 As <unk> continued to <unk> and <unk> in the Late <unk> ( 260 @.@ 4 - 251 @.@ 0 <unk> ) , 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 <unk> period , these animals dominated the freshwater ecosystems , evolving in a range of both small and large forms . During the Early <unk> ( 251 @.@ 0 - 245 @.@ 0 <unk> ) one group of successful long @-@ <unk> fish @-@ <unk> , 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 <unk> 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 <unk> . 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 . 
