The most familiar pelycosaurs are the sailbacked Dimetrodon and Edaphosaurus. Evolving during the Early Permian, each of these animals developed a large sail on its back; the sail was held upright by extremely elongated extensions of the neural arch of the back and tail bones. This large sail must have looked somewhat like a handheld fan, as it was covered by a membrane of vascularized skin—skin with blood vessels running through it. The function of the sail was probably to help regulate the body temperature of the animal. The sailbacked pelycosaur was most likely a cold-blooded, or ectothermic, creature; in the morning, it could warm itself more quickly than other animals by facing its sail toward the sun. Sunlight striking the sail would warm the blood circulating inside the sail membrane; that warmed blood then would circulate throughout the animal's body. This would have given the animal either a head start on catching prey or a jump start on escaping from a predator, whichever the case may have been.
Although having sails gave Dimetrodon and Edaphosaurus a similar appearance, they had very different lifestyles. Found in some productive fossil locations, including Texas, these two animals probably lived in the same habitat. Their interaction would have been strictly on an "as needed" basis, however, because Dimetrodon was a fierce predator and Edaphosaurus a plant eater.
Dimetrodon was about 11.5 feet (3.5 m) long, with a sail that reached a height of about five or six feet (1.5 to 1.8 m). It had a large, muscular head with powerful jaws. Its teeth were serrated for slicing flesh, much like a steak knife. In addition, Dimetrodon had longer teeth in the front of its jaws for stabbing its prey. It walked very much like a crocodile, with a sprawling gait and its belly barely off of the ground. Being cold-blooded, Dimetrodon probably could not run at top speed for more than a short time, so its hunting tactics likely consisted of chasing slower animals, ambushing others, and perhaps also eating carrion.
Edaphosaurus was a plant eater that also adapted a vascularized sail on its back for thermoregulation, or the control of body temperature. Its skull was much smaller in proportion to its body than that of Dimetrodon. The shallow jaws of Edaphosaurus were lined with uniformly sized, chisel-like teeth that the animal used to snip vegetation. The roof of the edaphosaur mouth contained a cluster of rounded, knoblike teeth for crushing plant matter taken into the mouth. Edaphosaurus grew up to ten feet (3 m) long and had a gait similar to that of Dimetrodon. Another distinguishing feature of Edaphosaurus was the design of its sail. Compared to the sail of Dimetrodon, that of Edaphosaurus had many small spines or articulations on the rays of its supporting sail bones. This may have increased the speed with which blood could have circulated through the sail. One can imagine that this would have allowed Edaphosaurus to warm up and get moving a little more quickly than Dimetrodon—an advantage that may have saved Edaphosaurus from its predatory neighbor on more than one occasion.
The pelycosaur line of synapsids was long lived but did not survive the mass extinction at the end of the Permian Period.
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