pelycosaur

Pelycosaurs were a diverse group of amniote vertebrates that lived during the Permian period, approximately 299 to 252 million years ago. They are often considered among the earliest successful land-dwelling reptiles and represent a transitional stage in vertebrate evolution. While commonly referred to as "mammal-like reptiles," they are more accurately classified as synapsids, an extinct lineage that would eventually give rise to mammals.

A defining characteristic of many pelycosaurs is the presence of a single temporal fenestra, an opening in the skull behind the eye socket. This feature distinguishes them from other amniote groups like sauropsids (which include modern reptiles and birds) and anapsids (which lack temporal fenestrae). The size and shape of this opening varied among different pelycosaur species, and its function is thought to have been related to jaw muscle attachment, allowing for stronger biting forces.

Pelycosaurs exhibited a wide range of body forms and lifestyles. Some, like the small and generalized *Haptodus*, were likely insectivores or small carnivores. Others, such as the well-known *Dimetrodon*, are famous for their prominent dorsal sails. These sails, formed by elongated neural spines of the vertebrae, are believed to have served thermoregulatory purposes, helping the animals absorb or dissipate heat. Herbivorous forms also existed, with specialized teeth and jaws for processing plant matter. The group's diversity highlights their adaptability to various Permian environments before their eventual extinction at the end of the period.