Sharks are survivors of 5 mass extinctions. So how did 90% of them disappear 19 million years ago? This near-extinction also, grimly, relates to a drastically changing global environment.
Researchers at the College of the Atlantic discovered the event while researching ancient fish and shark populations. The near-extinction applied to “ancient sharks”. These sharks aren’t the same things as our current forms. There are still a few deep sea survivors of the ancient sharks, but not many.
Some things to understand here:
- The extinction event was discovered by measuring the ratio of shark scale fossils to fish. Before the event, there were 1 in 5 prior to the extinction. After the extinction, it was 1 in 100 fossils. That’s an almost unthinkable, massive drop.
- The ancient sharks were far more numerous than sharks are now. That means they were part of a huge ecology which could support large numbers of top predators. Predators need a lot of protein. There were obviously many fish and other prey.
- Numbers of sharks dropped rapidly. 90% of the population and 70% of shark species literally vanished.
- These sharks included the famous Megalodon, the monster “great white” of the past. Megalodon survived this extinction event, but many other sharks obviously didn’t.
- Shark populations still haven’t recovered since this extinction event.
Umm…ah… We don’t know
The net takeaway from this information is likely to be an “event”. Populations of predators don’t vanish for no reason. Typically, they vanish through shortage of food. It’s one of the perks of being at the top of the food chain. There’s no stated indication of a drop in prey populations in this case, though.
(There might be qualifiers to this extinction if there is data to indicate fluctuations in prey populations which could mean ocean food deserts were around for any length of time, which would have decimated shark numbers and breeding populations.)
The researchers think some major global even triggered this drop in shark numbers. Global populations don’t just happen to all vanish at the same time. Sharks are also super-tough. They survived the Permian and Cretaceous extinctions, and the Ice Age. They can and do eat anything they can bite.
Against this – Megalodon was around during this time. A super-predator needs big prey. That tends to support the fact that the rest of the marine ecology was in reasonably good condition. Megalodon could also eat other sharks, not much of a help if there was a famine Limited numbers of one large predator simply couldn’t wipe out so many fish of any species. (Admittedly having super-predators eating anything around wouldn’t help other sharks much.)
Sharks are highly mobile, and can be wide-ranging. Local fluctuations in prey populations wouldn’t have much impact. The sharks could easily follow moving populations. If ocean food was scarce, they could move inshore, as they have done for millions of years.
Some options:
- Oceanic conditions might play a role, but what sort of conditions? What would affect predators, but not affect everything else?
- Invasive species? Researchers don’t believe the sharks were out-competed. These species would have had to face monster sharks, too.
- Obsolescence? Did new prey species outpace the old sharks? Seems unlikely. Sharks hunt in bandwidths of prey; they’re rarely specialists, and not lost if a few prey species are inaccessible.
Maybe…?
A possible issue not yet mentioned is breeding cycle vulnerabilities. If there were huge numbers of sharks, there would be huge numbers of shark fry, too. An influx of new predators would inevitably target abundant shark fry, and perhaps eggs.
These new predators may not have competed directly, but indirectly. Shark fry have a pretty tough time anyway. They’re routinely eaten by sharks and anything else that can get them. An invasion of large numbers of something as big, voracious, and fast as tuna, for example, would be a major hit to any fry population. A nice established breeding cycle, undisturbed for millions of years, would be very exposed to major catastrophic dislocation.
The scenario is that prey were plentiful, but the sharks, being so numerous, became targets themselves. Reducing numbers of fry, in big enough numbers, would naturally eliminate future breeding populations.
To prove this, you’d have to find:
- Lower numbers of shark fry scale fossils
- Added numbers of credible active predators
- A clear statistical relationship between fry populations and subsequent breeding populations.
This mystery needs solving. The possible ecological issues and risks of invasive marine populations are obvious targets for investigation.
- What happens when top marine predators disappear?
- Do you get different predators, or an oceanic food chain meltdown through increases in other types of fish?
- What happens if you’re suddenly confronted with oceans full of vast numbers of ecologically “unfiltered” bottom feeders, soaking up pollutants?
- Do the prey fish eat themselves out of food?
- What if anoxic-zone predators, safe in low-oxygen zones, suddenly explode in numbers, causing fish population crashes? (Jellyfish alone could do that, exterminating fish fry.)
Shark numbers are in steep decline due to overfishing and the revolting shark fin trade. These are the ocean’s premier survivors. Whatever caused this could happen again. Possible outcomes are to put it mildly dubious for humans who get so much food from the seas.
