The synapsids constitute the second major group of reptiles after the diapsids. They are characterized by a single hole in the skull just behind each eye socket called temporal fenestrae. Synapsids include the mammals (thus including us), their reptilian ancestors (“mammal-like reptiles”) and all their closest extinct relatives . The oldest known synapsid belongs to a group called Ophiacodontids which flourished during the Late Carboniferous and Early Permian periods. These were a rather peculiar lot with their unusually large head compared to their body size. The skull was very tall and narrow. The slightly recurved teeth were sharp, numerous and tightly packed inside the jaws, two being a bit larger than the other forming the upper canines. These medium size animals were among the largest land carnivores of their time. The limbs were rather short, broad and bulky.
The ancestral ophiacodontid type can be seen in Archaeothyris florensis (Reisz, 1972) from the Late Carboniferous of Nova Scotia, Canada (Westphalian C; 308-311 MYA), which is by the way, the earliest known definite synapsid. It somewhat looked like a modern lizard but with already the group hallmark of a tall and elongated skull. Measuring about 50 cm, it was contemporary to other early reptiles such Palaeothyris and living in a swampy forest made of giant tree-like lycopsids (club mosses) and dominated by amphibians and giant arthropods.
Varanosaurus acutirostris (Broili, 1904) from the Early Permian of Texas, measured about 1.2 m in length and was an agile predator with a slender laterally compressed snout. It is known from decent material including the almost complete holotype skeleton. A second species, V. wichitaensis, has been described, based on isolated postcranial remains (Romer, 1937) and is virtually indistinguishable from V. acutirostris besides being smaller and geologically slightly older.
Body proportions reached its extremes in Ophiacodon (Marsh, 1878; Cope, 1878; Case, 1907; Romer, 1925; Romer & Price, 1940) the best known and most studied ophiacodont. Many skeletons of this odd-looking animal are known from the Early Permian of Texas, New Mexico, Kansas, Oklahoma, Colorado, Utah and Ohio and it is unclear how many of the described species are actually valid. For the story, the type species, Ophiacodon mirus was first described by Othniel Marsh in 1878 based on a mandible and vertebrae in the midst of the so-called “bone war” against his rival Edwin Cope. Marsh clearly intended to beat Cope, who had a paper in press, in the naming of this animal but his hastily written paper was so deficient that the name Ophiacodon was ignored by the paleontological community for over 30 years. In the meantime, Cope’s paper published just days after Marsh’s, described three species, Theropleura retroversa, T. uniformis and T. triangulata, based on isolated vertebrae. The type specimen of Ophiacodon was reinvestigated by Williston and a new complete skeleton described in the 1910s (Williston & Case, 1913) and it is much later that Theropleura was found to be synonymous with Ophiacodon although the species names were retained (Romer & Price, 1940). The following species are still recognized today: O. mirus, known from several skeletons including a nearly complete one from New Mexico and Oklahoma; O. retroversus, known from multiple materials from Texas and Oklahoma, including a near complete skelton; O. uniformis from several partial skeletons from Texas and Oklahoma; O. navajonicus frong fragmentary postcranial skeletons from New Mexico; O. hilli known from a fragmentary skeleton from Kansas; O. major from fragmentary materials from Texas. It appears that size difference might reflect different growth stage rather than species (Brinkman, 1988). Ophiacodon was originally thought to be a semi-aquatic animal but recent studies debunked all the supposed aquatic adaptation that the animal might have possessed and today (Felice & Angielczyk, 2014), Ophiacodon is viewed as a fully terrestrial predator. Specimens of Ophiacodon have sizes ranging from 1.5 m up to 3 m in length.
Other ophiacodontids are known from rather fragmentary remains and include the Late Carboniferous genera Clepsydrops and Echinerpeton from North America and Stereorhachis from France, the Early Permian Stereophallodon, Baldwinonus from North America. Protoclepsydrops haplous (Carrol, 1964) from Nova Scotia might also have been an ophiacodontid predating Archaeothyris but the remains are so fragmentary that it is difficult to tell for sure.
Brinkman, D. (1988). Size-independent criteria for estimating relative age in Ophiacodon and Dimetrodon (Reptilia, Pelycosauria) from the Admiral and lower Belle Plains formations of west-central Texas. Journal of Vertebrate Paleontology, 8(2), 172-180.
F. Broili. 1904. Permische Stegocephalen und Reptilien aus Texas. Palaeontographica 51:1-120
Carroll, Robert L. (1964). "The earliest reptiles". Journal of the Linnean Society of London, Zoology 45 (304): 61–83
E. C. Case. 1907. Revision of the Pelycosauria of North America. Carnegie Institution of Washington 55:3-176
E. D. Cope. 1878. Descriptions of extinct Batrachia and Reptilia from the Permian formations of Texas. Proceedings of the American Philosophical Society 17:505-530
Felice, R. N., & Angielczyk, K. D. (2014). Was Ophiacodon (Synapsida, Eupelycosauria) a swimmer? A test using vertebral dimensions. In Early evolutionary history of the Synapsida (pp. 25-51). Springer Netherlands.
R. R. Reisz. 1972. Pelycosaurian Reptiles from the Middle Pennsylvanian of North America. Bulletin of the Museum of Comparative Zoology 144(2):27-60
A. S. Romer. 1925. An ophiacodont reptile from the Permian of Kansas. Journal of Geology 33(2):173-182
A. S. Romer. 1937. New genera and species of pelycosaurian reptiles. Proceedings of the New England Zoölogical Club 16:89-95
A. S. Romer and L. I. Price. 1940. Review of the Pelycosauria. Geological Society of America Special Paper 28:1-538
Williston, S. W., & Case, E. C. (1913). Description of a nearly complete skeleton of Ophiacodon Marsh. Carnegie Institution of Washington Publication, 181, 37-59.