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Total Wipe-Out!

Incredibly, sharks have been swimming about in our oceans for around 440 million years! Evolving earlier than trees and before Saturn had rings... prevailing through several global extinction events (including the asteroid that killed the dinosaurs)... sharks are true survivors! However, recent research has shown that at one point, sharks came very close to being completely wiped out! So what happened? When did it happen? How did they manage to bounce back? And is there a lesson we can learn about the extinction-risk of our sharks today?


Scalloped hammerheads are amongst the most Critically Endangered oceanic sharks alive today (Pimiento & Pyenson, 2021)


On the Record

Sharks and their relatives have been around for a long, long time! Scientists suspect sharks first appeared in our oceans some 440 million years ago. They have subsequently adapted and changed, and survived numerous extinction events, until they eventually became the sharks we know and love today (Ebert et al, 2021).


Fossils of complete shark skeletons like this Lebachacanthus senckenbergianus treasured by the Urzeitmeer-Museum are very rare (Image Credit: Yohanes Wahyu Nurcahyo / WikimediaCommons)

We don't know this thanks to some secret time machine! Scientists must rely on fossils to uncover the secrets about ancient sharks and their relatives. But this is not an easy task! On top of the challenges associated with (literally) digg through the ancient past for clues about extinct animals, sharks and rays (known collectively as "chondrichthyans" or cartilaginous fishes) offer an additional challenge for palaeontologists... It is incredibly rare to find their fossils because of the make-up of their skeletons.


When an animal dies, the flesh decomposes quite quickly, but it takes a long time for bone to break down. This is why it is uncommon to see a fossil showing the shape of an animal's whole body, but more common to find a fossilised skeleton instead. However, as sharks and rays have skeletons made of cartilage, not bone, their skeleton also degrades relatively quickly. So it is also rare to find fossils of sharks. The only part of a their body that is prevelant in the fossil record is their teeth, as the decomposition is slowed by their hardened enamel surface (Ebert et al, 2021).



Something Old, Something New

However, recent research has uncovered a pioneering new method for studying ancient sharks. Scientists have learned that they can find microscopic structures called "ichthyoliths" in the fossil record. These are very small pieces of shark skin and teeth that become preserved in the seafloor as they are naturally shed throughout a shark's lifetime (Pimiento & Pyenson, 2021).

This is possible because a shark's skin is actually covered in tiny, modified-teeth called "dermal denticles". As these minute scales are armoured (like the enamel covering their teeth) they preserve quite well and can be retrieved by drilling out sediment cores from the ocean floor. Researchers can then look at the layers of the ichthyoliths through time and learn about the abundnace and distributions of ancient sharks, long dead (Pimiento & Pyenson, 2021).


Fossilised dermal denticles can be tell us a lot about ancient sharks (Sibert & Rubin, 2021)

Total Wipe-Out (Almost!)

Thanks to this new technique scientists have uncovered there was an enormous extinction event that hit sharks some 19 million years ago (during the Miocene period). After this wholesale mass extinction, shark abundance dropped by as much as 90% and there was a 70% reduction in shark diversity across many different lineages all around the globe (Pimiento & Pyenson, 2021; Sibert & Rubin, 2021).


The Megalodon lived between 23 and 3.6 million years ago, meaning they lived through the the Miocene mass extinction event that wiped out many other shark lineages (Image Credit: Herschel Hoffmeyer / Shutterstock)

We had no record of this mass extinction event until now and, as we have no evidence there was any major climactic change at this time, scientists are baffled by what could have caused so many species of sharks to die off (Pimiento & Pyenson, 2021; Sibert & Rubin, 2021).


What is fascinating about this discovery is that it seems "pelagic" species (those that live offshore, as opposed to "coastal species") never really recovered from this event. Whilst the modern shark lineages began to appear some 2 - 5 million years after the mass extinction, the loss of so many different species of sharks completely shifted the makeup of shark communities, and this has reverberated through evolutionary lineages all the way through to the present day. As a result, our modern sharks may only be a tiny fragment of what they once were (Pimiento & Pyenson, 2021; Sibert & Rubin, 2021).


"Pelagic shark communities never recovered from a mysterious extinction event 19 million years ago"

- Pimiento & Pyenson, 2021


Hexanchiform sharks like this bluntnose sixgill are amongst the oldest sharks lineages, dating back to the time of the dinosaurs (Image Credit: NOAA Ocean Explorer / WikimediaCommons)

History Always Repeats Itself

Discovering this enormous extinction event is a humbling reminder that sharks are not supernaturally resilient - they cannot survive anything and everything, and it very possible to push them to the brink of extinction.

Today, we are doing just that.


Modern human exploitation of the environment is so extreme and so unsustainable, that our era has been dubbed the "Anthropocene" - a geological era in time where human activity is the dominant influence on climate and the environment (Pimiento & Pyenson, 2021).


There are striking similarities between the Miocene mass extinction event and Anthopocene shark declines. Where shark abundance decreased by 90% 19 million years ago, today, thanks to massive overfishing, shark abundance has decreased by 71% since 1970! It is estimated that if we carry on as we are, we will drive 31% species of pelagic sharks to extinction and see a 77% decrease in shark diversity globally (Pimiento & Pyenson, 2021).


"The parallels between this ongoing crisis and the extinction of pelagic sharks more than 19 million years ago feels like déjà vu... except that this time we know that the decline of sharks is happening at a faster rate than at any other in the history of the planet"

- Pimiento & Pyenson, 2021


There is no more time to wait. We must act now to save our threatened sharks. Could we ever forgive ourselves if we, and we alone, were the cause of the second shark mass extinction? We must see this example from the past as an opportunity to learn and make every effort we can to change for the better - to work to drawing our threatened sharks back from the brink of extinction NOW.



To learn more about how sharks have survived other mass extinction events you can check out I Will Survive.


References

Ebert DA, Dando M& Fowler S (2021). Sharks of the World: A Complete Guide, Second Edition. Princeton University Press: UK. IBAN: 978-0-691-20599-1.


Feld K, Kolborg AN, Nyborg CM, Salewski M, Steffensen JF & Berg-Sørensen K (2019). Dermal Denticles of Three Slowly Swimming Shark Species: Microscopy and Flow Visualization. Biomimetics, 4:2, 10.3390/biomimetics4020038. Access online.


IUCN (2023). International Union for the Conservation of Nature Red List of Threatened Species. Access online.


Pimiento C & Pyenson ND (2021). When sharks nearly disappeared. Science, 372:6546, 1036-1037. Access online.


Sibert EC & Rubin LD (2021). An early Miocene extinction in pelagic sharks. Science, 372:6546, 1105-1107. Access online.


By Sophie A. Maycock for SharkSpeak.

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