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A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. [1] This is especially important where the descendant group is sharply differentiated by gross anatomy and mode of living from the ancestral group.
The following species are often understood to be transitional fossils, many of which can be found at List of transitional fossils. Subcategories.
"Missing link" is a recently-discovered transitional fossil. It is often used in popular science and in the media for any new transitional form. The term originated to describe the intermediate form in the evolutionary series of anthropoid ancestors to anatomically modern humans (hominization).
This lack of continuous fossils in the record is a major limitation in tracing the descent of biological groups. When transitional fossils are found that show intermediate forms in what had previously been a gap in knowledge, they are often popularly referred to as "missing links".
Figure 1:In mammals, the quadrate and articular bones are small and part of the middle ear; the lower jaw consists only of dentary bone.. While living mammal species can be identified by the presence of milk-producing mammary glands in the females, other features are required when classifying fossils, because mammary glands and other soft-tissue features are not visible in fossils.
The skull of Thrinaxodon is an important transitional fossil which supports the simplification of synapsid skulls over time. The most notable jump in bone number reduction had occurred between Thrinaxodon and Probainognathus, a change so dramatic that it is most likely that the fossil record for this particular transition is incomplete.
The fossil record includes well documented examples of both phyletic gradualism and punctuational evolution. [17] As such, much debate persists over the prominence of stasis in the fossil record. [ 18 ] [ 19 ] Before punctuated equilibrium, most evolution biologists considered stasis to be rare or unimportant.
A transitional fossil is any fossilized remains of a life form that exhibits traits common to both an ancestral group and its derived descendant group. [99] This is especially important where the descendant group is sharply differentiated by gross anatomy and mode of living from the ancestral group.