Size and Weight:
Shonisaurus was among the strangest of all ichthyosaurs ("fish lizards"), looking a bit like a cross between a dolphin and a whale with its thick, round body, long, narrow front and rear flippers, pointed mouth with teeth only on the front end, and double-pronged tail. At upwards of 50 feet long and 30 tons, this was one of the biggest animals on the planet during the late Triassic period, matched in size only by the giant marine reptiles and fish that lived tens of millions of years later, notably Liopleurodon and Leedsichthys.
Like other ichthyosaurs, Shonisaurus is believed to have evolved from land-dwelling lizards that returned to an aquatic lifestyle during the early Triassic period. That makes this family of marine reptiles roughly analogous to modern dolphins and whales, which also evolved from smaller, land-dwelling ancestors. (By the way, Shonisaurus is the state fossil of land-locked Nevada, which was submerged beneath a shallow body of water during much of the Mesozoic Era.)