Mammallike Reptiles – Synapsida

(https://en.wikipedia.org/wiki/Synapsid)

Animals of the subclass Syanpsida were well established in the Permian period, 275 to 225 million years ago. Some representatives of the subclass seem in many ways to be transitional forms between the stem reptiles and the more highly developed mammals.

The transition occurred between the Permaian and the end of the Triassic period. The most ancient Permian synapsids were clsely related to the anapsid cotylosaurs. In particular the are believed to have descended from the captorhinomorphs. During the late Triassic, some synapsid reptiles evolved that were so similar in structure to the mammals that it is often difficult to say whether a particular specimen is a reptile or a mammal. In the course of their evolution they developed typical mammalian characteristics, such as the reduction of bones in the skull and the lower jaw, and the specialization of the teeth into incisors, canines, premolars, and molars. The synapsids, or mammallike reptiles, include a group of extinct reptiles that had a temporal cheek opening in their skulls, corresponding to the lower of the two openings found in diapsids, or “two-arched” reptiles.

The subclass Synapsida is divided into two orders – Pelycosauria, which includes the oldest and most primitive synapsids, and Therapsida, which are more advanced reptiles. Before we discuss these orders, however, let’s take a detour to another, rather intriguing order of animals, the Mesosauria.



The Mesosaurs

The mososaurs were a small group of reptiles that lived about 250 million yearas ago in the Permian period. Their fossils have been found only in fresh-water deposits in southern Africa and in the Brazil in South America. Because they appeared so early they are believed to be the first reptiles completely adapted to life in the water. The fossils that have been found are crushed, so paleontologist have had a difficult time classifying them. Some believe that they were anapsids, others that they were syapsids.

The order mesosauria contains a single family, the Mesosauridae, and one genus, Mesosaurus. Paleontologist used to classify three separate genera, but now these have all been lumped together. Mesosaurs were small reptiles, rarely growing longer than 12 to 16 inches they looked something like lizards, with very long necks and bodies and long, flexible tails which they swung from side to side as they swam. Their legs were well adapted for life in the water, and they probably had webbed toes. Their skulls seem to have had a lower temporal opening on each side, but this has not been proven.

Their jaws had a great many tiny pointed, curved teeth, which indicates that mesosaurs ate a diet of fish. Their nostrils were placed far back near their eyes, a characteristic adaptation for life in the water.

The discovery of these animals in both southern Africa and South America is evidence of the theory of continental drift, the hypothesis first propounded by the German geophysicist Alfred L. Wegener. At one time, according to this theory, all the continents formed a asingle land mass which later broke up and drifted apart. It is unlikely that Mesosaurs, which lived in ponds, lakes, and marshes, could have traveled from one continent to the other by swimming the Atlantic Ocean. It is more realistic to believe that they inhabited a common continent during the Permian, which later broke up, forming Africa, South America, and perhaps other land masses.

The Pelycosaurs

The pelycosaurs were the first of the synapsids. They appeared about 300 million years ago in the Upper Carboniferous period and lived until the middle Permian period, about 250 million years ago. Abundant fossil materials, which are well preserved in North America and the USSR, substantiate their existence and their evolutionary development. A very small number of fragmentary remains have also been found in European and South American deposits.

Pelycosaurs retained many characteristics that related them to the early cotylosaurs. Their skulls, except for the clearly visible temporal opening, were very similar to those of the captorhinomorphs, which makes it likely that this group of stem reptiles were their ancestors.

The first pelycosaurs, mostly of the early Permian period, all belong to the suborder Ophiacodontia. The ophiacodonts include the genus Varanosaurus, which is one of the most primitive. This reptile was about 5 feet long, with an elongated shape similar to that of a long lizard, a long tail, and sprawled-out legs. Its skull, which was still primitive and unspecialized, was long and narrow with the eyes set far back. The jaws had many sharp reeth. All of these characteristics were more developed in the later and more highly evolved pelycosaurs.

A more highly evolved genus, Ophiacodon, was a Permian reptile about 8 feet long. It differed greatly form Varanosaurus. It had a deep skull with long jaws that bore many pointed teeth. Evidently Ophiacodon ate fish and lived along the banks of steams and ponds. The ophiacodonts as a group did not survive very long. They were extinct by the middle of the Permian period. However, they did give rise to groups of pelycosaurs which survived for a longer time. One line of development led to a genus of large, aggressive carnivores that lived on the land, called Sphenacodon. The other line led to large plant-eating forms called Edaphosauria.



The Sphenacodonts lived through the Permian period and continued the developments begun by the ophiacodonts. In the characteristics of the skeleton, the two groups are quite similar. goophiacodonts. They were of different kinds with large, daggerlike ones in the front part of their jaws and smaller ones along the sides of the mouth, their skulls were narrow and deep, with powerful jaw muscles that made the bite of these predatory carnivores very dangerous.

The most familiar genus of this group was Dimetrodon, an animal that was about 8 feet long and was one of the most dominant reptiles of its time. Skeletal remains of this animals have been found in Permian deposits in North America. One characteristic of its skeleton was quite strange. The spines of its back vertebrate were very long and high, being highest at the middle of the animal’s back. This spine supposedly supported a sort of “sail” of skin. No one knows what its function was, as there is no structure of this size in living reptiles. It may have helped to regulate the animal’s temperature by increasing the surface area of skin. It could have warmed or cooled the animal rapidly as it shifted in and out of the sunlight.

In the genus Sphenacodon, which lived in North America during the Permian period, these spines were not quite so long. Still they were well developed and very probably served as a sturdy base of attachment for the powerful muscles of the neck and back.

The Edaphosaurs were quite different from the sphenacodonts. The genus Edaphosaurus is typical of this group. It was a placid vegetarian. It had a large body with a comparatively small skull. Its teeth were not specialized for different functions and formed an uninterrupted line around its jaws. There were also teeth on the palate, or roof of the mouth.

Edaphosaurus was about 10 feet long. It had long spines on its back like Dimetrodon. These spines were much thicker, however, and had irregular pieces branching off the main spines.

Members of the genus Casea were a little over 3 feet long and had no spines. Neither did edaphosaurs of the related genus Cotylorhnchus, which had barrellike bodies and small heads.





The Therapsids

About the middle of the Permian period a second group of synapsid reptiles developed. This was the order Therapsida. These reptiles covered almost every continental area during the Permian and Triassics periods. They were especially well represented in the regions that are now Russia and South Africa. Their structure makes it clear that therapsids descend from the pelycosaurs. During the course of their evolution they tended to develop even farther along the lines that eventually led to mammals.

The therapsids developed, for instance, a constantly widening cheek opening that eventually got to be similar to that of mammals. In some highly developed forms a secondary palate developed under the first, which again is the case in mammals. It served to separate the nasal openings from the mouth. This arrangement was a useful development because it allowed these animals, as it allows us, to breathe at the same time as they were eating. In the lower jaw the dentary bone became the largest and most dominanat. In mammals this bone (now called mandible) forms the entire lower jaw. These animals also developed specialized teeth. They had incisors, canine, premolars, and molars.

The skeleton in therapsids also developed variations which were similar to the structure in mammals. The legs, for instance, were pulled in underneath the body instead of sprawling out as in more primitive reptiles. This leg position enabled the therapsids to run better.

During the Permian and Triassic periods therapsids gave rise to two groups, the anomodonts and the theriodonts.



The Anomodontia includes two main lines of development, along with smaller groups of lesser importance. Both of these were plant-eating. These were the dinocephalians and the dicynodonts.

The dinocephalians were the more primitive of the two groups. Although they had some typical therapsid characteristics, they also shared some features with the primitive pelycosaurs. They had large bodies and thiccck skull bones. They lacked a secondary palate and their dentary bone was not well developed. The genus Moschops, which was close to the size of a rhinoceros, was typical of this group.

The dicynodonts survived until late Triassic times – unlike the dinocephalians, which became extinct before the end of the Permiam period. They had already spread wide during the Permian period, inhabiting every continent. They were the most common reptiles in that early time. Their remains have been found in Africa, North America and in continental Europe.

The dicynodonts measured from about a foot to about 8 feet in length. They had strong and straight legs, a short tail and a highly specialized skull. They had large temporal openings, theriodonts included a variety of reptiles. Some slender and formed long arches, almost like the cheek bones in mammals. The skulls show that the animals had small teeth or none at all. Probably the animals with none had horny beaks, similar to the ones turtles have today.

The genus Kannemeyeria, which dates from early Triassic times, is one of the best-known dicynodonts.




The Theriodonts belong to the last and in some ways the most interesting of the suborders into which the therapsids are divided. This group includes certain reptiles that lived during the Permian period and whose remains are found in great numbers, mostly in the Karro formation in South Africa. They were small reptiles that followed a course of evolution that led some of them to become fully developed mammals.

The theriodonts included a variety of reptiles. Some of these animals had characteristics so similar to those of mammals that they have often been considered to be the first representatives of the class mammalia.

The genus Cynognathus is a typical representative of a group of theriodonts known as cynodonts. It was an animal about the size of a wolf. It had a skull similar to a dog’s – narrow and long, with a very large temporal opening behind the eye. Its lower jaw was made almost entirely of the dentary bone. Its teeth were well differentiated and were specialized for a diet of meat. The canines, for example, were greatly enlarged and obviously well adapted for piercing and tearing flesh. The overall resemblance of Cynognathus to mammals is striking. Some scientist believe that it was even covered with fur.

Most of e other groups of theriodonts represented lines of development that more or less paralleled that of the cynodonts. The ictidosaurs and the tritylodonts are especially interesting, because they are believed to be the actual ancestors of mammals. They lived in Europe, Asia, and Africa, during the Triassic period and the first part of the Jurassic. They show so many characteristics of mammals in their skulls and their teeth that they are classified as reptiles only because of a few technical anatomical points.

Some of the characteristics that make an animal a mammal cannot be seen in the fossilized remains of extinct animals. These include a constant body temperature, a high rate of metabolism, a furry coat, and young that are born alive and nourished with milk from the mother. Because it is impossible to tell whether or not the most advanced mammallike reptiles had these traits, we cannot tell exactly when and where the advanced reptiles actually became mammals. All we know is that in the late Triassic and early Jurassic, one or more lines of advanced theriodonts did evolve into true mammals.