Daniel Cerquozzi, age 10, of Bradenton, Florida, for his question:
Did mammals live in the days of the dinosaurs?
All of our modern animals had ancestors that lived way back in the days of the dinosaurs. However, the early mammals were not much like the furry, four footed dogs and cats that share our world. In fact, if we could return to those distant times, we might mistake them for smallish dinosaurs. What's more, scientists tell us that actually they were somewhat related to those mighty reptiles. The strange little old timers became more like the mammals we know after the dinosaurs departed.
It is not easy to trace back the remote history of the mammals. Scientists approach the problem from both the past and the present. They study the existing groups of mammals such as the egg laying platypus, the pouched kangaroo and the cats who nurse their babies on mother's milk. These differences suggest how the earliest mammals coped with life when they shared the world with the dinosaurs.
Most of the provable evidence comes from fossils. However, complete fossil skeletons are very rare and even fragments are hard to find. Then comes the tedious task of dating these ancient remains. But sometimes the rewards are well worth the trouble. For example, in the past couple of years, scientists found the 200 millionyear old remains of lystrosaurus in Africa, India and Antarctica. This suggests that in those days these continents were within walking distance of each other, for lystrosaurus was a four footed plodder about the size of a large modern dog. Ibis bony remains also shed new light on the mammal ancestors that lived during the days, of dinosaurs.
The saurus in this old timer's name classified him as a dinosaur relative. However, his teeth and his sturdy legs resemble mammal features. Several other mammal type reptiles lived 160 million years ago, when the dinosaurs were in their heyday. A ratty little fellow named melanodon lived in North America. But fossil bones do not reveal how or when these primitive ancestors became furry, warm blooded mammals who fed their infants on mother's milk.
Life to them must have been like sharing a corral with a crowd of assorted elephants, some of them hungry meat eaters. It taught them to become artful dodgers, able to cope with unpleasant surprises. About 60 million years ago, a global change occurred, perhaps in the climate. The little mammals adjusted to this challenge also and they survived. The dinosaurs were wiped out because they were unable to change with the times.
By this time, the survivors were very much like small modern mammals. Suddenly they inherited a new world and, naturally, they made the best of it. Some grew larger. All species developed highly specialized features to cope with their different life styles.
In the Age of Reptiles, dinosaurs outnumbered all the other backboned animals. The Age of Mammals waited for their departure, but mammals were never very plentiful. Of the 50,000 or so modern vertebrate species, about 20,000 are fishes and 8,600 are birds. We have only about 5,000 mammal species but they are the most advanced animals of their age.