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 aquatic reptiles evolved body plans (and behavioral patterns) strikingly similar to those of modern dolphins and bluefin tuna. (See a gallery of ichthyosaur pictures.)
Stay up to date on the latest dinosaur news--sign up for the free About.com dinosaur newsletter today!
Ichthyosaurs are similar to dolphins in another, perhaps even more telling way. It's believed that these undersea predators evolved from terrestrial reptiles that ventured back into the water in the early Triassic (to put it another way, amphibians didn't split off hundreds of millions of years ago into separate branches of land and aquatic reptiles; rather, aquatic reptiles evolved from terrestrial reptiles). Dolphins and whales, analogously, are descended from land mammals that gradually evolved an aquatic lifestyle.
Early Ichthyosaurs
From an anatomical perspective, it's relatively easy to distinguish early ichthyosaurs from more advanced genuses. Ichthyosaurs of the middle to late Triassic, 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 were true ichthyosaurs at all, and hedge their bets by calling them proto-ichthyosaurs or ichthyopterygians.) Most early ichthyosaurs were fairly small, but the gigantic Shonisaurus may have attained lengths of 60 or 70 feet.
Although the exact evolutionary relationships are far from certain, there's some evidence that Mixosaurus may have been a transitional form between the early ichthyosaurs and later breeds. As reflected in its name (Greek for "mixed lizard"), this aquatic reptile combined primitive features--a downward-pointing, relatively inflexible tail and short flippers--with a sleeker shape and (presumably) a faster swimming style. Unlike most ichthyosaurs, remains of Mixosaurus have been found all over the world, so it must have represented a significant advance over its predecessors.
Later Ichthyosaurs
The early to middle Jurassic (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 genuses like Stenopterygius. Besides their streamlined shapes, these reptiles were characterized by their solid ear bones (which probably 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, possibly because it had evolved the ability to feed omnivorously (a fossil of this ichthyosaur has been found containing the remains of birds and baby turtles). Why did the ichthyosaurs perish? The answer may lie in the evolution of speedier fish (which were better able to elude predation), as well as better-adapted aquatic reptiles like plesiosaurs and mosasaurs.
Ichthyosaur Lifestyles
Despite the resemblance of some genuses to dolphins or bluefin tuna, it's important to remember that ichthyosaurs were reptiles, and not mammals or fish. However, they did share similar adaptations to an aquatic environment. Like dolphins, most ichthyosaurs are believed to have given birth to live young, rather than laying eggs like terrestrial reptiles (the proof of this lies in the remains of some genuses, such as Temnodontosaurus, that contain fossilized fetuses).
Also, for all their fish-like features, ichthyosaurs had lungs, and not gills--and therefore had to surface regularly for air. It's easy to imagine schools of, say, Excalibosaurus frolicking above the Jurassic waves, perhaps sparring one another with the their swordfish-like snouts (an adaptation evolved by some ichthyosaurs to suit their feeding habits).
Here's a list of the major ichthyosaur genuses; just click on the links for more information.
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.
Mixosaurus This "mixed lizard" may be the the missing link of ichthyosaurs.
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.
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.
