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.

[Illustration: One of the Large Jurassic Dinosaurs (_Diplodocus

longus_)

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.

[Illustration: A Duck-billed Dinosaur of the Cretaceous Period

(_Trachodon mirabilis_)]

[Illustration: Stegosaur (_Stegosaurus stenops_)]

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.

[Illustration: A Sea Turtle of Cretaceous Time (_Protostega gigas_)

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.