Extinct animals hide their secrets well, but an exceptionally well-preserved fossil of an aquatic¹（水生的） reptile²（爬行动物） , with traces of soft tissue present, is providing scientists a new window into the behavior of these ancient swimmers. According to the study published in PLoS ONE's Nov. 16 issue, the fossil, characterized by a team led by Johan Lindgren of Lund University in Sweden, is from the mosasaur（沧龙） family, a group of reptiles³ that lived between 65 and 98 million years ago.

The fossil was found in Western Kansas, and was submerged under a shallow sea at the time of the mosasaur's existence. Previous analysis of mosasaur locomotion⁴ had been limited by a lack of soft tissue fossils, which was crucial for the scientists to truly understand the degree of aquatic adaptation that the creature had achieved. The new findings, which include scales and skin impressions, suggest that the mosasaur was able to minimize its frictional drag（摩擦阻力） in the water. Additional features suggest that it held the front of its body somewhat rigid⁵ during swimming, leading it to depend on the rear of its body and tail for propulsion（推进） .

According to Dr. Lindgren, this study provides "unique insights into the biology of an extinct group of marine⁶ lizards⁷ that became adapted to aquatic environments in a fashion similar to that of the preceding ichthyosaurs ('fish-lizards') and succeeding whales." Thus, these results may have implications for understanding how this group ultimately transformed from land-dwellers to pelagic（浮游的） cruisers in a relatively⁸ short period of geological time.