The Enormous and the Unobservable

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Caroline Mckeon

Deciphering the extinction of the world’s biggest shark.

When she isn’t writing for the guardian, featuring in documentaries or championing science communication, Dr Catalina Pimiento can be found in field sites and natural history museums, investigating the biggest shark that ever lived.

Catalina in the field
Paleontologist Dr Pimiento in the field Image Source

Megalodon (Carcharocles megalodon) went extinct 2.6 million years ago. Biology can be less than straight forward at the best of times, but how do you research ecology when your study species no longer exists?

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Size comparison of Megalodon vs today’s biggest sharks Image Source

To address biological questions guarded by geological time, palaeontologists must carry out detective work. Dr Pimiento uses fossils, modelling and comparisons with potentially analogous systems found today to gather evidence for theories on the ecology, evolution and extinctions of prehistoric sharks.

One of the particular challenges of researching Megalodon is that sharks, as elasmobranchs, have cartilaginous skeletons; generally all that fossilises are the teeth.

Dr. Clifford Jeremiah Inside Reconstructed Megalodon Jaws
Megalodon jaw and Great White jaw for scale Image source
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Megalodon teeth Image Source

Fortunately, the hydrodynamics of swimming mean many benthic aquatic creatures evolve convergent body plans, helping researchers to use the shape and size of teeth to extrapolate morphological information.

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Convergent evolution of general body plan for aquatic species with similar life strategies        Image Source

Along with other lines of investigation, these expected morphologies can lead to surprisingly detailed insights into of many aspects of Megalodon biology; from how they fed to why they went extinct.

Prior to Dr Pimiento’s research, prominent theories suggested a global temperature change was responsible for Megalodon extinction. Using a combination of climate and distribution modelling, Pimiento showed that though the timing of the two events correlated, ocean temperature was not the cause.

Like the Great Whites of today, Megalodon is expected to have been mesothermic; (able to partially regulate it’s body temperature), enabling it to survive in a range of ocean temperatures at the time of its demise.

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Map of ocean temperatures suitable for Megalodon in the Pliocene. Colour range represents a scale from unsuitable (white) to optimal (red) Image Source

However, this apparent advantage seems to be the reason behind its downfall. The high energy demand of maintaining a mesothermic metabolism could be why Megalodon, among other species, was unable to cope with ecological changes in the world’s oceans, caused by the fall in global sea levels following the temperature drop.

Importantly, Pimiento showed this Pliocene extinction event resulted in not only a loss of species, but loss of functional traits.

Lower trophic levels have higher functional redundancy; multiple species overlap in ecological roles. Higher trophic levels – containing apex predators like Megalodon – are not as robust; loss of a single species is more likely to lead to a larger overall loss of ecosystem function. Losing one apex predator could be as bad as losing multiple species further down the food web, potentially destabilising an ecosystem towards ecological collapse.

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Functional diversity in molluscs Image Source

No apex predators have yet been lost from modern oceans, but fauna is still recovering from a “recent” extinction event, and so is vulnerable.

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A pod of Orcas – important apex predators in today’s marine environment Image Source

More than a fascinating puzzle about a charismatic species, the no-longer-mysterious extinction of Megalodon serves to highlight the complex and unexpected responses that species can have to changes in their environments, and how we must understand and protect our ecosystems, so as not to lose the oceanic giants of today.

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Humpback whale and calf Image Source

References

  1. Cover image source: https://www.nationalgeographic.com/magazine/2016/06/sharkspeciesfamilytreeoceanecosystempredator/
  2. Pimiento, C., MacFadden, B. J., (2016). Geographical distribution patterns of carcharocles megalodon over time reveal clues about extinction mechanisms. Journal of Biogeography, 43(8), 1645-1655. doi:10.1111/jbi.12754
  3. Pimiento C, Clements CF (2014) When Did Carcharocles megalodon Become Extinct? A New Analysis of the Fossil Record. PLoS ONE 9(10): e111086. doi:10.1371/journal.pone.0111086
  4. Pimiento, C., Griffin, J. N., Clements, C. F., Silvestro, D., Varela, S., Uhen, M. D., & Jaramillo, C. (2017). The pliocene marine megafauna extinction and its impact on functional diversity. Nat Ecol Evol, 1(8), 1100-1106. doi:10.1038/s41559-017-0223-6

 

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