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.
From an anatomical perspective, it's relatively easy to distinguish early ichthyosaurs from more advanced genera. The Ichthyosaurs of the middle to late Triassic period, such as Grippia, Utatsusaurus and Cymbospondylus, tended to lack dorsal (back) fins and the streamlined, hydrodynamic body shapes of Jurassic ichthyosaurs. (Some paleontologists doubt that these creatures were true ichthyosaurs at all, and hedge their bets by calling them proto-ichthyosaurs or "ichthyopterygians.") Most early ichthyosaurs were fairly small, but there were exceptions: the gigantic Shonisaurus, the state fossil of Nevada, may have attained lengths of 60 or 70 feet.
Although the exact evolutionary relationships are far from certain, there's some evidence that the appropriately named Mixosaurus may have been a transitional form between these early ichthyosaurs and later breeds. As reflected in its name (Greek for "mixed lizard"), this marine reptile combined some primitive features of early ichthyosaurs--a downward-pointing, relatively inflexible tail and short flippers--with the sleeker shape and (presumably) a faster swimming style of their later descendants. Unlike the case for most ichthyosaurs, remains of Mixosaurus have been found all over the world, a clue that it must have been especially well-adapted to its environment.
The early to middle Jurassic period (about 200 to 175 million years ago) was when ichthyosaurs truly ruled the seas. This was the age of the poster-fish of the ichthyosaurs, Ichthyosaurus, which is represented today by hundreds of fossils, as well as closely related genera like Stenopterygius. Besides their streamlined shapes, these marine reptiles were distinguished by their solid ear bones (which conveyed subtle vibrations in the water created by prey) and large eyes (the eyes of one genus, Ophthalmosaurus, were four inches in diameter!)
By the end of the Jurassic, most ichthyosaurs had gone extinct--though one genus, Platypterygius, survived into the early Cretaceous period, possibly because it had evolved the ability to feed omnivorously (a fossil specimen of this ichthyosaur has been found containing the remains of birds and baby turtles). Why did the ichthyosaurs go extinct? The answer may lie in the evolution of speedier prehistoric fish (which were better able to elude predation), as well as better-adapted marine reptiles like plesiosaurs and mosasaurs.
However, a recent discovery may throw a monkey wrench into accepted theories about ichthyosaur evolution. Malawania plied the oceans of central Asia during the early Cretaceous period, and it retained the primitive, dolphin-like body plan of genera that lived tens of millions of years before. Clearly, if Malawania could prosper with such a basal anatomy, ichthyosaurs may not have been "out-competed" by other marine reptiles after all, and we'll have to search for other reasons for their disappearance.
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).
Here's a list of the major ichthyosaur genera; just click on the links for more information.
Acamptonectes This "rigid swimmer" was closely related to Ophthalmosaurus.
Brachypterygius A big-eyed cousin of Ophthalmosaurus.
Californosaurus Guess what state this ichthyosaur was discovered in?
Cymbospondylus A very large--and very ancient--ichthyosaur.
Eurhinosaurus This ancient ichthyosaur looked like a modern sawfish.
Excalibosaurus This ichthyosaur was named after King Arthur’s sword.
Grippia The best specimen of this ichthyosaur was destroyed in World War II.
Ichthyosaurus A remarkably fish-like lizard of the Jurassic era.
Malawania A dolphin-like ichthyosaur of the early Cretaceous period.
Mixosaurus This "mixed lizard" may be the the missing link of ichthyosaurs.
Nannopterygius The most common ichthyosaur in the fossil record.
Omphalosaurus This "button lizard" may or may not have been a genuine ichthyosaur.
Ophthalmosaurus An ocean-dwelling reptile distinguished by its large eyes.
Platypterygius One of the rare ichthyosaurs to survive into the Cretaceous period.
Shastasaurus This ichthyosaur fed on soft-bodied cephalopods.
Shonisaurus The largest ichthyosaur yet to be discovered.
Stenopterygius A close relative of Ichthyosaurus.
Temnodontosaurus A dolphin-shaped ichthyosaur that gave birth to live young.
Utatsusaurus The most ancient ichthyosaur yet discovered.