 Several <unk> of <unk> were recognized in the late 19th and early 20th centuries . Animals now regarded as <unk> were primarily <unk> , but some were classified in the <unk> . <unk> were small @-@ bodied and had simple conical teeth , while <unk> were larger and had complex , folded <unk> and enamel in their teeth . <unk> included only a few forms , such as <unk> from Europe and <unk> from North America , that had poorly developed bones , external gills , and no ribs . Some skeletons of <unk> were later found with long ribs , prompting its <unk> to <unk> ( although more detailed studies found it to be a <unk> ) . Soft tissue , such as scales and external gills , were found in many well @-@ preserved <unk> fossils from Germany . In the early 20th century , <unk> would be recognized as <unk> forms of <unk> lacking many of the typical features that define the group , and is no longer recognized as a distinct group . 
