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an extinct genus of predatory mammal-like reptiles of the order Pelycosauria. The Dimetrodon, which grew up to 2.7 m, had dorsal vertebrae with very long spiny appendages, projecting from the dorsal surface and serving, perhaps, to stretch the membrane (dorsal sail), the purpose of which is unclear. There were some ten species. Remains of the Dimetrodon are known from the Permian deposits of North America.