Pelycosaurs are an early group of reptiles that lived during the late Carboniferous to the early Permian periods, around 300 to 250 million years ago. They are often recognized by their distinctive sail-like structures on their backs, formed by elongated neural spines. These creatures were primarily herbivorous or omnivorous and played a significant role in the evolution of later reptiles. As ancestors to modern mammals, Pelycosaurs are important for understanding the transition from reptiles to mammals. They varied in size and shape, with some species resembling large lizards. Their adaptations, such as changes in jaw structure and teeth, paved the way for the development of more advanced vertebrates.