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Each period had its dominant forms. The Silurian was the great age of trilobites; the Devonian, the age of fishes; Mesozoic times swarm with the gigantic reptiles; and in Tertiary times the mammals are dominant. Each period and era has its root in that which preceded it.
The multitude of kinds which we find in the Mesozoic period indicates that the life was in a state more experimental than that to which it has attained. A host of forms on their way towards the specialization which has now been attained have been removed from the sphere, in the manner of a scaffolding from a completed structure.
Over the earth generally, however, they were superseded by the placental mammals, which suddenly break into the geological record in the early Tertiary, and spread with great vigour and rapidity over the four continents. Were they a progressive offshoot from the Mesozoic Marsupials, or Monotremes, or do they represent a separate stock from the primitive half-reptile and half-mammal family?
The reptilian egg, unlike that of the frog, is covered with a shell, packed away under the surface of the ground, and left to its own fate. If, as most geologists believe, the climate of the Mesozoic was distinctly warm, this habit of the parent of forsaking the egg was not a serious matter.
Its bones were a marvel of lightness, the entire skeleton, even in its petrified condition, not weighing more than five or six pounds. The sharp beak, a yard long, was toothless and bird-like, as its name suggests BIRDS. The earliest known birds are found in the Jurassic, and during the remainder of the Mesozoic they contended with the flying reptiles for the empire of the air.
New ranges of continental extent were opened to them, food was abundant, the climate was congenial, and they now branched into very many diverse types which occupied and ruled all fields, the land, the air, and the sea. The Mesozoic was the Age of Reptiles.
There is a great break here a leap from Archaean times on the east side to Mesozoic times on the west. The east side is millions of years the older. Here is the original Plutonic or Azoic rock which apparently has never been under the sea since it was first thrust up out of the fiery depths. The west shore, including the Palisades, belongs to a much later geologic era.
Numerous fossil remains of them have lately been discovered in the Mesozoic strata of Europe, Africa, and America. To-day there are only two surviving specimens of the group, which we place together in the family of the duck-bills, Ornithostoma. The Australian duck-bills are distinguished externally by a toothless bird-like beak or snout.
It is more difficult to identify Monotreme remains, but the fact that Monotremes have survived to this day in Australia, and the resemblance of some of the Mesozoic teeth to those found for a time in the young Duckbill justify us in assuming that a part of the Mesozoic mammals correspond to the modern Monotremes.
The two mammals of the Trias, also, appear to belong to Australian groups. Every one is aware of the many curious points of resemblance between the marine fauna of the European Mesozoic rocks and that which now exists in Australia.
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