Thanks to Spielberg's masterpiece "Jaws", when it comes to sharks, we often think of an agile and powerful figure that shuttles underwater, coupled with a terrifying mouth. But there is actually a certain stereotype among them, because most species of sharks are relatively mild-tempered and will not hurt people. You must know that there are more than 530 species of sharks in the world, and only a dozen of them have records of injuring people. Whale sharks, the largest living fish, can reach more than 10 meters in length. Although they are very large, they filter feed on food such as algae, krill, and small fish, which is completely different from the ferocious and majestic image of sharks in our impression. Related. But as the saying goes, "Good things don't go out, but bad things spread thousands of miles." Because these few terrifying sharks, represented by the great white shark, leave an overly shocking impression on us, we tend to ignore the gentle images of most other sharks.

Great White Shark. From British photographer Euan Rannachan

The famous novelist Hemingway once described the cruelty of sharks in his novel "The Old Man and the Sea": "These are the sharks with only thick and sharp blue heads, huge Eyes and aggressive, snapping, all-devouring mouths, they won't even let go of the boat's oars or rudder when they're hungry, and they'll bite off the turtles' feet while they sleep on the water. and fin-like limbs; if they are really hungry, they will even attack people in the water, even though there is no smell of fish blood or fish mucus on them, they are killers who do all kinds of evil." The huge and ferocious Megalodon in "The Meg" has raised our terrifying impression of sharks to a new level. Such a prehistoric giant shark that has been considered to have disappeared for a long time suddenly appears in front of the world like a nightmare. The impact of such a killing spree can be imagined. However, the giant shark in "The Meg" is not completely fictional. Such a shark did exist in prehistoric times. Each of its teeth was 15-16 centimeters in size.

Poster of the movie "The Meg"

Gai Zhikun, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, shows the fossilized Megalodon tooth (photo provided by Gai Zhikun)

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In fact, the sharks we are talking about currently refer to most members of the Chondrichthyes class in a broad sense, but in a narrow sense they only refer to the new sharks in the Elasmobranch subclass of cartilaginous fishes. The class Chondrichthyes, to which sharks belong, has a very long evolutionary history. They were derived from ancient placoderms more than 400 million years ago, and had already diverged from our ancestors more than 400 million years ago. The earliest cartilaginous fishes appeared in the Early Silurian more than 400 million years ago - Cartilaginous fishes such as Fanjingshan fish and Dislotyma cartilage. It was not until the Devonian period that more ecological niches appeared. Advanced rip sharks, but cartilaginous fishes were not very big during this period and basically lived in the shadow of sea scorpions or dinofishes. This can be reflected in the size, distribution and quantity of their fossils.

After that, 290 million years ago Years ago, a group of ferocious "sea monsters" appeared on the earth, swimming in the oceans around the world. The most representative ones are the scallopodon shark and the spiral tooth shark. From the Triassic to the Jurassic period, plate The real rise of the carcharhinids in the branch branch.

The pioneer of sharks, the shark, showed its prowess in the Triassic oceans. They are widely distributed. In addition to saltwater oceans, they have even spread their footprints to inland freshwater lakes to prey on smaller animals in those waters.

In the Cretaceous period, we can already see tiger sharks, whisker sharks, flat sharks, and cat sharks, which are real sharks that can still exist in our oceans today. At this time, there were two outstanding representatives of sharks, namely the chalk nose shark and the horned shark. The former may be similar to our today's great white shark, and the latter is similar to today's tiger shark. We can see tooth fossils embedded by horned sharks in many scattered animal bone fossils in the Cretaceous strata.

Two hungry horned sharks are preying on a dead hadrosaurus. Source: Wikipedia

In the new generation, the megalodon that appeared in the movies we are familiar with appeared suddenly and became the undisputed overlord of the oceans in the Third Age. This kind of shark with an average length of 14 meters ( It is as high as a 4-5-storey building! ), and its maximum bite force is estimated to be 20 tons. It may be one of the creatures with the strongest bite force ever discovered in the history of the earth. In front of it, whales, giant squid and other marine creatures that are already heavy enough appear vulnerable. People have been studying this shark for nearly 400 years, and some new technologies and methods used by paleontologists in recent years have further confirmed the dominance of the megalodon. In a cover article titled "Cenozoic megatooth sharks occupied extremely high trophic positions" published in Science Advances this year, paleontologist Kast According to the stable isotope method, the corresponding relationship between the nitrogen-15 isotope content in the enamel and dentin of modern marine animals and their position in the food chain is used (that is, the higher the nitrogen-15 ratio in the teeth, the more nutrients the organism consumes. The higher the level of the food chain, the higher the level of the food chain.) It was found that the nitrogen-15 ratio in the enamel of the megalodon is much higher than that of modern great white sharks, polar bears, and orcas, indicating that the trophic level of the megalodon is very high. He was indeed at the top of the entire food chain at that time.

Relationship table between the size and impact level of megalodon and the nitrogen isotopes of its tooth enamel (from Emmaet, al. 2022)

Also published in "Science Advances" this year In another journal, paleontologist Jack Cooper calculated a complete model of the megalodon by combining the three-dimensional data of the spine and teeth of the ear-shaped megalodon with the reconstruction of the soft skull of the living great white shark. The calculated results show that the ear-shaped megalodon is more than 15 meters long, weighs up to 6156 kilograms, swims faster, and has the ability to cross the ocean and actively hunt large prey.

The three-dimensional model of the megalodon reconstructed by JackCooper using the teeth of the megalodon and the skull of the living great white shark (from JackCooperet, al. 2022)

Comparison of the sizes of all sharks since the Mesozoic Era, including The gray represents the extant group (source: Kenshu Shimada, DePaul University)

However, the fate of cartilaginous fish is very unfortunate. It took them a full 400 million years before Megalodon took over. The dominance of the oceans, but this prosperity did not last long. Megalodon has lived on the earth for 20 million years, but suddenly became extinct during the Pleistocene 2 million years ago. This is likely to have a lot to do with the sudden cooling of the climate. Although this is the shark's misfortune, it is also the luck of us humans to a certain extent. Imagine if there were such monsters in the sea, then the terrifying plot of the movie "The Meg" might really be reflected in reality. .