What did prehistoric sharks look like?

Show notes

This week we’re going on an epic journey back in time, long before dinosaurs roamed the earth. Around 450 million years ago the earliest ‘sharks’ appeared, and since then the Chondrichthyes – the sharks, rays, skates and chimaeras – have evolved, diversified and survived, outliving many other groups of animals and even making it through five mass extinctions. Host Isla travelled to the Natural History Museum in London to meet with curator of fossil fish Emma Bernard, to learn all about the extensive evolutionary history of sharks and meet some of the weird (and in some cases gigantic) species that roamed ancient seas.

Before hopping in our time machine, Emma tells us about her most memorable ocean experience [3.00] and how she got into palaeontology [5.17]. She never lost her childhood passion for dinosaurs and remembers her first mind-blowing experience being allowed ‘behind the scenes’ in a museum, which led her on a path into museum curation. She specialises in paleo-ichthyology – the study of fossil fish – and for her masters thesis studied the evolutionary transition from fish to mammals [7.36]. Did you know that some of your ancient ancestors were fish??

Our adventure into the prehistoric ocean begins by meeting some of the first sharks, or ‘shark-like’ forms, 450 million years ago in the Ordovician period [12.56]. Fossilised dermal denticles – the same material that shark skin is comprised of – from this time have been found, although there is some debate among scientists as to whether they are from ‘true’ sharks. However, there is wide consensus that the earliest shark teeth date back to 420 million years ago, which is still over 200 million years before dinosaurs entered the scene!

The tricky thing about sharks is that they are made of a soft tissue, called cartilage, which doesn’t easily fossilise like our bones can [16.06]. Some thicker parts, like the vertebrae, can become fossils but only under very specific conditions. So, a lot of what we know about prehistoric sharks is from the only part of their body that is hard enough to fossilise: their teeth! As Emma explains, sharks will typically lose 20, 000 – 40, 000 teeth in their lifetime, and ancient shark teeth litter the sea floor. We can tell a lot from their size, shape and age, and also compare the fossils to their modern-day counterparts. It’s like trying to piece together a very old jigsaw, when many of the pieces are still missing!

Scientists believe that the earliest sharks were small and elongated, shaped almost like a torpedo, with a very similar dorsal and caudal (tail) fin to today’s sharks [22.01]. During the Devonian period (around 370-360 million years ago) several small extinction events gradually wiped out 75% of all species, which actually worked out pretty well for sharks [23.52]. Most other large predators had gone extinct, opening up new environments and allowing sharks as a group to diversify and thrive. During the Carboniferous there was around 40 different families of sharks, giving this period of time its nickname, “the golden age of sharks” [24.50]. This was the time that some extremely odd species appeared, including helicoprion the ‘buzz-saw shark’, a chimaera with a bizarre spiral of teeth inside its jaw [25.01]. Stenthacanthus was also around during this time, another species of chimaera with a dorsal fin that looked like an ironing board [27.24]. This was clearly the era of chondrichthyan experimentation!

As Emma explains, another incredible thing about sharks and their relatives is that they survived five mass extinctions. This includes the ‘Great Dying’, the greatest mass extinction event to have ever occurred where over 90% of all life on earth went extinct [31.36]. The sheer diversity of sharks, and their ability to adapt to many different environments, has helped them to survive to present day. Some deep-water lineages with varied diets were able to endure the Great Dying and make it to the Jurassic period – perhaps the most iconic period in evolutionary history – where giant marine reptiles roamed the seas [34.00]. Many of the ancestors of shark species that we know today came onto the scene in this period. Hybodus was a shark with many different types of teeth, including sharper front teeth for tearing and grasping, and flattened back teeth for crushing and grinding [36.47].

We of course can’t do an episode on prehistoric sharks without talking about megalodon – the largest predatory shark to have ever existed [43.49]. This 15-18m long shark had teeth as big as a human hand, with serrated edges perfect for hunting and eating large whales. Until relatively recently, it was widely believed that white sharks are descended from megalodon but as Emma discusses, subtle differences between their teeth alongside newly discovered fossils suggest that megalodon and white sharks had completely different lineages [46.16]. In fact, the ancestors of white sharks may have even been in competition with megalodon. So, if white sharks are still around, could the meg still exist today? The evidence says no, and Emma rounds off our conversation by explaining why [51.35].

About our guest

Emma Bernard

Emma is a paleo-ichthyologist and curator of fossil fish for the Natural History Museum in London. After realising a passion for fossils and ancient life as a child, she went on to study earth sciences at the University of Glasgow. It was there that she experienced museum collections for the first time, and was fascinated by the volume and diversity of fossils behind the scenes. Emma then gained a masters degree in Paleobiology from the University of Bristol and a further masters in Museum Studies from the University of Leicester, and now has over twelve years of experience as a dedicated museum professional. Her work has taken her all over the world, collecting fossils in Morrocco, America, France, and her home country.

Twitter: @Emma_Bernard

Instagram: @natural_history_museum

Website: https://www.nhm.ac.uk/our-science/departments-and-staff/staff-directory/emma-bernard.html