Whale-Sized Marine Reptiles Once Ruled the Seas

The Swiss Alps might seem like a strange place to go searching for ocean life. The landlocked country is far from the ocean, the highest peaks in the range reaching over 14,000 feet above sea level. And yet, as published this year in the Journal of Vertebrate Paleontology, rock layers in those mountains have yielded the partial remains of whale-sized reptiles that swam the seas about 205 million years ago. Among the biggest of the shark-shaped ichthyosaurs, these ocean-dwelling saurians were some of Earth’s first giants–part of an evolutionary drama that paleontologists are just beginning to understand.

The as-yet-unnamed ichthyosaurs from the Swiss Alps are not alone. Paleontologists have also uncovered giant ichthyosaurs from fossil sites in Nevada, British Columbia, and England in the past 20 years. Many of those species match or exceed the largest ichthyosaurs previously known, reaching lengths that rival the blue whale for the biggest oceangoing animals of all time. These new finds are rewriting the story of when and why ichthyosaurs evolved into giant species.

So far as paleontologists have been able to piece together, the earliest ichthyosaurs were about six feet long and evolved about 249 million years ago during the early part of the Triassic Period. This was a time when life on Earth was still recovering from the world’s worst mass extinction, with reptiles proliferating and diversifying into new forms and niches. Starting from land-dwelling ancestors, ichthyosaurs quickly became adapted to life in the water and swam with snake-like motions of their bodies—and some got big very fast. Late last year paleontologists described Cymbospondylus youngorum, an ichthyosaur that was likely more than 50 feet long—about the size of a humpback whale—that lived about 244 million years ago. While five million years is long time in absolute terms, a close reading of the fossil record indicates that ichthyosaurs evolved giant sizes in a fraction of the time it took whales to do the same.

But while many of the largest whales are filter feeders that often seek out small prey like krill, giant ichthyosaurs were far from gentle giants. “The big ichthyosaurs were all predatory and filled many more niches than whales today,” says University of Manchester paleontologist Dean Lomax. Many of these giant ichthyosaurs were more like sperm whales or orcas, apex predators that had mouths full of sharp teeth. They swam seas that were swarming with ancient cephalopods, such as coil-shelled ammonites, but the big ichthyosaurs were certainly capable of feeding on sharks, smaller marine reptiles, and other ichthyosaurs, as well.

Big ichthyosaurs could be found in various waters at different times between 249 and 90 million years ago, but the Triassic—from 251 to 201 million years ago—was clearly a heyday for the biggest of the big. “Ichthyosaurs reached their peak size during the Mid- to Late-Triassic, with top estimates ranging from 82 to more than 98 feet,” Lomax says. Paleontologists typically find only bits and pieces of the largest ichthyosaurs, such as isolated teeth or vertebrae, but relatively complete giants have also been found. Shastasaurus sikanniensis from British Columbia reached lengths of 68 feet, as long as the largest known sperm whale. Such finds have led paleontologists to some critical questions about how and why these big ichthyosaurs evolved. We know ichthyosaurs got big, and fast, but what allowed them to do so?

Being terrestrial creatures ourselves, we sometimes think of giant size as something exceptional or something that requires a special explanation. Big animals are heavy, after all, and need a great deal of food. But life in the water is different, and being big has some distinct advantages. In a 2019 study of ichthyosaur shapes and sizes, Natural History Museum London paleontologist Susana Gutarra Díaz found that bigger bodies offer a significant bonus in the water. By studying the relationship between body size, drag while moving through the water, and the energy needed to swim, Gutarra Díaz and coauthors found that living large allowed ichthyosaurs to evolve into a wider array of shapes than would otherwise be possible.

“As animals become increasingly aquatic,” Gutarra Díaz says, “the bodies and limbs become streamlined.” Such adaptation lets water flow more smoothly over an animal’s body while it swims. That’s why many different animals in the seas—such as sharks, whales, and ichthyosaurs—have overall similar body shapes. But efficiently moving through the seas isn’t just about evolving an ideal shape for swimming. Animals that swim have to cope with drag, or the effects of water resisting an animal moving through it. The greater the drag, the more energy an animal has to expend to overcome it and move through the water. Being big allowed some ichthyosaurs to get around the problem.

A 50-foot-long ichthyosaur would have had more internal volume compared to its surface area than a 10-foot-long ichthyosaur. That means that big ichthyosaurs had more muscle mass compared to their surface area, or a greater proportion of their bodies able to make powerful movements to overcome the drag on their skin. To put it another way, a big ichthyosaur wouldn’t have to work quite so hard to quickly swim a hundred feet as a smaller ichthyosaur with less muscle compared to their surface area. Size is more important than shape, which explains why some of the big ichthyosaurs don’t look quite as “fast” as some of their smaller relatives. Many big ichthyosaurs did not have the classic “tear drop” body shape often expected for supremely-adapted aquatic animals.

Such biomechanical trade-offs offer a view into what allowed ichthyosaurs to grow so large, but what circumstances drove these reptiles to become leviathans is another matter. Paleontologists are still working to understand ocean productivity during the ancient past, how food webs, nutrient cycles, and other natural phenomena interacted to allow the evolution of creatures and ecosystems that might seem unfamiliar to us. But even in the absence of such direct evidence about ocean food webs, there may be a simpler answer to the question.

“I think one of the key reasons why ichthyosaurs reached such giant size,” Lomax says, “is because nobody else was doing that.” Especially in the wake of a mass extinction that wiped out about 95 percent of sea-dwelling species, the oceans of the earlier parts of the Triassic were a stage for recovery—an open field of possibilities for various reptilian creatures that started to become adapted to life in the water. Ichthyosaurs were among the first to dive in a sea that did not yet have giants in it. “Prior to the appearance of giant ichthyosaurs, the top predators in the seas were large fish and invertebrates,” Lomax notes, creatures which only reached much more modest lengths of 20 feet or so. “Ichthyosaurs changed the game.”

Riley Black | | READ MORE

Riley Black is a freelance science writer specializing in evolution, paleontology and natural history who blogs regularly for Scientific American.