediments,鈥� said Elizabeth Sibert, a Hutchinson postdoctoral fellow at Yale University and the lead author of the study, in an interview with 麻豆传媒. 鈥淚t was very stable, it was one shark fossil for every five fish fossils for 40 million years and this event disrupted 40 million years of vertebrate community stability.鈥�
Sibert came to this discovery after attempting to get a better understanding of how the shark and fish species respond to global change and how the species鈥� responses compare to one another.
She collected fish and scale samples from the last 85 million years to help get a sense of what the variability of those populations looked like.
What she ended up finding was a sudden, unexplained drop in shark populations that occurred roughly 19 million years ago.
鈥淲e actually got some sediments samples from a location that鈥檚 been in the same general environment for the entire last 80 million years and we got a record of shark and fish abundance through that time and what we found was that, to everybody's shock and surprise, there was a big drop off in shark abundance about 19 million years ago and there鈥檚 nothing that we know about in earth鈥檚 history that鈥檚 a big deal 19 million years ago,鈥� says Sibert.
Sibert says that this mass extinction of sharks was two times as big as the extinction the species faced in the Cretaceous-Paleogene mass extinction, which wiped out the dinosaurs and roughly three-quarters of the plant and animal species on the Earth.
鈥淚 like to say that we stumbled upon this entirely accidentally,鈥� says Sibert. 鈥淲e went looking during a time where we thought things were going to be incredibly stable and boring and yet we found this massive change, the biggest change in the last 66 million years and this change is really the only change of this nature for sharks and fish in the open ocean.鈥�
Leah Rubin, co-author of the study and an incoming PhD student at the State University of New York College of Environmental Science and Forestry, says that this discovery is interesting because there is no clear indication as to why exactly sharks faced this level of decline.
鈥淭he crazy thing about this extinction is that there are not any notable climate events or other extinctions that are happening at the same time so it鈥檚 really interesting to think about what would impact sharks but not other fish,鈥� says Rubin in an interview with 麻豆传媒 . 鈥淎nd that鈥檚 the million dollar question: why? We don鈥檛 have that answer and honestly, it鈥檚 kind of exciting to say that this paper sparked a lot more questions.鈥�
Sibert says that this discovery is not only important to better understanding the evolution of sharks, but can help inform modern scientific discussion around shark and other marine life extinction.
鈥淭o me the thing that鈥檚 really compelling here and the thing that鈥檚 really worth taking away is that predators, large marine organisms, things like sharks and in today's oceans things like whales, and tuna and swordfish and these other large migratory organisms, play a really important role in marine ecosystems and if you lose 90 per cent of them, which really looks like we鈥檙e currently doing, that can really completely change how an ecosystem works,鈥� says Sibert.
Around the world, , with some studies suggesting that around 100 million sharks die annually due to overfishing, shark finning and habitat destruction.
鈥淭he truth is that we do have sharks going extinct right now and so having this data can give us a foundation for understanding and predicting what's going to happen in the future so that we can protect communities that rely on fisheries for their source of protein, their livelihood and just generally, for us to have some more autonomy in going forward and knowing what's going to happen,鈥� says Rubin.
Sibert says that the extinction the species experienced 19 million years ago reflected a massive change that she fears will happen again if shark populations continue to decline.
鈥�19 million years ago we saw this big tipping point in marine ecosystem structure and I think that today with what we鈥檙e currently doing with overfishing and whaling and shark finning and all of these things that are removing the large predators from the ecosystem, I worry that we are rapidly heading towards a new tipping point that might send the oceans to a new and previously unseen ecosystem structure and state,鈥� Sibert says.
Sibert says that the next step is to learn more about the extinction to figure out why exactly it happened, how it affected the evolution of sharks and what other species were impacted during this time as well.
