Sharks are renowned for their strength, size and predatory nature, but were the ancient cartilaginous fish always like this? The authors of the new study examined many modern sharks and their extinct ancestors and found that these fish lived near the bottom in the Mesozoic. But the drastic warming of ocean waters due to global warming — up to 28 degrees Celsius at the surface — forced them to move to shallower depths, change their fins, grow larger and become the effective predators we know.

Among the inhabitants of the modern biosphere of the Earth there are both very young and very old. For example, primates appeared relatively recently, and sharks or, for example, sponges have existed for hundreds of millions of years. The authors of a new article in the journal Current Biology He became interested in the history of sharks – rather primitive cartilaginous fish that appeared more than 400 million years ago. The skeleton of such animals has no bones – it consists entirely of cartilage, which is reflected in the name of the class. Cartilaginous fish include, in addition to sharks, rays, chimeras and their extinct relatives.

At the same time, modern shark groups, whose representatives we can observe alive, appeared 200 million years ago. That is, at the beginning of the Mesozoic – the era of the dominance of dinosaurs and other giant reptiles.

To reconstruct the evolution of fish, scientists collected data on 490 species of modern sharks (90 percent of the total). Biologists divided them into benthic (living near the bottom), pelagic (living in the water column and actively swimming) and benthopelagic (something in between) based on the depth of their habitat. Contrary to expectations, it turned out that most modern sharks (70 percent) live near the bottom, while only 13 percent of the sharks we are used to seeing near the surface.

This suggests that modern shark groups initially appeared on the bottom of the seas and only later mastered shallow waters. To test the hypothesis, biologists used data on living and extinct animals to create a model showing changes in some of the characteristics of sharks during their evolution.

It turns out that the ancestors of modern sharks were indeed benthic fish. But later many of them began to actively move closer to the surface. The first and most significant episode of the “rise” of sharks occurred about 122 million years ago; these were fish from the order of lamno-like. For 20-30 million years, depending on their sample, carcharin-like sharks independently followed each other, then (already in the Cenozoic) three more groups – the so-called glowing sharks, and later – dwarf spiny sharks and whale sharks.

What prompted demersal fish to drastically change their lifestyle? Obviously, global warming, which broke climate records during the Cretaceous thermal maximum: the average temperature of the ocean’s surface layer then exceeded 28 degrees Celsius. By the way, now this value is less than 10 degrees.

The problem for sharks was that warming water reduced the concentration of dissolved oxygen. Avoiding the sea, many (but not all) of the ancestors of modern sharks moved higher, where the water was warmer, more crowded, and where conditions were generally completely different.

Biologists have shown that the transition to a pelagic lifestyle had a noticeable effect on the body arrangement of sharks – their morphology. In particular, their fins, initially short and rounded (compared to their width), became increasingly elongated, which increased maneuverability. The body of the sharks grew as a whole, as estimated by the length of the fish from the snout to the base of the tail. Accordingly, their muscle strength, assessed by the arrangement of the muscles and the number of tail strokes per unit of time, also increased. As a result, sharks learned to swim faster; experiments on modern cartilaginous animals show that increasing water temperatures actually make them better swimmers.

Finally, the shark migration also increased the diversity of their new habitats. Other forms appeared that actively dominated the existing ecological niches. Thus, the appearance of more phlegmatic demersal fishes, which could afford to lie on the bottom without risking drowning, changed dramatically. Large and aggressive super predators appeared, constantly on the move and feeding on a wide variety of prey.

The authors emphasize that the temperature factor played a decisive role in the evolution of sharks, creating them in their modern form. The result may be useful in estimating the status of shark populations under modern global warming conditions. However, this is still far from the Cretaceous period, which triggered rapid evolutionary transformations of cartilaginous fish.