Chapter II: Part 2

Good dinosaur material has been taken from the Canon City and Morrison districts. The formation is to be found on the lower west slope of the Dakota hogback. It consists of continental deposits of the stream and lake types. There is considerable sandstone in this formation and a little limestone is to be found here and there, but the most characteristic feature is in the shales. When freshly exposed, the shales are delicately tinted with gray, green, and maroon, a bronze-green being rather prominent. This formation is highly variable in character, with much of the clay often buried under the valley floor. In addition to the bones of reptiles, there are plant fossils, usually of poor quality, and fresh-water gastropods more or less abundant in some localities.

_Lykins._

Outcrops are not prominent, owing to the small amount of weather-resisting materials. The sandy clays are commonly of a deep red color mottled with spots of light gray. A white limestone is sometimes present near the middle of these deposits, and gypsum beds are included locally. The formation is often indicated only by red soil in the depressions between ridges. Few fossils have been reported.

_Lyons._

This formation is usually prominent as the eastern wall of the uplifted Red Rocks series. In some localities it forms a ridge of pink or white sandstone distinctly separated from the older sediments to the west. Very few fossils are found.

_Fountain._

Exposures usually are brown to red in color, though sometimes a dirty white. The prominent rocks are rather coarse sandstone, commonly with a gritty texture due to the angular character of the sand or gravel from which they were made. These are the westernmost of the Red Beds and the oldest of the uplifted sedimentary rocks bordering the foothills in most of our area. Fossils have been found in the formation, but it is practically barren for the territory here considered.

* * * * * * * *

This geological section also illustrates a method of dating crustal movements and the birth of mountain ranges, for the folding of the strata along the flanks of the Rocky Mountains has a great deal of significance in this connection. The sedimentary layers were originally deposited over much of the present mountain area in a horizontal position, and only those formations in existence at the time could be distorted by the upheavals which produced the new elevations. Of the series generally involved in the movement the Laramie beds are the youngest. Since these beds had not been formed until near the close of the Cretaceous period it is to be assumed that the mountains must be of more recent date, younger than the topmost of the deformed beds and at least as old as the lowermost of the undisturbed formations overlying them.

Some disturbance is evident also in the Arapahoe and Denver beds which overlie the Laramie, but this is believed to have occurred sometime after the occasion of the first great uplift. Volcanic materials in these beds lead to the belief that the sediments were deposited during a period of volcanic activity brought on by the crustal folding which terminated the Mesozoic era. Hence the conclusion arises that the age of the Denver and Arapahoe beds must coincide closely with some of the earlier stages in the history of the mountain system. This interval is often referred to as Post-Laramie time.

BEFORE THE AGE OF REPTILES

THE PRE-CAMBRIAN COMPLEX

The rocks of Pre-Cambrian time have been buried deeply under the accumulation of younger sediments, and the resulting pressure in many places has been tremendous. In addition to the effects of pressure there also is recorded in these ancient formations the repeated movements of the materials since they were first deposited. Vertical and side adjustments of parts, with relation to other parts, have distorted the original arrangement of the rock particles to such an extent that ordinary fossils would eventually become unrecognizable. These crushing, grinding, and kneading forces working through millions of years alone would account for the absence of fossils from the older deposits. Frequently the rocks have become so changed in form that their original character can only be conjectured, and because of this change they are known as metamorphic rocks.

A few beds of Archeozoic age remain in nearly their original condition, but they are either without fossils or they have produced very questionable and unsatisfactory specimens. The existence of life during these early stages of earth history is indicated largely by chemical rather than fossil evidence. Much of the ancient limestone has been converted into marble, but it is not unreasonable to believe that plants and animals were instrumental in the production of this type of rock as they are today. Certain varieties of iron ore deposits are now being built up by the aid of plants, and similar ores in the ancient rocks may have had a like origin. The presence of great quantities of carbon, in the form of graphite, may be regarded also as a sign of life, for this substance is accumulated on a large scale by living plants, and may be retained in a solid form after the partial decay of the plant tissues.

So far as the direct evidence goes, there is no sign of any creature of large size or of such complicated structure as the common plants and animals of today. The chemistry of the mineral deposits is not entirely convincing as to the presence of life, but it is regarded as highly probable that microscopic, single-celled plants and animals, comparable to modern algae and protozoa, were in existence during Archean time. Throughout later eras there is unmistakable evidence of gradual development from simpler to more elaborate life-forms and the Archeozoic is commonly regarded as a time of preparation during which simple organisms of some kind were becoming adapted to early conditions which could not support life on a higher plane. The importance of the work done by such lowly creatures in the preparation of suitable environments for more advanced modes of living is overlooked almost entirely.

During the next era, the Proterozoic, the record of life becomes somewhat clearer. Fossils are hardly to be regarded as abundant but there were several well-defined types of animals which left shells and other parts composed of mineral matter. Among these may be mentioned the Radiolaria, Foraminifera, Bryozoa, and Sponges. Radiolaria produced delicate, often lace-like shells of many patterns adorned with the radiating filaments or spines which have suggested the name for this group. Foraminifera produced minute shells, sometimes many chambered, and often bearing a confusing resemblance to the work of snails. Common chalk is composed almost entirely of such shells and fragments of them.

Sponges and Bryozoa are animals of slightly higher organization. They are many-celled instead of one-celled and the cells have special work to perform, which is a most important step in the direction of the specialization which characterizes the structural and life pattern of later arrivals. The Bryozoa lived in moss-like colonies which have been important rock-makers; the fossil forms bear some resemblance to corals. Sponges are too well known to require description although the familiar article of commerce is merely the framework of once-living animals. They represent the earliest organization of true animal bodies even though in appearance they may have a resemblance to plants.

Actual plants of this era were of the algae class, aquatic in habit as were their animal neighbors, the first to leave a record in the form of fossils. This record, obscure and distorted, has long been a source of perplexity to investigators. Without well-defined floras and faunas to guide them, and with rocks frequently in chaotic relationships, early geologists were content to regard it all as a “Pre-Cambrian complex.” Recent studies have contributed a great deal of information not available some years ago. It is quite possible that more advanced types of life were in abundance before the close of the second era, but material on which to base sound opinion is still scarce.

Rocks of Pre-Cambrian age are plentiful in the foothills region west of Denver. The schists, gneisses, and quartzites exposed for some miles immediately beyond the red-beds are part of this great complex. The Idaho Springs formation is known to be one of the oldest in this district, although its exact age has not been determined. Other formations are recognized among the metamorphic rocks of the region but none has contributed to our knowledge of early life.

CAMBRIAN LIFE

There can be no mistake as to the prolific development of life in Cambrian seas, for fossils of this age are to be found in many parts of the world, where ancient sea bottoms now form part of the land surface. Invertebrate animals appear to have made much progress, but plants were either scarce or too small and delicate to be productive of fossils. It is probable, however, that seaweeds and other algae were flourishing along with the invertebrates, because animal life is directly or indirectly dependent on the existence of plants. The latter sustain themselves by taking carbon and nitrogen from air, water, and soil, but animals must obtain their requirements by eating plants or eating each other. They cannot obtain what they need from the inorganic world without this help from the vegetable kingdom.

One group of animals stands out prominently above all its contemporaries. Known as the trilobites they were by far the most distinguished and most characteristic of Cambrian invertebrates. Trilobites inhabited the warmer seas of this period and several later ones, but were extinct by the end of the Paleozoic era. Hundreds of species have been described, most of them under four inches in length. Well-known distant relatives now living are the shrimps, and other crustaceans. The name Trilobite has reference to the three lobes which are apparent in the form of the upper surface, the central lobe forming a broad ridge extending along the back. Beneath the outer lobes on each side there was, during life, a row of short, jointed legs used for swimming and walking, but these delicate appendages are seldom preserved in the fossils.

Second in importance among the animals of the period were the brachiopods or lamp-shells, not true mollusks although they were provided with similar shells composed of calcium phosphate or calcium carbonate. Shells are of two parts (bivalved) as in the case of clams, but the valves are above and beneath the body instead of on the right and left sides, which is the arrangement among mollusks. Although abundant as individuals, there were only a few species during the earlier part of the period; the number of species increased, however, and the race became very persistent. About seven thousand species have been described, and the race is not yet extinct although the number of living species is relatively small.

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.

Some of the larger and better known plants of past ages are shown as
reconstructed by artists. Finer details of the reconstructions often
have to be neglected because of uncertainties due to the scattered
and fragmentary character of the fossil record.]

LEAF IMPRESSIONS
Carboniferous Ferns
Strap-leaf Conifer (Cordaites)
MODERNIZED TYPES
Sequoia Cone and foliage
Miocene Fossils (Florissant Shales)
Maple
Willow
Eocene palm (Denver Beds)
HORSE TAIL RUSHES
Restoration (Calamites)
Fossils Leaves and Stem
CYCADS
Restoration
Fossil Trunk
CLUB MOSSES
Restoration (Sigillaria)
Fossils Trunk Impressions
SCALE TREES
Restoration (Lepidodendron)
Fossil Leaf scars

Also included among the larger trees were the Cordaites or large-leaved evergreens, tall and slender, seed-bearing but not true conifers as yet. Leaves were strap-shaped or grass-like, the larger ones having a length of six feet and a width of six inches. Trunks were woody, resembling pine, but with a central pith. The flowers were small and resembled catkins in form.

Ferns and fernlike plants were so numerous that the period has been known as an age of ferns. Earlier knowledge of these forests was based on fossils of a fragmentary character from which an accurate association of parts could rarely be obtained. A general relationship with the ferns was apparent, but careful study of additional material has given us a rather different view of Carboniferous plant life and we note a highly diversified array of forms with many suggestions of modern tendencies. The flora as a whole may be regarded as highly specialized for the conditions which prevailed at the time and were not to continue through subsequent periods. Warm temperatures and abundant moisture were essential especially to spore-bearing types, and the cold, arid conditions of the next period put an end to many of the groups, or greatly reduced their prominence.

This could be regarded equally well as an age of insects, for some of these invertebrates acquired the greatest size they have ever had, particularly the dragonflies with a wing-spread of more than two feet in one of the largest fossils so far discovered. Cockroaches numbering upwards of five hundred species have been named. Though large they are hardly to be regarded as giants, lengths of three or four inches being about the limit.

Some of the insect types of today quite evidently existed among the inhabitants of Carboniferous forests, but it is apparent that there were also some antiquated forms which may have descended from the trilobites. Although some authorities regard this as the period in which insects originated, there are others who maintain that definite beginnings are not established so readily on present evidence. Spiders are believed to have made their appearance at this time.

Four-footed vertebrates resembling salamanders were prominent animals of the Carboniferous swamps. At first adapted to a life in water and later to land conditions, they are known as amphibians, the name being based on the ability to live in two different kinds of environment. Common living representatives of this group are the toads and frogs, but these tailless forms are not known among fossils of the Paleozoic era and are almost unknown throughout the Mesozoic. The Age of Amphibians, as we apply that phrase, was definitely not an age of toads and frogs.

These primitive land animals were of different types, ranging from much smaller sizes up to the length of a crocodile. Most of them had short legs, and feet which were suitable for locomotion upon land, but many of the creatures probably spent most of their lives in the water. Tails were usually high and flattened as if for swimming, sometimes long, at other times greatly shortened in proportion to the body. Heads were generally large, jaws long, and mouths wide.

Before the close of the period true reptiles appear, but this race of animals is destined to make a more spectacular advance than the amphibians and will be discussed in connection with Mesozoic life. The amphibians, however, are regarded as being the ancestors of the reptiles as well as the higher quadrupeds which follow them. Although living reptiles are readily distinguished from living amphibians there is a different situation with regard to these primitive forms, for among the fossils it becomes increasingly difficult to separate the two groups as new material is investigated.

Invertebrates had their ups and downs during the period. Trilobites became scarce, and brachiopods for a time were the most abundant of the shelled animals but later declined rapidly. In favorable localities the crinoids established a wonderful record for new species before the period had advanced very far. Hundreds of species of Carboniferous invertebrates are known, and in many of the rocks of the period they are the only fossils to be found, for the vertebrates were still unable to venture far from the swampy districts, and much inland territory was too well drained to support either the floras or faunas then existing.

In the Colorado area there are both marine and continental formations but the great coal-making forests and their inhabitants were limited to other localities. As a consequence this region is not famous for Carboniferous fossils.

PERMIAN HARDSHIPS

For a time there was no great change in North America following the opening of the Permian. Then began a series of mountain-making movements and continental uplifts which drained the swamps, lakes, and inland seas. With the passing of the vegetation which had established itself in and around these areas much of the animal life followed. It is probable that a considerable proportion of the marine life survived, much more than is indicated by the fossil record, but the receding seas carried the survivors into territory which is now inaccessible to fossil hunters.

After Middle Permian time the climate everywhere seems to have been cold and dry. By the end of the period there had been accomplished more geographical change throughout the world than at any time since the beginning of the Paleozoic era. Traces of the crustal movements which produced new mountain ranges can be followed in Europe, Asia, and North America. The Appalachian region was raised to a great height, possibly in excess of three miles. A major disturbance of this character is known among geologists as a revolution, and to this particular one the name “Appalachian Revolution” has been given.

The elevation of continents necessarily changes their coast lines. This, in turn, influences ocean currents which have an important bearing on climatic conditions. In addition to this, the elevation of mountain systems not only rearranges the distribution of hot and cold winds over the land areas but it may produce barriers to the migrations of floras and faunas, confining them to areas in which it is no longer possible to live. When the effect upon plants and animals is considered, it is easier to understand why a line is drawn across the geological time chart at such a point and an era of prehistory is regarded as closed.

During the Permian period there was recorded in the rocks more widespread glacial action than ever before or since. With less inland water to provide the necessary evaporation there was a marked decrease in rainfall, and arid or semi-arid conditions replaced the hospitable climate that had been such an important factor in the prolific life of the Carboniferous. The struggle for existence became intense, but hardier types of plants and animals, with greater ability to adapt themselves to adverse conditions, established themselves here and there, as ancestral forms became extinct. Most of the large spore-bearing trees died out and seed-producing varieties began to acquire prominence, among them the coniferous evergreens. Ferns, however, proved their adaptability by producing some new forms which became prominent in Permian floras.

The prehistoric amphibians have been divided into three orders, one of which includes all the larger forms. This group, known as the labyrinthodonts, continued on through Permian time but began to show backward tendencies, with dwindling limbs and a return to life in the water. Among the larger land varieties are typical fossils ranging from about fifteen inches to five feet in length. In outward appearance they differed from Carboniferous amphibians. One of the other orders, including a great diversity of smaller forms, became extinct during this period, leaving no known descendants. The third order is regarded as the oldest, and probably the ancestral group from which the modern newts and salamanders originated.

The most successful of Permian land animals were the peculiar reptiles that learned to live in drier regions. Like the horned toad and Gila monster of our arid southwestern United States, the larger Permian reptiles were four-footed animals. In size and shape they were not greatly different from amphibians then living. An exception to this rule, among some of them, is the development of long, bony spines above the vertebrae of the back. A fairly common fossil of this type, found in Texas and known as _Dimetrodon_, had a total length of six feet, about half of this being in the tail. The tips of the spines adorning the back reached a height of three feet or more and there was probably a covering of skin over these bones, which would produce a sail-like structure or “fin” of large size. Its use has not been explained but it provides an easy name for these odd creatures—the “fin-back lizards.”

Rock deposits produced in arid regions usually have characters which are not difficult to recognize. Gritty texture, irregular bedding, red color, and gypsum are common features. Formations of Permian age are to be found in Colorado but better fossil deposits have been discovered in Kansas and Texas.

The Mesozoic Era produced many types of reptiles besides the
dinosaurs. Two of the marine forms are shown in this illustration,
both from Cretaceous beds of the western United States. Plesiosaurs
were the giants of the seas in their time, lengths of forty to fifty
feet being not uncommon. A long flat tail provided locomotion for
the mosasaurs whereas the plesiosaurs resorted to the peculiar limb
structures known as flippers or paddles.]

THE AGE OF REPTILES

The Mesozoic, or era of middle life, was a long stretch of time during which there was marvelous development among the reptiles. Many strange types were produced and most of them became extinct before the end of the era. The reptilian stock branched out in many directions. Types emerged which differed from one another so widely that their mutual relationships have become obscure. Hideous and fantastic creatures suggesting sea serpents and dragons were worldwide in distribution. Reptiles of the air and seas acquired large size and weird forms, but greater advances were made upon land.

The flying reptiles or pterosaurs flourished in Jurassic times with some of the larger varieties surviving until near the close of the Cretaceous. Although these winged lizards were the first of the vertebrates to fly they are not to be confused with birds. They were without feathers, and the earlier forms were provided with long tails bearing a flattened rudder-like tip. One of the best known of this type had a length of about eighteen inches. Its jaws were long and provided with sharp teeth. The wings were membranes attached to body and legs, stretched and manipulated by means of greatly elongated fingers. In later types there was a reduction in tooth equipment and length of tail. _Pteranodon_, found in Kansas, had a wing spread of twenty-five feet, a large toothless beak, a short body, and a mere stub of a tail. It was one of the last of these winged monsters.

Several types of marine reptiles appeared during this era, among them the plesiosaurs which first appeared in Triassic seas. These peculiar animals were serpent-like with regard to the character of head, neck, and tail, but in other respects were quite different, the short barrel-shaped body being provided with four large paddles corresponding to the usual limbs of quadrupeds. Fossil remains of these animals are common in many Jurassic and Cretaceous deposits, some of the largest exceeding forty feet in length. Mosasaurs, also marine carnivores, inhabited shallow Cretaceous seas throughout the world and are especially abundant as fossils in the Kansas chalk beds. These were elongated forms with a resemblance to salamanders in some respects but provided with long pointed jaws and sharp teeth. Swimming was accomplished largely by the tail though probably aided to some extent by four webbed paddles or flippers. The ichthyosaurs were more fish-like in construction, as the name implies. The limbs were short and broad, and there was usually present a well-developed tail-fin as well as a large fin on the back. They were especially abundant in Jurassic time. Fossils are fairly common in marine deposits of western North America. Mosasaurs and ichthyosaurs were about half as long as the plesiosaurs.

DINOSAURS

Most spectacular of the prehistoric reptiles were the dinosaurs, a large group of animals varying greatly as to size, form, and habits. They were adapted for a life on land though many of them probably spent much of their time partly submerged in the waters of lakes and streams. There is little that can be said of the group as a whole other than that all of them were reptiles. Further than that it is necessary to regard them as belonging to several different subdivisions of the Reptilia. Classification has been difficult and the names used for the various subdivisions are often misleading to the layman who tries to understand the terminology.

Ancestral reptiles were five-toed and five-fingered but among the dinosaurs there were many departures from the standard formula. Three or four of the digits were commonly well developed, the others when present being shortened or reduced to mere rudiments. Early in the history of dinosaurs there was a division of the stock into two main branches, each of which includes a variety of types and sizes, and is again subdivided. The two main groups are best recognized by the construction of the bony framework which comprises the pelvic girdle or hip region of the skeleton. In order to avoid technical difficulties, however, the remaining discussion of these interesting reptiles will be confined to a few names and descriptions which serve to illustrate roughly the great amount of variation that developed from the comparatively simple ancestral pattern. The plan according to which the dinosaurs are usually classified is barely suggested by the types described.

The meat-eaters were active creatures provided with powerful jaws and teeth. They were unarmored, moved about on their hind feet, and during their time were the most highly advanced of all animals. _Tyrannosaurus_ with a length of forty-five feet or more, and _Deinodon_, nearly as large, were among the greatest of these. Both lived in the Cretaceous period. Their teeth were simple but strong, knife-like, curved, and finely serrated. Skulls were large and the forelimbs were reduced almost to a state of uselessness. Large carnivores lived also during Jurassic time and even as far back as late Triassic. Early Triassic forms were of smaller size.

More primitive flesh-eating dinosaurs of the Triassic and Jurassic periods were delicately proportioned and lightly built bipeds bearing some resemblance to birds. _Struthiomimus_, which means ostrich-resembling, was about the size of the bird which provides the name. It was slender in the limbs, three-toed, long necked, long tailed. The skull was small, forelegs long for a biped. Unlike most dinosaurs it was toothless. All these bird-like carnivores were small as compared with other contemporary forms. Compsognathus, of Germany, and one of the smallest of all dinosaurs, had a length of less than three feet, including the long tail.

This magnificent specimen, exhibited by the Denver Museum of Natural
History, has a length of seventy-five feet six inches. Two years
were required to complete the task of removing the bones from the
matrix rock and preparing them for mounting. Diplodocus was one of
many large reptiles which inhabited western North America a hundred
and fifty million years ago. The skeleton was obtained from the
Morrison beds of eastern Utah. The same formation is exposed in many
Colorado localities, including the foothills west of Denver, where
it acquired its name from the town of Morrison.]

In Jurassic time there became prominent a group of large dinosaurs which were more equally developed as to fore and hind limbs. They were sluggish creatures, quadrupedal in their manner of locomotion, vegetarians in regard to their diet. Some of them reached enormous proportions and it is believed that they resorted to life in the water in order to get part of the weight off their feet. _Diplodocus_ and _Brontosaurus_ are the names of well-known giants in this group. They had long necks and tails, very small skulls, were the largest of all land animals and are known to have reached a length of eighty feet or more. Some estimates, based on measurements of incomplete skeletons, have exceeded one hundred feet, but these extremes are somewhat questionable. _Diplodocus_ was the more elongated of the two, with much of its length in the whip-like tail. Our mounted skeleton has a length of seventy-five feet six inches, measured along the vertebrae. Its height at the pelvis is twelve feet six inches.

The teeth of these large quadrupeds are of a slightly broadened and blunted form which has caused some speculation as to their possible use. It has even been suggested that the animals were fish-eaters but this seems impossible in view of the great size and general characteristics of the group. Although they differ extremely in some respects, they are regarded as being more closely related to the carnivores than to the herbivores of the second great branch of the tribe.

The unquestioned herbivores, constituting this second branch of the dinosaurian race, also include both bipeds and quadrupeds. The better known plant-eaters were large animals but not such monsters as _Tyrannosaurus_ or _Brontosaurus_. Of the bipeds, _Trachodon_ is perhaps best known. It is one of the duck-billed dinosaurs which had an average length of about thirty feet. The duckbills were unarmored, active animals, good swimmers as well as runners. They were prominent and widely distributed during late Cretaceous time. Many skeletons have been found in western North America. Natural casts and impressions of mummified remains indicate that the hides were scaly and the feet provided with webs between the toes. The bill was broad, flat, and toothless, but the sides of the mouth were provided with a large number of simple teeth closely arranged in parallel rows. The fine skeleton exhibited in our hall is thirty feet six inches in length. Near relatives of _Trachodon_, such as _Corythosaurus_ had hollow, bony crests, combs, or tubular structures on top of the head. These may have been of some service in connection with breathing while feeding under water.

Among the quadrupedal vegetarians an interesting family is represented by _Stegosaurus_, a late Jurassic dinosaur having a length of about twenty feet. These creatures had heavy limbs, all used in walking, an arched back, and almost no brain at all. A double row of large flattened plates standing upright and extending from the rear of the skull nearly to the tip of the tail provided some protection for the back of the animal, but otherwise there was no defensive armor. Several long spikes at the end of the tail probably served as weapons. The mounted skeleton in our collection was obtained from Garden Park, near Canon City, Colorado, a district which has long been famous for dinosaur remains.

The ankylosaurs were more completely armored with closely set bony plates fitting neatly over the body. They were of about the same size as the stegosaurs but the body was broad and somewhat flattened. These armored quadrupeds apparently lived only during the Cretaceous period, after the disappearance of the stegosaurs. Their tooth equipment was very poor and in a few cases entirely lacking. _Ankylosaurus_ and _Nodosaurus_ are good examples of the type. They have been described as animated tanks and are sometimes referred to as having the appearance of enormous horned toads.

Among the last of the dinosaurs to come and go were the horned quadrupeds known as the Ceratopsia. Their entire history appears to have been confined to the Upper Cretaceous and the closing stages of the reptilian era in America. _Triceratops_ and _Monoclonius_ are well-known representatives of the group. Besides the horns, which appeared above the eyes or near the center of the nose, there was a broad, flattened, backward extension of some of the skull bones which produced a great frill or collar reaching over the neck as far back as the shoulders. This frill, combined with the large skull, gave the animal the appearance of being nearly one-third head. _Triceratops_ had three horns, _Monoclonius_ only one. The average length of the animals was slightly under twenty feet.

Although very little is known about the ancestry of the horned dinosaurs a valuable discovery in Mongolia may throw some light on the subject. A small dinosaur with a well-developed frill, but no horns, once inhabited the region of the present Gobi desert, and in recognition of the apparent relationship it has been named _Protoceratops_. In addition to numerous skeletons, several nests of eggs were found in association with the bones. Until this discovery was made, dinosaur eggs had been practically unknown. A reproduction of one of these nests is among our exhibits.

This marine animal belongs to a group which became extinct near the
close of the great reptilian era, but a few related forms still
survive. Their weight is greatly reduced by the peculiar
construction of the shell, and the front feet are enlarged for use
as oars, an excellent illustration of the manner in which a land
type can become adapted to life in the sea.]

With the possible exception of a very few short-lived survivals dinosaurs were extinct before the opening of the Age of Mammals, many of them for millions of years. Along with them went other types of ancient reptiles, and the cause of their extinction is a problem which may never be solved. Conditions remained favorable for the turtles, which made their first appearance during Triassic time, and for the crocodiles, which date back to the Jurassic period. Snakes were only at the beginning of their history as the era closed. The survival of these modern forms suggests that they were favored to a greater extent than the dinosaurs during a prolonged period of changing conditions the full details of which are unknown to us.

In general it is to be expected that disaster would first overcome the highly specialized creatures, such as the dinosaurs, which had become more delicately adjusted to the particular environments in which they lived. It appears that some of them had been too progressive up to a certain point, but not sufficiently adaptable to get beyond that stage, or fortunate enough to make their advances in directions that could be followed, through fluctuations in the matter of food supply, predatory enemies, climate, and other factors which bear upon success and failure.

The reptilian era closed with exceptional volcanic activities in many parts of the world, but these cannot account for the disappearance of the highly diversified and abundant reptilian life. The eruptions were merely incidental to movements and readjustments in large masses of rock comprising the earth’s crust or surface. Such crustal folding and elevations always have been of serious consequences to both plants and animals because of their effect upon drainage and climate. There were disturbances of this kind in western North America in late Jurassic time, with folding and uplift in the region of the Sierras and probably extending from Mexico to southern Alaska. A great trough to the east of this elevated district was produced in the course of these movements and provided access to the sea from south to north. During the Cretaceous period there were repeated invasions and retreats of the sea by way of this great depression, consequent upon slight changes in the elevation of the floor. Hence there are numerous marine formations in Colorado and adjoining states, some of them rich in fossils.

Before the close of the Cretaceous period the sea had made its final departure from this region, and the Mesozoic era was terminated by revolutionary disturbances which brought about the uplifting of a new mountain system. The Rocky Mountains may be regarded as part of this system and to have had their birth at this time. The Rockies, however, show unmistakable signs of repeated elevation, with intervals of erosion during which there was great reduction of their total height. What we see of them today is the result of more than fifty million years of continuous geological activity.

PLANT LIFE AND CLIMATE

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.

COAL AND FOSSIL FOOTPRINTS

The abundance of plant life in the Colorado area during the Cretaceous period is indicated by the extent of coal deposits of this age. About one-fourth the area of the state is underlain by coal seams varying in thickness from a few inches to fifty feet or more, most of it being Cretaceous. In the northern Colorado district the coal-bearing formation is the Laramie. Near Denver there is some coal in the Arapahoe formation which overlies the Laramie and is of later age.

Coal mines often produce excellent plant fossils, and occasionally other evidence of prehistoric life. In a mine near Canon City, Colorado, a series of natural casts of dinosaur feet was taken from the overlying rock after the coal had been removed. One of these, in the Denver Museum of Natural History, is seen to consist of sandstone inside a very thin layer of dark clay. Flattened against the lower surface is the carbonized stem of a Cretaceous plant which grew in the swamp where the coal deposit was formed.