Some idea of the Mesozoic climate is obtained from the character and
distribution of the plant life. Triassic floras are not large and there
is very little fossil evidence for the earlier half of the period. It is
quite possible that arid or desert conditions prevailed for a time in
much of North America, as at the close of the Paleozoic era. Plant life
was at first not abundant, and conditions were unfavorable for the
production of fossils. In Upper Triassic rocks of Virginia, however,
there are signs of swampy conditions, with rushes and ferns
predominating. Adjoining forest areas were well timbered with large
coniferous evergreens which show no annual growth rings, as similar
trees do in regions where cold winters alternate with warm summers. This
suggests, for that time and place at least, a uniformly warm climate,
lacking seasonal variations. Warm temperature or subtropical climates
are indicated again by some of the Jurassic and Cretaceous plants, but
intervals of lower temperatures and variable climates are also apparent.
Palms, figs, and other trees, very similar to modern types now living
only in warmer regions, were widely distributed in late Cretaceous time,
and their range was extended into regions which have since become too
cold to support such growths.
The trend toward modern forms in the plant world was gradual, but
throughout the era there were occasional novelties that attract the
attention of botanists. Ferns and horsetail rushes, reminiscent of the
Paleozoic forests, soon began to lose their prominence as the
seed-bearing trees gained the ascendency. Mesozoic time could well be
called the age of cycads, because of the striking performance of this
plant group. Different varieties flourished in the three periods, with
the Jurassic standing out as the time of greatest abundance.
To the uninitiated, the usual cycad fossils resemble “petrified
pineapples,” but these are merely the scarred stems or trunks of small
to medium-sized trees with a tufted arrangement of leaves at the top,
and usually without branches. Foliage and habit of growth suggest
something more like large ferns or low-growing palms, with short, thick
trunks seldom more than fifteen feet tall and many of them under three
feet. The leaves are rarely found entire or attached to the trunks, but
occasional discoveries indicate a leaf-length of about ten feet.
Although they are classed among the first and lowest of seed-bearing
plants, and in this respect are related to the conifers, their
appearance was quite unlike that of the modern cone-bearing evergreens.
More nearly resembling the common conifers of today were the sequoias,
of early Mesozoic origin and far more abundant during Cretaceous time
than they are at present. The maidenhair trees, now represented by a
single species of _Ginkgo_ which is cultivated principally in China and
Japan, were never very prominent but are of interest as an ancient
family that persisted throughout the Mesozoic and down to our own time.
Before the close of the Cretaceous period the flowering plants had
greatly outnumbered the spore-bearing groups, such as the ferns and
horsetails which were formerly so abundant. We know little of early
flowers, however, except in connection with trees, the large gayly
colored blossoms of the type now conspicuous in woodlands, meadows, and
gardens being later arrivals and poor subjects for preservation as
fossils.
Cretaceous floras were surprisingly modern in character, far in advance
of the animal life. Poplars, plane trees, magnolias, palms, figs, oaks,
and buckthorns were abundant at the close of the Cretaceous, as
indicated by fossils of the Laramie formation, which is the surface rock
in many localities near Denver. Also abundant in various places at this
time were walnut, hazelnut, laurel, tulip, maple, beech, birch,
breadfruit, ivy, holly, and many other well-known trees and shrubs.
Sedges and grasses, which became so important to the herbivorous mammals
of the next era, made their first appearance in Cretaceous time but were
then inconspicuous.