New fossil reveals early and rapid evolution of giant Mesozoic ichthyosaurs

Although whales are now the largest of Earth’s creatures, they were not the first ocean giants to ply the seas. In a new study, researchers report 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. While it took whales about 90% of their 55-million-year history to evolve into the ocean giants we know today, ichthyosaurs evolved to similar sizes in the first 1% of their 150-million-year history on Earth. The findings suggest that Triassic marine food webs could support such massive creatures, despite the absence of many primary producers following the Permian extinction 252 million years ago. Ichthyosaurs were among the first vertebrates to thrive in the oceans after the Permian extinction and persisted through most of the Mesozoic (250 Ma – 66 Ma). Here, Martin Sander and colleagues describe the well-preserved, 244-million-year-old remains of a new and very large species of ichthyosaur – C. youngorum – which were discovered in the dry mountains of northwestern Nevada. According to the skull – which measures nearly 2 meters in length – the creature is estimated to have been more than 18 meters long when alive, rivaling the size of some of the largest modern whales. However, what makes this find more notable is the species’ age. According to Sander et al., 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 8 million years after the emergence of the group, suggesting a rapid evolution in body size. “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,” write Lene Delsett and Nicholas Pyenson in a related Perspective, ”especially if we want to sustain the presence of the surviving ocean giants among us that contribute to our own well-being.”