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Name: Ichthyosaurus (Greek for "fish lizard")
Habitat: Oceans worldwide
Historical Period: Early Jurassic (200-190 million years ago)
Size and Weight: About six feet long and 200 pounds
Distinguishing Characteristics: Streamlined body; pointed snout; fishlike tail
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 two otherwise dissimilar creatures inhabiting the same ecological niches to evolve the same general features.)
One odd fact about Ichthyosaurus is that it possessed thick, massive ear bones which likely conveyed subtle vibrations in the surrounding water to this marine reptile's inner ear, an adaptation that doubtless aided Ichthyosaurus in locating and eating fish as well as avoiding encroaching predators). Based on an analysis of this reptile's coprolites (fossilized excrement), it seems that Ichthyosaurus fed mainly on fish and squids.
Various fossil specimens of Ichthyosaurus have been discovered with the remnants of babies nestled inside, leading paleontologists to conclude that this undersea predator didn't lay eggs like land-dwelling reptiles, but gave birth to live young. This was not an uncommon adaptation among the marine reptiles of the Mesozoic Era; most likely the newly born Ichthyosaurus emerged from its mother's birth canal tail-first, to give it a chance to slowly acclimate to the water and prevent accidentally drowning.
Ichthyosaurus has lent its name to an important family of marine reptiles, the ichthyosaurs, which descended from an as-yet-unidentified group of terrestrial reptiles that ventured into the water during the late Triassic period, about 200 million years ago. Unfortunately, not a whole lot is known about Ichthyosaurus compared to other "fish reptiles," since this genus is represented by relatively scant fossil specimens. (As a side note, the first complete Ichthyosaurus fossil was discovered in the early 19th century by the famous English fossil hunter Mary Anning, the source of the tongue-twister "She sells sea shells by the sea shore.)
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. Unfortunately, very few ichthyosaurs managed to survive past the end of the Jurassic period, about 150 million years ago, and the last known members of the breed seem to have disappeared about 95 million years ago during the middle Cretaceous (about 30 million years before all the marine reptiles were rendered extinct by the K/T meteor impact).