CRINOIDS CEPHALOPODS Coiled types Ammonite Scaphite Straight-shell type Baculite TRILOBITE BRACHIOPODS BIVALVES Inoceramus Oyster GASTROPODS Snail-like Univalves PROTOZOA UNICELLULAR FORMS Radiolaria (Microscopic) Fusulina limestone Foraminifera (Enlarged) MULTICELLULAR FORMS Cup coral Reef coral Sponge Bryozoa THE ORDOVICIAN RECORD 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. SILURIAN EVENTS