Contents Updated: Thursday, August 05, 1999
During the Triassic period the confrontation between the mammal-like reptiles and the forerunners of the dinosaurs was head on. In Bakker's words:
A titanic ecological battle was waged between the advanced pre-mammals, led by the dicynodonts and the cynodonts, and the Erythrosuchidae [crimson crocodiles, early predecessors of the crocodiles and dinosaurs] and their descendants... Two mighty evolutionary dynasties collided in direct competition.
What happened? The fastest and most efficient predators of the early Triassic, the warm-blooded cynodonts and their therapsid kin, lost to the supposedly cold-blooded crimson crocodiles and to their descendants, the thecodonts.
The only relatives of the therapsids surviving till today are the duck-billed platypus and the spiny anteater. Significantly enough these animals are only partly warm-blooded. A platypus's typical body temperature is only 30 degrees Celsius, compared with 37 degrees Celsius for most mammals, and it is subject to wide fluctuations. It also lays eggs and has no nipples but instead has refined sweat glands that ooze milk into its fur for the neonate to lick. It is just what one might expect from a form intermediate between reptiles and mammals.
The therapsids probably became warm-blooded to protect themselves against the advent of colder conditions. Then, for the same reason they developed a furry exterior. But why did the crimson crocodiles and thecodonts become warm-blooded? The answer seems to be precisely because they were adapting for an active, bipedal mode of living—and, as we have seen that would not have been possible without internal heat regulation.
Thecodonts evolved from swamp dwelling forms which developed strong rear limbs for swimming and steering themselves in the water, just like frogs and crocodiles. The swamps began to dry up in the middle of the Triassic period and many of a previous dominant life form, the amphibians, died out.
The thecodonts' ancestors were somewhat more adaptable, necessarily learning to spend more time out of the water until they became independent of it. On dry land they found that their strong hind limbs allowed them to rear up and catch the insects that lived in profusion around the drying swamps. They found that they could run on their hind legs for short distances, improving their chances of survival.
Simultaneously, through natural selection, they became warm-blooded thereby gaining the energy needed for running and hunting. Finally they brought their legs into a better position beneath their bodies to provide more efficient propulsion to exploit their faster metabolism. With long hind legs and an energy system to match, the thecodonts were ready to take on the world. Thus the thecodonts ousted the therapsids from their advantageous position.
They, in turn, were to succumb. The dinosaurs proper, descendants of the thecodonts, appeared on the scene by the end of the Triassic. The invasion of the thecodonts and their successors, the dinosaurs, put paid to the ambitions of the mammal's precursors, the therapsids, and those also of the mammals themselves for a hundred million years or so.
Dinosaurs did not appear until 200 million years after the first vertebrates crawled on to the land. As we have seen, they were preceded by a series of other types of land animals including a period of several tens of million years about 250 million years ago when the pre-mammals dominated.
They were warm-blooded but they were not the first warm-blooded animals. They were the culmination of a series of mighty dynastic struggles among previous warm-bloods. Bipedalism and possibly superior lungs gave them an advantage that others, including the mammals, could not match. In only five million years, short in geological terms, the dinosaurs sprinted to power.
It has taken about five million years from the emergence of mankind to world dominance!
For 140 million years mammals were forced into less desirable habitats especially where it was cold and barren. They lived in burrows, terrified of the dinosaurs and pterosaurs. Insignificance was an advantage. Tiny creatures like shrews and mice were less likely to attract attention. Because mammals at this time were very small—the largest was no bigger than a hedgehog—and rocky terrain is not suitable for fossilization, fossilized remains of these mammals are uncommon. Only their tiny teeth are commonly found.
Though small and insignificant, mammals were not free from danger. By day, though large dinosaurs would not have been interested in them, the smaller predatory dinosaurs were. And besides dinosaurs, true lizards were still sunning themselves motionlessly on the rocks waiting for a small animal to come unwarily by.
Mammals were nocturnal. They had to forage by night when it was too cold for lizards to be active, limiting their enemies to small nocturnal dinosaurs. The mammals' niche was quite lowly for the millions of years of the supremacy of the dinosaur. They could not compete with a superior animal.
All was changed by the mass extinctions which ended the Cretaceous age. The mammals inherited the earth and eventually one of their species came to do remarkable things. It was the thinking mammal—mankind.