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MAMMAL NAMES | 'Largest Animals Ever'? Part II:

Ireland's Blue & Fin Whales VS Ancient Sea Monsters

The giant shark Megalodon that lived during the Pliocene (c. 5.3–2.6 million years ago) is a contender for the title of 'second largest animal ever'

Last month we measured the claims of Ireland’s blue and fin whales to the titles of ‘largest animal ever’ and ‘second largest animal ever’ against some stiff competition in the form of the largest dinosaurs to ever live on land – the mighty sauropods. However, we found that, on the balance of all evidence, even the gargantuan dimensions of these colossal land creatures came up short when compared to those of these giant baleen whales that are still swimming through Ireland’s seas as you read this, at least in terms of weight. We realised, then, that to find goliaths that could truly threaten these gigantic whales’ claims to the greatest titles in the animal kingdom, we would have to turn our eyes to the domain they rule so imperiously today – the sea. So, this month we will immerse ourselves in the waters of ancient oceans, scanning the waves for the very greatest of the ancient sea monsters.

Our first port of call is to go back to that wonderful part of the past we explored last month, the Age of Dinosaurs, albeit to an earlier stage in this great age than we encountered before. Our investigations of the largest dinosaurs to ever live introduced us to forms that lived during the Jurassic period (c. 201–145 million years ago) and the Cretaceous period (c. 145–66 million years ago). However, to meet the greatest sea reptiles that ever lived, we must go back to that period which preceded them, the Triassic, that spanned from around 252 to 201 million years ago.

During the Triassic, Ireland was just one part of a gigantic landmass that included almost all the lands of the Earth – the supercontinent Pangaea ('Pangaea' meaning ‘all land’). In the later part of this period, if you set out from Ireland and travelled southwards overland for thousands of kilometres you would eventually reach Argentina, which, for the dinosaurs, was the birthplace of the giants.

Map of Pangaea with approximate location of Ireland marked with red dot

Last month we saw that many of the largest sauropods that ever lived roamed Argentina during the Cretaceous, but an earlier, independent experiment in gigantism amongst the larger group they belong to, the sauropodomorphs, also took place in this land in the Triassic. Although it was long believed that the dinosaurs did not reach weights of 10 tonnes or more until the Jurassic, in 2018 it was discovered that a form that did weigh around 10 tonnes stomped across Argentina in the later Triassic, around 215 million years ago, and it has been named Ingentia prima, a Latin name which essentially means, quite suitably, ‘first giant’.

The dinosaurs, then, were larger during the later Triassic than was once thought, although it is clear that they still had quite a way to go before they would reach the earth-shaking sizes attained by the very largest in later periods. However, around the time Ingentia prima roamed lands far to the south, in the seas around Ireland there could be found a race of reptiles that had already reached the largest sizes that would ever be reached by a marine reptile.

These were the ichthyosaurs, a group of reptiles whose exact relationships to the dinosaurs and other reptiles is as yet unknown. What is clear, though, is that these marine reptiles had evolved from four-legged terrestrial ancestors who had returned to the sea, a path taken millions of years later in the evolution of the whales, as we saw in a previous post – a superb example of convergent evolution. And, just like the whales who emulated them, the ichthyosaurs evolved an extremely fish-like shape – indeed this is where the name ‘ichthyosaur’ comes from, simply meaning ‘fish-lizard’.

Excellent illustration depicting the convergent evolution of fish-like shapes in the ichthyosaurs and the whales from terrestrial ancestors (Museum of Northern Arizona)

Although some of the ichthyosaurs, including a few fossil forms found in Co. Antrim in Ireland’s northeast, only measured around two to three metres in length, making them about the same size as a bottlenose dolphin, other forms could be far, far larger, as very recently it has been discovered that they may have reached dimensions comparable to those of the largest of the living baleen whales. And, as it turns out, these gigantic forms, the very greatest of the ichthyosaurs, are known to have darkened the ancient seas off the Irish coast.

Up until this year, the largest known ichthyosaur in the world was a form called Shonisaurus sikanniensis, fossils of which have been found in British Columbia in Canada. This was a gigantic creature, measuring up to 21 metres in length, but in 2018 a new study revealed that ichthyosaurs powering through the seas around Britain may have been significantly larger.

This study reported the discovery of a bone from the lower jaw of an ichthyosaur which had been found at a place called Lilstock, in Somerset in southwest England, and which dates to somewhere within the last seven million years of the Triassic (c. 208–201 million years ago). Comparing the size of this bone to that from other large ichthyosaurs led the authors of this study to conclude, albeit tentatively, that the Lilstock giant ichthyosaur was larger than Shonisaurus sikanniensis, measuring, by some estimates, between 22 and 26 metres in length. As they put it themselves in the study, a length of 26 metres is ‘approaching the size of a blue whale’.

What is more, this ichthyosaur fossil find in Lilstock spurred the reappraisal of a number of other fossils found in southwest England that were previously attributed to dinosaurs or other land reptiles. These, from Aust Cliff in Gloucestershire, are now thought to belong to ichthyosaurs also, and one of these ichthyosaurs could be even larger than the Lilstock form – as much as 30 per cent larger or more. Thus, if we take the Lilstock ichthyosaur to measure 22–26 metres in length, this Aust ichthyosaur would stretch to an incredible 28.5–34 metres, making it possibly longer than a blue whale!

Being known from only bone fragments, though, these estimated lengths are, of course, far from certain. And, for the same reason, making any estimate of the weight of these massive marine reptiles would be fraught with hazard. However, these fossils do suggest that these sea dragons may have been enormous beasts.

We cannot say if they would truly have threatened the claims of the blue whale and fin whale to their heavyweight titles, and the fact they were likely long, slender animals, as Shonisaurus sikanniensis was, argues against them being true contenders for the blue whale’s crown at least. Yet the fin whale may need to look over its shoulder, and if more complete fossils are found in the future that confirm the upper size estimates for these giant ichthyosaurs, this cetacean inhabiting Ireland’s modern seas could well have to cede its title of ‘second largest animal ever’ to them.

It is truly fascinating to picture marine reptiles of such gargantuan size inhabiting Ireland’s ancient seas, while early dinosaurs zipped across its surface and early pterosaurs flew through its skies. However, we do not have to travel all the way back to the Age of the Dinosaurs to find mighty contenders for at least the second greatest title in the animal kingdom. In fact, we need travel back only a few million years to the Pliocene Epoch (c. 5.3–2.6 million years ago) to meet an ancient sea monster that could truly take a bite out of the fin whale – the giant shark, Megalodon.

Today, the largest predatory shark in the oceans is the famous, or unfairly infamous, great white shark, Carcharodon carcharias. Megalodon may have been a close relative of the great white, as some consider it too to be a member of the genus Carcharodon, Carcharodon megalodon, but as others insist in placing it in a different genus, considering it to be Carcharocles megalodon, this is why it is often referred to simply as Megalodon alone. Whatever its true relation to the modern great white shark, though, it is very clear that it was an order of magnitude larger than this modern predator.

The largest great white shark ever recorded measured 6.4 metres in length and just over 3.3 tonnes in weight. However, Megalodon could measure, by one estimate, up to 17–20 metres long (so not quite as large as the 25 metres it is depicted in the 2018 film, the Meg), and as it was more robustly built than the great white shark, it seems to have reached staggering weights. Estimates of its weight vary from just under 50 tonnes to over 100 tonnes! Thus, if it reached the upper weight estimate (just over 103 tonnes), it could possibly wrest the title of ‘second largest animal ever’ from the fin whale if the maximum weight of this cetacean turned out to be closer to the 90-tonne estimate than the 117-tonne one.

What a sea monster we are left to picture in the form of Megalodon, then. A giant predatory shark three times longer than the largest living great white sharks and 15 to 30 times heavier than them or more. On top of this, they had a bite force 10 times more powerful than a great white, and each one of their teeth was as large as a human face, hence the name ‘Megalodon’, meaning ‘big tooth’.

So, Megalodon would have been the undisputed master of the Pliocene seas. It is amazing, then, to discover that claims have been made that it terrorised the seas around Ireland, as fossils of Pliocene age from four separate sites in Britain have been attributed in the past to Megalodon. However, a 2014 study deemed these fossils to be unreliable, meaning that the closest reliable fossils to Ireland all come from locations that are more southerly, namely, Portugal, Spain, and Italy.

It is unclear, then, whether this giant shark ever did venture into the shallow waters around Ireland to feast on the beasts hugging its shores. However, its presence in the Pliocene seas may well have had a profound impact on the ancestors of some of the marine creatures inhabiting Ireland’s seas today, not least the blue and fin whales, who may, in fact, owe their gigantic size in part to this giant shark.

The same 2014 study mentioned above has noted that the baleen whales did not reach the gigantic dimensions of modern forms like the blue and fin whale until the Pleistocene Epoch (c. 2.6 million years ago–11,700 years ago), and that the baleen whales that lived at the same time as Megalodon were relatively small-bodied forms. Thus, although it is not absolutely definite that Megalodon preyed on the baleen whales that shared its Pliocene waters, it may well have, and it seems that its extinction around the Pliocene/Pleistocene boundary, about 2.6 million years ago, could be what facilitated the rise of the baleen whales to become the incredible leviathans they are today.

L: Photo showing just how gigantic the teeth of Megalodon were. R: Evocative illustration of Megalodon chasing two small baleen whales

In the end, then, the baleen whales’ ascension to the halls of the titans may have been partly due to the demise of other gargantuan sea monsters, allowing them to become unquestionably the largest beasts in modern seas. But whereas the fin whale can certainly claim the title of ‘second largest animal on Earth’ today, our investigations have revealed that its claim to the greater title of ‘second largest animal ever’ is not as strong, and it is possible that this title may belong to a more ancient marine monster.

However, the claim of the blue whale to the title of ‘largest animal ever’ stands on firmer ground. Although some ancient behemoths of land and sea may have been longer than this living whale, the most reliable evidence indicates that none were as heavy as this aquatic colossus. And this is the most amazing fact: that in Ireland’s seas today there lives the largest animal that has EVER lived.

Thus, though we have lamented in previous posts the loss of the greatest land leviathans in the megafaunal extinctions which began late in the Ice Age, our modern seas still contain enormous wonders that no other age of the Earth can match. And yet, one more point needs to be made regarding the blue whale’s claim to the greatest title in the animal kingdom. Whereas when we talked about the largest land mammal to ever live, the gigantic 22-tonne elephant Palaeoloxodon namadicus, we spoke of the bulls being the largest forms, when it comes to the blue whale, like all other baleen whales, it is the cows that grow to the greatest sizes. So, when conferring the title of ‘largest animal ever’ on the blue whale, we must remember one very important thing: that we are crowning not a king but a queen.

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