Sidor, Christian Department of Biology, University of Washington, Seattle, Washington.
- Additional Readings
The clade that includes all living and extinct species that share a closer relationship with modern mammals than with modern reptiles. The evolutionary history of synapsids is usually described in terms of three major radiations. The oldest synapsids are Middle Pennsylvanian in age (approximately 320 million years ago [Ma]) and belong to the earliest, “pelycosaur” radiation. Pelycosaur-grade synapsids are known primarily from Texas, Oklahoma, and surrounding states, although several fossils have been discovered in Europe. By the Middle Permian (260 Ma), a group of advanced synapsids, the therapsids, had achieved nearly complete distribution across the supercontinent of Pangea. As with their pelycosaur forebears, therapsids diversified into both carnivorous and herbivorous species, some growing up to 3–4 m (10–13 ft) in length.
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