Ichthyosaurs were streamlined, dolphin-shaped reptiles, but their backgrounds differed from many reptiles living around them: These creatures’ ancestors went back to the sea after living on the land. Some scientists believe that ichthyosaurs were the first major group of reptiles to return to the sea. At first, they no doubt stayed close to the shoreline, just as seals and walruses do today. But after millions of years, the creatures went into the oceans, spread, and eventually became totally fish-shaped.

The oldest ichthyosaur fossils, and the most primitive so far, are 240-million-year-old fossils found in Japan. The primitive ichthyosaur, measuring about 9 feet (2.7 meters) long, probably lived its entire life in the water. Its shape was not yet like a dolphin; and its pelvis bone was still attached to the vertebrae, similar to those of land animals and dissimilar to the later ichthyosaurs. The primitive ichthyosaurs also had fins that were similar to the limbs of land reptiles (with splayed fingers). In other words, this primitive fossil shows the first step the ichthyosaurs took from the land to the oceans.

There was one other strange characteristic of the ichthyosaurs: For unknown reasons (as evident in the fossil record), about 135 million years ago, the animals began to fade away, becoming totally extinct between 90 and 100 million years ago. This was much earlier than the demise of the dinosaurs about 65 million years ago.