Dinosaurs | Page 3

some would not be very noticeably so at first glance.
Among molluscs, the Ammonites, related to the modern Pearly
Nautilus, are an example of a race very numerous and varied during all
the periods of the Reptilian Era, but disappearing at its close, leaving
only a few collateral descendants in the squids, cuttlefish and nautili of
the modern seas. The Brachiopods were another group of molluscs, or
rather molluscoids for they were not true molluscs, less abundant even
then than in previous ages and now surviving only in a few rare and
little known types such as the lamp-shell (Terebratulina).
Insects. The Insect life of the earlier part of the Age of Reptiles was
notable for the absence of all the higher groups and orders, especially
those adapted to feed on flowers. There were no butterflies or moths,
no bees or wasps or ants although there were plenty of dragonflies,
cockroaches, bugs and beetles. But in the latter part of this era, all these
higher orders appeared along with the flowering plants and trees.
Plants. The vegetation in the early part of the era was very different
both from the gloomy forests of the more ancient Coal Era and from
that which prevails today. Cycads, ferns and fern-like plants, coniferous
trees, especially related to the modern Araucaria or Norfolk Island Pine,
Ginkgos still surviving in China, and huge equisetae or horsetail rushes,
still surviving in South American swamps and with dwarfed relatives
throughout the world, were the dominant plant types of that era. The
flowering plants and deciduous trees had not appeared. But in the latter
half of the era these appeared in ever increasing multitudes, displacing
the lower types and relegating them to a subordinate position. Unlike
the more rapidly changing higher animals these ancient Mesozoic
groups of plants have not wholly disappeared, but still survive, mostly
in tropical and southern regions or as a scanty remnant in contrast with
their once varied and dominant role.
There is every reason to believe that upon the appearance of these
higher plants whose flower and fruit afforded a more concentrated and
nourishing food, depended largely the evolution of the higher animal
life both vertebrate and insect, of the Cenozoic or modern era.

FOOTNOTES:
[Footnote 1: The records of Egypt and Chaldaea extend back at least
sixty centuries.]
CHAPTER II.
NORTH AMERICA IN THE AGE OF REPTILES.
ITS GEOGRAPHIC AND CLIMATIC CHANGES.
North America in the Age of Reptiles would have seemed almost as
strange to our eyes in its geography as in its animals and plants. The
present outlines of its coast, its mountains and valleys, its rivers and
lakes, have mostly arisen since that time. Even the more ancient parts
of the continent have been profoundly modified through the incessant
work of rain and rivers and of the waves, tending to wear down the land
surfaces, of volcanic outbursts building them up, and of the more
mysterious agencies which raise or depress vast stretches of mountain
chains or even the whole area of a continent, and which tend on the
whole so far as we can see, to restore or increase the relief of the
continents, as the action of the surface waters tends to bring them down
to or beneath the sea level.
Alternate Overflow and Emergence of Continents. In a broad way these
agencies of elevation and of erosion have caused in their age-long
struggle an alternation of periods of overflow and periods of
continental emergence during geologic time. During the periods of
overflow, great portions of the low-lying parts of the continents were
submerged, and formed extensive but comparatively shallow seas. The
mountains through long continued erosion were reduced to gentle and
uniform slopes of comparatively slight elevation. Their materials were
brought down by rivers to the sea-coast, and distributed as sedimentary
formations over the shallow interior seas or along the margins of the
continents. But this load of sediments, transferred from the dry land to
the ocean margins and shallow seas, disturbed the balance of weight
(isostasy) which normally keeps the continental platforms above the
level of the ocean basins (which as shown by gravity measurement are

underlain by materials of higher specific gravity than the continents). In
due course of time, when the strain became sufficient, it was readjusted
by earth movements of a slowness proportioned to their vastness. These
movements while tending upon the whole to raise the continents to or
sometimes beyond their former relief, did not reverse the action of
erosion agencies in detail, but often produced new lines or areas of high
elevation.
[Illustration: Fig. 2.--North America in the Later Cretacic Period. Map
outlines after Schuchert.]
Geologic Periods. A geologic period is the record of one of these
immense and long continued movements of alternate submergence and
elevation of the continents. It begins,