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Historic Background of Reptiles (8th Part)

© Janat Khatoon

There is no missing link between mammals and reptiles, nor any single fossil type which, as Archaeopteryx does for birds, stands out clearly as half retile, half mammal. If each mammalian feature could be traced to its point of origin, we might hope to put a finger on the first mammals. As it is, the ease rests mostly on bones and teeth, and relying on such skeleton and dentition that today we associate with mammals. We can only deduce the developing pattern of the less soled attributes not likely to be preserved in the rocks. Fortunately there are good clear differences in the skeletons of reptiles and mammals. One such is the structure of the jaw joint. Two bones enter this joint in both mammals and reptiles, but they are different bones in the two animals. Furthermore, while in the mammal the lower jaw is a single bone, in the ancestral reptiles several bones are involved. Fossils from the late Triassic show various stages of reduction of jawbones, and a number of other trails that we now associate with mammals made their appearance at the same time.

By the Jurassic there were true mammals in the world. They were all small insecure-looking creatures, and when we consider the kinds of breasts they shared the earth with we may justifiably wonder how they were able to survive at all. There could be no better illustration of the vagaries of extinction and survival. After dominating the scene for 80 million years the dinosaurs have gone, and the hairy, frisky, rat-small creatures that cowered about their feet have inherited the earth.

The Age of Reptiles is an ideal time from which to draw graphic illustrations of the three great evolutionary processes: extinction, survival and adaptive radiation, the last being the tendency for a stock of living things to evolve in such a way as to exploit all the available opportunities of its environment. From their modest beginnings in the Triassic, the dinosaur line spread and split and sent out adaptive versions of itself into endless specialized roles and niches. The whole spectrum of Mesozoic life, furthermore, shows a similar trend. But because the adaptive radiation of dinosaurs was the most spectacular show put on by vertebrate animals prior to the hydrogen bomb, the completely unexplained cutting off of all dinosaur lines in the Cretaceous is one of the most impressive cases of extinction that we know about. And when extinction happens to a vast, diverse and seemingly successful group like the dinosaurs, it demands an explanation. But though a great deal of thought has been given to the matter, it still remains largely an impressive mystery.

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