Pelycosaur

Pelycosaurs (meaning "basin lizards") were the earliest synapsids. They were not dinosaurs or reptiles. They are an informal group which were once called the "mammal-like reptiles". 
The term is only used informally, if at all, in the modern scientific literature.

These tetrapods appeared during the Pennsylvanian and went extinct at the end of the Permian period. They were the dominant land animals for some 40 million years, and were almost wiped out by the P/Tr extinction event. A few survived until the lower Triassic to form the therapsids. The therapsids led to the mammals. 

Because the term "Pelycosaur" does not include their descendants, it is not used much now.Botha-Brink J. and Modesto S.P. 2007. A mixed-age classed 'pelycosaur' aggregation from South Africa: earliest evidence of parental care in amniotes? Proceedings of the Royal Society B, 274(1627): 2829–2834.  

The most notable of these land animals was Dimetrodon, the top predator of its time.

 Characteristics 
Pelycosaur fossils have been found mainly in Europe and North America, although some small, late-surviving forms are known from Russia and South Africa.

At least two pelycosaur clades independently evolved a tall sail of elongated vertebral spines. In life, this would have been covered by skin, and possibly was a thermoregulatory device and/or a mating display. 
Unlike lepidosaurian reptiles, pelycosaurs lacked epidermal scales. 

In 1940 the group was reviewed in detail and every species known at the time described (and many illustrated) in an important monograph. Well-known pelycosaurs include the genera Dimetrodon, Sphenacodon, Edaphosaurus, and Ophiacodon.

 Possible relatives 
The pelycosaurs were possibly related to the goronopsids. In the Walking with Monsters- Life Before Dinosaurs, the series showed that Dimetrodon evolved into a gorgonopsid. This told many to believe that pelycosaurs are possibly related to the gorgonopsians and the cynodonts.

 Classification 

References

Category:Synapsids