How Mega was Megalodon?
Megalodon... Once a term which was rarely used by the general public, has now become a household name. Thanks to their appearance in popular films, likeThe Meg, many more people are now aware of these prehistoric sharks. However, as is so common in monster-movies loosely based in fact, Megalodons (full name Otodus megalodon and previously Carcarocles megalodon) in popular media are often exaggerated: faster, bigger, more teeth... scarier. So how big was the Megalodon in real life? And how on Earth do you figure out the maximum size of an extinct animal anyway?
Sharks are their relatives are collectively known as "chondrichthyans" because their skeletons are made up of cartilage, not bone. As cartilage breaks down relatively quickly once an animal has died, it is actually quite rare to find shark specimens in the fossil record. The only part of sharks that we commonly find fossilised are the teeth because the hard, mineralised out-layer does not degrade as rapidly as the rest of the body.
Therefore, when we try to determine the true size of the Megalodon, often all we have to go on are the teeth. The largest Megalodon tooth that has ever been found in the fossil record was a whopping 18cm long, which tells us that Megalodons must have been the largest predatory fish that ever lived. But can we estimate their actual size?
Yes, scientists can look at living shark species to estimate what size Megalodon might have been been, because they have found equations which describe the relationship between the size of a tooth and the whole length of the shark (known as "total length" or TL). This is known as "allometry" (Corn et al, 2016). For the calculations to be accurate, scientists must answer two questions: how should we measure the tooth and what living shark shall we use as our model?
Firstly, it is important to know which tooth measure you need to use because for some sharks you must take the measurement from the top to the base of the tooth at a right angle, for others you must take a slanted measure down the side of the tooth, in order to get an accurate estimate of the total body length (Corn et al, 2016).
Secondly, and potentially even more problematic, is deciding which modern day shark is most likely to be most similar to the Meg. For this, a closest living relative is probably the best bet. Yet, here we run into trouble... because there is some debate about which extant sharks are the Meg's next of kin.
In the past, it was thought that Megalodon's closest living relative was the great white shark (Carcharodon carcharias). However, in recent years, scientists have discovered that Megalodon is not the ancestor of the today's great white. In fact, it is now thought that the ancestors of the Meg and of the great white diverged around 100 million years ago. Instead it is thought that the Megalodon eventually gave rise to the mako sharks (Isurus oxyrinchus and I. paucus), making them it's closest living relatives and the great white, just a distant cousin (Cooper et al, 2020).
Using the allometry measurements of these sharks has allowed scientists to make more accurate reconstructions of how mega the Megalodon might have been... they estimated Meg was up to 16 metres long! That is three times the size of today's biggest great whites and even bigger than the largest shark alive today: the whale shark (Rhinocodon typus) (Cooper et al, 2020). Megalodon truly earned it's name!
So why did such an incredibly successful species go extinct? Megalodons had a large distribution throughout all oceans around the globe between 23 and 2.6 million years ago. They predated on other sharks and large marine mammals - ancestors of todays whales. In the past it was thought that Megalodon went extinct due to a prehistoric climate change event, which meant that the waters were just too cold for them to survive. However, recent research suggests that temperature was not directly involved, but several other indirect factors drove them to extinction. First, with the shift in climate, the range contraction of Megalodon into temperate waters meant that prey availability was dramatically reduced. Whatsmore, the emergence of new species which were competitors for the same prey sources, meant that Megalodon simply did not have enough to eat... They likely all starved to death (Pimiento et al, 2016).
One of those new species that outcompeted Megalodon into extinction was none other than the great white shark. It was exactly the same features of the great white's body that make them so successful today which made them so perfectly evolved all those millions of years ago: they have "regional endothermy". This means they are not 'cold-blooded' like the Megalodon, but maintain an internal body temperature around their eyes, brain and muscles above that of the surrounding water. This allowed them to survive in colder waters in the past, and is what gives them their incredible thermal tolerance and cosmopolitan distribution to this day (Pimiento et al, 2016). Meg might have been big, but great white pack heat!
To learn more about shark endothermy, check out Sharks Packing Heat.
Cooper JA, Pimiento C, Ferrón HG (2020). Body dimensions of the extinct giant shark Otodus megalodon: a 2D reconstruction. Scientific Reports, 10, 14596.
Corn KA, Farina S, Brash J & Summers A (2016). Modelling tooth–prey interactions in sharks: The importance of dynamic testing. Royal Society Open Science, 3:8.
Pimiento C, MacFadden BJ, Clements CF, Varela S, Jaramillo C, Velez‐Juarbe J & Silliman BR (2016). Geographical distribution patterns of Carcharocles megalodon over time reveal clues about extinction mechanisms. Journal of Biogeography, 43:8.