 Recently , the theory that <unk> were the ancestors of all <unk> has gained wide support . The skull morphology of some small <unk> has been compared to those of modern frogs and <unk> , but the presence of <unk> , <unk> teeth in small , <unk> or immature <unk> has been cited as the most convincing argument in favor of the <unk> origin of <unk> . <unk> in <unk> and many <unk> <unk> , <unk> teeth have <unk> tips and bases . During the development of most <unk> , teeth begin to <unk> at their tips . <unk> normally proceeds downward to the base of the tooth , but <unk> from the tip stops abruptly in <unk> teeth . <unk> resumes at the base , leaving an area in the center of the tooth <unk> . This pattern is seen in living amphibians and fossils . 
