AN ADVANCED MAMMAL LIKE REPTILE (Thrinaxodon)
Thoracic rib cage
(lower jaw) Paired canines
Thrinaxodon walked like a mammal, with its legs directly beneath its body. Its teeth and jaws were powerful, with a high coronoid process on the lower jaw for
Forward-pointing feet strong muscle attachment. The rib cage was probably closed off by a muscular diaphragm, which allowed the lungs to expand and contract for efficient breathing.
primitive therapsid (Procynosuchus)
Dentary (lower jaw) Synapsid opening earliest mammal (Morganucodon)
Dentary Coronoid process
Eye socket and synapsid opening pelycosaur (Varanosaurus)
A mammal's strong, biting jaws resulted from progressive changes in the skull and lower jaws of their ancestors. In the earliest mammal-like reptiles, the pelycosaurs, the lower jaw was made of several bones, the dentary being the largest. In the more advanced therapsids, the skull was higher and the synapsid opening larger, for longer jaw muscles. The dentary bone was also larger. In the mammals, the eye socket and synapsid opening merged, and the lower jaw was formed entirely of the dentary. This had a high coronoid process which formed an extra site for muscle attachment. The small bones at the rear of the therapsid's lower jaw had moved into the mammal's middle ear, to form part of a chain along which sound was transmitted.
Eye socket primitive therapsid (Procynosuchus)
Dentary (lower jaw) Synapsid opening earliest mammal (Morganucodon)
ier body retains a greater amount of heat than a small one, providing a thermal reservoir that reduces the effects of fluctuating environmental temperature.
However, the later therapsids possessed other features that would finally end their dependence on the sun as a source of energy. By eating more food more frequently, and by digesting and metabolizing it more quickly, they could rely on food as their source of energy.
This new method of controlling body temperature required many changes, including better jaws and teeth, better locomotion, better breathing control, and better external insulation to maintain the body's internal temperature.
An innovation among the mammal-like reptiles was the evolution of teeth of different sizes and shapes (called a "het-erodont" dentition). Even the earliest pelycosaurs had 3 kinds of teeth in their jaws. The food-grasping incisors at the front were separated from the biting cheek teeth at the back by several long, stabbing canines. In normal reptilian fashion, all these teeth were renewed periodically by waves of replacement along each jaw; the even-numbered teeth were replaced in one wave, and the odd-numbered teeth in the next.
The more advanced therapsids had departed from this reptilian "all-change" method of tooth replacement. Instead, teeth were replaced only a few times during the aninmal's life. This was a major development because it allowed each tooth to remain in the jaws for a longer period, so that the opposing crowns of the upper and lower teeth could develop a complex pattern of crests and valleys. These could then meet in a precision bite, and present a rough pavement on which food could be cut, crushed and ground to a pulp before being swallowed. Well-chewed food could be digested more quickly, which resulted in a more rapid release of the vital nutrients needed to fuel the body.
The jaws and skull also changed. First, the size of the synapsid openings increased, allowing longer jaw muscles to develop. Second, the back of the skull became longer, and the sides bowed out, to produce more space for the muscles.
Third, there was an improvement in the structure of the lower jaw. Previously, some of the jaw muscles were attached to the large dentary bone at the front of the jaw, while others were attached to a number of smaller bones at the back. These smaller bones were progressively reduced in size among the therapsids (and lost completely from the jaws of mammals, below), so removing the potentially weak junction between the bones of the lower jaws. The dentary itself also developed a high flange (the coronoid process) at the back, which provided an attachment point for larger, more powerful jaw muscles (see p. 184).
Another small, but spectacular, change in connection with the jaw bones of mammal-like reptiles provided better hearing in mammals. The 2 bones that had formed the joint between the skull and the lower jaw in mammal-like reptiles retreated into the middle ear of mammals. Here, they linked up with the already present stapes ("stirrup") bone, to form a chain of 3 bones (called the hammer, anvil and stirrup because of their shapes). Sound is transmitted along this chain from the outer eardrum to the fluid-filled canals of the inner ear.
The progressive integration of these 3 small bones into the ear can be traced in therapsids of the Triassic. In fact, these changes can still be seen, telescoped in time, during the development of a modern mammal embryo. They provide as good a proof of evolutionary change as the Arcfiaeo£>teryx-link between reptiles and birds (see p. 173).
Mammals also developed a pair of fleshy ear-flaps on either side of their heads. It is not certain precisely when these external ears evolved, but they have been shown on some of the later therapsids illustrated on p. 191.
While all these changes were occurring in the jaws and teeth of mammal-like reptiles, other skeletal changes were making the limbs of these creatures more efficient. The primitive pelycosaurs of the Late Carboniferous, such as Archaeothyris, moved in the old reptilian style — bending the body from side to side, with the limbs sprawled out horizontally. By the Early Permian, sphenacodonts, such as Dimetrodon, had evolved a new type of locomotion. The shape of the bones in the hips and hindlimbs, and of the joints between the vertebrae, show that the stride of the hindlimbs was accompanied by an up-and-down flexure of the backbone (see p. 185).
From Dimetrodon onward, the evolutionary story of the mammal-like reptiles is of a steady and rapid increase in this flexing of the limbs and body in the vertical, rather than the old-fashioned horizontal, plane. In addition, the feet changed position, from projecting to the sides to pointing forward, and the toes became shorter and of similar lengths.
This whole system had far more po tential for fast movement, since a longer stride and a faster swing of the legs could be achieved simply by elongating various bones of the limbs and feet.
Other features of the advanced therap-sid reptiles strongly suggest that they were warm-blooded, like their descendants, the mammals. For example, the abrupt reduction in the extent of the ribs in such cynodonts as Thrinaxodon suggests that the whole front part of the body cavity, which houses the lungs and heart, had been closed off by a muscular sheet of tissue, the diaphragm. This development set off a chain of events. It allowed larger lungs to be filled and emptied more rapidly and more frequently, which, in turn, resulted in more oxygen entering the bloodstream. This permitted the tissues to use the oxygen faster — to speed up digestion or to increase muscular exertion, when, for example, chasing prey or avoiding predators.
Because the tissues of a warmblooded animal require a constant supply of oxygen, it cannot stop breathing for more than a short time. This need conflicts with the need to keep food in the mouth while it is being chewed up. This problem was solved by some of the advanced therapsids (the theroceph-alians and the cynodonts) by the development of a secondary palate — a shelf of bone that separated the air passage from the mouth. This structure is yet another pointer to support the theory that the advanced mammal-like reptiles were already warm-blooded.
There is no way of knowing whether the mammal-like reptiles were covered in hair or fur. Such insulation is not necessary in larger animals (and many of the therapsids were large), since they have a proportionately smaller surface area through which heat may be lost.
It is significant, however, that the final transition between the mammal-like reptiles and the mammals, at the end of the Triassic, was accompanied by a marked reduction in size.
The first mammals, such as Mega^os-trodon (see pp.198, 200) were tiny, shrewlike creatures. There is no doubt that, as they watched from the trees through the long millennia of the reign of the dinosaurs, they were kept warm during that vigil by a covering of fur. And this simple, furry pelt may have been the vital feature that enabled the mammals to survive the catastrophe that exterminated the dinosaurs (see p. 93), and left these tiny creatures free to inherit the Cenozoic world.
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