As in other eras, the invertebrates fluctuate with the periods.
Characteristic forms appear, become more or less prominent, then in many
cases decline or disappear. Variations among the mollusks are
particularly helpful in the identification of rocks which originated in
the Cretaceous seas. Clam-like bivalves of the genus _Inoceramus_, the
straight-shelled ammonids known as _Baculites_, and oysters, are locally
common in some of the formations exposed a few miles west of Denver.
The ammonids, or “ammonites,” were extremely abundant throughout the
world during Mesozoic time. Their shells were chambered like those of
the pearly nautilus, a related cephalopod mollusk inhabiting tropical
seas at the present time. While only four species of the Nautilus tribe
are living today, thousands of species of ammonids swarmed the
prehistoric seas. Many new forms came into existence in Triassic time
but few survived the period. A pronounced revival occurred in the
Jurassic, only to be followed by a decline and eventual extinction at
the close of the Cretaceous. Ammonites measuring three or four inches
across the diameter of the coiled shell were about average size, but
diameters up to three or four feet were not uncommon. Externally the
shells were ornamented with ribs, knobs, and spines; inside was a pearly
lining. The partitions were thin and composed of the same pearl-like
substance as the lining. Each partition becomes wavy as it approaches
the shell, and the line of union has a distinct pattern which is seen in
specimens which have lost the outer shell layer. This wavy suture line
becomes more complicated in the later members of the race, and the
peculiar markings developed by the repeated partitions provide a
convenient method of identification.
The belemnites or ink-fishes, regarded as ancestors of the cuttlefishes
now living, comprise another group of carnivorous mollusks. These,
however, had lost the external shell, and the usual fossil is part of an
internal shell or “skeleton,” known as the guard. This limy structure
has the form of a pointed cigar, and is seldom over a foot long although
the total body-length of the larger animals was commonly about six to
eight feet. Several hundred species have been described, the majority
being of Jurassic age. They declined rapidly toward the close of the
era.
The invertebrate life of the Mesozoic was strongly dominated by
mollusks, with cephalopods in first place, the bivalve pelecypods and
the single-valve gastropods or snail-like forms sharing subordinate
positions. The dominating trilobites, sea-scorpions, and tetracorals of
the Paleozoic had disappeared, while the brachiopods and crinoids were
greatly modified and more like the forms which live today.
Crinoids became moderately abundant at various times, but in many ways
different from their relatives of the preceding era. Some of the largest
known species, with stems estimated as fifty feet long, have been found
in lower Jurassic rocks. A great abundance of microscopic life is
indicated by the frequent occurrence of chalk in the Cretaceous
formations. Corals of the modern reef-building type (hexacorals) were
common in the warm seas of a large part of the world.
The insects of the early Mesozoic are represented by few fossils
although it is evident some new forms were becoming established. The
warm climates prevailing throughout much of the world appear to have
been a favorable factor in the progress of insect life. In addition to
the older cockroach and dragon-fly types may be mentioned the arrival of
grasshoppers, cicadas, caddis-flies, beetles, and ants.
Several hundred species are found in Jurassic rocks, and by the end of
the Cretaceous period most of the insect families now known to us were
probably in existence. The record is seriously obscured by the erosion
of rocks which so frequently marks the end of a period, also by the
small size of the subjects, and by the exceptional conditions required
for the production of such delicate fossils. Among the last of the
familiar insects to appear were the bees and butterflies. These
evidently came in with the more advanced types of flowering plants that
produce the nectar on which many insects feed. It is probable, too, that
without the arrival of these insects and their service in the
pollination of flowers, the floras of today would be rather different
from what they are.