pelycosaurs

Pelycosaurs are an informal grouping of early synapsids, often described as pelycosaur-grade amniotes, that lived during the late Carboniferous to early Permian periods, roughly 320 to 270 million years ago. They are among the earliest amniotes to branch from the lineage that would eventually include mammals and are part of the broader transition from primitive reptile-like ancestors to more mammal-like reptiles. They were widespread before therapsids became the dominant land vertebrates in the Permian.

Pelycosaurs are not reptiles. The term covers a diverse array of primitive synapsids that share certain skull features and dental patterns with later mammal-like animals. They occupied a range of ecological roles, including carnivores and, in some lineages, herbivores. A distinctive feature of several forms is a large sail along the back formed by elongated neural spines, a trait famously seen in Dimetrodon and in Edaphosaurus, the latter of which is interpreted as an adaptation for display or thermoregulation rather than flight or speed.

Anatomy and diversity: Pelycosaurs typically possessed sprawling or semi-erect limbs and differentiated dentition. Their skulls show the single temporal opening characteristic of synapsids. Some were large predators, while others evolved grinding teeth and beaked jaws for herbivory, reflecting a range of feeding strategies within a relatively compact time frame in Earth's history.

Evolution and legacy: By the middle to late Permian, pelycosaurs declined as therapsids diversified and eventually came to dominate terrestrial ecosystems in many regions. Fossils have been found in North America, Europe, and parts of Russia and Africa, indicating a wide geographic distribution. Notable genera include Dimetrodon (carnivorous with a prominent sail) and Edaphosaurus (herbivorous with its own sail).