The Secret History of Sharks! Shark Evolution and Adaptations
SOW NOTES
Sharks have been on this earth for over 400 million years. Palaeontologist and author of The Secret History of Sharks, John Long, has always been fascinated with the extraordinary adaptations that have enabled sharks to persevere through some dramatic twists and turns in evolutionary history; surviving hostile climates, armoured predators and five mass extinctions. It was an obsession that began in childhood [06.26]. “I realised that where I lived there were fossil sites nearby where you could find fossil shark’s teeth that were five or six million years old,” John recalls, “…that just got me hooked. And every weekend we’d be down there looking for sharks’ teeth and whale bones and fossil penguin bones and all sorts of remains of an ancient marine ecosystem, and it just totally fascinated me. So I determined that when I finished school, I wanted to be a palaeontologist.” This passion led John all the way to a PhD in the evolution of fishes, which has blossomed into a lifelong career piecing together the story of sharks and other ancient fishes from the tiniest of clues.
Unlike bony fishes, sharks rarely fossilise well [10.19]. Their skeletons are made of cartilage, which decays quickly after death. Palaeontologists who specialise in sharks, like John, must rely on tiny fragments – like teeth, scales and fin spines – that are harder and less likely to break down over time. More often than not, what we know of ancient sharks comes from these minute parts of their body: “It’d be true to say that 99% of the entire fossil record of sharks is based on teeth and scales and spines!” Occasionally, however, something extraordinary is uncovered; a complete or near-complete fossil of the shark’s entire body. “…That 1% where you get articulated complete sharks is truly spectacular. And it’s those special fossils that gives us the window into their biology and anatomy,” says John. These rare finds allow scientists to reconstruct not just what ancient sharks looked like, but how they lived — what they ate, how they moved, and how they interacted with their environment.
But these “special” fossils don’t appear in the fossil record until the Devonian period, almost 50 million years after scientists believe sharks first came on the scene. So how do we know sharks were around before then [14.45]? The answer is still somewhat murky. The origins of sharks remain one of the biggest mysteries in vertebrate evolution. “[Understanding] the origin of sharks is like being a detective at a murder mystery where there’s just not enough clues,” laughs John. This is because the earliest potential evidence we have is from microscopic scales. “The scales are quite characteristic of modern sharks,” John explains. “We don’t even know if they had jaws or teeth, but we know they’re closer to sharks than to any other group of fish based on the structure and the shape of these tiny little placoid scales.” In fact, these early denticles – which date back to an astonishing 465 million years ago – resemble those of a modern-day nurse shark.
“Mako sharks and nurse sharks have three riblets on the surface. It’s really the fact that they have a flat surface part of the crown that sticks out of the skin and a base that sticks into the skin. And that base has got like a curved neck kind of thing. And modern sharks have the same sort of structural shape that other extinct groups of fishes don’t have.”
Over time, these fragments become more complex, hinting at early diversification. But even then, scientists are often working with little more than “crumbs” of evidence. It’s not until the Devonian period – also known as the ‘Age of Fishes’ – that sharks appear more clearly in the fossil record. And it’s during this time [that] they develop their first “superpower”: the ability to continuously replace teeth [24.00].
This gave sharks a major advantage. Always having fresh, sharp teeth at the front of their mouth allowed them to compete with other predators and exploit a much wider range of prey. The Devonian is also the first time we start to see diversity in their tooth morphology. “By the middle parts of the Devonian, we’re getting teeth that are more complicated, with more cusps on a single root…we’re starting to get teeth that are more robust and rounded, and they’re for crushing prey,” says John. “By the end of Devonian, we’ve even got sharks teeth that are serrated for the first time…like we get on a white shark or other species like hammerheads and so on, for eating flesh and cutting through that.”
As sharks diversified, they also increased in size and expanded into new environments [30.00]. These included freshwater river systems. “This is a real turning point in shark evolution,” notes John. And it wasn’t long before sharks experienced another defining moment in history: the extinction of the placoderms. Placoderms were extremely large, armoured fishes that dominated the Devonian oceans. They were the top predators of the time, and many preyed on sharks. But a mass extinction event at the end of the Devonian wiped out the placoderms, allowing sharks to take centre stage for the first time.
The Carboniferous period is often described as the “golden age” of sharks [32.50]. In the absence of major predators, sharks rapidly diversified and, for a time, ruled the oceans. This was a period of intense experimentation with sharks evolving a wide variety of unusual body forms and feeding strategies. “The diversity of shark tooth types and shark body shapes just goes through the roof… this crazy psychedelic time in the diversity of sharks,” explains John. Some species developed crushing teeth for hard prey, while others evolved bizarre structures like the spiral-shaped tooth whorls of the so-called ‘buzzsaw sharks’. How species like Helicoprion used this peculiar dental mechanism exactly is unknown, but some scientists have made educated guesses. “A shark expert in Idaho has been studying these for many years, and his studies…found no wear on the tips of these teeth,” John says, “so they’re most likely eating soft prey like squid. And this tooth whorl, which was hinged on a lower jaw that was quite kinetic, quite movable, [so Helicoprion] could push this jaw out and slash it back, hooking or jagging a squid on it back into its mouth.” What a way to go, if you’re a squid!
However, this ‘golden’ chapter of shark history didn’t last forever. At the end of the Permian came a mass extinction event known as “the great dying”, so called because a staggering 96% of marine life was wiped out [43.25]. It was the end of the road for the likes of the buzz-saw sharks, and many other of the whacky, experimental sharks and chimaeras that had dominated the previous 60 million years. But, miraculously, some sharks made it through the greatest extinction event that has ever occurred.
“The true success story of sharks across this extinction boundary are the tiny little hyperdonts,” explains John. “These are a really boring group of sharks that look a lot like Port Jackson sharks or horn sharks today…And they were mostly small. And after this massive environmental chaos of the Great Dying, they stayed small for about 140 million years. Most sharks were tiny, under about a metre in length.” They may have been small, but what made hyperdonts so successful was their dentition. “They didn’t have one kind of tooth in their mouth. They had several different kinds, like a Swiss Army knife kind of dentition, where you can have pointed sharp teeth at the front for grabbing fish, or crinkled teeth at the back that can grind up clams or crabs or hard shell prey.”
These little sharks paved the way for most of the groups of sharks we know today [49.05]. The Jurassic period – when T. rex was roaming the earth – saw the beginnings of the neosqualians, who represent all the modern sharks now living in our oceans. They had a new, super resilient kind of tooth with three layers of enameloid, an incredibly hard and durable substance. This allowed sharks to once again become successful, giving rise to the lamniforms (mackerel sharks), angel sharks, and the sharks’ close relatives, the rays and skates. “And by the end of the Cretaceous, the end of the age of dinosaurs, most of all the major families of sharks have already appeared in the fossil record,” John says. And, following the extinction event that wiped out the dinosaurs, sharks went on to achieve even greater physiological feats:
“From 66 million years on to today, we see the rise of the carcharhinid sharks, which were very much in the background in the age of dinosaurs. And now they suddenly take to the fore, and they overcome, they over diversify…and they become the dominant shark group today with over 300 species,” John explains. This is when we start to see the rise of the megatooth sharks – gigantic lamnids that just get bigger and bigger. “…the genus Ototus jumps in and you’ve got the shark called Ototus obliquus living straight after the dinosaur era that was up to eight to nine meters in length… and very quickly, as whales appeared in the ocean, you start getting serrated teeth adapted for eating mammal-type prey. And then as whales evolved and got bigger and bigger and bigger, so did the sharks. So, it became a bit of an arms race that culminated in Megalodon.”
Although the super-predator megalodon no longer cruises the seas, sharks remain as one of the most successful groups of animals of all time [56.06]. They have endured multiple mass extinctions – events that wiped out so many others. Even as other dominant groups disappeared, sharks persisted, reshaping themselves again and again across millions of years. Their resilience can be attributed to the key adaptations they have evolved, that mark significant chapters in their evolutionary history; tough, reinforced scales, a light and flexible cartilaginous skeleton, super-powered sensory systems and, of course, their teeth. The latter is perhaps the most powerful. “I think the most important thing is their teeth,” says John, “and their ability to generate teeth of so many different shapes and kinds and different tissues. Every single shark on the planet today can be identified by its teeth alone. They’re that unique and special.”
ABOUT OUR GUEST
PROFESSOR JOHN LONG, FLINDERS UNIVERSITY
Prof John Long‘s research focuses on the early evolution of fishes, including sharks. He has worked the Gogo fossil sites in the Kimberley for 40 years and found many fossils of significance, including the evidence for the origins of sex in vertebrates, breathing and limb structures in stem tetrapods, and papers on early vertebrate soft tissues. He recently reported on the world’s oldest reptile tracks from Australia in Nature (May, 2025). John has participated in 4 expeditions collecting fossils in the remote mountains of Antarctica. He has published over 200 scientific papers, 160 popular science articles, and some 30 books (including several children’s books). He has discovered and named more than 90 new species of prehistoric creatures (mostly fishes, but also a dinosaur and a plesiosaur). He was elected President of the Society of Vertebrate Paleontology 2014-2016, and has won the research medals of The Royal Society of Victoria and The Royal Society of South Australia, as well as the 2019 Bettison and James Award for lifetime achievement.
