Why were prehistoric marine reptiles so huge?

The ocean would have been a scary place 200 million years ago. There were plenty of apex predators like the Shonisaurus, a 30-ton marine reptile with a beak like a dolphin. And Dakosaurus, a one-ton prehistoric crocodile that made both fish and pterosaurs feared. But among the most famous prehistoric marine reptiles was the elasmosaurus, with its long, elegant neck, mouth full of sharp teeth, and enormous body. If you saw one heading your way, you’d be sure to change course.

These curious creatures looked like something out of a science fiction novel, and they’ve long left scientists wondering how they came to be. But new research published in the journal Communications Biology may have answers. The researchers suggest that their huge size and long neck seemed to evolve simultaneously because their enormous body size dampened the drag their long necks created in the water.

“The rapid rates of evolution of neck proportions in long-necked elasmosaurs suggest that large trunks may have released hydrodynamic stresses [caused by their] neck,” according to the study’s authors.

larger than life

Elasmosaurs were a species of plesiosaur that swam similarly to a modern sea turtle – a method scientists call “underwater flight”. They waved their large fins as they whipped the water. Their elongated fins had evolved from legs as they moved out of land and into open waters.

We already know from modern aquatic tetrapods like whales and dolphins that larger size is an evolutionary advantage on the high seas, says Susana Gutarra Diaz, lead author of the study and researcher at the Natural History Museum in London. Although they can expend more energy as they grow in size, the energy expended per unit body mass is smaller, she says. In addition, there are also hydrodynamic advantages.

“As their size increases, the force they have to overcome to move through the water decreases relative to their size,” says Gutarra Diaz.

The purpose of their long necks is more of a mystery, though Gutarra Diaz is sure it has to do with hunting and feeding. One theory is that their long necks allowed for a sneakier attack on prey, with their huge trailing proboscis. Elasmosaurs, for example, had one of the longest necks, 43 feet long, with a tiny head and sharp teeth – perfect for catching small, slippery fish. Still, she says there’s a lot to learn about how they used their necks and how flexible they were while swimming and feeding.

“What we do know is that it would have been very expensive to have a long neck and a small body,” says Gutarra Diaz.

Evolutionary advantage

Research definitely moves the ball forward, according to Robin O’Keefeassociate professor of biological sciences at Marshall University who was not involved in the study.

“The test of a good article is that it answers questions and raises new ones, and it does both,” O’Keefe says. “The study makes a well-founded assumption that large body size is an evolutionary release. So when you get into extra large size, you can afford to have a really long, trailing neck.

O’Keefe says that while this is a strong guess, there’s still a lot we don’t know. He argues that the sample size was quite small and, more importantly, that this evolution did not happen everywhere. In other parts of the world, huge plesiosaurs did not have long necks, leading one to wonder why long necks were not common across the world.

“The only place we see super long necks is in the western interior, in other parts of the world their necks were conservative relative to their height,” says O’Keefe.

Clearly there was an evolutionary advantage to the super-long necks of elasmosaurs, but we’re far from sure why and how they benefited these prehistoric beasts. But whatever the reason, these huge reptiles continue to intrigue us 66 million years later. And researchers like Gutarra Diaz and O’Keefe will continue to unravel the ancient mysteries of these long-extinct mega sea beasts.

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