Ophiacodon has a large skull with a deep snout. It has the longest skull of any early synapsid, reaching up to 20 inches in one specimen. The jaws are lined with many small teeth. It was larger than most other tetrapods of its time, ranging from 5.2 to 9.8 inches in length and 57 to 506 pounds in weight.
Specimens of Ophiacodon vary greatly in size. These differences in size were once used to distinguish species, but are now recognized as variations related to the ages of individuals. Smaller bones often have more poorly developed joint surfaces than larger bones, implying that they come from juvenile individuals while the larger bones come from adults. Analysis of the microscopic anatomy of bones suggests that differences in size represent different growth stages rather than different species.
- Ophiacodon hilli
- Ophiacodon major
- Ophiacodon mirus
- Ophiacodon navajovicus
- Ophiacodon retroversus
- Ophiacodon uniformis
Remains of Ophiacodon have been found in a couple of places in England and France as well as in Nova Scotia and in several states in the United States, such as Arizona, Colorado, Kansas, New Mexico, Ohio, Oklahoma, Texas and Utah.
Ophiacodon most likely lived on land, but paleontologists have sometimes thought that it was semi-aquatic. An aquatic habitat for Ophiacodon was first proposed by E. C. Case in 1907, although he later dismissed the idea. Anatomical features suggesting that it spent much of its time in the water include broad claws that seemed to be adaptations for baddling, thin jaws and numerous small teeth that seemed to be adapted for eating fish, and weakly developed bones, which are seen in many other secondarily aquatic tetrapods. In 1940, Alfred Romer and Llewellyn Ivor Prince proposed that hind limbs with a greater length than forelimbs was another aquatic adaptation of Ophiacodon, supposedly because the hindlimbs would have been used to propel it through water. Several of these features are no longer thought to be evidence of an aquatic lifestyle; for example, broad claws are seen in most early tetrapods, even those that are known to have been almost exclusively terrestrial, and the long hind limbs of Ophiacodon would not have been an effective means of propulsion because the feet were still relatively small and had little surface areas of which to form a paddle. Analysis of the vertebrae of Ophiacodon indicate that it was most likely terrestrial and spent little time in water.