Part 8

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.

Since the shape of dinosaur feet is unmistakable we can only assume that a large reptile of this type walked over the surface of swampy ground in which a great thickness of decaying vegetation had accumulated. A layer of mud settled over the top and became sufficiently firm to retain the mold of the feet as the animal moved along. Any plant material either on the mud or included in it was pushed to the bottom of the impressions and flattened out by the weight of the huge creature. Then sand was washed into the footprints from some nearby source during a heavy rainstorm.

Following these events there was probably a subsidence of the area, and a great thickness of rock-making sediments was built over the ancient swamp. The buried vegetation gradually became converted into coal, the sand consolidated into a firm sandstone, and the mud produced the shales forming the roof of the present mine, which is now at an elevation of a mile above sea level as a consequence of the general uplifting of the Rocky Mountain region during late Cretaceous and subsequent time.

When the coal was removed, the hard sandstone casts separated readily from the softer shales surrounding them. A small amount of the shale adheres to the sandstone, and some of the flattened vegetation, now in the condition of coal, still remains attached.

MESOZOIC INVERTEBRATES

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.