mammal-like reptiles

mammal-like reptiles The mammal-like reptiles together with the mammals form a monophyletic group (ancestor and all its descendants) termed the synapsids. They are all characterized by having a single skull opening behind the orbit. The earliest synapsids appeared in rocks of Late Carboniferous age at the same time as the first members of the main reptile group, the diapsids. Throughout their history the synapsids were quadrupedal (four-footed) terrestrial organisms that showed steady improvement both in their locomotor ability and the differentiation and specialization of their teeth. These advances helped them to become the dominant terrestrial carnivores and herbivores of Permian and Early Triassic times. They consist of two successive groups, the pelycosaurs that occur from the Late Carboniferous through the Permian, and the therapsids that occur through the Permian and Triassic and into the Jurassic, and which gave rise to the mammals.

The pelycosaurs (Fig. 1) were initially all carnivorous, but they subsequently developed herbivorous forms. From relatively small ancestors, pelycosaurs developed into animals up to 3.5 m in length. Reconstructions of their skeletons show that the massive front part of the body was supported on sprawling front limbs while the hind limbs had a greater range of movement although they were still sprawling. The ankles were poorly defined and could not provide much forward thrust as the feet were extended laterally. The spine was stiffened to allow the movement of the hind limbs to be converted into forward thrust with little lateral body movement. This method of progression circumvented a problem that constrains the endurance of many quadrupedal reptiles. David Carrier of the University of Michigan has shown that the twisting of the body that occurs as most amphibians and reptiles run compresses each lung alternately and prevents the animal from breathing while running. Lizards, therefore, progress in short rushes with frequent stops to breathe. The stiff spine and sprawling gait of the pelycosaurs, although resulting in a more ponderous progression, was actually an improvement, allowing them to keep running for longer periods.

A characteristic pelycosaur was Dimetrodon, a large predator from the Permian of Texas. This animal possessed tall neural spines supporting a web of skin forming a ‘sail’ along its back. Living reptiles are ectothermic, that is, they need external sources of heat to regulate their body temperature. Because of this their internal temperature can fluctuate with the external temperature. To counteract this, living reptiles bask in the sun and orient their bodies for maximum heat absorption. This behaviour suggests that Dimetrodon and related forms with similar ‘sails’ may have used them as surfaces to pick up heat rapidly when needed, or to radiate it when they became too warm. The pelycosaurs included large herbivores such as Edaphosaurus that also possessed large sails supported by neural spines.

Drier conditions in the Late Permian that resulted from the amalgamation of the continental blocks contributed to the extinction of the pelycosaurs. However, a second group of synapsids, the therapsids (Fig. 1), evolved from them in the Late Permian and became important Triassic tetrapods consisting of several carnivorous and herbivorous lineages. One group of therapsids, the dicynodonts, were herbivores with a worldwide distribution. They were particularly abundant in the Late Permian, where they constitute 90 per cent of the therapsid diversity. They are characterized by a short snout and the loss of almost all their teeth, which were replaced by a turtle-like beak used for cropping vegetation. Ranging from rat- to cow-sized, and with specializations for a variety of modes of herbivory, their success was due, at least in part, to their ability to replace the cutting surface of the jaws continuously, an improvement on the intermittent tooth replacement of most reptiles. Another group of therapsids, the dinocephalians, included both carnivorous and herbivorous forms, some of which grew to very large size and had large flanges and crests projecting from their skulls. Herbert Barghusen from the University of Chicago has suggested that these were used in head-butting behaviour to establish dominance within a group.

The theriodonts include the ancestors of the mammals. They were specialist carnivores, some quite large, in which the teeth were well differentiated and the jaw musculature had been rearranged to provide a more precise occlusion of the teeth. This resulted in a much more efficient biting and chewing mechanism, the presence of which suggests that these animals had a high metabolic rate, although probably not up to mammalian grade; they are sometimes reconstructed with mammalian characteristics such as hair, for which there is no fossil evidence. The major skeletal character that separates them is the presence of a typical reptilian jaw articulation between the quadrate and the articular. This is lost in mammals, in which the joint moves to the dentary and the squamosal.

Although the synapsids were dominant in the Permian and in the Early Triassic, they were replaced during the Triassic by the diapsid archosaurs, particularly the dinosaurs. These were animals that had developed bipedality (moving on their hind limbs), an advance that eliminated the special problems of breathing while running that are inherent in quadrupedal reptiles, and one that led to their dominance through most of the Mesozoic while the mammal-like reptiles dwindled.

D. K. Elliott

Bibliography

Carroll, R. L. (1988) Vertebrate paleontology and evolution. W. H. Freeman, New York.
Colbert, E. H. and and Morales, M. (1991) Evolution of the vertebrates. Wiley-Liss, New York.