MAMMALIA.—The history of those warm-blooded animals, the mammals, commences in the early part of the Mesozoic period. It was then that the skull began to assume the characters seen in the modern quadrupeds, and their well-formed limb-bones, and fusion of the three bones on each side of the pelvic arch to form the innominate bone, also show relationship to the later types. The earliest ancestral mammalian forms seem to be related to the theromorphic reptiles, predominant in the Permian and Trias. The mammals first to make their appearance were probably related to those of the Monotreme and Marsupial orders. More nearly related to the former is the group of mammals of the Mesozoic period, the Multituberculata.
Multituberculata.—
This group comprises the Triassic Tritylodon (South Africa and Germany); the Upper Jurassic Bolodon (England and United States); the Upper Jurassic to Lower Cainozoic Plagiaulax (England, United States and France); and the Lower Eocene Polymastodon (New Mexico). The molar teeth are ridged longitudinally, and carry numerous tubercles, hence the name of the group, and resemble the deciduous teeth of the Duck-billed Platypus (Ornithorhynchus).
Monotremata.—
The Monotremata are represented at the present day in Australia and New Guinea by the Echidna or Spiny Anteater, and by the Ornithorhynchus or Duck-billed Platypus of Eastern Australia and Tasmania. These egg-laying mammals show relationship towards the reptiles both in structure and in methods of reproduction.
A Pliocene species of Ornithorhynchus (O. maximus) has been recorded from the Deep-leads of Gulgong, New South Wales, and the same beds have yielded the remains of Echidna (Proechidna) robusta. Remains of another species, Echidna, (P.) oweni, have been described from the Pleistocene Cave-breccias of the Wellington Valley Caves, New South Wales; and Ornithorhynchus agilis is found in deposits of similar age in Queensland.
Marsupials.—
The Marsupials or pouched mammals belong to the sub-class Metatheria. They are divided into Diprotodontia and Polyprotodontia, accordingly as they possess a single pair of incisor teeth in the lower jaw, or many front teeth, hence the names of the two sub-orders. A later classification of the Marsupials is that of their division into syndactyla and diadactyla.
The diadactyla have the second and third toes separate, and are represented by the family Dasyuridae or Native Cats. These are polyprotodont. They are the most archaic of the marsupial group. Remains of Dasyurus, both of extinct and still living species are found in Pleistocene Cave-breccias in Victoria and New South Wales. The Tasmanian Devil (Sarcophilus ursinus) (Fig. 138, 139) and the Tasmanian Wolf (Thylacinus cynocephalus), still living in Tasmania, have left numerous remains on the mainland, in Victoria and New South Wales. Of the latter genus an extinct species is T. major from the Pleistocene of Queensland ([Fig. 140]).