Dimetrodon ( or ; ) is an extinct genus of sphenacodontid synapsid that lived during the Cisuralian to early Guadalupian epoch of the Permian period, around 294.1–271 million years ago. With most species measuring long and weighing , the most prominent feature of Dimetrodon is the large neural spine sail on its back formed by elongated spines extending from the vertebrae. It was an obligate quadruped (it could walk only on four legs) and had a tall, curved skull with large teeth of different sizes set along the jaws. Most fossils have been found in the Southwestern United States, the majority of these coming from a geological deposit called the Red Beds of Texas and Oklahoma. More recently, its fossils have also been found in Germany and over a dozen species have been named since the genus was first erected in 1878.
Dimetrodon is often mistaken for a dinosaur or portrayed as a contemporary of dinosaurs in popular culture, but it became extinct by the middle Permian, some 40 million years before the appearance of dinosaurs. Although reptile-like in appearance and physiology, Dimetrodon is much more closely related to mammals, as it belongs to the closest sister family to therapsids, the latter of which contains the direct ancestor of mammals. Dimetrodon is traditionally assigned to the paraphyletic group "pelycosaurs", a term now considered obsolete and replaced by terms such as "primitive synapsids" or "basal synapsids"; the name "mammal-like reptiles" is also traditionally used for non-mammalian synapsids due to some of the features shared with modern mammals such as tooth specialization and endothermy, but that term is now also defunct. The Dimetrodon skull has a single opening (temporal fenestra) behind each eye, a feature shared among all synapsids, unlike the skulls of reptiles and birds, both of which belong to the clade Sauropsida, which had diverged from the synapsids by the Late Carboniferous.
Dimetrodon was probably one of the apex predators of the Cisuralian ecosystems, feeding on fish and tetrapods, including reptiles and amphibians. Smaller Dimetrodon species may have had different ecological roles. The sail of Dimetrodon may have been used to stabilize its spine or to heat and cool its body as a form of thermoregulation. Some recent studies argue that the sail would have been ineffective at removing heat from the body, due to large species being discovered with small sails and small species being discovered with large sails, essentially ruling out heat regulation as its main purpose. The sail was most likely used in courtship display, including threatening away rivals or showing off to potential mates.
History of discovery
Earliest discoveries
The earliest discovery of Dimetrodon fossils were of a maxilla recovered in 1845 by a man named Donald McLeod, living in the British colony of Prince Edward Island (modern Canada). These fossils were purchased by John William Johnson, a Canadian geologist, and then described by Joseph Leidy in 1854 as the mandible of Bathygnathus borealis, a large carnivore related to Thecodontosaurus, although it was later reclassified as a species of Dimetrodon in 2015, as Dimetrodon borealis. Although Bathygnathus was named first, a petition to conserve the genus Dimetrodon and suppress the genus Bathygnathus was submitted to the International Commission on Zoological Nomenclature (ICZN) in 2015, which was approved in 2019.
First descriptions by Cope Fossils now attributed to Dimetrodon were first studied by American paleontologist Edward Drinker Cope in the 1870s. Cope had obtained the fossils along with those of many other Permian tetrapods from several collectors who had been exploring a group of rocks in Texas called the Red Beds. Among these collectors were Swiss naturalist Jacob Boll, Texas geologist W. F. Cummins, and amateur paleontologist Charles Hazelius Sternberg. Most of Cope's specimens went to the American Museum of Natural History or to the University of Chicago's Walker Museum (most of the Walker fossil collection is now housed in the Field Museum of Natural History).
Sternberg sent some of his own specimens to German paleontologist Ferdinand Broili at the Ludwig-Maximilians-Universität München, although Broili was not as prolific as Cope in describing specimens. Cope's rival Othniel Charles Marsh also collected some bones of Dimetrodon, which he sent to the Walker Museum. The first use of the name Dimetrodon came in 1878 when Cope named the species Dimetrodon incisivus, Dimetrodon rectiformis, and Dimetrodon gigas in the scientific journal Proceedings of the American Philosophical Society.
The first description of a Dimetrodon fossil came a year earlier, though, when Cope named the species Clepsydrops limbatus from the Texas Red Beds. (The name Clepsydrops was first coined by Cope in 1875 for sphenacodontid remains from Vermilion County, Illinois, and was later employed for many sphenacontid specimens from Texas; many new species of sphenacodontids from Texas were assigned to either Clepsydrops or Dimetrodon in the late 19th and early 20th centuries.) C. limbatus was reclassified as a species of Dimetrodon in 1940, meaning that Cope's 1877 paper was the first record of Dimetrodon.
Cope was the first to describe a sail-backed synapsid with the naming of C. natalis in his 1878 paper, although he called the sail a fin and compared it to the crests of the modern basilisk lizard (Basilicus). Sails were not preserved in the specimens of D. incisive and D. gigas that Cope described in his 1878 paper, but elongated spines were present in the D. rectiformis specimen he described. Cope commented on the purpose of the sail in 1886, writing, "The utility is difficult to imagine. Unless the animal had aquatic habits and swam on its back, the crest or fin must have been in the way of active movements... The limbs are not long enough nor the claws acute enough to demonstrate arboreal habits, as in the existing genus Basilicus, where a similar crest exists."
Early 20th century descriptions
In the first few decades of the 20th century, American paleontologists E. C. Case authored many studies on Dimetrodon and described several new species. He received funding from the Carnegie Institution for his study of many Dimetrodon specimens in the collections of the American Museum of Natural History and several other museums. Many of these fossils had been collected by Cope but had not been thoroughly described, as Cope was known for erecting new species on the basis of only a few bone fragments.
Beginning in the late 1920s, paleontologist Alfred Romer restudied many Dimetrodon specimens and named several new species. In 1940, Romer coauthored a large study with Llewellyn Ivor Price called "Review of the Pelycosauria" in which the species of Dimetrodon named by Cope and Case were reassessed. Most of the species names considered valid by Romer and Price are still used today.
New specimens In the decades following Romer and Price's monograph, many Dimetrodon specimens were described from localities outside Texas and Oklahoma. The first was described from the Four Corners region of Utah in 1966 and another was described from Arizona in 1969. In 1975, Olson reported Dimetrodon material from the Washington Formation of Ohio, which has been given a tentative assignment of D. cf. limbatus. A new species of Dimetrodon called D. occidentalis (meaning "western Dimetrodon") was named in 1977 from New Mexico. The specimens found in Utah and Arizona probably also belong to D. occidentalis.
Before these discoveries, a theory existed that a midcontinental seaway separated what is now Texas and Oklahoma from more western lands during the Early Permian, isolating Dimetrodon to a small region of North America, while a smaller sphenacodontid called Sphenacodon dominated the western area. While this seaway probably did exist, the discovery of fossils outside Texas and Oklahoma show that its extent was limited and that it was not an effective barrier to the distribution of Dimetrodon.
In 2001, a new species of Dimetrodon called D. teutonis was described from the Lower Permian Bromacker locality at the Thuringian Forest of Germany, extending the geographic range of Dimetrodon outside North America for the first time.
Description
Dimetrodon was a quadrupedal, sail-backed synapsid that most likely had a semi-sprawling posture between that of a mammal and a lizard and also could walk in a more upright stance with its body and the majority or all of its tail off the ground. Most Dimetrodon species ranged in length from , and are estimated to have weighed between . The smallest known species, D. teutonis, was about long and weighed . The larger species of Dimetrodon were among the largest predators of the Early Permian, with D. grandis measuring and weighing some ; D. limbatus is estimated at and some . The closely related Tappenosaurus, known from skeletal fragments in slightly younger rocks, may have been even larger at an estimated long.
Skull The skull of Dimetrodon is tall and compressed laterally, or side-to-side. The eye sockets are positioned high and far back in the skull. Behind each eye socket on each side is a single hole called an infratemporal fenestra. An additional hole in the skull, the pineal foramen (or "third eye") along the midline between the parietal bones, can be seen when viewed from above. The back of the skull (the occiput) is oriented at a slight upward angle, a feature that it shares with all other early synapsids. The upper margin of the skull slopes downward in a convex arc to the tip of the snout. The tip of the upper jaw, formed by the premaxilla bone, is raised above the part of the jaw formed by the maxilla bone to form a maxillary "step". Within this step is a diastema, a gap in the tooth row.
A single large opening on either side of the back of the skull links Dimetrodon to mammals and distinguishes it from most of the earliest sauropsids, which either lack openings or have two openings. Features such as ridges on the inside of the nasal cavity and a ridge at the back of the lower jaw are thought to be part of an evolutionary progression from early four-limbed land-dwelling vertebrates to mammals. On the inner surface of the nasal section of the skull are ridges called nasoturbinals, which may have supported cartilage that increased the area of the olfactory epithelium, the layer of tissue that detects odors. These ridges are much smaller than those of later synapsids from the Late Permian and Triassic, whose large nasoturbinals are taken as evidence for warm-bloodedness because they may have supported mucous membranes that warmed and moistened incoming air. Thus, the nasal cavity of Dimetrodon is transitional between those of early land vertebrates and mammals. Another transitional feature of Dimetrodon is a ridge in the back of the jaw called the reflected lamina, which is found on the articular bone, which connects to the quadrate bone of the skull to form the jaw joint. In later mammal ancestors, the articular and quadrate separated from the jaw joint, while the articular developed into the malleus bone of the middle ear. The reflected lamina became part of a ring called the tympanic annulus that supports the ear drum in all living mammals.
Teeth The size of the teeth varies greatly along the length of the jaws, lending Dimetrodon its name, which means "two measures of tooth" in reference to sets of small and large teeth. One or two pairs of caniniforms (large, pointed, canine-like teeth) extend from the maxilla. Large incisor teeth are also present at the tips of the upper and lower jaws, rooted in the premaxillae and dentary bones. Small teeth are present around the maxillary "step" and behind the caniforms, becoming smaller further back in the jaw. On the roof of the mouth, towards the back, the pterygoid bones each bear a large flange which supports a row of palatal teeth.