 As <unk> continued to flourish and diversify in the Late Permian ( 260 @.@ 4 - 251 @.@ 0 Mya ) , a major group called <unk> became more dependent on life in the water . The vertebrae became weak , the limbs small , and the skull large and flat , with the eyes facing upwards . During the Triassic period , these animals dominated the freshwater ecosystems , evolving in a range of both small and large forms . During the Early Triassic ( 251 @.@ 0 - 245 @.@ 0 Mya ) one group of successful long @-@ snouted fish @-@ eaters , the <unk> , even adapted to a life in the sea , the only known amphibians to do so with the exception of the modern crab @-@ eating frog . Another group , the <unk> , included medium- and large @-@ sized animals 2 @.@ 3 to 4 m ( 7 @.@ 5 to 13 @.@ 1 ft ) in length , with large and flat skulls that could be over a meter long in the largest forms such as Mastodonsaurus . These animals spent most or all their lives in water as aquatic predators , catching their prey by a sudden opening of the upper jaw and sucking in fish or other small animals . 
