The monotremes or egg-laying types of mammals such as the duck-bill and
spiny anteaters which now inhabit Australia are almost unknown as
fossils. Marsupials, the next higher living group, which includes the
opossum and kangaroo, appeared at the end of Cretaceous time along with
the placentals or higher mammals which dominate the history of the
Cenozoic era. Nevertheless, there are a few teeth and jaws from rocks of
Triassic and Jurassic age to indicate that small mammals, from the size
of mice to slightly larger than rats, existed throughout most of the Age
of Reptiles. There is no complete skeleton of any of the earlier forms,
and little is known of their relationships either with living orders of
mammals or with probable ancestors among the reptiles. The record
becomes somewhat clearer toward the end of the era but it is obscured
again by the great disturbances which followed.
Looking back among earlier land animals for the origin of the first
mammalian stock it is necessary to go as far as Permian or even
Carboniferous time. Reptiles then living had many structural features in
common with mammals, and mammal-like forms continued to flourish until
late in the Triassic. An interesting group of such animals, named
therapsids, was one of the earliest reptilian stocks to appear, and is
well known from fossils found in the Red Beds of Texas and New Mexico,
in Europe, South Africa, and Asia. Quite a variety of types is included
in this group, with many advances in dentition, and modifications of the
skull, limbs and pelvic construction which strongly suggests a
relationship to the mammals.
[Illustration: Murals Over Fossil Exhibits, Hall of Mammals
Top: Eocene; Protylopus, Tanyorhinus, Patriofelis, Uintatherium,
Turtle, Crocodile, Eohippus.
Middle: Upper Oligocene; Mesohippus, Merycoidodon, Hoplophoneus,
Metamynodon, Poebrotherium, Trigonias.
Bottom: Pliocene; Teleoceras, Turtle, Synthetoceras, Amebelodon,
Teleoceras.]