Fossils or organisms that show the intermediate states between an ancestral form and that of its descendants are referred to as transitional forms. There are numerous examples of transitional forms in the fossil record, providing an abundance of evidence for change over time.
Pakicetus (below left), is described as an early ancestor to modern whales. Although pakicetids were land mammals, it is clear that they are related to whales and dolphins based on a number of specializations of the ear, relating to hearing. The skull shown here displays nostrils at the front of the skull.
A skull of the gray whale that roams the seas today (below right) has its nostrils placed at the top of its skull. It would appear from these two specimens that the position of the nostril has changed over time and thus we would expect to see intermediate forms.
Note that the nostril placement in Aetiocetus is intermediate between the ancestral form Pakicetus and the modern gray whale an excellent example of a transitional form in the fossil record!
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Aetiocetus skull image courtesy of Tom Deméré, San Diego Natural History Museum; Gray whale skull image provided by Pearson College and Race Rocks
Understanding Evolution © 2015 by The University of California Museum of Paleontology, Berkeley, and the Regents of the University of California