Edaphosaurus, (meaning "pavement lizard" for dense clusters of teeth) is a genus of extinct edaphosaurid synapsid that lived in what is now North America and Europe around 300 to 280 million years ago, during the late Carboniferous to early Permian periods. The American paleontologist Edward Drinker Cope first described Edaphosaurus in 1882, naming it for the "dental pavement" on both the upper and lower jaws, from the Greek edaphos ("ground"; also "pavement") and sauros ("lizard").
Edaphosaurus is important as one of the earliest known large plant-eating (herbivorous) amniote tetrapods (four-legged land-living vertebrates). In addition to the large tooth plates in its jaws, the most characteristic feature of Edaphosaurus is a sail on its back. A number of other synapsids from the same time period also have tall dorsal sails, most famously the large apex predator Dimetrodon. However, the sail on Edaphosaurus is different in shape and morphology. The first fossils of Edaphosaurus came from the Texas Red Beds in North America, with later finds in New Mexico, Oklahoma, West Virginia, and Ohio. Fragmentary fossils attributed to Edaphosaurus also have been found in the Czech Republic and in Germany in Central Europe.
Edaphosaurus species measured from 0.5 metres (1.6 ft) to almost 3.5 metres (11.5 ft) in length and weighed over 300 kilograms (660 lb). In keeping with the tiny head, the cervical vertebrae are reduced in length, while the dorsal vertebrae are massive, the tail is deep, the limbs are short and robust and the ribs form a wide ribcage. Like most herbivores, Edaphosaurus would have had a capacious gut and symbiotic bacteria to aid in the breakdown of cellulose and other indigestable plant material. Like its more famous relative Dimetrodon, Edaphosaurus had a sail-like fin that was supported by bones of the vertebral column. Edaphosaurus differs from Dimetrodon in having cross-bars on the spines that supported its fin.
The head of Edaphosaurus was short, relatively broad, triangular in outline, and remarkably small compared to its body size. The deep lower jaw likely had powerful muscles and the marginal teeth along the front and sides of its jaws had serrated tips, helping Edaphosaurus to crop bite-sized pieces from tough terrestrial plants. Back parts of the roof of the mouth and the inside of the lower jaw held dense batteries of peg-like teeth, forming a broad crushing and grinding surface on each side above and below. Its jaw movements were propalinal (front to back). Early descriptions suggested that Edaphosaurus fed on invertebrates such as mollusks, which it would have crushed with its tooth plates. However, paleontologists now think that Edaphosaurus ate plants, although tooth-on-tooth wear between its upper and lower tooth plates, indicates only "limited processing of food compared to other early plant-eaters such as Diadectes, a large non-amniote reptiliomorph (Diadectidae) that lived at the same time. Early members of the Edaphosauridae such as Ianthasaurus lacked tooth plates and ate insects.
The sail along the back of Edaphosaurus was supported by hugely elongated neural spines from neck to lumbar region, connected by tissue in life. When compared with the sail of Dimetrodon, the vertebral spines are shorter and heavier, and bear numerous small crossbars. Edaphosaurus and other members of the Edaphosauridae evolved tall dorsal sails independently of sail-back members of the Sphenacodontidae such as Dimetrodon and Secodontosaurus that lived at the same time, an unusual example of parallel evolution. The function(s) of the sail in both groups is still debated. Researchers have suggested that such sails could have provided camouflage, wind-powered sailing over water, anchoring for extra muscle support and rigidity for the backbone, protection against predator attacks, fat storage areas, body temperature control surfaces, or sexual display and species recognition. The height of the sail, curvature of the spines, and shape of the crossbars are distinct in each of the described species of Edaphosaurus and show a trend for larger and more elaborate (but fewer) projecting processes over time. Romer and Price suggested that the projections on the spines of Edaphosaurus might have been embedded in tissue under the skin and might have supported food-storage or fat similar to the hump of a camel. Bennett argued that the bony projections on Edaphosaurus spines were exposed and could create air turbulence for more efficient cooling over the surface of the sail to regulate body temperature. Recent research that examined the microscopic bone structure of the tall neural spines in edaphosaurids has raised doubts about a thermoregulatory role for the sail and suggests that a display function is more plausible.
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