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There's an important concept in biology known as "convergent evolution": creatures that occupy the same evolutionary niche tend to adopt roughly the same forms. The ichthyosaurs (pronounced ICK-thee-oh-sores) are an excellent example: starting about 200 million years ago, these marine reptiles evolved body plans (and behavioral patterns) strikingly similar to those of modern dolphins and bluefin tuna. (See a gallery of ichthyosaur pictures.)
Ichthyosaurs (Greek for "fish lizards") were similar to dolphins in another, perhaps even more telling way. It's believed that these undersea predators evolved from archosaurs (the terrestrial reptiles that preceded the dinosaurs) that ventured back into the water during the early Triassic period. Analogously, dolphins and whales can trace their descent to ancient, four-legged prehistoric mammals that gradually evolved in an aquatic direction.
Despite the resemblance of some species to dolphins or bluefin tuna, it's important to remember that ichthyosaurs were reptiles, and not mammals or fish. All of these animals did, however, share similar adaptations to their aquatic environment. Like dolphins, most ichthyosaurs are believed to have given birth to live young, rather than laying eggs like contemporary land-bound reptiles (the proof of this lies in the remains of some ichthyosaurs, such as Temnodontosaurus, that contain fossilized fetuses).
Also, for all their fish-like characteristics, ichthyosaurs possessed lungs, not gills--and therefore had to surface on a regular basis for gulps of air. It's easy to imagine schools of, say, Excalibosaurus frolicking above the Jurassic waves, perhaps sparring with one another using their swordfish-like snouts (an adaptation evolved by some ichthyosaurs to spear fish).