Having swum in the world’s oceans for over 450 million years and survived five global mass extinctions, sharks are ancient creatures that evolved long before dinosaurs walked on land.
It wasn't until the Jurassic and the Cretaceous that sharks started to look more like the sharks that we see today. At the end of the Cretaceous, an extinction event caused by an asteroid impact wiped out 75 percent of all species.
The very first sharks are a mystery. All we know of them are these tiny little scales a fraction of a millimetre long. But we have no teeth, we have no body fossils.
So the very first sharks that are known from complete skeletons are not till the early Devonian about 410 million years ago.
And 359 million years ago, bang, the whole oceans changed. The placoderms, dominant fishes of the day, went extinct and sharks finally had the chance to succeed.And after that they radiated. It was the golden age of sharks. Many diverse and strange and bizarre sharks appeared.
Helicoprion was the world's first mega predator, the first gigantic fish to grow to something like 30 feet, maybe 8 or 9 meters in length. It was huge. We call it the Buzzsaw Shark because it had a coily row of teeth that were razor sharp, serrated like steak knives and grew continuously out the lower jaw in a circular whorl.
These animals would have been swimming around in the oceans, feeding on a variety of hard prey. And they used these teeth in a really weird fashion. So the upper jaw is kind of like a groove and as the teeth kind of pressed into it, it would have split apart these hard organisms, and then the lower jaw kind of pulled back to rotate the soft inner parts of these prey species into the oral cavity.
So this is a fossil from New South Wales. It's one of the few examples of a complete shark that we have in the fossil collection here at the Australian Museum. It's a type of Xenacanth shark. So these lived in fresh waterways, so rivers and estuariesin Australia in the Triassic, which is around 200 million years ago.
You can see all of the different features here. So we have the skull of the shark preserved. We have the vertebral column here. We even have the claspers. So they tell us what sex the shark was. We can tell it's a male based on the presence of these features, as well as the fin rays are also present.
Sharks have lived for a huge amount of time.They've survived the five big mass extinction events. And so we sometimes think of sharks as survivors. But it's not that they weren't impacted by these events. So every time there was kind of global turmoil, every time the oceans were thrown into these periods of change, sharks were impacted.
We saw the extinction of many species over time, and we've seen sharks change in their ecology. We've seen them change how they looked through time as well.
Fishes are the basic body plan... It was simply the changing proportions of that body plan that led to the evolution of reptiles and mammals and eventually to us humans.
The first jawless fish, called agnathans, are our common ancestor. They are still around today in the form of lampreys and hagfishes.
Fossil agnathans date back about 518 million years; a time when almost all life lived in the oceans. Agnathans later evolved into bony fish, amphibians, dinosaurs and even birds.
Early shark ancestors
Some of the sharks cruising the world’s oceans more than 300 million years ago looked somewhat like sharks as we know them. Others were very strange indeed.
Akmonistion lived 323-330 million years ago. The above illustration shows what scientists think this ancient relative of sharks looked like.
We know from fossil remains that males had a distinctive anvil-shaped dorsal fin. This was covered in small spikes which were versions of the denticles, or scales, which cover the skin of most sharks. This fin probably meant Akmonistion couldn’t swim particularly fast.
At about 60cm in length, this shark was relatively small. It probably ate small fish. Small fins and teeth suggest it might have been a bottom dweller.
Recent thinking is that Akmonistion may be more closely related to modern chimaeras or ‘ghost sharks’, than to sharks themselves.
Falcatus falcatus was a 25- 30cm shark that lived about 325 million years ago. From fossils, we know it lived in warm shallow seas in what’s now the middle of the North American continent.
Males had a prominent fin spine that curved forwards over the head. No one is really sure what this was used for, but one fossil shows a female biting the fin of a male, which suggests it played a role in mating.
One of the problems for palaeontologists studying ancient sharks is that they don’t have much to go on. Because sharks’ skeletons are made of cartilage, they don’t fossilise well. Luckily, the hard tooth-like scales that cover a shark’s body, called denticles, do.
What we know about early sharks comes from tiny fossil denticles found in rocks. The oldest are around 450 million years old.
How does a shark tooth become a fossil?
Stage 1: A shark sheds a tooth while chomping on some prey. The tooth falls to the seafloor. Any fleshy parts rot away leaving behind the hard tooth remains.
Stage 2: Sediment buries the shark tooth and over time a thick bed of sedimentary rock accumulates, trapping the tooth deep within the ground.
Stage 3: Gradually, the tooth turns to stone as it's replaced by other minerals, which leach in from the surrounding sedimentary rocks
Stage 4: The rock with the fossil shark tooth is exposed by weathering and erosion or dug out of the ground by palaeontologists.
Fossils give us a glimpse into the past worlds in which sharks lived.
Evolving through the ages
For hundreds of millions of years sharks have been adapting to changes in oceans and rivers.
Dramatic cycles of oceans cooling and warming, different types of prey becoming extinct, algal blooms, acidic oceans, competition with other large predators ... sharks have been through it all and survived.
The largest fish that ever lived, Megalodon (Otodus megalodon) swam in our oceans between about 20 million and 3.6 million years ago. It evolved from a now-extinct group known as the mega-toothed sharks (Otodontidae). Luckily for us it became extinct long before humans evolved.
Scientists think Megalodon grew up to 20m long. That’s more than three times longer than the largest White Shark. Even though they look like White Sharks, Megalodon’s closest living relative is the Shortfin Mako Shark.
Megalodon means ‘giant tooth’ and you can see why. The largest recorded tooth is 17.8cm long.
Adult Megalodons ate whales and fish. The only predators larger than Megalodons were large predatory relatives of sperm whales. Megalodon teeth have been found in seas (and in places which used to be under the sea) around every continent except Antarctica.
We actually have genealogy to sharks, we have genealogy to everything. We all trace our whakapapa or genealogy back to the same origins. They are part of us because they are part of our environment.
Golden age of sharks
Like all living creatures, sharks are part of the ecosystems in which they live. They have been shaped through time by what’s happening in their environment. About 359 million years ago an extinction event killed off many other types of fish and gave sharks the chance to diversify and and rule the oceans. This period is called the ‘golden age of sharks’. A whole variety of different species appeared, including Akmonistion, various species of Helicoprion, and Falcatus.
Early sharks looked very different to the ones that are around today. Xenacanths were just one of the weird and interesting sharks that evolved after each mass extinction. Reaching a maximum length of 2m, Xenacanths had a sharp dorsal spine behind the head which might have helped protect them from predators.
Late in their evolutionary history Xenocanths lost this spine; maybe because of the danger posed to mothers giving birth to baby sharks with spines, or because it hampered their manoeuvrability.
Xenacanth sharks lived in freshwater, estuarine and coastal marine environments between about 350 and 200 million years ago. Dispersed throughout the world, they survived extreme climatic conditions with seasonal changes to river levels. Today their fossils are found in Australia, Brazil, India, Oman, Unites States of America, United Kingdom and Germany.