You might be forgiven for mistaking Ichthyosaurus for the Jurassic equivalent of a bluefin tuna: this marine reptile had an amazingly fishlike shape, with a streamlined body, a finlike structure on its back, and a hydrodynamic, two-pronged tail. (The resemblance can be chalked up to convergent evolution, the tendency for creatures inhabiting the same ecological niches to evolve the same general features.)
One odd feature of Ichthyosaurus is that it possessed thick, massive ear bones, which likely onveyed subtle vibrations in the surrounding water to this marine reptile’s inner ear (an adaptation that doubtless aided Ichthyosaurus in locating and eating fish). Also, because various fossil specimens of Ichthyosaurus have been discovered with the remnants of babies nestled inside, it’s believed that this marine reptile didn’t lay eggs like most other reptiles, but gave birth to live young.
As you may have guessed from its name, Ichthyosaurus has lent its name to an important family of marine reptiles, the ichthyosaurs, which descended from terrestrial reptiles that ventured into the water during the late Triassic period. Before they faded from the scene (supplanted by better adapted plesiosaurs and pliosaurs), in the late Jurassic period, the ichthyosaurs produced some truly massive genera, most notably the 30-foot-long, 50-ton Shonisaurus.