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   ocr: Tree( ofFishes D lower sides of their heads. Thus as a group they have an ancestry distinct from that of rays. Some traits not as easily observed as these have been discovered by studying DNA. Cladograms are diagrams that illustrate how closely animals in a group are related, which share common ancestors, and the sequence of the group through time. Arrangement of organisms in a cladogram depends on shared features that only a particular group possesses and on which of the group first developed such features. The diagram thus reflects a group's evolutionary history. 3 à