When you think of a shark, you probably imagine a large, powerful predator gliding through the open oceans. Species like the great white shark, tiger shark and bull shark dominate the media, with rare attacks on humans fuelling widespread fears and influencing policy, such as the increased use of shark nets in Australia.
Yet these three charismatic predators represent less than 0.6 percent of all living sharks. The more than 500 shark species in existence today show an astonishing diversity: from gigantic 20-metre-long whale sharks to hand-sized bioluminescent lantern sharks, from flat angel sharks to hammerhead sharks, as well as sawsharks, goblin sharks and wobbegongs.
But how did this extraordinary diversity evolve? A new study led by me examined the evolution of body shapes in sharks, from their prehistoric ancestors more than 400 million years ago to the present day.
A time before the dinosaurs
The diversity of forms visible today did not emerge overnight; the shark lineage dates back to a time before the dinosaurs. Normally, scientists use fossils to trace changes in body shape and size in the evolutionary family tree of different animal groups. However, this is impossible with sharks.
The reason: shark skeletons are made of cartilage rather than bone. Unlike mammals, birds or reptiles, there are therefore hardly any complete fossils of ancient sharks. Instead, countless isolated fossilised teeth are found.
As a result, scientists have known very little about how, when and why the diversity of shark body types visible today developed. Instead of using fossils, we gathered information on body shapes from scientific illustrations of more than 400 living shark species. Using a statistical method called “Ancestral State Reconstruction,” we estimated the body shapes of prehistoric sharks.
Additionally, we collected data on the preferred habitats of different shark species and how environmental conditions have changed since the first sharks appeared.
Prehistoric sharks were bottom-dwellers
Our analyses suggest that prehistoric sharks likely lived benthically. That is, they stayed on or near the seabed. Pelagic sharks, which roamed the open oceans and resembled today’s large predators like great white sharks, tiger sharks or bull sharks, did not emerge until the Jurassic period, 145 to 201 million years ago at the earliest.
This means: for the first half of their existence, sharks were confined to habitats near the seabed.
“Interestingly, we found that in three of the four conquests of the open ocean by sharks, a change in body shape occurred, including the evolution of a deeper body and more symmetrical tail, which took place shortly before the habitat shift.”
The timing of these changes suggests that historical climate change, including rising sea levels and tectonic shifts, played a crucial role in creating new pelagic habitats that these sharks could colonise.
In other words: as the climate changed, so did the habitats of prehistoric sharks, enabling the evolution of new body shapes. Coincidentally, these deeper bodies with more symmetrical tails proved better suited to life in open water.
By looking back in time, we can better understand how prehistoric ecosystems functioned and predict how they might respond to future human-induced climate change. These findings also show: not all sharks are the same. Most sharks, both prehistoric and living, are small bottom-dwellers, not large, dangerous apex predators.
So the next time you think of a shark, don’t forget the prehistoric bottom-dwellers that shaped the seas long before the first dinosaurs.
