When you imagine a shark, you may think of a torpedo-shaped, streamlined creature with a prominent dorsal fin, a large mouth ringed by sharp, triangular teeth and a crescent-shaped tail. Jaws, basically.
Actually, the shark group of fish are widely varied. The Epaulette shark (Hemiscyllium ocellatum) can walk on land, the frilled shark (Chlamydoselachus anguineus) is flatter-bellied, adapted to hunt in the deep-sea, the Tasselled Wobbegong (Eurcrossorhinus dasypogon) is a carpet shark that resembles an old patterned rug and the goblin shark (Mitsukurina owstoni) is eel-like with a long dagger-shaped snout.
These are just a handful of over 500 species of shark that we know about today, each well-adapted to its particular environment.
Yet in the past, there were many more: fossil records suggest more than 3,000 types of shark and their relatives existed at one time. And some of the ancestors of modern sharks were even weirder and more wonderful than those swimming today.
Their long history starts in the late Silurian period, about 450 million years ago. It was a time when sea levels were high and coral reefs began to form. The Earth's climate was warm and stable. Molluscs, crinoids and trilobites were some of the only living creatures on the Earth before scorpions and centipedes appeared on the land.
The teeth will grow on the inside of the mouth and move forward as they get bigger
Around this time, sharks too appeared, evidenced by the oldest known shark scales found in Siberian deposits.
Jawed and bony fish began to diversify, including the evolution of a group of fish called acanthodians, or "spiny sharks". These extinct fish looked like small sharks but had varying numbers of fins.
"It appears that sharks arose from within those," says Charlie Underwood of Birkbeck University of London, UK. "Where they end and sharks begin is quite up for debate. Certainly we know that some of these acanthodians have teeth that formed in a very similar way to sharks. The teeth will grow on the inside of the mouth and move forward as they get bigger, in a sort of conveyor belt. Among these are the earliest sharks."
Fast forward 50 million years to the Early Devonian, a warm and arid time on Earth when forests spread across the land, seed-bearing plants first appeared and the planet underwent great geological change.
Everything we think we know about shark evolution is from the teeth
This is when we have the first remains of shark teeth, from the Leonodus shark. These teeth are both small (4mm) and two-pronged, but they offer few clues as to what the Leonodus shark actually looked like. They are similar to the teeth of another shark called Xenacanthus that appeared millions of years later in the Late Devonian, leading to speculation that Leonodus, like Xenacanthus, lived in freshwater.
It may seem that teeth are not much to go by, but everything we think we know about shark evolution is from the teeth, says Lisa Whitenack of Allegheny College in Pennsylvania, US. From teeth, she says, we can learn about what environment the shark lived in, what they ate and how they are related to other sharks.
But we have to wait until 380 million years ago for the next clue to shark evolution. That comes from the braincase of Antarctilamna, a so-called lamnid shark from Antarctica. Its head, fin, spines and teeth suggest that it was eel-like.
Early sharks that seemed to have something that allowed them to thrive
There's a reason the Devonian period is referred to as the 'Age of Fishes'. It was the time when they diversified greatly. A skeleton of the now extinct shark Cladoselache, shows just how much. It was very different from its eel-like ancestors. It was a 2m-long, torpedo-shaped shark with equal-sized dorsal fins, a short stout spine in the front, five fill slits and large eyes. It took its prey tail-first, indicating it could easily outswim its meals.
At this time, a school-bus sized group of fish called Dunkleosteus, also swam the seas. These were giant, heavily-armoured fish and may have competed for similar prey. This could have been just the trigger sharks needed to evolve further. Other armour-plated fish existed too, but it was early sharks that seemed to have something that allowed them to thrive, while these other giants died out.
Enter the golden age of sharks, 360 million years ago during the Carboniferous period. The largest predators of the sea at this time were the Chondrichthyans (cartilage fish). They had their skeletal jaws and tough scaly skin to thank for that. The enamel on their teeth was also frequently replaced.
You end up with the bottom lower jaw having a big circular blade sticking out
This group included rays (close relatives of sharks), skates and a bizarre branch called the chimaeras, which featured species such as ratfish, ghost sharks and spook fish. It was within this last group, the chimaeras, that extremely weird and wonderful sharks appeared, says Underwood. "By the Carboniferous period, the majority of shark-like things are on the chimaeras branch, rather than the branch towards modern sharks."
Prehistoric sharks certainly looked much stranger than the modern sharks we share the planet with today, even weirder than the Port Jackson, with its strange patterning and smoothed, numerous fins.
The Stethacanthus (above), for example, had an anvil-shaped dorsal fin on its back. "No one really knows what it used it for," says Christopher Bird, of the National Oceanography Centre, Southampton, UK and Shark Devocean blog. It is one of many evolutionary mysteries in the shark world.
Another was the spiral-shaped tooth structure, called a tooth whorl, of the Helicoprion (see below). These were dinner-plate sized and likely sat at the tip of the lower jaw. Some of these tooth spirals were 40cm across.
They could eat, crunch or suck prey into their mouths
"As they grow and move into the mouth position, rather than falling out, the teeth just stay stuck to each other," explains Underwood. "The shark doesn't lose teeth as they move outside the mouth. So you end up with the bottom lower jaw having a big circular blade sticking out and behind that... crushing teeth. It's a very strange arrangement."
These bizarre traits aside, ancient sharks actually had the same basic features as the sharks we know today.
More innovation occurred at the start of the Jurassic period, 213 million years ago, when 12 new groups evolved. Sharks with flexible jaws started to appear. This meant they could feed on things that were bigger than themselves, says Bird. "They were able to exploit the newly emerging habitats as the world was changing."
Their protruding jaws came to good use. They could eat, crunch or suck prey into their mouths. "Sharks in the Jurassic period often had teeth with a flat-ridged surface to make it easier to crunch on crunchy things," says Whitenack.
As environments changed, sharks evolved different features. A tail fin allowed sharks to swim faster for long distances to pursue prey. Most sharks evolved a mouth under their snout, although a few species have mouths at the front of their snouts such as the frilled shark and angel shark.
Sharks were certainly tenacious. The creatures that thrived during this period survived right into the Cretaceous, often defined by its end. Sixty-five million years ago most of the dinosaurs were wiped out. Many other animals died too but sharks lived on.
And why wouldn't they? They had already survived four other catastrophic mass extinctions. Their bodies were clearly well adapted to survive.
Some evolved the ability to glow in the dark
What's more, they could exploit the fact that so many other creatures were wiped out. It was during these "recovery stages following historic mass extinctions" that the biggest number of new species appeared, says Bird.
Following the asteroid that wiped out the dinosaurs, for example, there was a second wave of deep-sea sharks. "The sharks are able to recolonise the water. We start seeing the cookie-cutter sharks and lantern sharks move in after this post-crisis event," says Bird.
These also exploited new habitats following extinction events. They even managed to survive during times when the ocean lost its oxygen - including one such event in the Cretaceous period, when many other, larger, species died out. As a refuge, sharks moved deeper underwater, says Bird. And while there, they had another cunning trick. Some evolved the ability to glow in the dark.
The end of the Cretaceous gave sharks the opportunity to flourish. Not all survivors were successful though, including one giant of the sea, once thought to be a direct relative of the great white shark.
About 16 million years ago the Carcharodon megalodon first appeared. It could grow up to 16.8m and weighed 25 tonnes. Its mouth would gape open an impressive 2m, showing its 15cm-long teeth, perfect for eating everything else big in the ocean. It made the great white shark look like a goldfish in comparison.
We don't know why the megalodon went extinct. One idea is that climate change disrupted the availability of prey. It was big, so needed to eat a lot. Any tiny change could therefore have threatened its survival. It's likely that many factors combined to cause this giant to disappear two million years ago.
Other survivors from the Cretaceous lived on to become the sharks we know today. Hammerhead sharks for example, are among the most recent to appear in the fossil record and are assumed to be one of the last modern shark orders to evolve.
Their t-shaped heads increase lift as the sharks swim through the water, allowing them to make sharp turns. It also helps them sense more of their environment.
And we now have greater insight into how their strange-shaped heads evolved. Genetic techniques allow us to peer back in time at the evolution of modern-day sharks. In one such experiment in 2010, scientists looked at the DNA of eight species of hammerhead to build a genetic family tree going back thousands, possibly even millions, of generations.
"Our study indicates the big hammerheads probably evolved into smaller hammerheads, and that smaller hammerheads evolved independently twice," said Andrew Martin of the University of Colorado at Boulder, at the time of the study.
"As the sharks became smaller, they may have begun investing more energy into reproductive activities instead of growth."
Recently, it has become clear that we may not even know how many sharks live in the ocean. An elusive shark called the megamouth (Megachasma pelagios), was only discovered several decades ago. In 1976 a US research vessel off the coast of the Hawaiian island of Oahu hauled up a shark nearly 5m in length, with a great fleshy mouth surrounding broad jaws.
Their very survival is under threat
Since then, 49 have been found all over the world. Usually they are dead when caught but one living specimen has given scientists some idea of its environment and habits. Its soft cartilage and flabby tissue suggests a slow-steady swimmer that filter-feeds on shrimp, sea jellies and small crustaceans.
But despite new species still being discovered, the very survival of sharks is under threat. Many are endangered and their biggest threat? Us. Climate change, pollution and habitat destruction are all factors affecting their numbers.
The main threat to their survival is overfishing. Humans kill many species in large quantities for meat and fins. Several are now on a list that seeks to protect endangered species from international trade (the CITES list), and includes open-water predators such as basking, whale and great white sharks, which are caught in vast quantities for meat.
Even deep-sea sharks are vulnerable. Despite the incredible features they have evolved to help them succeed, their reproduction rate is slow. That means if any are killed, the knock-on effect is huge.
Deep-sea shark species can't recover, explains Bird. They don't have the potential to reproduce offspring quicker than they're being taken out. These sharks are often targeted for their liver oil. It contains a molecule called squalene, sought after by the cosmetics industry for its moisturising properties.
We're a new predator in the ocean
The International Union for Conservation of Nature (IUCN) now estimates that a quarter of sharks and rays are threatened with extinction. Although sharks have survived several mass extinctions, the rate at which their populations are being reduced by human activity is extreme and many species are not protected. In 2014, scientists said improved management of fisheries and trade is "urgently needed" to promote population recovery.
If their rate of decline continues, the future of sharks is uncertain. "We're a new predator in the ocean," says Bird. Sharks were once top predator but "we're decimating their populations. One day, they may not be able to bounce back and recover."