Part 6

Cambrian life evidently included representatives of all the great divisions of invertebrates; sponges, jelly-fishes, worms, and primitive corals have been reported. At the end of the period there was an elaborate molluscan fauna. The closing of the period in North America was apparently a gentle elevation of continental areas and a consequent withdrawal of the sea.

Only a few prominent types have been selected from thousands of invertebrates known to zoologists. The forms illustrated are of frequent occurrence as fossils.]

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

Since land floras and faunas had not yet become conspicuous the fossil record for this period is limited to those areas which were invaded by the sea. Apparently there was no such invasion of the present Colorado region, for rocks of this age are not in evidence. If they exist at all they are restricted to localized districts which are deeply buried under sediments of later periods. There may have been no Silurian deposition in this area, or such rocks may have been produced only to be destroyed by elevation and consequent subjection to weathering and erosion during a long interval of time, in which they were completely removed. In the region of the Colorado Rockies there is no evidence of returning seas until late Devonian time.

In other parts of the world, however, there was extensive deposition of rock-making sediments in seas which were inhabited by algae and invertebrates of the types previously described. Among the common animals of the time there were still numerous species of brachiopods, trilobites, corals, crinoids, and bryozoans. In addition to the primitive cone-shaped, cup corals there were several advanced types but the habit of building large reefs was not yet established.

“Sea scorpions,” really large crustaceans, flourished during Silurian time, and late in the period there appeared a race of true scorpions which lived on dry land or between high and low tides along the seashore. These were smaller and much like modern descendants but probably they did not wander far from the ocean shores where an abundance of food was available. These little scorpions, the largest measuring only two and a half inches in length, are the oldest air-breathing land animals of the fossil record.

It was not until the period was well advanced that fishes became numerous, and much of our knowledge of the beginning of an “Age of Fishes” has been obtained from European fossils. Although fishes are classed with the vertebrate or backboned animals there are large groups which do not have bony skeletons but are provided instead with a simple framework of cartilage. Among the earlier and more primitive types were the ostracoderms or bony-skinned fishes with no internal bones and only a small amount of bony substance in the armor-like plates and scales which covered the forward portion of the body.

The ostracoderms comprise a small group of fishes about which very little is known. They appear to have been inhabitants of fresh-water streams as well as lagoons bordering the seas, and may have been related to the small sharks of the time. They lived during the Ordovician, Silurian, and Devonian periods, and left no descendants now recognized among living creatures. A much larger type of armored fishes is known as the arthrodires, a name which refers to a pointed neck and an arrangement of the armor plates to permit a movement of the head. These were the most ferocious fishes of the Silurian and Devonian seas, some of them reaching a length of twenty feet though most were much smaller. Their jaws were provided with formidable shearing and crushing plates instead of teeth.

DEVONIAN PROGRESS

The Devonian is one of the most outstanding of all periods from the viewpoint of life development. Dominance of the fishes is its greatest achievement, the invertebrates remaining about as they were and the higher vertebrates barely in evidence, but life on a large scale was no longer confined to the seas. Fresh-water fishes became prominent and land plants well established. The first forests appeared, with fern-like plants predominating although woody trees of several types and considerable size were included. It is quite possible that extensive land areas had been well supplied with vegetation during earlier times, but the delicate tissues of plants are far less likely to be preserved than the limy parts of animals. The fossil record, therefore, cannot be expected to reveal more than a suggestion of the progress made at this level of living. The story of plant life becomes much clearer in the next period when conditions were more favorable for the production and preservation of plant fossils.

Land animals of the time are almost unknown. A few snails and scorpions have been found, and some footprints made by early amphibians. Insects probably were in existence although the evidence is not quite clear on this point. The increasing number of fresh-water fishes, however, may be regarded as a sure indication that inland conditions were becoming more favorable for plant and animal inhabitants of all kinds.

The extent of development among the fishes cannot be accurately indicated by naming a few types, for it is mainly in the number of species and genera within the larger groups that progress is seen. In general it may be stated that the fishes of the period had not yet acquired the bony skeleton and typical form of familiar modern species. Skeletons were of cartilage, partly hardened in some instances by lime. Armor plates were customary with certain races but were not present among all fishes. Neither were these armored forms exceptionally large, as compared with living sharks. Although occasional giants appeared, the majority were small. Many were sluggish creatures with poorly-developed jaws, living as scavengers on sea and stream bottoms. Tail fins were usually unbalanced as in the sharks, or pointed and rounded rather than evenly forked.

The great tribe of true bony fishes, such as the cod and perch, which includes more than ninety percent of the fishes living today, was not yet in existence. About one-third of the many kinds of fishes then living were related to the sharks, a group which is relatively insignificant in recent years. Nearly one-fourth of the total belonged to a tribe of enamel-scaled fishes, now represented only by a few sturgeon and gar-pike.

Lung fishes have never been a large group but it is noteworthy that they have had existence since Middle Devonian time. Living members of the race, inhabitants of Africa and South America, make a practice of burrowing into the mud of stream channels during dry seasons and are provided with lungs which enable them to breathe air in the manner of higher vertebrates. They survive the complete drying-up of the streams and live for months without water. Other forms, with less development of lungs, frequent stagnant pools and come to the surface occasionally for a breath of air. All are provided with gills also, which enables them to obtain their oxygen as other fishes do. They are believed to be a connecting link between the fishes and the early amphibians. More accurately, perhaps, they should be regarded as holding an intermediate position without being directly ancestral to any higher type of vertebrate animal.

Still dominant among the invertebrates were the brachiopods, on the whole averaging a little larger in size, and otherwise indicating congenial times for that type of organism. They reached the peak of their development during this period. Trilobites were declining although a few new and strangely ornamented varieties made a brief appearance. Crinoids apparently found living conditions less favorable during Devonian time, but in a later era they again became prominent. Corals were favored only at times and in certain localities. Along with the crinoids they appear to have suffered from the presence of an unusual amount of mud in the waters of their customary habitats. Both had a preference for clear water as indicated by the absence of fossils from limestones containing more than a very small percentage of muddy sediments. Crustaceans, similar to the sea-scorpions and better known as eurypterids, became prominent among fresh-water animals. Some were unusually large for creatures of this class, lengths of several feet being recorded from fragments. Gastropod mollusks came into prominence in localities where living conditions were favorable. Bivalves continued to thrive but the cephalopods had a rather meager development considering the heights they were to achieve in subsequent periods.

In western North America the large expanse of territory known as the Great Plains was evidently well above sea level during this entire period, for no beds of this age are found in eastern Colorado. West of the Front Range, however, there was some deposition of marine sediments during late Devonian time. Formations of this age are exposed near Salida and Glenwood Springs, on the White River Plateau, and in the San Juan region.

The Carboniferous period gets its name from the vast deposits of coal which were developed during that time in many parts of the northern hemisphere. Depressed land surfaces bordering the continents, and extending well into the interior of present boundaries, supported dense growths of vegetation and provided the swampy conditions most favorable to coal production. Varieties of plants which are now of small size and lowly position in the botanical world acquired the proportions of large trees.

CARBONIFEROUS FORESTS

Best-known fossils of the period are carbonized portions of the larger trees, and impressions left in the muds and sands of ancient bogs. Forest trees of several kinds reached the height of a hundred feet, with a trunk diameter of two to six feet. This size often is exceeded in modern forests, but by trees of an entirely different type. Considering the amount of development among the plants of earlier periods, Carboniferous forests provide an outstanding spectacle of advancing life.

Quite common among the larger trees were two varieties of club-mosses, also known as scale trees. They were cone-bearing evergreens with only slight resemblance to modern conifers. Instead of seeds they produced spores, a method of reproduction which is practiced among ferns. The trunks were marked from bottom to top with uniform patterns of cushions and scars indicating the points at which leaves were attached during the earlier stages of growth. In the Lepidodendrons the rows of scale-like cushions wind spirally upward while among the Sigillaria there is a vertical arrangement of leaf-scars which resemble the imprints of a seal, these impressions being in straight and parallel rows on a surface which may be either ribbed or smooth. The leaves of scale trees were stiff and slender, and arranged in grass-like tufts at the top.

Calamites, related to our horsetail rushes, were somewhat smaller than the scale trees. Their trunks consisted of a thin, woody cylinder with a pithy interior, and were marked at intervals by nodes which gave them the “jointed” appearance of a bamboo stem. Leaves were arranged in circles around the nodes of main stem or branches. Spore-bearing cones appeared at the tips of the stems.