The Succession of Faunas

Cretaceous.

The marine fauna of the Cretaceous exhibits in the main a continuation of that of the Jura; but certain families of Alcyonaria make their appearance. The echinoids exhibit the incoming of some new groups of which the most important is that of the heart urchins. The brachiopods are still further reduced in variety and the Polyzoa become still more abundant, largely on account of the dominance of the Cheilostomata. Many new families of lamellibranchs and gastropods are introduced, but the ammonoids become less varied, disappearing altogether at the close of Cretaceous times. Before they do so they branch out into curious forms in which the shell comes unrolled to a varying ex tent, whilst certain others exhibit a return to a simplicity of structure which vividly recalls that of certain Middle Triassic ceratites.

Amongst the fish, the elasmobranchs are with few exceptions modern in type. Even such specialised forms as the goblin sharks are represented by species belonging to an existing genus. None the less, certain Tertiary selachians, the eagle rays and sawfish have not yet appeared though their ancestors are found in Upper Cretaceous rocks. The Mesozoic "ganoids" are rare and little varied, the great bulk of the bony fish being teleosts belonging to the more primitive families, although in some cases to genera which still exist. Amongst the reptiles the dinosaurs and ptero dactyls reach their largest size to vanish with the end of the period, whilst the seas contained mosasaurs in addition to ichthyo saurs, plesiosaurs and Chelonia. In fresh waters in the Upper Cretaceous an aquatic rhynchocephalian Champsosaurus makes its appearance and the first snakes are found in the Upper Creta ceous limestones of Istria and in Patagonia. The few known Cretaceous birds belong to a super-order characterised by the possession of teeth, certain of them being adapted for powerful flight, whilst the remainder were aquatic with hind-legs modified to form paddles and the wing even more reduced than that of a penguin.

The majority of the Cretaceous mammals belong to the same groups as those which existed during the Jura, but a few forms can be shown to be closely related to the Opossums and others recently found in Mongolia are of great importance because they have every appearance of being the ancestors of the placental mammals.

Tertiary.

The Tertiary period has seen the rise and develop ment of all the higher groups of animals. In the sea the modern corals belonging to the Hexacoralla become dominant. Amongst the Echinodermata the asteroids show extremely little change in type, and the ophiuroids pass on little altered. The echi noids are, however, of very different appearance, such animals as Clypeaster and the cassidulids becoming very abundant and the group of the Spatangidae branching out into a great variety of forms. The brachiopods are reduced to the still existing families and become rare both as species and as individuals. The Mollusca on the other hand become even more varied and abundant, new families of lamellibranchs and gastropods arising and few dying out. The most striking change, however, is the complete disappearance of the ammonites and belemnites; the Octopods known from the Upper Cretaceous becoming very abundant in the recent seas. Amongst the arthropods, the decapods become increasingly abundant whilst it has been demonstrated conclusively that all the orders of insects were in existence early in Tertiary times.

Tertiary fish represent a development of those present in Upper Cretaceous times, and are on the whole of little interest. The reptiles become reduced to those orders which still live. The Chelonia, lizards and snakes undergo a rapid evolution branching out to fill many different types of habitat, whilst the Rhyncho cephalia and crocodiles present no new modifications. Thus the most characteristic Mesozoic reptiles, dinosaurs, pterodactyls, ichthyosaurs, plesiosaurs and mosasaurs die out during or at the end of the Cretaceous period. It is an interesting point however that the marine Chelonia and champsosaurs live on into Tertiary times.

The most important feature of the Tertiary faunas is how ever the "adaptive radiation" which takes place amongst the birds and mammals, leading up in the latter group to man himself. For the details of this process reference should be made to the articles MAMMALIA and ORNITHOLOGY.

The knowledge of the distribution of animals in time sum marised above is founded on an immense material. Many millions of fossils have been examined by palaeontologists during the past century, and the conclusions resulting from their investigations cannot be seriously modified by the unexpected discovery of ani mals before or after the period to which we believe them to have been restricted. The whole story of the history of life may be summed up thus :—In the Lower Cambrian, representatives of all phyla of invertebrates capable of ready preservation as fossils, are present, but within each phylum they exhibit little range in structure and on morphological grounds are regarded as of low organisation. In succeeding periods the variety of structure ex hibited by the members of each phylum becomes greater, a fact which is expressed by the steady increase in the number of fami lies which are recognised. The higher families come in steadily one after the other and certain groups achieve a dominant posi tion, vast numbers of species and of individuals being found all over the world. These dominant groups die out not by a sudden extinction but by a gradual decrease of the numbers of families, species and individuals. Amongst the vertebrates that group which from its structure has always been regarded as the most primitive, the Cyclostomata, is the first to appear and is followed successively by the fish, Amphibia, reptiles, mammals and birds, these groups making their entrance in the order which would have been expected from their structure. The whole phenomenon of the geological occurrence of animals provides the most impor tant single piece of evidence which exists for the truth of the the ory of evolution. Darwin in the Origin of Species expressed his belief that it was to the evidence of fossils that we must turn for support of the evolution theory, and since his day a large part of the energies of palaeontologists have been devoted to the study of the relationships of extinct animals and to an attempt to trace the actual lines of descent which they exhibit. The phylogenies which have been drawn up by palaeontologists are of diverse types; they may merely illustrate the evolution of group from group, or they may display the mode in which one species has arisen from another, but the mode by which they have been derived is the same in all.