Extensive land areas must have subsided again early in the Ordovician
period for marine sediments were laid down over a large part of the
North American interior, and three epochs or subdivisions of the period
have been based on as many invasions of the sea. In these ancient
deposits the record of life continues to show new forms. Nothing of a
very spectacular sort is recorded other than a great increase in the
number of species among types that were established in earlier periods.
Trilobites were at their best, brachiopods continued to flourish, and
the mollusks made new progress, especially the cephalopods, a group
which includes our cuttle fishes and squids. Some of these predatory
creatures attained large size and were no doubt masters of the sea.
Typical forms were provided with tapering chambered shells that
occasionally reached a length of twelve or more feet. Most of the shells
were straight and trumpet-like or but slightly curved. Some were closely
coiled and in this respect more like the well-known nautilus now in
existence.
The bryozoans became very common in the later part of the period and
corals made slight advances. Somewhat of a novelty at this time were the
crinoids, commonly known as “stone lilies” although not plants at all.
They have been described as starfishes with back turned downward and a
thick stem attached beneath. Where they lived in great abundance the
limestone deposits may consist almost entirely of their stems. Crinoids
continued to produce a variety of forms throughout several of the
succeeding periods.
The brachiopods were commonest of all animals representing this period,
however, and their wide distribution, together with their known
preference for warm waters, is taken to be an indication of mild
temperatures prevailing over a large portion of the earth. Land plants
are indicated by spore-bearing forms related to the ferns and mosses.
Impressions of such plants have been found in Europe but, since most of
the known rocks of this age were formed in seas, the marine algae are
more abundant as fossils.
In the Colorado area, rocks of Ordovician time are exposed only in
mountainous areas where they have been lifted high above their original
levels. They are not especially rich in fossils although they have
produced some fish remains which are of interest in that they suggest an
age of vertebrates which is just ahead.