The Evolution of Sharks

In September 2003, Randall Miller, a paleontologist from the New Brunswick Museum, found the oldest intact shark fossil ever, taking us back 410 million years. This example of Doliodus problematicus is only 9 inches long but shares the essential shark characteristics we know and love such as bony jaws, teeth which just keep on growing back in, and a skeleton made not of bone, but of cartilage. The jury is out on the exact connections involved, but the New Brunswick fossil is an example of one of the earliest, most primitive of sharks.

410 millions years ago we were well into what is often called the ‘Age of the Fish’. This was the Devonian Period which falls roughly between 418 and 360 million years ago. This is long before mammals, at least 150 million years before the first dinosaurs. The land formed only two masses, Laurasia and Gondwanaland. The seas spread out over great shelves, forming the relatively warm, shallow areas that really give life rocket fuel. The oceans were heaving and the early sharks were did well for themselves.

Sharks have had a chequered history, sometimes burgeoning and diverse, at others, after cataclysmic events, down to a handful of species. This change in fortunes has happened 5 times since our Doliodus was around. The incredible variations they have displayed over the millenia are witness to the elegance and adaptive power of their basic construction.

Most of what we know about sharks has been learned from shark teeth. These have come down to us in abundance, being the hardest structures in a sharks body. Palaeontologists, by studying the teeth of live sharks, have learned to read the fossils so that they can tell the difference between inter and intraspecies variations.

Doliodus’ immediate ancestors are referred to as ‘jawed fish’, dating from the late Ordovician to the early Silurian, around 455-450 million years ago. Until recently it was thought that Leonodus, who appears in the fossil record about 400 million years ago, was probably the first true shark, but of course our little guy calls this into question. Around the same time as Leonodus we had the first Cladoselache who, although he had some variations (such as a mouth on the front of his head instead of his jaw being being ‘underslung’), was the earliest species that could definitely be identified as a shark. He, however, has not presented us with an articulate fossil.

Burgeoning life also meant fierce competition. These little sharks weren’t the scariest thing in the water, they had to compete with monsters like the ‘sea scorpion’, a giant euryptid, but they held their own and continued to thrive throughout the Devonian.

The beginning of the Carboniferous, with it’s dropping sea levels, was so advantageous to sharks that the next 40 million years is often called ‘The Golden Age of Sharks’. This is where the ancient shark afficionado really gets his kicks. This lot varied from most modern sharks in that their jaws were shorter and more rounded, not as strong, which suggests they weren’t as fiercesome as they are today. But this was a time when life was a feast so our toothy friends were able to burst forth and multiply into an array of weird and wonderful forms. Here are some of the most jaw dropping:

-Stethocanthus grew to about 11ft long. Instead of a dorsal fin he had something which paleontologists believe resembled a brush. He also had an arrangement of teeth on the top of his head!

-Heliocanthus. We’re not sure how big he was. When their fossil teeth were first discovered they were thought to be some kind of creature like an ammonite. The teeth are arranged in a characteristic whorl, which started under the jaw and spiralled outwards as the teeth aged.

-Falcatus. These were only 6 inches long. The males had a dorsal fin spine which grew into a long-forward growing appendage, thought to be relevant to courtship.

These are just the most popular, there were hundreds of weird variations, including giant sharks with 3 metre wide mouths.

As the Carboniferous matured, this explosion of diversity slowed and stagnated. For about 100 million years sharks stayed more or less the same. Having said that, this is the period when the first Xenocanths, eel-like sharks who lived in fresh water, came in to their own.
It was not until the next cataclysm (at the juncture of the Triassic and Jurrasic Periods) provoked the next great adaptive radiation that sharks began to take the forms we think of as modern sharks. Now, at last, we have arrived in the time of the dinosaurs. The early sharks shared the seas with ichthyosaurs and plesiaurs. The word cataclysm is closely related to the word catalyst, and this is how it has always been for sharks. The early Jurassic is also the time when the first rays, called guitar fishes, appeared. Another example was the Hybodus who lived between 165 and 150 million years ago. This specimen had two kinds of teeth for chomping on slippery food or for crunching the shells of sea urchins. He was about 6ft long and looked like a typical shark.

There were 3 more of these extinction events, punctuating the Triassic and Jurassic periods. Sharks survived. Even when the dinosaurs, along with 70% of the Earth’s population, were wiped out 64 million years ago at the end of the Cretacous Period, sharks survived.

We are rapidly approaching the present day and the sharks have gotten very familiar:

‘Oceans in the Cretaceous period (140 to 65 million years ago) were dominated by goblin, sand, probable, nurse, cow and angel sharks.’ (Reefquest)

By the early Eocene, 50 million years ago, we also find fossil records of sand tiger sharks (Ondontaspis robusta) and the first ‘ground sharks’, the Carcharinids developing alongside the first whales. These are the forerunners of blue sharks and cat sharks.

Everybody’s favourite shark, the Great White, is now thought to have evolved from Isurus hastalis, a Maku shark whose teeth resembled a Great White’s and lived about 65 million years ago. Previously the Megalodon was the prime candidate. Megalodon was descended from a mackerel shark who lived between 60 and 45 million years ago. It reached its maximum size about 12 million years ago. At about 50ft in length, with 6″ teeth, he could almost have swallowed our Great White whole.

By the time we reach the Miocene Period the direct ancestors of all the sharks we know today had made their appearance.

The story of the shark is also a story about the changing tides of geological time. The basic structure of a shark is one of nature’s most successful, adapting and ultimately profiting by the most destructive events since time began.

If you would like to find out more about sharks and their ancestors, here are some good places to look at:

BBC Walking with Dinosaurs web-site
New Scientist
National Geographic
‘Prehistoric Sharks’ by Duncan Graham-Rowe
ReefQuest Centre for Shark Research