As a group, sharks are older than trees. But what was the first shark? Where did it live? And how did it give rise to a dynasty of aquatic superpredators that would, slowly but surely, take over the world’s oceans?
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What are sharks?
The term ‘shark’ refers to a collection of predominantly predatory fish that belong to a wider group known as the cartilaginous fish. The members of this group are characterised by their strange skeletons, which are made almost entirely of cartilage rather than bone.
While this group isn’t nearly as diverse as its sister-group, the bony fish, it does contain a number of fish other than sharks, such as rays, skates, and the lesser-known chimaeras, or ‘ghost sharks’.
Contrary to popular opinion, not all extant sharks are giant killing machines. The smallest known shark, the dwarf lanternshark, is only 17cm in length and primarily eats small shrimp. The largest known shark, the 12m-long whale shark, also eats small shrimp and, like the majority of shark species, poses no significant threat to humans.
Still, sharks are some of the most feared animals around today, with species such as the great white, the tiger shark, and the bull shark haunting the thoughts of many swimmers and surfers.
When did sharks first appear?
The earliest confirmed modern sharks (those that look like and are closely related to species living today) first appeared in the Early Jurassic, around 200 million years ago.
However, there’s evidence of shark-like fish living a lot earlier, perhaps as early as the Late Ordovician (around 450 million years ago). This early evidence of sharks is based on several finds of scales that look quite similar to those of known sharks. So far, only the scales of these mysterious, shark-like fish have been found, so palaeontologists are still debating what they really belong to.
While physical evidence of these early, enigmatic sharks may be limited to just scales, geneticists have discovered that, by roughly 420 million years ago, the cartilaginous fish had begun to split into the distinct groups that we know of today.
This discovery was made by examining the DNA of living sharks, skates, rays, and chimaeras, and it suggests that there may have been sharks swimming in the world’s oceans around the same time that the first plants had started to colonise the land.
The first sharks that we’d recognise as sharks today appeared in the Middle Devonian (~380 million years ago) and are represented by the likes of Antarctilamna, Cladoselache, and Doliodus. They may be only distantly related to today’s sharks, and Cladoselache may actually be more closely related to chimaeras than true sharks, but they were active predators and had torpedo-shaped bodies and characteristically pointy dorsal fins – the kind of traits we associate with sharks today.
How many prehistoric shark species were there?
During the following periods, the Carboniferous and the Permian (359-252 million years ago), countless numbers of species evolved. This was a golden evolutionary age for sharks and a time when many species experimented with some truly bizarre body plans.
Some strange species from this time include: Helicoprion, the ‘buzzsaw shark’ whose teeth were attached to a spiral-shaped structure that resembled a spiky octopus tentacle; Stethacanthus, a shark with an ironing board-shaped dorsal fin covered in tiny barbs; and Xenacanthus, an eel-like shark that, unlike most extinct and extant species, lived in freshwater.
By the Early Cretaceous (~145 million years ago), lamniform sharks had evolved and had begun to take over the world’s oceans. The lamniformes are better known as the ‘mackerel sharks’ and include some of today’s most familiar species, such as the great white, as well as several lesser-known species, such as the goblin shark and the megamouth shark.
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How have sharks evolved over time?
The earliest sharks looked a little different to modern species. These differences are most apparent when looking at the mouths of early sharks and, specifically, their teeth.
A lot of early sharks had conical, non-serrated teeth, while later sharks had (and still have) triangular, serrated teeth. The major benefit of conical, needle-like teeth is that they help to grip fast and slippery prey, such as squids and small fish. However, conical teeth aren’t great at tearing huge chunks of meat out of the flanks of large prey animals; this is where the triangular, serrated teeth of later sharks have an evolutionary edge.
The jaws of sharks have also changed with time. By the Early Jurassic, the ancestors of modern groups had evolved more flexible, protruding jaws that allowed them to eat large prey, including prey often bigger than themselves. Their bodies also became increasingly more streamlined during this time, allowing them to swim faster and therefore hunt a larger variety of prey.
It hasn’t exactly been a straightforward evolutionary journey for sharks, though. The asteroid impact that wiped out the dinosaurs 66 million years ago also killed off many species of sharks, particularly the large species that lived close to the water’s surface. The survivors of this particular extinction event were predominantly small and lived in deep water, feeding primarily on small fish. A few million years later, sharks began to increase in size and giant species appeared once more. It was at this time that the largest, most ferocious sharks that have ever lived started to appear.
How have sharks survived so long?
As animals go, few groups have lived for a longer period of time than sharks. If the mysterious scales found in Early Ordovician-aged rocks belong to a shark, or at least an early ancestor of sharks, then the group has been around for nearly 450 million years!
To put that into perspective, trees appeared 390 million years ago; anatomically modern humans, on the other hand, emerged just 300,000 years ago.
In some form or another, sharks have survived all five of Earth’s major mass extinctions. They suffered major losses during each extinction, but as a group they pulled through and diversified on the other side. There’s no single adaptation that makes sharks hardier than other animals; instead their longevity and resistance to extinction is likely as a result of several different factors.
As a group, sharks have relatively flexible diets, eating pretty much anything they can fit into their mouths. This means that, during times of environmental upheaval, they can easily transition to more readily available prey. Those that have more specialised diets and rely on very specific foodstuffs, meanwhile, are a lot more likely to face extinction during these calamitous periods.
The sheer diversity of sharks is another factor that has helped them pull through extinction events. These extinction events are largely indiscriminate, affecting species across a countless number of different environments, but sometimes there are a handful of niches that are left virtually untouched, or are at least less affected than others.
The abyssal zone, or deep water, is often less affected by extinction events than other underwater environments and it’s here where researchers think a lot of species of sharks survived and weathered Earth’s big five mass extinctions. After several million years of ‘hiding’ in the deep, these surviving species are thought to have evolved, their descendants slowly but surely returning to niches closer to the water’s surface.
It’s this diversity that is arguably the reason why sharks have survived so long and what has ultimately allowed them to dominate the world’s oceans for hundreds of millions of years. It’s also what will no doubt allow them to continue to dominate for millions of years to come.
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Main image: illustration of Helicoprion sharks. Credit: Getty