Shark: I have walked sideways in the sea for tens of millions of years

Thanks to Spielberg's masterpiece "Jaws", when we mention sharks, we often think of a figure that is agile and powerful, shuttling back and forth underwater, plus a terrifying bloody mouth. But there are actually certain stereotypes, because most species of sharks are relatively gentle and will not hurt people. You should know that there are more than 530 species of sharks in the world, and only a dozen of them have a record of hurting people. For example, the whale shark, the largest living fish, can reach a length of more than 10 meters. Although it is very large, they filter food such as algae, krill, and small fish, which is completely different from the fierce and majestic image of sharks in our impression. However, as the saying goes, "Good news doesn't go out, but bad news spreads a thousand miles." Because these few terrifying sharks represented by the great white shark have left us with an overly shocking and terrifying impression, we often ignore the gentle image of most other sharks.

Great white shark.

Image courtesy of British photographer Euan Rannachan

The famous novelist Hemingway once described the brutality of sharks in his novel The Old Man and the Sea: "These sharks have only thick, sharp blue heads, huge eyes, and snapping, all-devouring aggressive mouths. When they are hungry, they will not even let go of the oars or rudders on the boat. They will bite off the feet and fin-like limbs of turtles when they are asleep on the water; if they are really hungry, they will even attack people in the water, even though people do not have the smell of fish blood or fish mucus. They are killers who do all kinds of evil." The huge and ferocious megalodon in The Meg released in recent years has raised our impression of the horror of sharks to a new height. Such a prehistoric giant shark, which has been thought to have disappeared for a long time, suddenly appeared in front of the world like a nightmare and started a killing spree. The impact it brought can be imagined. However, the giant shark in The Meg is not completely fictional. Such sharks did exist in prehistoric times, and its mere teeth were 15-16 cm in size.

Movie poster of The Meg

Gai Zhikun, a researcher at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences, displays a megalodon tooth fossil (Photo provided by Gai Zhikun)

In fact, the sharks we are talking about now refer to most members of the class Chondrichthyes in a broad sense, but in a narrow sense, they only refer to the new sharks in the subclass Elasmobranchs of cartilaginous fish. The class Chondrichthyes to which sharks belong has a very long evolutionary history. They were born from ancient armored fishes more than 400 million years ago, and they had parted ways with our ancestors more than 400 million years ago. The earliest cartilaginous fishes appeared in the Early Silurian period more than 400 million years ago - Fanjingshan fish and Diplodocus of the class Acanthopterygii of cartilaginous fishes. It was not until the Devonian period that the slit-mouthed sharks with a more advanced ecological niche appeared. However, cartilaginous fishes were not large in size during this period, and basically lived under the shadow of sea scorpions or terrestrial fishes. This can be reflected in the size, distribution and number of their fossils.

One of the earliest cartilaginous fishes - the newly molded Fanjingshan fish.

Image from Plamen et al, 2022

Ecological restoration map of the Silurian-Devonian period. The blue fish near the middle of the water surface are early cartilaginous fish - sticklebacks.

Image credit: M.Hattri

Then, 290 million years ago, a group of ferocious "sea monsters" appeared on the earth and swam in the oceans around the world. The most representative of them were the lobulate tooth shark and the spiral tooth shark, which belong to the same cartilaginous fish as modern sharks. It was not until the Triassic to Jurassic period that what we call sharks in a narrow sense - the true carcharhiniformes in the subclass Elasmobranch - truly rose.

The forerunner of true sharks, the hybodonts, were very active in the Triassic oceans. They were widely distributed, and in addition to saltwater oceans, they even spread their footprints to inland freshwater lakes to prey on smaller animals in those waters.

By the Cretaceous period, we can already see tiger sharks, wobbegongs, monk sharks, cat sharks, and other real sharks that still exist in our oceans. At this time, there were two outstanding representatives of sharks, namely the Cretaceous sharpnose shark and the horn shark. The former may be similar to the great white shark we have today, and the latter is similar to the tiger shark of today. We can see tooth fossils embedded in many scattered animal bone fossils in the Cretaceous strata.

Two hungry horn-scaled sharks attack a dead hadrosaur.

Image source: Wikipedia

In the Cenozoic Era, the Megalodon, which we are familiar with in movies, appeared out of nowhere and became the undisputed overlord of the Tertiary ocean. This prehistoric giant shark, with an average length of 14 meters (as high as a 4-5-story building if hung up), and a maximum bite force estimated to be 20 tons, 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 squids and other marine creatures that are already heavy enough seem 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 used the stable isotope method and the correspondence between the nitrogen-15 isotope content in the enamel and dentin of modern marine animals and their position in the food chain (that is, the higher the nitrogen-15 ratio in the teeth, the more nutrients the organism has consumed and the higher its level in the food chain). He found that the nitrogen-15 ratio in megalodon tooth enamel was much higher than that of modern great white sharks, polar bears, and killer whales, indicating that the megalodon's nutritional level was very high and it was indeed at the top of the entire food chain at the time.

Table of the relationship between the size and impact level of Megalodon and the nitrogen isotopes of its tooth enamel

(Image from Emma et al. 2022)

In another journal published this year in Science Advances, paleontologist Jack Cooper combined the three-dimensional data of the spine and teeth of the ear-shaped megalodon with the soft skull reconstruction of the living great white shark to calculate a complete megalodon model. The calculated results show that the ear-shaped megalodon was more than 15 meters long and weighed up to 6,156 kilograms. It also swam faster and had the ability to cross the ocean and actively prey on large prey.

Jack Cooper reconstructed a 3D model of Megalodon using Megalodon teeth and the skull of a living great white shark.

(Image from Jack Cooper et al. 2022)

A schematic diagram comparing the sizes of all sharks since the Mesozoic Era, with the gray ones representing the living groups (Image source: Kenshu Shimada, DePaul University)

However, the fate of cartilaginous fish is very unfortunate. It took them 400 million years to let the megalodon take the throne of the ocean overlord, but this prosperity did not last long. The megalodon lived on the earth for 20 million years, but suddenly became extinct in the Pleistocene 2 million years ago, which may have a lot to do with the sudden cooling of the climate. Although this is the misfortune of sharks, it is also the luck of us humans to a certain extent. Imagine if there are such monsters in the sea, then the horror plot of the movie "Megalodon" may really come true.

References:

[1]Andreev, PS, Sansom, IJ, Li, Q. et al. Spiny chondrichthyan from the lower Silurian of South China. Nature609, 969–974 (2022).

[2]Nicholas D. Pyenson,Paul L. Koch,Oh, the shark has such teeth: Did megatooth sharks play a larger role in prehistoric food webs?, Science Advances, 8, 25, (2022).

[3]Cooper JA, Hutchinson JR, Bernvi DC, Cliff G, Wilson RP, Dicken ML, Menzel J, Wroe S, Pirlo J, Pimiento C. The extinct shark Otodus megalodon was a transoceanic superpredator: Inferences from 3D modeling. Sci Adv. 2022

[4]Kenshu Shimada, Martin A. Becker & Michael L. Griffiths (2021) Body, jaw, and dentition lengths of macrophagous lamniform sharks, and body size evolution in Lamniformes with special reference to 'off-the-scale' gigantism of the megatooth shark, Otodus megalodon, Historical Biology,