Shonisauruspopularis

Overview: The first of the marine reptiles, ichthyosaurs, appeared in the Triassic, and by the late Triassic they had become huge! One of the largest was Shonisaurus, discovered in Nevada in 1920 and named by Charles Camp in 1976. It was a predator that would eaten anything from fish to other marine reptiles, and it grew larger than many modern whales, making it one of the largest marine reptiles of all time!

Discovery: Shonisaurus popularis was discovered in the Luning Formation of Nevada in 1920, but excavation was not begun until 1954. In the 1950s and 1960s, these excavations, led by Charles Camp, produced 37 specimens out of what is now Berlin-Ichthyosaur State Park. In 1976, Camp named these large ichthyosaurs Shonisaurus popularis, with a genus name meaning “lizard from the Shoshone Mountains”, and a species name meaning “common”, likely referring to the large number of skeletons which had been discovered. In 2004, a second larger species, Shonisaurus sikanniensis, was discovered in the Pardonet Formation of British Columbia, Canada. This second species has at times been classified in the closely related genus Shastasaurus but is now considered safely Shonisaurus.

Evolution, Description, and Ecology: Shonisaurus popularis lived between 137 and 127 million years ago in a warm shallow that sea that covered what is now Nevada. It was a member of the Ichthyosauria, the first group of marine reptiles to evolve, and among the best suited to life at sea. They outwardly resembled dolphins with a fish/dolphin shaped body, a long snout, and a dorsal fin. Additionally, like dolphins, they gave birth to live young instead of laying eggs. However, they differed from dolphins by moving their tails side to side instead of up and down and in having four flippers instead of just two. Shonisaurus was a member of the family Shastasauridae which were very large and unlike later ichthyosaurs, more eel-like than the highly dolphin-like ichthyosaurs that would follow. Like all ichthyosaurs, Shastasaurus was a predator, and it likely hunted fish, ammonites, and other marine reptiles, perhaps including smaller ichthyosaurs and the strange long-necked tannystropheids. Shonisaurus grew between 13.5–15 meters (44–49 ft) long and is estimated to have weighed 21.6–29.7 tonnes (23.8–32.7 tons), similar in size to a modern humpback whale. It was one of the largest known marine reptiles, exceeded only by fellow early ichthyosaurs Cymbospondylus youngorum (17 m/56 ft), Shonisaurus sikanniensis (21 m/69 ft), and BRSMG Cg2488, an unnamed shastasaurid from Lilstock, England (22 m/72 ft), as well as possibly the mosasaurs Tylosaurus proriger (possibly up to 15.8 m/52 ft) and Mosasaurus hoffmanni (possibly up to 17.1 m/56 ft), though the size estimates for these mosasaurs are less certain and may have been exaggerated. Shonisaurus could be distinguished from other ichthyosaurs, not just by size, but also by its elongated flippers.

For years, scientists have been trying to figure out why so many Shonisaurus were found at the discovery site. One theory is that it represents a beaching event. However, recent research published in 2022, which revealed the remains of baby Shonisaurus among the adults, posits that Berlin-Ichthyosaur State Park was a birthing ground for the ichthyosaurs, to which numerous ichthyosaurs would migrate annually, much like modern whales. They also discovered that the ichthyosaurs in the park didn’t all die at once but accumulated over millennia as Shonisaurus migrated to the same location year after year.

Extinction and Legacy: No fossils of Shonisaurus popularis are known after 227 million years ago. However, the larger Shonisaurus sikanniensis lived until 210 million years ago, and it may be descended from S. popularis, perhaps even replacing it as the apex predator of the seas covering western North America. Giant ichthyosaurs continued to rule the world’s oceans until the middle Jurassic when they were replaced by the pliosaurs. Shonisaurus popularis can be seen in the Nevada State Museum in Las Vegas, Nevada, USA, and in situ fossils can be seen at Berlin-Ichthyosaur State Park, in Nye County, Nevada. Shonisaurus popularis was made the state fossil of Nevada in 1984.