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Transitional forms

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!

Horse evolution tree Our understanding of the evolution of horse feet, so often depicted in textbooks, is derived from a scattered sampling of horse fossils within the multi-branched horse evolutionary tree. These fossil organisms represent branches on the tree and not a direct line of descent leading to modern horses.

But, the standard diagram does clearly show transitional stages whereby the four-toed foot of Hyracotherium, otherwise known as Eohippus, became the single-toed foot of Equus. Fossils show that the transitional forms predicted by evolution did indeed exist.

As you can see to the left, each branch tip on the tree of horse evolution indicates a different genus, though the feet of only a few genera are illustrated to show the reduction of toes through time.



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Fossil evidence

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Homologies


Aetiocetus skull image courtesy of Tom Deméré, San Diego Natural History Museum; Gray whale skull image provided by Pearson College and Race Rocks

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Evo examples
Learn more about transitional features in Understanding macroevolution through evograms, a module exploring five examples of major evolutionary transitions in the fossil record.