 <unk> became a commonly used name at the turn of the century . <unk> included both <unk> and <unk> in the group . Cope 's <unk> and <unk> fell out of use . In 1919 , British paleontologist D. M. S. Watson proposed that the evolutionary history of these large amphibians could be seen through changes in their vertebrae . <unk> forms in the <unk> graded into <unk> forms in the <unk> , and finally into <unk> in the <unk> . More importantly , Watson began using the term <unk> to refer to these groups . The name <unk> was rarely used in the decades that followed . Swedish paleontologist <unk> <unk> @-@ <unk> removed <unk> from the group , narrowing its scope to <unk> and <unk> . His classification of <unk> was based heavily on characteristics of the skull rather than the vertebrae . 
