Ancient Arctogaea

ANCIENT ARCTOGAEA Inspection of the foregoing lists which represent the existing fauna of the various zoo-geographical regions, will show that although they are distinct from one another and mingle only incompletely in the transition regions, certain animals such as the tapirs, may occur in widely separated areas not now connected by a practicable land-bridge. The whole distribution of these mammals is inexplicable on the basis of the present distribution of land, water and mountain ranges; it can only be understood in the light of the history of the evolution of mammals, now known through the work of three generations of palaeontologists in very considerable detail. Perhaps the simplest method of ex plaining this history is to take the mammalian orders in suc cession and discuss the time and place of origin of their more important families and genera.

Monotremata.

It is clear that the monotremes are a group of very ancient origin, probably Triassic, but that the existing forms are highly specialised and closely related to one another. The only fossil forms known are Pleistocene and belong to the existing genera.

Marsupialia..

Polyprotodont marsupials of the family Didelphidae occur in the Upper Cretaceous of North America. Their remains are found in Eocene, Oligocene and Miocene of North America and Europe and they have inhabited South America throughout the Tertiary. The South American carnivorous bor hyaenids appear to be autochthonous, whilst the Australian dasy urids have no relatives outside Notogaea. The pseudo-diproto donts, the coenolestids, are represented in South America from the Oligocene to the present day, and have no other relatives. The true diprotodonts have always been Australian; Wynyardia from the Middle Miocene of Tasmania is the oldest known form.

Edentata.

The group Edentata includes three completely distinct orders which have probably no close relationship with one another. These are the Xenarthra, the Tubulidentata and the Pholidota. The earliest certainly known members of the Xenarthra occur in the Colpodon beds of South America, per haps Oligocene, and in the Notostylops beds of the Eocene. Very important is the occurrence in the Lower and Middle Eocene of North America of animals (Metacheiromys) which are plausibly regarded as aberrant armadillos without body armour. In the Basal Eocene and in the Lower and Middle Eocene of West and North America there occur animals, the Ganodonta, which may be ancestral to the Gravigrada, the giant ground sloths. If these

relationships be substantiated it will follow that North and South America were connected by a land-bridge at the beginning of Tertiary time. It is none the less certain that the later evolution of the Xenarthra took place in an isolated South America not again connected with the northern hemisphere until Pliocene times.

The Tubulidentata include only the single genus Oryctero pus, which on grounds of its comparative anatomy is probably of ungulate derivation. Extinct species of this genus, or of one closely allied, occur in Pontian (Lower Pliocene) rocks in Persia, Samos, Greece and the south of France, but no earlier forms are certainly known, a humerus from the Oligocene of France being not really determinable.

The Pholidota, including the genus Manis, are represented as fossils only by certain very doubtfully determined fragments from the European Oligocene and Miocene.

Insectivora.

The order Insectivora includes a large number of forms only distantly related to one another. The Menotyphla at present represented by the elephant shrews of Africa and the tree-shrews of Malay, probably also covers a number of forms whose remains are found in Europe, Mongolia, and North America in rocks of various ages from the Basal Eocene into the Oligocene. It must be admitted however, that the evidence on which these forms are referred to the group, is very slender.

The Zalambdodontidae including the solenodons of the Antilles, Potamogale and the golden moles of Africa, together with the Madagascan tenrecs include not only a perfectly characteristic skull from the Basal Eocene of North America but other forms extending as high as the Oligocene. Xenotherium from the Oligo cene of North America is supposed to be a golden mole, and Necrolestes from the Upper Miocene of Patagonia has also been placed in the neighbourhood of that group; in neither case however is the systematic position quite certain. At any rate the group is of such antiquity that its members have had the whole Tertiary period in which to wander over the world and may hence reach any part of it which has been connected by land within that period.