As a matter of course, the remains of Mammals are scanty, and occupy but a small space in the geological record, since the greater number of the Mammalia are terrestrial, and the greater number of the stratified fossiliferous deposits are marine. The Mammals, too, are the most highly organised of the entire sub-kingdom of the Vertebrata ; and therefore, in obedience to the well-known law of succession, they ought to make their appearance upon the globe at a later period than any of the lower classes of the Vertebrata. Such, in point of fact, is to a great extent the case ; and if the geological record were perfect, the law would doubtless be carried out to its full extent.
It is in the upper portion of the Triassic rocks - that is to say, not long after the commencement of the Mesozoic or Secondary epoch - that Mammals for the first time make their appearance; three or four species being now known in a zone of rocks placed at the summit of the Trias, just where this formation begins to pass into the Lias. The earliest of these - the oldest known of all the Mammals - appears at the upper part of the Upper Trias (Keuper) and also at its very summit (Penarth Beds), and has been described under the name of Microlestes antiquus. The nearest ally of Microlestes amongst existing Mammals would seem to be the Marsupial and insectivorous Myrmecobius, or Banded Ant-eater of Australia. As only the teeth, however, of Microlestes have hitherto been discovered, it is impossible to decide positively whether this primeval Mammal was Marsupial or Placental.
The next traces of Mammals occur in the Stonesfield Slate (Lower Oolites), and here four species, all of small size, are known to occur. Most of these were Marsupial, but it is possible that one was placental. They form the genera Amphi-lestes, Amphitherium, Phascolotherium, and Stereognathns. After the Stonesfield Slate another interval succeeds, in which no Mammalian remains have hitherto been found; but in the fresh-water formation of the Middle Purbeck - at the top, namely, of the Oolitic series - as many as fourteen small Mammals have been discovered. These constitute the genera Plagiaulax, Spalacotherium, Triconodon, and Galestes. In the Jurassic rocks of North America, as in those of Europe, small Mammals (belonging to the Marsupials, and referable to the family of the Didelphidae) have been discovered. Another gap then follows, no Mammal having hitherto been discovered in any portion of the Cretaceous series (with doubtful exceptions).
Leaving the Mesozoic and entering upon the Kainozoic period, remains of Mammals are never absent from any of the geological formations. From the base of the Eocene rocks up to the present day remains of Mammals commonly occur, constantly increasing in number and importance, till we arrive at the fauna now in existence upon the globe.
The more important forms of fossil Mammals will be spoken of in treating of the separate Mammalian orders.