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Conservation status: Fossil

Dimetrodon grandis skeleton at the
National Museum of Natural History
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Superclass: Tetrapoda
Class: Synapsida
Order: Pelycosauria

Cope, 1878

Suborders and families





The pelycosaurs (from Greek pelyx meaning "bowl" and sauros meaning "lizard") were smallish to large (up to 3 meters or more) primitive Late Paleozoic "mammal-like reptiles." They appeared during the Late Carboniferous and reached their acme in the early part of the Permian period, remaining the dominant land animals for many millions of years. A few stragglers continued into the late Permian.

Pelycosaurs are not dinosaurs, although they are popularly grouped with them. They were the earliest and most primitive synapsids, and went extinct before the Triassic period, when the dinosaurs arose. The pelycosaurs appear to have been a group of reptiles that had direct ancestral links with mammals, having differentiated teeth and a developing hard palate.

Well-known pelycosaurs include the genera Dimetrodon, Sphenacodon, Edaphosaurus, and Ophiacodon.

Paleozoic era (542 - 251 mya)
Cambrian Ordovician Silurian Devonian Carboniferous Permian


Pelycosaurs are synapsid amniotes. Amniotes are animals whose embryos are surrounded by an amniotic membrane that encases it in amniotic fluid. Reptiles have traditionally been defined as including all the amniotes except birds and mammals. Synapsids are tetrapods (four-legged vertebrates) that have a pair of holes in their skulls behind the eyes. The synapsids are considered to have eventually evolved into mammals and are often referred to as mammal-like reptiles.

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 evolved (often considered independently acquired) a tall sail, consisting of elongated vertebral spines: The Edaphosauridae and the Sphenacodontidae. The Edaphosauridae are a family of mostly large (up to 3 meters or more) herbivorous pelycosaurs and are the earliest known herbivorous amniotes (and along with the Diadectidae, the earliest known herbivorous tetrapods). The Sphenacodontidae are a family of small to large, carnivorous pelycosaurs; during the later part of the early Permian these animals grew progressively larger (up to 3 meters or more), to become the top predators of their environments. In life, the sail of these families are assumed to have been covered by skin, and possibly functioned as a thermoregulatory device or for mating display.

In 1940, the group was reviewed in detail and every species known at the time described (and many illustrated) in an important monograph by Alfred Sherwood Romer and Llewellyn Price.

Pelycosauria is a paraphyletic taxon because it excludes the therapsids. For that reason, the term is not used in some modern texts. Eupelycosauria is used to designate the clade that includes most Pelycosaurs along with the Therapsida and mammals. In contrast to "Pelycosaurs," this is a monophyletic group. Caseasauria refers to a pelycosaur side-branch or clade that did not leave any descendants.


Dimetrodon (/daɪˈme.tɹəʊˌdɒn/) was a predatory synapsid (mammal-like reptile) genus that flourished during the Permian. Fossils of Dimetrodon have been found in North America and Europe. It is a member of the Sphenacodontidae family mentioned above.

Dimetrodon was a dominant carnivore, the largest known of its day. It grew to up to 3 meters (10 feet) in length. The name Dimetrodon means "two-measure teeth," referencing its large skull with two different types of teeth (shearing teeth and sharp canine teeth). Dentition showing this differentiation of teeth is called heterodonty. It walked on four side-sprawling legs and had a large tail. Dimetrodon may have moved in a manner similar to present-day lizards.

The most distinctive characteristic of Dimetrodon was the spectacular sail on its back. The sail is speculated to have been used to regulate body temperature; the surface area would allow it to warm up or cool off more efficiently. In this way, it could have easily picked off slower prey that was still warming up. The sail may also have been used in mating rituals and to warn off other predators. The sail was supported by neural spines, each one sprouting from an individual vertebra. Bramwell and Fellgett (1973) calculate that a 200 kg Dimetrodon would heat up from 26°C to 32°C in 205 minutes without a sail and in only 80 minutes with a sail.

As a synapsid, Dimetrodon was distantly related to humans and all other modern mammals. Synapsids were the first tetrapods to evolve differentiated (or heterodont) teeth. Whereas reptiles hardly chew their food, simply gulping it down, synapsids like Dimetrodon had teeth that could be used to help shear meat into smaller pieces for easier ingestion. These "two-measure teeth" are considered eventually to have given rise to the various kinds of teeth present in modern mammals.


Sphenacodon was a pelycosaur that fossils show to be about 10 feet in length. As its name suggests, Sphenacodon belongs to the family Sphenacodontidae. Sphenacodon's vertebral spines were long, and probably acted as attachment points for massive back muscles, allowing the animal to lunge powerfully at its prey. Though the spines were long, it did not have a sail like Dimetrodon. However, it was closely related to Dimetrodon and other sphenacodontids. Sphenacodon lived during the early Permian in the North American region.


Edaphosaurus (Greek: earth lizard; from edaphos, "ground"; and sauros, "lizard") was a primitive herbivorous pelycosaur reptile. It is a member of the Edaphosauridae family.

Along with the Diadectidae, Edaphosaurus is one of the earliest known plant-eating animals. It had a remarkably small, short and shallow skull, a wide body and thick tail. On its back is a sail, different in shape to that of its contemporary, Dimetrodon, the vertebral spines being shorter and heavier and bearing numerous small cross bars.

The earliest known species are known from fragmentary remains of small animals from the late Carboniferous. Successive species increased in size during the middle Permian period, until they attained about 3.2 meters in length, as represented by the species Edaphosaurus cruciger and Edaphosaurus pogonias. These large species are distinguished by the cervical and anterior thoracic neural spines bearing large club-like sidebars.

Edaphosaurus pogonias is also the type species, a large early Permian form whose fossils are known from the Permian red beds of Texas (United States). However, it is not known for certain if all these species attributed to this genus actually belong there. The name Naosaurus claviger is given to an earlier, smaller species that is usually included under Edaphosaurus.


Ophiacodon is a large synapsid pelycosaur. It is at least two meters in length when full grown. As judged from the fossil evidence, the largest ones were 2.5 meters, and the smaller ones were 1.5 meters. The size of Ophiacodon size increases as time progressed during the early Permian epoch until its extinction at the end of the epoch.

It is a specialized member of the Ophiacontidae family lineage. Being the most famous ophiacodontid in the family, its fossils were found in North America. The skull was deep, with long jaws, and full of sharp teeth. Ophiacodon may have eaten fish in streams and ponds, although the high narrow skull would seem to mitigate against such a lifestyle. It is related to other ophiacodontids, such as Archaeothyris.

Taxonomy and Phylogeny

  • Class Synapsida
      • Suborder Caseasauria
        • Family Eothyrididae
          • Eothyris
        • Family Caseidae
          • Casea
          • Cotylorhynchus
          • Ennatosaurus
      • Suborder Eupelycosauria
        • Family Varanopseidae
          • Varanosaurus
          • Mycterosaurus
          • Mesenosaurus
          • Varanops
        • Family Ophiacodontidae
          • Archaeothyris
          • Ophiacodon
        • Family Edaphosauridae
          • Edaphosaurus
          • Ianthasaurus
        • Sphenacodontia
          • Haptodus
          • Palaeohatteria
          • Pantelosaurus
          • Cutleria
          • Sphenacodontoidea
            • Family Sphenacodontidae
              • Ctenospondylus
              • Dimetrodon
              • Secodontosaurus
              • Sphenacodon
              • Tetraceratops

ISBN links support NWE through referral fees

  • Carroll, R. L. 1988. Vertebrate Paleontology and Evolution. WH Freeman & Co.
  • Colbert, E. H. 1969. Evolution of the Vertebrates. John Wiley & Sons Inc (2nd ed.)
  • Reisz, R. R. 1986. Handbuch der Paläoherpetologie—Encyclopedia of Paleoherpetology, Part 17A Pelycosauria. Verlag Dr. Friedrich Pfeil. ISBN 3-89937-032-5
  • Romer, A. S., and L. I. Price. 1940. Review of the Pelycosauria. Geol. Soc. Amer. Spec. Papers 28: 1-538.
  • Romer, A. S., 1947, revised ed. 1966. Vertebrate Paleontology. Chicago: University of Chicago Press.


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