The first giants on Earth: how whale-sized reptiles "ruled" the seas

Huge reptiles swam in the seas about 205 million years ago. They were called one of the largest sharks

ichthyosaurs and the first giants on Earth.

These ichthyosaurs from the Swiss Alps have not yet been givenname, but they are no longer alone. Over the past 20 years, paleontologists have also found giant ichthyosaurs in fossils in the US, Canada, and England. Many of these species match or even surpass the largest ichthyosaurs previously known. In length, they are similar to the blue whale, which grows to about 33 m.

With these findings, paleontologists are changing their understanding of when and why ichthyosaurs grew so quickly and dramatically.

Have these reptiles always been this huge?

Previously, paleontologists have established that the very firstichthyosaurs were only 180 cm long and evolved about 249 million years ago at the beginning of the Triassic period. It was a time when life on Earth was still recovering from the world's worst mass extinction, and reptiles were multiplying and expanding their range.

Starting with their land ancestors, ichthyosaursquickly adapted to life in the water: they swam like snakes, wriggling and leaning forward with their whole body. Later, they began to quickly adapt to the new environment and grow.

At the end of 2021, paleontologists describedCymbospondylus youngorum, an ichthyosaur that was probably over 15m long - about the size of a humpback whale - and lived about 244 million years ago. Five million years is a long time, but not for evolution. Nevertheless, ichthyosaurs managed to change dramatically. In comparison, it took whales much longer to grow in size the same way.

What did the giant ichthyosaurs eat?

Usually the largest whales feed on krill whenfilter the water. But giant ichthyosaurs looked for prey differently. “All large ichthyosaurs were predators and had a much more varied diet than modern whales,” says University of Manchester paleontologist Dean Lomax.

Many of these giant ichthyosaurs were largersimilar to sperm whales or killer whales - predators with a mouth full of sharp teeth. They swam in seas that teemed with ancient cephalopods, ammonites, which had a ringed shell. Ammonites are extinct relatives of living cephalopods such as squid and octopus. Their soft body was encased in a spirally coiled shell.

But the largest ichthyosaurs could certainly prey on sharks, small marine reptiles, and other ichthyosaurs.

Large ichthyosaurs could be found in different water bodies from 249 to 290 million years ago, but the Triassic - from 251 to 201 million years ago - is the period of their heyday.

“Ichthyosaurs were at their best inmid to late Triassic. According to the maximum estimates, they reached from 25 to 30 m, ”says Lomax. Paleontologists usually find only fragments of the largest ichthyosaurs, such as individual teeth or vertebrae. But it was possible to find relatively intact remains.

Shastasaurus sikanniensis found inCanada, reached a length of 21 m. The largest known sperm whale was the same length. All these findings have raised a logical question for paleontologists: how and why ichthyosaurs evolved so quickly.

How did ichthyosaurs manage to grow to such a size?

People think of huge creatures as something exceptional, so the researchers tried to understand how such a leap in development occurred.

In 2019, London's Natural History Museum paleontologist Susana Gutarra Diasvo examined the shapes and sizes of ichthyosaurs. She has found that being big in the water is an advantage.

The team studied the relationship between body size,resistance when moving through water and the energy needed for swimming. As a result, Diaz and co-authors found that due to their huge size, ichthyosaurs were able to evolve into completely different forms. If they were small, this would not have happened.

“As the animals begin to spendmore and more time in the water, their bodies and limbs become streamlined,” she says. This adaptation allows water to flow more smoothly over the animal's body during locomotion. This is why different sea creatures, such as sharks, whales and ichthyosaurs, have generally similar body shapes.

But not only the shape of the body allows you to quickly andswim efficiently. Animals that live in water must cope with its resistance or impact. The greater the resistance, the more energy it takes to overcome it and move on. Ichthyosaurs were large and were able to get around this problem.

This means that large individuals had moremuscle mass compared to their surface area, as well as a large part of the body could make powerful movements to overcome the force of water. In other words, a large ichthyosaur would not have to work as hard to swim a kilometer fast as a smaller ichthyosaur, which has less muscle relative to its surface area.

Size is more important than shape - this explains whysome large ichthyosaurs do not look as fast as other small marine animals. They did not have the classic streamlined body shape that looks like a teardrop.

All these biomechanical compromises helpunderstand why it was beneficial for ichthyosaurs to grow up. But what circumstances caused these reptiles to become huge leviathans is another matter. Paleontologists are still working to understand what was happening in the oceans of that time: how food webs interacted, what were nutrient cycles and other natural phenomena that allowed creatures to evolve.

However, the answer to this question can be very simple:

“I think one of the key reasons ichthyosaurs got to such gigantic sizes is because no one else has done it,” says Lomax.

Especially after the mass extinction, whichdestroyed about 95% of the species living in the sea. The oceans of that time were undergoing a recovery phase, which means that it was an open field of opportunity for various reptiles that were just beginning to get used to life in the water.

Ichthyosaurs were among the first to dive into the sea, which did not yet have any giants.

“Before the appearance of large ichthyosaurs, the mainpredators in the seas were large fish and invertebrates. They were relatively small. Only 6 m long or more. But ichthyosaurs have changed the rules of the game,” Lomax concluded.

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