Our oceans, seas, and even rivers are populated by over 500 species of sharks. They are incredibly efficient hunters and have a fierce reputation yet some of them are endangered and could face extinction in the near future. This sounds like a depressing but modern dilemma that is faced by many other animal species thanks to the activities of we humans. Yet, recent research has shown that the ancient sharks faced a similar threat to their survival over 19 million years ago and they very nearly didn’t make it!
There is evidence that a hugely significant “extinction event’” took place that altered the destiny of sharks forever. What we cannot say is whether this was a rapid event that lasted just a day or an ongoing challenge that could have lasted thousands of years.
Shark Extinction Research
Sharks are an ancient species and have been on the earth for around 400 million years. It is reasonable to assume that, over this time, they would have faced their fair share of challenges. Thanks to some groundbreaking research we have learned about one particular challenge that almost killed every last one of them.
Scientists have been investigating some tiny fossilized shark scales (denticles) and fish teeth that are only visible with a microscope. These fascinating finds are called ichthyoliths and they are found deep under the ocean floor in sediments that have been settling for hundreds of millions of years. This is like a record of what was happening on earth at the time each layer settled – every layer corresponds to a certain period in the earth’s history.
This particular research has been reported by Elizabeth Sibert who is a postdoctoral fellow at Yale University’s Institute for Biospheric Studies and her colleague Leah Rubin, who was an undergraduate at the College of the Atlantic in Bar Harbor in Maine when the research was conducted. They analyzed sediment that had been collected from sites in the North Pacific and South Pacific oceans that were not influenced by ocean circulation or currents. Their research examined both the quantity and diversity of shark fossils.
They compared sediment content over the years and pinpointed a time, around 19 million years ago when numbers of a particular type of shark fossil dropped by 90%. The next challenge was to find out if diversity had also declined. To do this they grouped the tiny fossils into 80 different categories of shapes and tracked the numbers of each one. They discovered that 70% of the shapes had disappeared completely! Clearly, this “event” had affected both numbers and diversity in a very serious way leaving the researchers “shocked.”
Inevitably, comparisons are drawn with the catastrophic loss of the non-avian dinosaurs that occurred 66 million years ago (the Cretaceous period) and that we now know was triggered by a large asteroid hitting the earth.
What Caused the Mass Shark Extinction?
This fascinating research raises an important question. What could have caused such a dramatic downturn in the fortunes of ancient sharks? At present, we do not know the answer but there are a few different interpretations of the results of the research.
Could It Have Been a Localized Event?
Scientific research is open to interpretation and startling findings like this often trigger a debate in the paleontology community. Romain Vullo, a paleontologist at the French National Center for Scientific Research (CNRS), has put forward a potential alternative explanation. He notes that this pattern has not been seen so far in other shark fossil records from around the world. Speaking to Live Science, he said, “…further data from other regions in the world would be required to confirm the interpretation of the authors.” Because this phenomenon has only been seen in sediment from the South Pacific, it could represent a local occurrence that left the shark population elsewhere intact. This means it was not a global extinction event.
However, Sibert defends the research saying “While we don’t have good data from this very specific time interval all over the world, we do have a lot of ‘before’ extinction snapshots and ‘after’ extinction snapshots from all over the world. Before the extinction, there are lots of shark scales, and after, there are not.”
Could the Effect Have Been Worse in Some Regions?
Another interpretation is that sharks suffered losses globally during this time but fared worse in some locations compared to others. Sibert explains that “it is possible that this extinction was strongest in the open ocean environment, and not in the coastal environment.” This would help to explain why these shark fossils are not found in sediment younger than 19 million years.
More research is needed to establish if coastal species and perhaps other animals and ecosystems were also affected at the same time.
At the moment, scientists seem to agree that there is no evidence of a severe environmental change that could have accounted for the extinctions. There is no known global climate event at the time but that does not mean that we won’t discover that there was one as research progresses.
Severe climate changes and asteroid impacts are potential causes of this type of event.
Species can become extinct because they are hunted by predators. Sadly, humans have hunted several species to extinction but obviously, humans were not around at this time.
Also, the rapid increase in species like the tuna, billfish, and seabirds (potential prey for sharks) was not seen until several million years after this event.
It really is a mystery that will be researched over the coming years. Up until now, the sediment relating to this time period has not been found in many sediment cores.
As Catalina Pimiento (University of Zurich and Swansea University, UK) and Nicholas D. Pyenson,(Smithsonian Institution, Washington, D.C., and Burke museum in Seattle, Washington) wrote in Science journal when the findings were published, “Our view of the ancient oceans is constrained by the environments recorded in the rock record, which are often limited to shallow-water deposits that provide little insight into the oceanwide history of pelagic [oceanic] faunas.”
Lessons for Today
Sadly, there are important lessons for us to learn from these findings that are relevant to the precarious position of many species today. Over the last half a century, the number of sharks in our oceans has declined by 70%. There is no mystery about what is causing this. It is due to human activity, specifically over-fishing, climate change, and the warming of the oceans. A quarter of all shark species are under threat of extinction.
This is not just a problem for sharks. They make up part of a complex ecosystem that is finely balanced. Any large change in the numbers of one organism could have catastrophic repercussions for many others and most of these changes are not reversible.
We can see that the extinction event cataloged in this research changed the nature of the oceans forever. Sharks have never recovered from this event, even though it happened 19 million years ago.
Sadly, we are now facing similar losses if we cannot halt and reverse the environmental damage that we have caused.
New research into previously unexplored sediment from deep below the ocean floor has indicated that sharks have previously suffered a mass extinction event. Even though there is some debate regarding the exact nature and extent of the event that happened around 19 million years ago, it appears to have altered the diversity of sharks forever.
We need to learn the lessons of how profoundly extinctions can affect the delicate ecosystems of our planet and find ways to protect every species on our earth. If we don’t, sediment cores from this century are going to tell an equally shocking story!
FAQs (Frequently Asked Questions)
When did this extinction event take place?
The sediment that showed this event took place dates from around 19 million years ago.
What caused the extinction event?
That is not clear. Scientists are still trying to work that out.
What can cause an animal to become extinct?
There are several factors that can work alone or together to drive a particular species to extinction. It can be a natural event such as an asteroid hitting the earth. This can cause such a dramatic change in climate that certain animals cannot survive. Extinction can also be caused by loss of habitat (and food), predation and climate change caused by human activity.
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- Science, Available here: https://www.science.org/doi/10.1126/science.aaz3549
- , Available here: https://www.livescience.com/mysterious-shark-extinction-19-million-years-ago.html