In an Evolutionary Race to Dominate the Seas, Ichthyosaurs Were the Winners

About 246 million years ago, a sea lizard with a skull the size of a grand piano died in the ancient ocean that is now Nevada. It was an ichthyosaur, and its body was most likely the size of a modern sperm whale.

Although ichthyosaurs and whales are separated by a few hundred million years, they have a lot in common. Both descend from lineages of animals that returned to the sea after stints on land. Both evolved giant bodies that made them the largest creatures in the seas when they lived. Both birthed live young.

But it took whales 45 million years of living in the ocean to evolve their most giant body sizes. This new species of giant ichthyosaur appeared only three million years after the first ichthyosaurs took to the seas, suggesting the sea lizards evolved big bodies at a breakneck speed. This early giant lived before small dinosaurs were common on land; the terrestrial world would not see a giant this size for about 40 million more years, with the emergence of sauropods in the Jurassic.

A group of scientists describe the new ichthyosaur, which they named Cymbospondylus youngorum, and reconstructed its food webs in a paper published on Thursday in the journal Science.

“It is definitely a surprise,” said Benjamin C. Moon, an ichthyosaurus researcher at the University of Bristol in England who was not involved with the research. “It’s not a long time to go from pretty much just in the water to suddenly dominating in such massive sizes.”

The ichthyosaur was first discovered in 1998 in Fossil Hill, Nev. But excavations did not begin until 2011 because the bones rested in steep mountains, making it difficult to transport equipment to the site, said Lars Schmitz, a paleontologist at Scripps College in California and an author of the paper. “It’s very strenuous,” Dr. Schmitz said. “It was a huge effort to get it out of the field.”

To Dr. Schmitz, the fossil’s large size was humbling, even half-buried — the reptile’s humerus dwarfed his rock hammer. “It makes you feel very small,” he said.

An artist’s reconstruction of Cymbospondylus youngorum in the Triassic ocean, present-day Nevada.Credit…Stephanie Abramowicz/Natural History Museum of Los Angeles County

In 2015, the researchers finished excavating all that remained of the ichthyosaur — its skull, shoulder and arm bones — and sent the fossil to be prepared at the Natural History Museum of Los Angeles County. “It was mind-blowing seeing it,” said Jorge Velez-Juarbe, an associate curator of marine mammals at the museum and another author of the paper.

Based on the size of its skull, the authors estimate the ichthyosaur very likely grew as long as 55 feet. Dr. Moon said this might be a slight overestimate and suggested a more conservative 45 to 50 feet. “The same ballpark of modern day whales,” they said. “There was nothing else as big as these things around.”

The ichthyosaur swam in the seas of the Triassic Era shortly after the most severe mass extinction in Earth’s history, which killed off 81 percent of marine life. The researchers had one question: “How did it become so big?” Dr. Schmitz said.

In modern oceans, many giant whales are filter feeders, straining krill and other plankton through the plates of their mouths. But this abundance of modern plankton, which enabled whales to become so large, did not exist when the ichthyosaurs lived, which might suggest those ancient oceans did not have enough energy to support such a large predator.

Eva Maria Griebeler, an evolutionary ecologist at Johannes Gutenberg University Mainz in Germany and an author of the paper, examined fossils gathered from the Nevada site to reconstruct the food webs of the ichthyosaur’s ancient seas. She and other researchers consulted teeth and stomach content, as well as size differences between food web members, to understand who ate whom, Dr. Griebeler said. The ichthyosaur’s bluntly pointed teeth suggest it fed on fish and squid, and perhaps even smaller marine reptiles.

“Count the number and size of the predators at the top, and the number and sizes of their prey and see whether these numbers add up,” Dr. Moon said, explaining the model.

Removing the excavated skull fossil required helicopter support.Credit…Martin Sander

Dr. Griebeler’s model found that the abundance of ammonites alone provided enough energy to support the giants. They did not feed directly on the ammonites, but they ate other creatures that crushed the shelled cephalopods: a shorter, less diverse food web that still offered the same energy input as modern oceans. “It’s this astonishing thing,” Dr. Griebeler said. “This food web has a completely different structure than extant ones.”

Lene Liebe Delsett, a paleontologist at the Smithsonian National Museum of Natural History who was not involved with the research, praised the study’s food web model as a “first step” toward understanding the Triassic ocean environment. “There’s still so much we don’t know about these early ecosystems,” she said.

And how did ichthyosaurs manage to balloon in a paltry three million years when whales took 45 million years? Dr. Velez-Juarbe said he could not think of any other marine vertebrates that evolved large body sizes as quickly as the ichthyosaurs did. But the authors offer a number of possible explanations, including that the reptiles’ large eyes and endothermy may have made them better hunters. Or perhaps the mass extinction offered life an opportunity to diversify, reducing the number of competing predators.

Dr. Delsett, who wrote a perspective in Science accompanying the new paper with Nick Pyenson, also a paleontologist at the Smithsonian, believes research on extinct marine giants can offer insight into the conservation of whales.

“They lived through one mass extinction and survived; they lived through climate change,” Dr. Delsett said of the ichthyosaurs. “If you can understand marine evolution, it is easier to take better care of the oceans today.”

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