A typical ichthyosaur looks like this (see note for derivation and pronunciation of "ichthyosaur", as well as its usage in this page).
Yes, just like a fish. The strange thing is that they were not fish at all: they were reptiles like lizards, snakes, and crocodiles. You can easily tell this by looking at the skull and fins. A similar case is known for mammals too: think about dolphins and whales.
Ichthyosaurs swam in the Mesozoic ocean when dinosaurs walked on land. To be precise, they appeared slightly earlier than dinosaurs (250 million years ago versus 230 ma) and disappeared again earlier (90 ma versus 65 ma).
The earliest ichthyosaurs are known from the Lower Triassic strata of Canada, China, Japan, Spitsbergen, and possibly Thailand. These first ichthyosaurs were not yet very much fish-shaped. Rather, they looked like a lizard with fins, just like in the figure below (fins were added from other specimens).
Ichthyosaurs diversified very quickly once they appeared. Several different body plans emerged in the Early and Middle Triassic. But, if you simplify the matter, you can see that there was a general transition from lizard-shaped body plan to fish-shaped one through the evolution of ichthyosaurs, as in the figure below.
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Last updated on November 15, 2000