Early giant reveals faster evolution of large body size in ichthyosaurs than in cetaceans

Description Early marine giant The largest animals to have ever lived occupied the marine environment. Modern cetaceans evolved their large size over tens of millions of years in response to the increased productivity of cold marine waters. However, whales were not the first marine giants to evolve. Sander et al. describe a 244-million-year-old fossil ichthyosaur that would have rivaled modern cetaceans in size (see the Perspective by Delsett and Pyenson). The animal existed at most 8 million years after the emergence of the first ichthyosaurs, suggesting a much more rapid size expansion that may have been fueled by processes after the Permian mass extinction. —SNV A 244-million-year-old fossil of a massive ichthyosaur suggests that there was a burst of gigantism in the group after the Permian extinction. INTRODUCTION The iterative evolution of secondarily marine tetrapods since the Paleozoic offers the promise of better understanding how the anatomy and ecology of animals change when returning to the sea. Recurring patterns of convergence in the geological past may suggest predictability of evolution when transitioning from full-time life on land to full-time life in the ocean. Ichthyosaurs (fish-shaped marine reptiles of the Mesozoic) and today’s cetaceans (whales, dolphins, and porpoises) are two of the most informative lineages to exemplify secondary returns to the sea. 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. 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. RATIONALE The integration of fossil and extant data can improve understanding of aquatic adaptation and gigantism as patterns of convergent evolution, particularly when interpreted in an ecological context. Our paleontological fieldwork in the Fossil Hill Member (Middle Triassic, Nevada, USA) provided the basis for the marine reptile data and resulted in finds of giant ichthyosaurs as part of the pelagic Fossil Hill Fauna. We compiled data for both fossil and living whales from the extensive literature. Together, these data provide the basis for computational analyses of maximum body size and its evolution over time. Modeling of energy flux in the Fossil Hill Fauna helps in understanding how the Fossil Hill ecosystem could have supported several large to giant tetrapod ocean consumers so early in ichthyosaur evolutionary history. RESULTS We describe an ichthyosaur with a 2-m-long skull from the Fossil Hill Fauna as a new species of Cymbospondylus. At present, this is the largest known tetrapod of its time, on land or in the sea, and is the first in a series of ocean giants. The Fossil Hill Fauna includes several other large-bodied ichthyosaurs in the Cymbospondylus radiation. The body-size range in this Triassic fauna rivals the range seen in modern whale faunas, from a total length of about 2 m in Phalarodon to more than 17 m in the new species. As preserved in the fossil record, the Fossil Hill Fauna represents a stable trophic network and could even have supported another large ichthyosaur if it bulk fed on small, but abundant, prey such as ammonoids. In absolute time, the new ocean giant lived 246 million years ago, only about 3 million years after the appearance of the first ichthyosaurs. Our research suggests that ichthyosaurs evolved large body size very early on in the clade’s history, comparatively earlier than whales. CONCLUSION Ichthyosaurs and cetaceans both evolved very large body sizes, yet their respective evolutionary pathways toward gigantism were different. Ichthyosaurs seem to have benefited from the abundance of pelagic conodonts and ammonoids after the recovery from the end-Permian mass extinction, even in the absence of modern primary producers. Cetaceans took different routes, but all appear to be related to trophic specialization, including the loss of teeth in baleen whales (Mysticeti) and the evolution of raptorial feeding and deep diving in toothed whales (Odontoceti). Ichthyosaurs evolved large body sizes earlier in their history than cetaceans. The Fossil Hill Fauna of the Middle Triassic of Nevada, USA, is critical for recognizing this pattern. It features the first ocean giant among tetrapods, only 3 million years after ichthyosaurs first appeared. Whales took comparatively longer to attain similarly large body sizes. CREDIT: STEPHANIE ABRAMOWICZ Body sizes of marine amniotes span six orders of magnitude, yet the factors that governed the evolution of this diversity are largely unknown. High primary production of modern oceans is considered a prerequisite for the emergence of cetacean giants, but that condition cannot explain gigantism in Triassic ichthyosaurs. We describe the new giant ichthyosaur Cymbospondylus youngorum sp. nov. with a 2-meter-long skull from the Middle Triassic Fossil Hill Fauna of Nevada, USA, underscoring rapid size evolution despite the absence of many modern primary producers. Surprisingly, the Fossil Hill Fauna rivaled the composition of modern marine mammal faunas in terms of size range, and energy-flux models suggest that Middle Triassic marine food webs were able to support several large-bodied ichthyosaurs at high trophic levels, shortly after ichthyosaur origins.