Dimetrodon

Kingdom: Animalia

Phylum: Chordata

Superclass: Tetrapoda

Order: Pelycosauria

Family: Sphenacodontidae

Genus:Dimetrodon

Species:Dimetrodon limbatus

Introduction

Dimetrodon is perhaps the most recognizable primitive member of the synapsids, which is believed to be the hereditary line that eventually gave rise to the mammals. Synapsids are characterized by the presence of temporal openings in the skull below and posterior to each eye socket. These openings both decreased the effective mass of the skull and accommodated the attachment of the jaw muscles to produce stronger jaws. Dimetrodon itself is most recognizable by the large sail-like dorsal fin extending like a fan from the back of the neck to a point on the tail behind the lower hips.

Although Dimetrodon has been made popularly familiar as a dinosaur, often as the first creature described in children's books about dinosaurs, it is not a member of the dinosaur family at all. Dimetrodon actually lived in the Early Permian period, and so predates the dinosaurs by tens of millions of years. It shares the feature of a prominent dorsal “sail,” with its contemporary pelycosaur, Edaphosaurus. Two Cretaceous dinosaurs, Spinosaurus and Ouranosaurus, also featured a pronounced dorsal structure reminiscent of Dimetrodon's sail, and for many years, paleontologists believed that they sported a sail structure similar to Dimetrodon. More recent research of these Cretaceous dorsal structures indicates that these dorsal structures were slightly different and probably looked more like a hump than a sail.

The widely separated geographic regions in which Dimetrodon fossils have been found lend support to the theory that all of the planet's land masses were once part of the supercontinent known as Pangaea. Tectonic activity occurring over millions of years slowly pulled this giant land mass in different directions, fracturing and refracturing it into the various segments that have formed the continental masses that exist today. The effect can be seen on a small scale in the way the solid crust that forms on molten lava fractures and moves about according to the motions of the underlying fluid material. The various locations in which the fossils of animals such as Dimetrodon have been found are therefore believed to have been part of the same geographical region of Pangaea when those animals were alive some 290 million years ago.

Classification

Dimetrodon is a pelycosaur, of the Sphenacodontidae family, which lived in the Permian period of geological time, between 251 and 299 million years ago. The popular association of Dimetrodon with the dinosaurs is entirely erroneous, however, because true dinosaurs are not recognized from the fossil record until about 50 million years after the period in which Dimetrodon and its relatives flourished. The original classification of Dimetrodon as a dinosaur was based on a stereotypical popular image of dinosaurs as oversized primitive reptiles like gigantic lizards and crocodilians. In fact, Dimetrodon is not even a reptile, as the early synapsids are generally believed to be more closely related to modern mammals than to dinosaurs or other reptiles, despite their reptile-like appearance. (Though early classifications did often list it as a "mammal-like reptile.") However, Dimetrodon is not a mammal either.

Classification of plants and animals, including dinosaurs, is generally made according to the system devised by Carolus Linnaeus, a Swedish naturalist who lived in the early eighteenth century. In 1735, Linnaeus published a scientific book called System of Nature, in which he classified plants and animals by class, order, genus, and species. Each step in the classification identified a group of decreasing diversity or variation, with the species being the most specific identifier of any living organism. Linnaeus’ work marked a revolution in the way in which scientists viewed the living beings of the world.

Biologists since Linnaeus’ time have expanded his system of classification by the introduction of cladistics. Cladistic classification is based on the concept of evolution and ties different groups of animals together through common ancestral forms. Thus, the broad classification of the Chordata includes all animals with a spinal structure or spinal chord, and the narrower classification of the Synapsida includes all animals with spinal chords and the synapsid skull structure. This path is typically illustrated with the use of a cladogram, which is essentially just the “family tree” diagram for a creature. Such cladistic analysis is amenable to being adjusted according to new information that becomes available and is also conducive to the development of new theories and the refinement of older ones.

Within the synapsids, Dimetrodon is classed as a member of the Sphenacodontidae according to the structure of its head and dentition. The Sphenacodontidae are characterized by relatively long and narrow, but massive, jaws armed with large canine teeth and knife-like incisors at the front of the jaw, and smaller shearing teeth along the cheeks. The type species of the genus Dimetrodon is Dimetrodon limbatus, though numerous other species have been described.

Anatomy

Dimetrodon was a carnivorous quadruped that resembled an oversized lizard. Adult Dimetrodons grew to a length of 3 meters (10 ft) and body height of up to 1 meter (3 ft). Dimetrodon's most prominent feature was a large semi-circular, fin-shaped structure that extended from the base of the skull to the base of the tail just behind the rear hips. The fin was formed by the elongated spines of the vertebrae and was believed to have been covered by a more or less fleshy membrane of skin that was richly supplied with blood vessels. This “sail” added another 1 meter (3 ft) to the overall height of the creature. Microscopic examination of the internal structure of fossilized Dimetrodon bones reveals a lack of blood vessels consistent with cold-blooded or ectothermic status.

The name Dimetrodon is a combination word meaning “two measures (or forms) teeth,” due to the nature of its dental equipment. The blunt, square-shaped mouth of Dimetrodon was equipped with both large canine teeth at the front of the mouth and well-developed shearing teeth along the sides of the jaws. The large canines enabled Dimetrodon to fix its prey with a secure grip and to deliver a killing bite, while the shearing teeth enabled it to bite through and shear off mouth-sized chunks of flesh from its prey. The synapsid structure of Dimetrodon's skull accommodated the extra-large muscles that powered the biting force of the jaws, which is perhaps the main reason it was likely a top predator of its time.

The actual structure and function of Dimetrodon's sail is the subject of some controversy. Because Dimetrodon is believed to have been cold-blooded, many paleontologists have postulated that the sail may have served as a thermoregulatory structure that enabled Dimetrodon to maintain a fairly constant body temperature. The structure of the spinal vertebrae does not indicate that the sail could be folded down to the body, but that it was held erect at all times. Thus, by turning so that sunlight could impinge directly on the skin of the sail, Dimetrodon would have been able to harvest heat from the sun to raise its body temperature when needed. Alternatively, flushing the sail with blood when overheating would have enabled Dimetrodon to eliminate excess heat by transferring it to any cooling wind that happened to be blowing. However, some later studies cast doubt on this hypothesis, especially as species have been identified with relatively small sails that would have had little thermoregulatory effect.

It is also possible that Dimetrodon's sail was used to communicate with other Dimetrodons as part of the mating process or as a threat display to exert dominance over rival Dimetrodons and other creatures. This may never be known for certain, as the dorsal sail of the Dimetrodon is presumably found on fossils of both males and females of the species. However, some researchers have proposed that sexual dimorphism was present in Dimetrodons, so the sail may have evolved through sexual selection.

Intelligence

Dimetrodon had a massively built skull that accounted for about 14 percent of its overall length. The skull exhibits a large nasal structure indicative of an acute sense of smell. The brain case itself, however, is very low and sloping, especially over the eyes. This suggests a small brain with a disproportionately large frontal lobe for the sense of smell.

Animal intelligence may be estimated using a measurement called the encephalization quotient (EQ). It essentially compares the size of an animal's brain to the average brain size of animals having similar body mass. It is presumed that the level of intelligence corresponds to the EQ, at least in general terms, such that the higher the EQ, the higher the animal's level of intelligence. Conversely, the lower is the EQ, the lower is the intelligence of the animal.

No EQ value for Dimetrodon has been estimated. It may, however, have been on a par with that of modern crocodiles and alligators.

Reproduction and Population

It can be argued that Dimetrodon was very successful, surviving as a genus for as much as 20 million years. Though not a reptile, the reproductive manner and life span of Dimetrodon may very well have been the same as that common to reptiles such as the crocodilians of the present day. That is, Dimetrodon was most likely oviparous and may have laid large clutches of eggs in self-heating nests, guarding them until the hatchlings emerged some time later. It may also have nurtured its young until they had grown and developed to the point of having to move on and find their own territorial niches. However, it is not known whether or not Dimetrodon was viviparous, giving birth to live young instead of laying eggs.

Diet

Dimetrodon was undoubtedly wholly carnivorous, as it lacked teeth that would enable it to grind up plant material for digestion, and was also one of the largest predators of its time. An adult Dimetrodon would have typically weighed between 150–250 kilograms (330–550 lb). Its prey would probably have included such contemporary creatures as Edaphosaurus, a sail-backed herbivore approximately the same size as Dimetrodon, and Sphenacodon, another carnivorous pelycosaur about the same size as Dimetrodon but without a dorsal sail fin. Studies have also suggested that Dimetrodon largely ate aquatic creatures, including sharks and amphibians. Dimetrodon's large, inward-pointing canines and extra-large jaw muscles would have provided a very secure hold on its prey, protecting Dimetrodon from injury while procuring its next meal.

It is also possible that Dimetrodon was a carrion eater as well as an active hunter. This is suggested by the overly developed nasal and brain structures, typically observed in other scavengers such as present-day carrion vultures. An inordinately well-developed sense of smell favors their location of a carrion meal from a distance.

Behavior

Dimetrodon displays the stereotypical lizard-like posture associated with reptiles. Its four legs are approximately equal in length front to back, extending laterally outward from the shoulder and hip joints. This is quite unlike true dinosaurs, in which the legs are positioned directly below the corresponding joints to support the weight of the animal. This also provides superior balance and a higher capacity for speedy movement that is lacking in Dimetrodon. It is believed that Dimetrodon was not a fast-moving animal, but rather moved in much the same manner as present-day crocodilians—spending much of its time resting on its belly with legs splayed out to the side, or walking with its body elevated on all four legs and alternating placements of the front and back feet.

As a carnivore, or possibly a scavenger-carnivore, Dimetrodon is thought to have lived a fairly solitary lifestyle.

Habitat and Other Life Forms

The Permian habitat was very different from the environment faced by true dinosaurs millions of years afterward. The Permian climate began continuing the glacial period that began during the preceding Carboniferous age. The climate became gradually warmer and drier as ferns and conifers became the dominant terrestrial plants. Some of the plant forms that appeared in the Permian age, such as the ginkgo, still exist in the present time.

The Permian age was replete with life forms other than Dimetrodon, although they were relatively primitive in form and structure. Other known creatures of the Permian age include Cacops, Platyhystrix, Seymouria, Phlegethontia, Diplocaulus, Pantylus, Diadectes, Labidosaurus, Mesosaurus, Araeoscelis, Ophiacodon, Sphenacodon, Edaphosaurus, and Casea.

Research

The fossil remains of Dimetrodon were first unearthed by Donald McLeod in 1845 near French River in Nova Scotia, Canada, although they were not identified as Dimetrodon until 1963. The animal was first named by Edward Drinker Cope, a nineteenth-century American paleontologist, for fossils found in Texas in the late 1800s. Dimetrodon fossil remains have been found in various locations in Canada and the United States. The first specimen found outside North American was discovered in Germany and described in 2001.

Dimetrodon is relatively well-represented in the fossil record, and research continues as new specimens are discovered and older fossils and reevaluated. Much modern research focuses on classification, especially amid ongoing dispute over taxonomy in general. Most importantly, by the end of the twentieth century there was widespread recognition that Dimetrodon and other primitive synapsids are not reptiles, as they were often previously classified. Another area of ongoing research is the function of Dimetrodon's dorsal sail.

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