ANCESTORS OF THE MAMMALS
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.
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.
THE AGE OF MAMMALS
The striking feature of life development in the Cenozoic era is the great progress and expansion over the earth of the mammalian races. The division of the era into periods, however, was based largely on a study of fossil mollusks. In the Paris basin of France, it was noticed by the geologists of a century ago that the youngest of the sedimentary beds contained the greatest number of recent or still living species. Successively downward into the older beds the percentage of recent species decreased until there were practically no living species represented in the oldest rocks of the series. From the percentage of recent forms among prehistoric ones it was proposed that the following division be made: Eocene, meaning dawn of the recent; Miocene, meaning less recent; and Pliocene, meaning more recent. Sometime later it was suggested that another period be added, and to this was given the name Pleistocene, meaning most recent. In 1854, the older Miocene formations were segregated and referred to a newly provided Oligocene period, this name meaning little of the recent.
Early geologists grouped the rocks in three great divisions, applying the names Primary, Secondary, and Tertiary. To these was added afterwards the name Quaternary, which applied to the youngest formations of the earth. Only two of these terms remain in common use at present: it is a frequent practice to refer to the combined Eocene, Oligocene, Miocene and Pliocene periods as the “Tertiary” division of Cenozoic time; to the Pleistocene and Recent periods as the “Quaternary” division. The geology of some remote future may be clearer with regard to the full significance of this subdivision of the Age of Mammals into two parts. It may be that a great era was concluded at the end of Pliocene time as others have been concluded, by the usual earth disturbances and climatic changes and by the decline of animals once prominent in the faunas of the world. Events of such character have registered their occurrence but may eventually prove to have been a series of minor events not comparable with the revolutionary changes that terminated other great time divisions. The favored practice of including ourselves and our times in the Cenozoic is based on a trend of opinion which holds that no great era has been ended since the Age of Reptiles was concluded.
Skulls of the clumsy, six-horned uintathere and the early, hornless titanothere form part of this Eocene display. In the mural these animals and the little “three-toed” Eohippus, smallest of horses, are pictured with a contemporary turtle and crocodile.