Saturday, 5 April 2014

It’s Worse in the Water

by Rob Cooper

The vast majority of the surface of planet earth is covered by water and considering this it should be no surprise that whilst terrestrial (living on land) creatures can get rather frighteningly large, the denizens of the depths are always one step ahead. In this article I’ll examine ten of the most fearsome, peculiar and benign ocean creatures with the hope of showing just how fantastic the diversity of ocean live has been over time.

1. Dunkleosteus
Back in the Devonian a peculiar group called the Placoderms developed to truly monstrous sizes. Characterised by their armoured bodies and bony plates instead of the teeth the Placoderms reached their most terrifying in the killer whale sized Dunkleosteus. Strangely for such a large predator the 8 metre long Dunkleosteus could suck prey towards its mouth by rapidly opening its jaws (in less than 1/15th of a second). Unhealed bite marks on the head guard of younger Dunkleosteus also lend evidence to a theory of cannibalism.

2. Helicoprion
If you thought Dunkleosteus had strange teeth you may be surprised to learn far stranger tooth morphologies existed in the prehistoric seas. Helicoprion is a fantastic example of this having a ‘toot whorl’ that looked remarkably like a circular saw inspiring many strange artists’ impressions of the creature. The function of the peculiar teeth has yet to be determined but it is thought that the successive rings of teeth are analogous to growth rings in trees and that teeth successively pushed up to replace those that were lost, similar to how its modern day relatives sharks replace their teeth today.

3. Megamouth Shark
To continue from shark like animals to true sharks we next encounter a very rare modern shark colloquially known as the ‘Megamouth shark’. Since its discovery in 1976 only 55 specimens of this elusive shark have been seen or caught. Like its larger relatives, the basking and whale sharks, the Megamouth has an enormous mouth full of very small teeth that are ideal for sieving plankton from the ocean. As it follows the movement of plankton down to over 200 metres in depth it has small photophores (light emitting organs) to attract plankton and small fish to its 1.3 metre wide mouth.

4. Megalodon
It would be an insult not to mention the mightiest of all ocean predators in this article and Megalodon certainly fits that title handsomely. Well respected as the largest and most powerful predator in the history of all life (with one competitor) megalodon is now estimated at lengths of at least 14-18 metres and up to 20 metres and weights of slightly over 100 tonnes. Megalodon has the largest jaws ever discovered in the animal kingdom large enough for a full grown man to walk through without bending down and an estimated bite force of over 180,000 Newtons. This, along with other palaeontological evidence, suggests that Megalodon preyed on large whales until its extinction 1.5 million years ago.

5. Livyatan 'the great whale'
But what animal was big and ferocious enough to rival Megalodon? Strangely enough the answer comes from a contemporary of Megalodon’s the titanic Livyatan. Livyatan was a raptorial sperm whale which lived between 12-13 million years ago in the Eocene period. Akin to megalodon Livyatan remains have been found alongside those of baleen whales, sharks, dolphins, porpoises and many other sea creatures supporting the hypothesis of Livyatan being an apex predator alongside Megalodon. Unlike modern sperm whales Livyatan had the largest teeth of any animal yet discovered (bar the tusks of elephants and walrus) at up to 36cm in length. Livyatan was thought to ambush prey swimming at the surface from beneath much like the modern great white shark.

6. Leedsichthys 'monster fish'
From two rather fear inducing giants to a rather more benevolent one. Meet Leedsichthys, the largest bony fish to have ever lived. Leedsichthys ranged from around 10 to nearly 15 metres in length so probably rivaled the modern whale shark in size although a heavier bone skeleton likely made Leedsichthys considerably heavier. A strange tendency in large oceanic animals is that of eating remarkably small prey and Leedsichthys was no different and likely fed on plankton either by pumping water through its huge mouth or actively swimming to filter small organisms from the water around it.

7. Giant Manta Ray
Benevolent giants still exist today in the form of the majestic manta ray. Gliding through the waters like marine birds these gentle giants can have a wingspan of up to seven metres and weigh over a tonne. Manta’s are also filter feeders and typically herd their tiny prey into tight balls and swim through at speed with their large rectangular mouths open. On occasion Manta’s have been observed somersaulting through particularly tight balls of prey presumably to catch more prey.

8. Predator X
Leaping back 155 years to the late Jurassic period we come to a group of marine reptiles called the Pliosaurs. Pliosaurs were characterised by short necks and immensely powerful large jaws, the largest specimens having estimated bite forces comparably or far greater than Tyrannosaurus rex. Predator X was one of the largest of all Pliosaurs reaching between 12-15 metres long and possibly weighing up to 45 tonnes. Strangely, compared to modern day marine species, Pliosaurs used their four flippers for locomotion and had no tail fins to speak of; two of these fins would be used for normal locomotion whilst the other two would be used to create a burst of speed when ambushing prey.

9. Plesiosuchus
Crocodiles are often said to be living fossils and quite rightly so for modern crocodilians have hardly changed at all for hundreds of millions of years. However back in the Jurassic period, something very strange happened in crocodilian evolution… They began to adapt fully to living in the sea. At around 6.8 metres in length Plesiosuchus was one of the largest marine crocodiles and like its kin had shed the heavy, restricting armour of its forebears in favour of a streamlined body. The skull of Plesiosuchus shows many similarities to modern killer whales implying it regularly predated on large marine reptiles of the time.

10. Shastasaurus 'ocean giant'
On the topic of marine reptiles, it would be immensely uncharitable of me to finish this article without mentioning the largest marine reptile yet discovered. Shastasaurus was a primitive ichthyosaur, a group of marine reptiles that in their later stages would bear many similarities to dolphins yet the primitive forms were quite unique. The prey base and feeding method of Shastasaurus remains a mystery as it seems to have had no teeth and the suggestion that it fed on soft bodied molluscs is generally not supported by ichthyosaur skull morphology, what this strange animal was specialised for we may never know but that hardly makes it a less thrilling creature to imagine.

The oceans simultaneously offer some of the most fearsome and most benevolent creatures that planet earth has ever harboured. From gentle filter feeding giants to titanic ambush predators lurking beyond the reach of light the oceans are a place where life has fully explored the limits of size. No dinosaur has even come close to the largest baleen whales of today and whilst we may be thankful that many of their huge predators such have Megalodon have finally passed on; we do ourselves a great service by remembering just how rich the oceans are and how important they are as an ecosystem to us and other terrestrial species.