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Pelycosaurs (from the Greek pelyx meaning "bowl" and sauros meaning "lizard") are an order of tetrapods that emerged during the Late Carboniferous, flourished during the Early Permian, then were replaced by their descendants, the therapsids, during the Late Permian. This primitive group was the dominant form of terrestrial life for about 40 million years, 3/5ths of the duration of dominance of mammals, and was among the first amniotes (non-amphibian tetrapods) to evolve. It is one of the most primitive groups of tetrapods, after basal tetrapods and early amphibians.
Pelycosaurs ranged in size from a large rat to a wolf to bigger than an elephant. Being the dominant form of terrestrial life, they evolved to exploit numerous roles, including herbivores, insectivores, and carnivores. Some resembled robust lizards, others were more foreign, such as the iconic Dimetrodon grandis, an apex predator 3 1/2 meters (11 feet) in length, with huge vertebral extensions that created a large skin-covered sail. This animal had a somewhat dragon-like appearance. Other pelycosaurs shared the back sail, which is believed to have been used to increase the temperature of the animal. Another pelycosaur, Cotylorhynchus, was a titanic barrel-shaped herbivore with a small head and a total length exceeding 6 m (20 ft). It would have been longer and heavier than the largest elephants alive today.
Pelycosaurs were among the first amniotes, meaning they had well-developed eggs, allowing them to venture far from the water — amphibians must remain moist and lay their eggs in water. The pelycosaurs were the first successful amniote group, capable of colonizing continental interiors that had been reached by plants and insects tens of millions of years earlier. However, the time when the pelycosaurs were successful, the Early Permian, was also a time of great deserts, as all the world's continents had merged into the supercontinent Pangaea, which included practically all the world's land mass. The interior of Pangaea would've been the biggest desert the world had ever seen. For this reason, art that depicts reconstructions of pelycosaurs often show them in a desert-like background, although many groups of pelycosaurs probably lived in forests.
Pelycosaurs are occasionally lumped in with dinosaurs, especially when simplifying matters for children, but adults should know that there is no relationship between dinosaurs and pelycosaurs except that they both were the dominant form of life on Earth for a period. Despite their occasionally reptilian appearance, Pelycosaurs were synapsids, not sauropsids (reptiles), and actually the ancestors of mammals. One group of pelycosaurs, the therapsids, went on to acquire very mammalian features, and after more than a hundred million years, produced the first true mammals. All pelycosaurs died out before the end of the Permian, sparing them from the massive extinction at its end.