Order Pelycosauria

The first order of synapsids to arise in the Late Carboniferous to Early Permian, around 310 Million years ago, were the pelycosaurs, making up around 70% of all the tetrapod genera alive in the Early Permian. The pelycosaurs are split into six families, and resemble modern iguanids, but with shorter limbs. They diverged from other lineages through adaptations to feeding on large prey, the size of their skulls increasing disproportionately to their bodies, and having enlarged teeth and prominent canines compared with contemporary insectivores. This made them the first carnivorous amniotes (see characters and anatomy).


The skeleton of a Dimetrodon, with extremely long neural spines to support the sail.

Reproduced with kind permission from Benton and Harper, 1997

The most startling thing about the pelycosaurs is the large sail structure that ran the length of the back in many genera, particular examples being Dimetrodon and Edaphosaurus. The fossil remains of many pelycosaur groups show us that long neural spines extended out from the backbone, creating struts to support a thin layer of skin with a very large surface area. This was richly supplied with blood vessels, and acted as a thermoregulatory device in these cold-blooded animals, allowing them to 'sunbathe' as soon as the sun came up, warming the blood, and allowing them to be active far earlier in the morning than those with no sails. The sail could also be pointed away from the sun, allowing cooling control when the animal overheated.

Calculations suggest that in a 200 kg Dimetrodon, a body temperature increase of 6 degrees C could be achieved in 205 minutes without a sail, or 80 minutes with. This was such an effective strategy that it has been seen to evolve independently at least three times within the pelycosaurs, and be seen in several unrelated dinosaurs such as the spinosaurs and ouranosaurs.


The skeleton of Ophiacodon, the largest and best known of the earliest pelycosaurs.

Reproduced with kind permission from Benton and Harper, 1997


The Ophiacodontidae include the oldest known pelycosaurs, characterised by a long narrow skull, and a low skull table. The largest and best known is Ophiacodon, which could reach lengths between 1.5 and 3 metres, and is thought to have fed mainly on fish and tetrapods rather than insects.

Edaphosauridae and Caseidae

These families were herbivorous. The edaphosaurids had long neural spines, along with short transverse crossbars to aid the support of the sail, and a very short, broad skull with peg like teeth for eating plants. The palate was also completely covered in small, peg-like denticles to increase the efficiency of its herbivorous lifestyle. The shape of the body is also typical of herbivores - there are short limbs because there is no need to chase prey, and a large ribcage that expands out sideways to increase its volume, and help in the digestion of tough plant material.

The caseids were the most diverse and widespread herbivores in the late Early Permian. They had no long spines, and the denticles on the palate were very small and restricted to the upper surface of the mouth. The teeth at the edge of the mouth looked like those in modern lizards, with ridged spatula-like crowns for breaking down plant material. They too had barrel shaped ribcages to contain the extensive gut systems needed, and the jaw joint was centred below the tooth line, an adaptation that increases the power within the mouth to the cheek teeth, the region where the most force is needed for grinding.

Sphenacodontidae, Eothyrididae and Varanopidae

The Sphenacodontidae were the dominant terrestrial carnivores of the Early Permian, and had greatly enlarged canines. This family included genera such as Dimetrodon and Sphenacodon, both reaching lengths of more than 3 metres, and many had large thermoregulatory sails. Partly because they have the most gracile limbs, they are thought to have given rise to the Order Therapsida, from which the mammals arose. The least known groups are the eothyrids and varanopids, whose positions within the pelycosaurs are unsure. They have some ancient, and some more modern traits, but were both small, agile and powerful groups of predators.

Return to the Major Subgroups page

Return to the Synapsida home page