Synapsid

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Synapsida, or mammal-like reptiles, are believed to have led to the evolution of mammals. Synapsida rose to power during the Late Carboniferous (Pennsylvanian) period and continued to be the predominant land vertebrate through the Permian and Triassic periods. Prior to the development of the synapsid type skull, reptiles had a low, flat head with jaw muscles that were arranged in such a way to allow their jaws to snap together, but not to bite together in a hard pressing motion. Thus their diets and actions were very limited. With the development of the synapsid skull many other environmental niches were opened up, drastically changing the way that the environment lived and evolved, such as plants which required the evolution of better defensive characteristics.

Classification

Classification of the synapsida have changed much over the past couple of years and is still hotly debated. Synapsida are classified as

Morphology and Evolution

The Skull

The shape and morphology of the skull took a different evolutionary turn in the synapsids and their subsequent mammalian descendants. One of the biggest changes, that paved the way and forced several other changes to take place, was the development of an antorbital fenestra ("fenestra" is latin for opening or window) behind the eye socket, called the synapsid opening. As the synapsids evolved, so did the size of this opening. It is thought that the jaw muscles were attached to this opening, which in turn aided in relieving the stress that biting puts on the back of the skull, and coupled with the elongation of the back of the skull, allowed for a stronger, more precise bite.

Dentition

The mammal-like reptiles developed the heterodont shaped dentition with sharp edged incisors for cutting, pointed canines for tearing, and cheek teeth with flat crushing surfaces for chewing. Though they still retained the typical reptilian characteristic of replacing old and worn teeth with new teeth, as the synapsids evolved they gained the ability to keep their teeth longer. Because they were able to keep their teeth longer they were capable of developing more specialized dentition and a better precision bite. This allowed for their food to be chewed down to a pulp, easier to swallow, and thus better to digest. This, in turn, allowed for the nutrients from the food to be released and used by the body much more quickly.

The Limbs

With the sphenacodonts, such as the Dimetrodon, the limbs of these later pelycosaurs started to move differently than that of reptiles or earlier synapsid. Reptiles move with their limbs facing out horizontally and moving in a side-to-side motion. Starting with the sphenacodonts, synapsids moved in an up-and-down motion as seen through the evolved structure of the joints, hips, and hind limbs. The synapsids mark the beginning of the vertical locomotion, the up-and-down movement which required the limbs to flex more and with more ease and helped the synapsid to move much quicker. Reptiles also have outward facing feet whereas the synapsid, starting with the sphenacodonts, have more forward projecting feet.

Warm Bloodedness

The ability to regulate body temperature is a distinctive characteristic in mammals, or endotherms. Ectotherms, or reptiles, lack this ability and must rely mostly on the sun in order to warm their bodies and get their energy. This is a time consuming, and even potentially dangerous, activity, which requires that the reptile be in the sun, usually in an open space, in order to get the most warmth quickly. With the development of better locomotion, insulation, and breathing control, the later therapsids were able to get their energy from other sources and reduce their dependence on the sun as an energy source. Early synapsids, such as the pelycosaurs, were believed to have been ectotherms, due to the large nature of their bodies and the sails on their backs, which has been suggested worked in the same way as solar panels, by absorbing the heat and thereby allowing for temperature control.

Pelycosauria

Pelycosauria was the earliest and most primitive of the synapsida reptiles, and thought to have roamed the earth as early as 300 million years ago, the Archarothyris, during the Late Carboniferous.