Ancient sharks, which are no longer considered a "living relic", were of great variety.
If there is a creature in the world that causes us a primal fear of the inhabitants of the deep sea, it is a shark with giant teeth. Experts call it Otodus megalodon (big tooth). This 15-meter distant relative of the modern white shark was the largest predatory fish of all time. In exceptionally large individuals, the mouth is filled with serrated teeth the size of a human hand. Everything from Peter Benchley's Jaws to the big-budget Meg: The Monster of the Deep keeps us fearing this shark, even though it lived more than 2.6 million years ago.
All this sensational pursuit kept us from learning more about this imposing, whale-eating shark. For example, where did this megalodon come from? But to get a proper idea of its origin, we need to look at how sharks appeared.
It is difficult to pinpoint exactly when sharks became sharks. Nowadays it is quite easy to look at modern fish and say: this is a shark, but this is not. But the further we plunge into the depths of time, the more the dividing lines blur. However, paleontologist Kenshu Shimada of De Paul University says sharks, as cartilaginous fish in the broadest sense of the word, are approximately 400 million years old.
Many early sharks are known to us only for their scales and teeth. One of the earliest sharks called Leonodus is best known for its split teeth. Comparison with later and better studied sharks suggests that Leonodus was more like an eel in shape than the sharks we know today. But there are not very many fossils.
Found in the Canadian province of New Brunswick, a skeleton helps to understand what these first sharks looked like. Called Doliodus problematicus, this fish is one of the oldest sharks ever found by scientists. It has a wedge-shaped head and spines protruding from its fins and belly. "Some modern sharks still have fin spines," says Shimada. "But due to the significant decrease in the number of these appendages, modern sharks are less 'prickly' in appearance compared to their ancient ancestors." Perhaps these thorns protected them from other toothed fish that lived in large numbers in the same ancient seas.
It's a shame the other sharks haven't survived as well. Although they have a very long history (almost twice as long as dinosaurs), “the vast majority of shark fossils have only sporadic teeth,” Shimada notes. But there are also some exceptions. The 318 million-year-old Bair Gulch Limestone in Montana contains shark fossils with great detail. However, almost everything we know about ancient sharks is from their teeth. A whole group of ancient sharks called cladodont had very strange teeth in the form of a long central incisor surrounded by smaller, sharp appendages. They look like a terrible crown, and were more suitable for capturing slippery prey than for chewing it.
However, we can learn a lot about the nature of these ancient swimmers from what they left behind. Even though they are called "living relics" that have survived unchanged, we know that ancient sharks were very diverse in shape, size and color. The eel-like xenacanths were adorned with thorns that made them resemble a unicorn, and the stetacanths had a sort of bristly comb on their heads. Finds in places like Beer Gulch help to understand the way of life of these strange forms. One of the fossils found there is the remains of two 15 cm Falcatus sharks. It looks like they died while mating. The female's jaws dug into the male's spike-like head ornament. Perhaps this can give us some idea of the mating habits of ancient individuals.
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Even the familiar species are being revised. “A good example is the Cretoxyrhina mantelli giant shark found in Kansas,” says Shimada. The fact that the teeth of this predator are similar to the teeth of the modern mako shark led paleontologists to hypothesize a connection between this ancient giant and a modern sea demon, moving at high speed. But then they changed their point of view. “The small amount of skeletal remains allowed us to more accurately assess the size of the body of this species, its shape, structure of teeth and even patterns of growth,” says Shimada, noting that Cretoxyrhina mantelli was a unique shark, different from the modern Mako. Measuring about seven meters in length, this "Ginsu shark" was a larger predator,and in its size and way of life it was more similar to the modern great white shark, although it did not feed on seals, but on marine reptiles such as mosasaurs and plesiosaurs.
Having studied this information, we can now turn our eyes to the megalodon. As is the case with most ancient sharks, we can mostly get an idea of the origin of the megalodon by our teeth. According to the paleontologist Catalina Pimiento of the University of Swansea, this famous shark is still debated in the scientific world about various details, but she is convinced that "the megalodon belongs to the extinct Otodontidae family", which comes from more ancient form Cretalamna.
Such disputes over classification are important not only for scientific communication, but also because by identifying the closest relatives of the megalodon, we can form new ideas about where sharks came from and how they behaved. For example, when scientists considered the megalodon a relative of the great white shark, they transferred the behavior of this predator to its larger relative. Now that scientists have moved the megalodon away from the great white shark, having found out that it is closer to other "megalodon" sharks, they have new questions about the predator, which seemed very familiar to them.
As paleontologists have found out, sharks, called megalodons, appeared about 20 million years ago. In this regard, the question arises, what events of that period could accelerate the evolution of such a terrifying fish. “During that time period, there were many changes in the environment, including global warming,” says Pimiento. This may be related to the rapid evolution of many new marine mammals, which were the main source of food for the megalodon. And the abundant food became the reason for such a frightening size of this predator. And of course, he was present in the ancient seas from the very beginning, occupying an important place there. “The body size of this species has not changed very much over time,” says Pimiento. That is, Magalodon has always been a giant. At that time, there were other large sharks that became the ancestors of the great white,but they were very much inferior in size to the megalodon.
The Megalodon was big and strong, and it did him good. This shark swam in the seas for over 17 million years, gradually bringing its prey to extinction. In this she was helped by changes in temperature and sea level, which have the most serious impact on the ocean habitat. We got the teeth and vertebrae that allow us to properly assess this powerful predator from a distance of 2.6 million years.
Brian Switek