Shark evolution has been in motion for hundreds of millions of years, with around 500 species now swimming the oceans. Although sharks are under more and more pressure, mainly from human activity such as hunting, their history shows they are able to adapt rapidly to environmental changes. This flexibility in shark evolution could be a benefit in the midst of climate change, which is already causing problems for many ocean dwellers who lack this adaptability.
Are Sharks Dinosaurs?
Factoring just how long they’ve been around, some people may wonder — are sharks dinosaurs?
According to Live Science, sharks have hunted through the oceans for around 450 million years, while dinosaurs only arrived around 240 million years ago. Furthermore, dinosaurs mostly didn’t survive the many extinction events that sharks survived (albeit in reduced numbers). Despite their primordial and sinister appearance (thanks Jaws!), they’re not even close relatives. A Harvard news article notes that chickens, ostriches and alligators still remain the closest relatives to Tyrannosaurus rex.
The Natural History Museum of London describes how the “golden age” for sharks arrived pre-dinosaur times, roughly 359 million years ago. At the time, sharks dominated the oceans following a mass extinction event that wiped out a lot of their competitors. CNN notes that these animals were actually the first vertebrate predators on the planet, firmly ahead of dinosaur activity.
Fossils Uncover the Secrets of Shark Evolution
Sharks are difficult to follow through the fossil record. Unlike dinosaurs, sharks have a cartilaginous skeleton rather than being made up of solid bone. Unfortunately, this structure doesn’t fossilize well because it doesn’t calcify. So, finding evidence of shark species throughout the ages is mostly limited to teeth and skin scales, which contain calcifying minerals such as calcium and bioapatite.
In fact, examining these denticle microfossils showed evidence of an extinction that wiped out around 90% of sharks 19 million years ago. Not only did open ocean populations of sharks disappear, but the species also became less diverse, according to a study published in Science.
Occasionally, an entire skeletal remnant is fossilized. On discovery, this can tell paleontologists a lot about ancient sharks. For example, a single fossilized vertebral column helped scientists determine the age and size at birth of the mighty Megalodon.
Shark Evolution for Species Success
From the fossil evidence available, scientists have been able to piece together some of the phases of shark evolution and see how each phase has helped sharks remain a top predator.
For example, being cartilaginous makes sharks relatively lightweight, which helps them conserve energy while swimming long distances. Tapered bodies and well-placed fins coupled with dermal denticles and tail fins that are larger at the top make swimming itself much more efficient and powerful. This means that sharks can undertake long migrations and swim at speed. As a result, they have few natural predators apart from humans.
They are also generalists, able to occupy a variety of ocean niches, and their teeth have evolved to cope with a broad flexibility in diet. For example, fossil evidence shows that, during the early Jurassic period, sharks developed a flexible, protruding jaw. This helped them tackle larger prey and eat animals bigger than themselves.
Sharks are optimal hunters with a lateral line sensory system that senses vibration from prey in the water. They also have the ability to pick up on electromagnetic fields which helps not only with food location but also with navigation.
Sharks also survived the Paleocene-Eocene Thermal Maximum, which saw a dramatic rise in both global carbon dioxide and temperature around 56 million years ago. Their continued survival can likely be credited to their ability to rapidly adapt to environmental change. For example, a rise in water temperature causes a rapid reduction in shark body size over the relatively short evolutionary period of 7,000 years. Certain shark species are also able to move between salt and fresh water with ease.
New Shark Discoveries, Ancient and Modern
New shark species continue to be discovered, both ancient animals in the fossil record and contemporary sharks swimming the oceans today.
For example, closer examination of an encased plaster specimen held by the Canadian Fossil Discovery Centre in Manitoba introduced the world to Dave — finally “freed” after 40 years in the archive. At around 15 feet in length, not only is Dave one of the largest fossilized shark specimens in the world, but he also might be a new species never seen before.
Closer examination of a 20-year-old fossil in storage has also revealed a new ancient shark species coming from the shallow primordial seas near the United Kingdom. SciTech Daily reports that the evidence comes from the teeth of this extremely rare shark skeleton fossil.
New species swimming around in today’s oceans continue to be discovered, too. For example, Smithsonian Magazine describes a new glowing kitefin discovered in the ocean depths around New Zealand. Meanwhile, the number of sixgill saw shark species increased to three this January after two new species of this ancient lineage were discovered off eastern Africa, each showing the typical and rather odd arrangement that Pop Sci describes as “long, flat snouts studded with teeth.” Sawsharks use their chainsaw-like snouts to dice their prey into ribbons before dining, in case you were wondering the purpose of a nose studded with teeth.
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