Transitional fossils are evidence for evolution, but often not recognized. Examples include Pikaia, a primitive notochord organism, Tiktaalik, a fishapod with basic wrist bones, and Archaeopteryx, a bird-dinosaur hybrid. Transitional fossils also exist between humans and chimpanzees.
Transitional fossils are the fossils of transitional life forms that represent an evolutionary bridge between two recognized groups. Transitional fossils are among the strongest evidence for Darwin’s theory of evolution and natural selection, but they are not sufficiently recognized. Many people believe that there are more gaps in the transitional fossil record than there actually are. In fact, hundreds of important transitional forms are known.
One of the obvious early transitional fossils is Pikaia gracilens, a simplistic lancet-shaped organism found in the Burgess Shale (505 million years ago) and the Flinders Ranges of Australia (560 million years ago). Pikakia is a transitional fossil between invertebrates and vertebrates (Chordata), showing a primitive notochord, precursor of a backbone. Pikakia averaged 1 cm (1 2/5 inches) and swam above the sea floor, where it likely ate microfauna and food particles. Pikakia fossilizes relatively poorly, with only 60 specimens found so far.
A later transitional fossil is the lobe-finned fish Tiktaalik, a proto-tetrapod that lived about 375 million years ago, during the late Devonian period. The flippers from Tiktaalik show basic wrist bones and simple digits, showing that they carried weight and that the animal at least engaged in limited forays onto land. Tiktaalik is considered a transitional fossil between fish and tetrapods. Because it shows characteristics of both, Tiktaalik and similar fossils have been dubbed “fishapods.”
Another transitional fossil, one of the most famous, is Archaeopteryx, transitional between birds and dinosaurs. Archeopteryx lived 155–150 million years ago, during the Late Jurassic. The first complete skeleton was announced in 1862, and the species became key evidence used to defend the theory of evolution in its infancy. Archeopteryx could grow up to 0.5m (1.6ft) in length, and despite its superficial resemblance to birds, it actually has more in common with small theropod dinosaurs.
Many transitional fossils have been found between humans and our immediate ancestors, chimpanzees. Because these species lived relatively recently, only the last few million years, these fossils tend to be better preserved than older ones. Homo habilis and Homo erectus are two common examples of such transitional fossils and tell us a lot about hominin evolution.
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