Ocean giants of the past remind us they may not always exist

A new fossil is providing scientists with a better understanding of how Mesozoic ichthyosaurs evolved. Their rapid evolution, followed by their abrupt disappearance, is proof that even the most powerful creatures in the world are not safe from the impacts of abrupt environmental change. 

Ichthyosaurs appeared slightly earlier than dinosaurs, and went extinct about 25 million years before dinosaurs were wiped out. The giant marine reptiles looked like a dolphin with large teeth. 

Whales are now the biggest animals on Earth, but some ichthyosaurs were even larger. In a new study published by AAAS, researchers describe the discovery of new and exceptionally large ichthyosaur fossils, which hint at an early and rapid burst in the evolution of extreme body size in Mesozoic oceans. 

“The notable resemblance in body shape and lifestyle of ichthyosaurs and cetaceans contrasts with their separation in time by nearly 200 million years, providing an often-cited example of convergent evolution,” wrote the study authors. 

“Ichthyosaurs arose 249 million years ago and populated the oceans for the next 150 million years. Cetaceans did not evolve until about 56 million years ago. As tail-propelled swimmers, ichthyosaurs and cetaceans evolved not only convergent body shapes but also large body sizes.”

For whales, it took about 90 percent of their 55-million-year history to evolve into the massive animals they are today. The researchers found that for ichthyosaurs, it took just one percent of their 150 million years to evolve to similar massive sizes.

The results of the study suggest that marine food webs during the Triassic were stable enough during the Triassic to support huge animals, despite the absence of many primary producers following the Permian extinction 252 million years ago. 

Led by Martin Sander of The Dinosaur Institute at the Los Angeles Natural History Museum, the research team analyzed the well-preserved, 244-million-year-old remains of a new and very large species of ichthyosaur known as C. youngorum. The fossils were discovered in the mountains of northwestern Nevada. 

The length of the skull, which measures nearly two meters, indicates the creature would have been more than 18 meters long. Even more notable is the relatively young age of this particular species. According to the experts, C. youngorum evolved merely 2.5 million years after the appearance of the oldest proposed ichthyosaur relative, which was less than a meter in size, and at most eight million years after the emergence of the group. This suggests that ichthyosaurs experienced a rapid evolution in body size. 

In a related Perspective, Lene Delsett and Nicholas Pyenson said : “Ichthyosaur history tells us ocean giants are not guaranteed features of marine ecosystems, which is a valuable lesson for all of us in the Anthropocene. especially if we want to sustain the presence of the surviving ocean giants among us that contribute to our own well-being.”

The study is published in the journal Science.

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By Chrissy Sexton, Earth.com Staff Writer