(Text by Brandon Sing/Shark Guardian. Illustrations by Julius Csotonyi. Images by various contributors)
The history and evolution of sharks goes back a long time, but let’s define “a long time”, because that phrase alone really does not do it justice. Put it this way: Dinosaurs lived during the Mesozoic era, which began about 245 million years ago. Sharks, on the other hand, were living in our oceans 200 million years before the dinosaurs! Sharks are some of the oldest creatures this planet has ever known. How can we possibly know or prove this? Fossil records. Interestingly, shark fossils are extremely rare because sharks are mostly comprised of cartilage, and cartilage disintegrates too fast to become fossilised. For this reason, most shark fossils are made up of just prehistoric teeth, fin spines, and scales. From these few remains, we have been able to piece together a pretty good picture of the sharks that plied the ancient seas.
Scientists studying 400 million-year-old shark fossils from Cleveland, Ohio, found that their specimens turned out to belong to an ancient 1.2-metre Cladoselache shark. As one of the first species of sharks, the Cladoselache shared only certain characteristics with modern-day sharks. While they had gill slits and torpedo-shaped bodies, Cladoselache sharks had no placoid scales covering their skin (also known as dermal denticles), which provide modern sharks with added protection and aid in swimming. Cladoselache sharks also did not possess claspers, the male anatomical structures used in mating that are found in present-day sharks and rays. It is actually not known how they reproduced, but neither the lack of claspers nor dermal skin hindered their survival, because Cladoselache sharks lived in the oceans for about 100 million years.
Stethacanthus was a prehistoric shark that lived from 345 to 280 million years ago. These 60- to 70-centimetre-long sharks preferred to swim in shallow, warm waters. The Stethacanthus shark possessed a fascinating dorsal fin that was in the same position as the dorsal fin of a modern shark, but the top was flat and covered in large scales. Scientists are unsure what these dorsal fins were used for. One belief is that this headgear was used to ward off larger predators and protected Stethacanthus from injury if attacked, much like the protrusions of today’s horn sharks.
From 16 million to 2.6 million years ago, megalodon ruled the waters of the Earth. Researchers have discovered that they had many physical and behavioural similarities to today’s great whites. While their exact size is still under discussion, megalodon was approximately 12 to 21 metres long. Megalodon teeth have been found in Africa, North America, South America, India, Australia, Japan, and Europe. It is believed that they fed on whales and other marine mammals. Why megalodon all died off remains a mystery. Some postulate that these sharks swam in warm waters, and a lowering of ocean temperatures may very well have led to their demise. Another plausible explanation is that their favoured prey began to migrate to colder waters, or even disappeared. Even though the megalodon is thought to have become extinct about 2.6 million years ago, it is still regarded as a “modern shark”.
Cladoselache and Stethacanthus are both types of sharks that existed during the Devonian period 419 to 359 million years ago, and based on the morphology and behaviour of present-day sharks, they are proof that these predators have evolved tremendously over time. Modern sharks all have tooth-like dermal denticles covering their skin, and skeletons made up of cartilage. In addition, the fins and gill slits of today’s shark species are all relatively similar. While one group of ancient sharks evolved to become rays and skates, modern sharks haven’t changed much for 140 million years.
EVOLUTIONARY TIMELINE OF SHARKS