2021年雅思阅读机经类11

of the primitive archosaurs. Their bodies，however，developed the external appearance of the phytosaurs because of their aquatic lifestyle. The crocodile is the only archosaur that survived the still unknown factors that wiped out most of the reptile class at the end of the Mesozoic period. Though modern crocodiles walk on 4 legs，their two legged ancestry is revealed by their hind legs which are longer than the front legs，making them slant forward when they stand. The crocodilian skull still carries a basically archosaurean shape. It has a rather long，pointed skull，especially in the fish eating species of crocodiles.

The biggest，most prominent change in the crocodile since its early days is to the palate. The palate is the flat bony part at the roof of the mouth. In phytosaurs，the nostril holes in the palate are located under the outer nostrils，which were shifted to the far back of their snout. However in crocodiles，the nostrils are located at the front of the snout. This caused a problem in keeping the breathing passages from filling with water. Millions of years of evolution solved this problem. A second palate was formed，channeling the air above the mouth and into the throat passageway，where it can be opened and closed by a special flap or valve of skin. Crocodiles are actually classified on the basis of how far back their secondary palate extends，ranging from those who have no secondary palate to those with a fully formed palate separating the air they breathe from the water in their mouths.

The first crocodilians were called Protosuchians，living during the late Triassic to early Jurassic times. The difference between these and modern crocodiles is in the legs. The Protosuchians had very strong overdeveloped legs，which were set at right angles to the body. These allowed the animal to carry itself higher off the ground and probably gave them much more speed than their modern day counterparts. These legs also lead scientists to believe that these reptiles mostly lived on land. Beginning in the Jurassic period，the crocodilians quickly filled the gap of the extinct phytosaurs，becoming large and fully aquatic reptiles. The Mesosuchians were the next evolution of the protosuchians and lived during the Jurassic period and beyond. These reptiles had not yet developed a secondary palate，but were much more adapted to aquatic life than its predecessor. They had strong armor plating，made from a series of paired large bones on their back with smaller plates on its undersides. They spread throughout the land and lived in fresh water，though some adapted for