Do you that sharks are some of the oldest creatures the Earth has ever known? Brandon Sing/Shark Guardian find out why they are regarded as the most well-adapted animals on Earth
(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.
CLADOSELACHE
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
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.
MEGALODON
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”.
SHARK EVOLUTION
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
420 MILLION YEARS AGO (MYA)
ORDOVICIAN PERIOD
(Image © Justin Gilligan)
Cladoselache: These sharks had neither dermal denticles covering their skin nor the male anatomical structures used in mating that are found in modern sharks and rays.
Ctenacanths: These early sharks are known for their fin spines much like the horn sharks of today (pictured on the left). Ctenacanth spines were combed like modern-day stingray spin
400 MILLION YEARS AGO
DEVONIAN PERIOD
It’s thought that at least 10 families of sharks existed during this time. They would later branch off to become other species.
345 MILLION YEARS AGO
CARBONIFEROUS PERIOD
Illustrations © Julius Csotonyi
Stethacanthus: The distinctive anvil-shaped dorsal fin of this species was a unique feature that no modern-day shark possesses.
The Carboniferous Period underwent a significant period of adaptive radiation giving rise to what is referred to today as the
“Golden Age of Sharks”. This continued until…
250 MILLIONS YEARS AGO
CARBONIFEROUS PERIOD
The Permian–Triassic extinction event killed 99 percent of all ocean species
190 MILLION YEARS AGO
JURASSIC AND CRETACEOUS PERIODS
THE FIRST MODERN-DAY SHARKS:
SEVENGILL SHARKS
Image © Warren Baverstock
Sevengill sharks: These sharks, also known as cow sharks, can be traced back 190 million years, to the early Jurassic. The group includes 37 species in 10 genera.
Hybodonts were sharks that lived in shallow seas 180 million years ago, and researchers once considered them to be candidates for the ancestors of modern sharks, though this thesis is now usually discounted. They lived in both fresh water and seawater, a tolerance shared with today’s bull sharks.
155 MILLION YEARS AGO
The lamnoids include some of the best-known sharks, such as the great white (pictured left) shark, thresher shark, basking shark, goblin shark, and sand tiger. Great whites have been around for some 11 million years.
The great white and megalodon coexisted for about 10 million years. It has been suggested that these mighty predators kept out of each other’s way by feeding on different prey (megalodon on whales, the great white on seals) and living in different areas (megalodon in warm water, the great white in cooler water).
65 – 35 MILLION YEARS AGO
FILTER FEEDERS (WHALE SHARK, BASKING SHARK AND MEGAMOUTH SHARK)
Image © Tom Burns
Sometime between 65 and 35 million years ago, several different orders evolved from being predators to filter-feeding plankton. The whale shark, the basking shark and the rare deep-water megamouth shark are the only known filter-feeding shark species today.
50 – 35 MILLION YEARS AGO
HAMMERHEADS – THE MODERN SHARK OF OUR TIME
Hammerhead sharks are the most evolutionarily advanced sharks, evolved to meet the demand of their changing prey and dietary preferences. Their heightened senses and behavioural tactics are like no other sharks. There are 10 known species of hammerhead sharks in our oceans today. Hammerhead teeth first appeared in mid-to-late Eocene deposits, from about 50 to 35 million years ago.
SHARK EVOLUTION FACT FILE
• The first fish appeared around 510 MYA
• Sharks appeared about 425 to 400 MYA
• Dinosaurs appeared about 230 MYA
• Some mammals appeared around the same time as dinosaurs
• Early modern humans emerged just 200,000 years ago
• The span of recorded history is only about 5,000 years
• Sharks have existed 100 times longer than humans
• Sharks have existed three times longer than dinosaurs, or twice as long if you include birds
• Sharks have survived five mass planetary extinctions
Read the rest of this article in Issue 5/2015, AA No. 83 of Scuba Diver magazine by subscribing here or check out all of our publications here.
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