Sphenacodontidae

The sphenacodontids were large carnivores Dimetrodon, Sphenacodon, Ctenospondylus, and Secodontosaurus (Reisz, Berman and Scott 1992). These reptiles were the dominant predators of their time, and their fossil record extends from the Latest Carboniferous through to the early Middle Permian.

All the sphenacodonts had a deep, narrow skull with massive jaws and a formidable array of teeth - long canines, daggerlike incisors and small, cutting cheek teeth. The name of the most well-known sphenacodontid, Dimetrodon, actually refers to this adaptation, it means "two measure teeth". The sphenacodonts were the first animals to develop such a specialized set of teeth, and were the first large terrestrial carnivores to evolve (all earlier large carnivores were eitehr fully or semi-aquatic).

Many of these forms (Dimetrodon, Sphenacodon, Secodontosaurus, and Ctenospondylus) are all large to very large predators that have tall neural spines that in life doubtless supporting a large "sail" or fin on the animal's back. This strange structure most certainly served as a thermoregulatory device. In the cool morning the creature would turn side on to the morning rays, thus soaking up heat and becoming more active before its rivals or prey did. During the middle of the day, or whenever the creature was in danger of overheating, it could turn head on to the sun, thus shading the fin and allowing excess heat to dissipate.

These animals had long limbs making them relatively agile, fast moving animals, especially when compared to their slower, bulkier herbivorous relatives. The later and larger types like Dimetrodon, Sphenacodon, and Ctenospondylus all have similarly constructed, massive skulls with extremely large anterior incisors and canines.

Not only were these animals of great ecological significance during their long reign, but they also have an important position on the evolutionary tree, being ancestral to primitive therapsids and ultimately to mammlas and man.

Haptodus

Notes:Haptodus is a relatively small pelycosaur that probably constiututes the basal or primitive ancestral type of the family. It shares many structural features of the skull and skeleton with the more specialised sphenacodontids, indicating they are closely related. Haptodus is known from the Late Pennsylvanian (Latest Carboniferous) and Early Permian of North America and Europe. Both small and medium sized individuals are known. These animals were clearly effective predators, like contemporary large tropical lizards, feeding on both arthropods and small vertebrates. Haptodus lacked the spectacular sail that characterised the bigger Sphenacodontids like Dimetrodon, Ctenospondylus, and Secodontosaurus. It is also more common in Europe (central equatorial Pangea), whilst the other forms are primarily American (west equatorial Pangea)

Sphenacodon

Notes: The vertebral spines of Sphenacodon's backbone were long, and probably acted as attachment points for massive back muscles, allowing the animal to lunge powerfully at its prey. Sphenacodon did not have the elongate spines and distinctive "sail" of other advanced sphenacodontids.

Notes:Distinguished from the contemporary S. ferocior by smaller size, more slender build, and less neural spine development.

Notes:A larger and less common contemporary of S. ferox, distinguished by more robust proportions and greater neural spine development.

Dimetrodon

Notes: Perhaps the most well-known prehistoric non-dinosaur reptile, Dimetrodon was a common large carnivorous pelycosaur of the early Permian of North America, immediately recognisable by the "fin" or "sail" of elongate vertebral spines running along its back, upto a metre in length, and in life covered with a layer of skin and blood vessels and serving as a heat-exchange mechanism. In size and build it was comparable to a modern-day alligator, except that it was a fully terrestrial (land-living) creature. Dimetrodon was the dominant predator in its environment for some twenty-five million years, during which time it evolved through about a dozen species, become steadily larger as time progressed (an example of "Cope's Law"). Dimetrodon legs, although strong, were short, so it may have hunted through ambushing its prey, the sail on the back also no doubt serving to help camoflague the creature when it hid among stands of bamboo-like Calamite plants.

The largest, most specialised and most spectacular of the pelycosaur carnivores, Dimetrodon remained the dominant carnivore in its environment for some twenty-five million years, before being ousted by the up and coming eotitanosuchian and dinocephalian theraspids during the Ufimian age. Curiously, despite its obvious adaptions, Dimetrodons remained confined to a limited geographical region (fossils are known only from the Witchita, Clear Fork and San Angelo beds of the Texas-Oklahoma region, and some fragments of a small early species from New Mexico). This limited area (the actual extent being unknown due to the fact that only in a few places are there fossil deposits) is here termed the "Dimetrodon sub-province".

In skull and general body form, Dimetrodon was very similar to contemporary types like Sphenacodon and Ctenospondylus. However, these latter two have blade-like (narrow/flattened in cross section) neural spines supporting the sail, whereas Dimetrodon has greatly elongated spines that are rounded in cross-section section.

Dimetrodon is such a well-known creature it's even featured in most popular books on dinosaurs (even though it is not a dinosaur, and not even related). There are also several webpages dedicated to it. I've listed a few technical references under each species.

Known species:

Notes: The last and largest of the long-skulled, stocky-bodied lineage of Dimetrodons. Apart from D. angelensis this was the largest species of Dimetrodon.

Secodontosaurus is an unusual form that, although similiar in body, differs in the shape of the head from other sphenacodonts. The skull is low and narrow, although the neural spine morphology (of the "sail" backbone) is similiar to that of Dimetrodon. The cranial modifications of Secodontosaurus indicates an adaptation to specialised feeding strategies, perhaps preying upon burrowing animals, or perhaps aquatic feeding habits