The Elements of Geology Part 27

Cephalopods are now to have their day. The archaic Orthoceras lingers on into the Tria.s.sic and becomes extinct, but a remarkable development is now at hand for the more highly organized descendants of this ancient line. We have noticed that in the Devonian the sutures of some of the chambered sh.e.l.ls become angled, evolving the Goniat.i.te type. The sutures now become lobed and corrugated in Cerat.i.tes. The process was carried still farther, and the sutures were elaborately frilled in the great order of the Ammonites. It was in the Jura.s.sic that the Ammonites reached their height. No fossils are more abundant or characteristic of their age. Great banks of their sh.e.l.ls formed beds of limestone in warm seas the world over.

The ammonite stem branched into a most luxuriant variety of forms.

The typical form was closely coiled like a nautilus. In others the coil was more or less open, or even erected into a spiral. Some were hook-shaped, and there were members of the order in which the sh.e.l.l was straight, and yet retained all the internal structures of its kind. At the end of the Mesozoic the entire tribe of ammonites became extinct.

The Belemnite (Greek, belemnon, a dart) is a distinctly higher type of cephalopod which appeared in the Tria.s.sic, became numerous and varied in the Jura.s.sic and Cretaceous, and died out early in the Tertiary. Like the squids and cuttlefish, of which it was the prototype, it had an internal calcareous sh.e.l.l. This consisted of a chambered and siphuncled cone, whose point was sheathed in a long solid guard somewhat like a dart. The animal carried an ink sac, and no doubt used it as that of the modern cuttlefish is used,--to darken the water and make easy an escape from foes.

Belemnites have sometimes been sketched with fossil sepia, or india ink, from their own ink sacs. In the belemnites and their descendants, the squids and cuttlefish, the cephalopods made the radical change from external to the internal sh.e.l.l. They abandoned the defensive system of warfare and boldly took up the offensive.

No doubt, like their descendants, the belemnites were exceedingly active and voracious creatures.

FISHES AND AMPHIBIANS. In the Tria.s.sic and Jura.s.sic, little progress was made among the fishes, and the ganoid was still the leading type. In the Cretaceous the teleosts, or bony fishes, made their appearance, while ganoids declined toward their present subordinate place.

The amphibians culminated in the Tria.s.sic, some being formidable creatures as large as alligators. They were still of the primitive Paleozoic types. Their pygmy descendants of more modern types are not found until later, salamanders appearing first in the Cretaceous, and frogs at the beginning of the Cenozoic.

No remains of amphibians have been discovered in the Jura.s.sic. Do you infer from this that there were none in existence at that time?

REPTILES OF THE MESOZOIC

The great order of Reptiles made its advent in the Permian, culminated in the Tria.s.sic and Jura.s.sic, and began to decline in the Cretaceous. The advance from the amphibian to the reptile was a long forward step in the evolution of the vertebrates. In the reptile the vertebrate skeleton now became completely ossified.

Gills were abandoned and breathing was by lungs alone. The development of the individual from the egg to maturity was uninterrupted by any metamorphosis, such as that of the frog when it pa.s.ses from the tadpole stage. Yet in advancing from the amphibian to the reptile the evolution of the vertebrate was far from finished. The cold-blooded, clumsy and sluggish, small- brained and unintelligent reptile is as far inferior to the higher mammals, whose day was still to come, as it is superior to the amphibian and the fish.

The reptiles of the Permian, the earliest known, were much like lizards in form of body. Const.i.tuting a transition type between the amphibians on the one hand, and both the higher reptiles and the mammals on the other, they retained the archaic biconcave vertebra of the fish and in some cases the persistent notochord, while some of them, the theromorphs, possessed characters allying them with mammals. In these the skull was remarkably similar to that of the carnivores, or flesh-eating mammals, and the teeth, unlike the teeth of any later reptiles, were divisible into incisors, canines, and molars, as are the teeth of mammals.

At the opening of the Mesozoic era reptiles were the most highly organized and powerful of any animals on the earth. New ranges of continental extent were opened to them, food was abundant, the climate was congenial, and they now branched into very many diverse types which occupied and ruled all fields,--the land, the air, and the sea. The Mesozoic was the Age of Reptiles.

THE ANCESTRY OF SURVIVING REPTILIAN TYPES. We will consider first the evolution of the few reptilian types which have survived to the present.

Crocodiles, the highest of existing reptiles, are a very ancient order, dating back to the lower Jura.s.sic, and traceable to earlier ancestral, generalized forms, from which sprang several other orders also.

Turtles and tortoises are not found until the early Jura.s.sic, when they already possessed the peculiar characteristics which set them off so sharply from other reptiles. They seem to have lived at first in shallow water and in swamps, and it is not until after the end of the Mesozoic that some of the order became adapted to life on the land.

The largest of all known turtles, Archelon, whose home was the great interior Cretaceous sea, was fully a dozen feet in length and must have weighed at least two tons. The skull alone is a yard long.

Lizards and snakes do not appear until after the close of the Mesozoic, although their ancestral lines may be followed back into the Cretaceous.

We will now describe some of the highly specialized orders peculiar to the Mesozoic.

LAND REPTILES. The DINOSAURS (terrible reptiles) are an extremely varied order which were masters of the land from the late Trias until the close of the Mesozoic era. Some were far larger than elephants, some were as small as cats; some walked on all fours, some were bipedal; some fed on the luxuriant tropical foliage, and others on the flesh of weaker reptiles. They may be cla.s.sed in three divisions,--the FLESH-EATING DINOSAURS, the REPTILE-FOOTED DINOSAURS, and the BEAKED DINOSAURS,--the latter two divisions being herbivorous.

The FLESH-EATING DINOSAURS are the oldest known division of the order, and their characteristics are shown in Figure 329. As a cla.s.s, reptiles are egg layers (oviparous); but some of the flesh- eating dinosaurs are known to have been VIVIPAROUS, i.e. to have brought forth their young alive. This group was the longest-lived of any of the three, beginning in the Trias and continuing to the close of the Mesozoic era.

Contrast the small fore limbs, used only for grasping, with the powerful hind limbs on which the animal stalked about. Some of the species of this group seem to have been able to progress by leaping in kangaroo fashion. Notice the sharp claws, the ponderous tail, and the skull set at right angles with the spinal column.

The limb bones are hollow. The ceratosaurs reached a length of some fifteen feet, and were not uncommon in Colorado and the western lands in Jura.s.sic times.

The REPTILE-FOOTED DINOSAURS (Sauropoda) include some of the biggest brutes which ever trod the ground. One of the largest, whose remains are found entombed in the Jura.s.sic rocks of Wyoming and Colorado, is shown in Figure 330.

Note the five digits on the hind feet, the quadrupedal gait, the enormous stretch of neck and tail, the small head aligned with the vertebral column. Diplodocus was fully sixty-five feet long and must have weighed about twenty tons. The thigh bones of the Sauropoda are the largest bones which ever grew. That of a genus allied to the Diplodocus measures six feet and eight inches, and the total length of the animal must have been not far from eighty feet, the largest land animal known.

The Sauropoda became extinct when their haunts along the rivers and lakes of the western plains of Jura.s.sic times were invaded by the Cretaceous interior sea.

The BEAKED DINOSAURS(Predentata) were distinguished by a beak sheathed with horn carried in front of the tooth-set jaw, and used, we may imagine, in stripping the leaves and twigs of trees and shrubs. We may notice only two of the most interesting types.

STEGOSAURUS (plated reptile) takes its name from the double row of bony plates arranged along its back. The powerful tail was armed with long spines, and the thick skin was defended with irregular bits of bone implanted in it. The brain of the stegosaur was smaller than that of any land vertebrate, while in the sacrum the nerve ca.n.a.l was enlarged to ten times the capacity of the brain cavity of the skull. Despite their feeble wits, this well-armored family lived on through millions of years which intervened between their appearance, at the opening of the Jura.s.sic, and the close of the Cretaceous, when they became extinct.

A less stupid brute than the stegosaur was TRICERATOPS, the dinosaur of the three horns,--one horn carried on the nose, and a ma.s.sive pair set over the eyes. Note the enormous wedge-shaped skull, with its sharp beak, and the hood behind resembling a fireman's helmet. Triceratops was fully twenty-five feet long, and of twice the bulk of an elephant. The family appeared in the Upper Cretaceous and became extinct at its close. Their bones are found buried in the fresh-water deposits of the time from Colorado to Montana and eastward to the Dakotas.

MARINE REPTILES. In the ocean, reptiles occupied the place now held by the aquatic mammals, such as whales and dolphins, and their form and structure were similarly modified to suit their environment. In the Ichthyosaurus (fish reptile), for example, the body was fishlike in form, with short neck and large, pointed head (Fig. 333).

A powerful tail, whose flukes were set vertical, and the lower one of which was vertebrated, served as propeller, while a large dorsal fin was developed as a cut.w.a.ter. The primitive biconcave vertebrae of the fish and of the early land vertebrates were retained, and the limbs degenerated into short paddles. The skin of the ichthyosaur was smooth like that of a whale, and its food was largely fish and cephalopods, as the fossil contents of its stomach prove.

These sea monsters disported along the Pacific sh.o.r.e over northern California in Tria.s.sic times, and the bones of immense members of the family occur in the Jura.s.sic strata of Wyoming. Like whales and seals, the ichthyosaurs were descended from land vertebrates which had become adapted to a marine habitat.

PLESIOSAURS were another order which ranged throughout the Mesozoic. Descended from small amphibious animals, they later included great marine reptiles, characterized in the typical genus by long neck, snakelike head, and immense paddles. They swam in the Cretaceous interior sea of western North America.

MOSASAURS belong to the same order as do snakes and lizards, and are an offshoot of the same ancestral line of land reptiles. These snakelike creatures--which measured as much as forty-five feet in length--abounded in the Cretaceous seas. They had large conical teeth, and their limbs had become stout paddles.

The lower jaw of the mosasaur was jointed; the quadrate bone, which in all reptiles connects the bone of the lower jaw with the skull, was movable, and as in snakes the lower jaw could be used in thrusting prey down the throat. The family became extinct at the end of the Mesozoic, and left no descendants. One may imitate the movement of the lower jaw of the mosasaur by extending the arms, clasping the hands, and bending the elbows.

FLYING REPTILES. The atmosphere, which had hitherto been tenanted only by insects, was first conquered by the vertebrates in the Mesozoic. Pterosaurs, winged reptiles, whose whole organism was adapted for flight through the air, appeared in the Jura.s.sic and pa.s.sed off the stage of existence before the end of the Cretaceous. The bones were hollow, as are those of birds. The sternum, or breastbone, was given a keel for the attachment of the wing muscles. The fifth finger, prodigiously lengthened, was turned backward to support a membrane which was attached to the body and extended to the base of the tail. The other fingers were free, and armed with sharp and delicate claws, as shown in Figures 336 and 337.

These "dragons of the air" varied greatly in size; some were as small as sparrows, while others surpa.s.sed in stretch of wing the largest birds of the present day. They may be divided into two groups. The earliest group comprises genera with jaws set with teeth, and with long tails sometimes provided with a rudderlike expansion at the end. In their successors of the later group the tail had become short, and in some of the genera the teeth had disappeared. Among the latest of the flying reptiles was ORNITHOSTOMA (bird beak), the largest creature which ever flew, and whose remains are imbedded in the offsh.o.r.e deposits of the Cretaceous sea which held sway over our western plains.

Ornithostoma's spread of wings was twenty feet. Its bones were a marvel of lightness, the entire skeleton, even in its petrified condition, not weighing more than five or six pounds. The sharp beak, a yard long, was toothless and bird-like, as its name suggests

BIRDS. The earliest known birds are found in the Jura.s.sic, and during the remainder of the Mesozoic they contended with the flying reptiles for the empire of the air. The first feathered creatures were very different from the birds of to-day. Their characteristics prove them an offshoot of the dinosaur line of reptiles. ARCHAEOPTERYX (ANCIENT BIRD) (Fig. 338) exhibits a strange mingling of bird and reptile. Like birds, it was fledged with perfect feathers, at least on wings and tail, but it retained the teeth of the reptile, and its long tail was vertebrated, a pair of feathers springing from each joint. Throughout the Jura.s.sic and Cretaceous the remains of birds are far less common than those of flying reptiles, and strata representing hundreds of thousands of years intervene between Archaeopteryx and the next birds of which we know, whose skeletons occur in the Cretaceous beds of western Kansas.

MAMMALS. So far as the entries upon the geological record show, mammals made their advent in a very humble way during the Trias.

These earliest of vertebrates which suckle their young were no bigger than young kittens, and their strong affinities with the theromorphs suggest that their ancestors are to be found among some generalized types of that order of reptiles.

During the long ages of the Mesozoic, mammals continued small and few, and were completely dominated by the reptiles. Their remains are exceedingly rare, and consist of minute scattered teeth,--with an occasional detached jaw,--which prove them to have been flesh or insect eaters. In the same way their affinities are seen to be with the lowest of mammals,--the MONOTREMES and MARSUPIALS. The monotremes,--such as the duckbill mole and the spiny ant-eater of Australia, reproduce by means of eggs resembling those of reptiles; the marsupials, such as the opossum and the kangaroo, bring forth their young alive, but in a very immature condition, and carry them for some time after birth in the marsupium, a pouch on the ventral side of the body.

CHAPTER XXI

THE TERTIARY

THE CENOZOIC ERA. The last stages of the Cretaceous are marked by a decadence of the reptiles. By the end of that period the reptilian forms characteristic of the time had become extinct one after another, leaving to represent the cla.s.s only the types of reptiles which continue to modern times. The day of the ammonite and the belemnite also now drew to a close, and only a few of these cephalopods were left to survive the period. It is therefore at the close of the Cretaceous that the line is drawn which marks the end of the Middle Age of geology and the beginning of the Cenozoic era, the era of modern life,--the Age of Mammals.

In place of the giant reptiles, mammals now become masters of the land, appearing first in generalized types which, during the long ages of the era, gradually evolve to higher forms, more specialized and ever more closely resembling the mammals of the present. In the atmosphere the flying dragons of the Mesozoic give place to birds and bats. In the sea, whales, sharks, and teleost fishes of modern types rule in the stead of huge swimming reptiles. The lower vertebrates, the invertebrates of land and sea, and the plants of field and forest take on a modern aspect, and differ little more from those of to-day than the plants and animals of different countries now differ from one another. From the beginning of the Cenozoic era until now there is a steadily increasing number of species of animals and plants which have continued to exist to the present time.

The Cenozoic era comprises two divisions,--the TERTIARY period and the QUATERNARY period.

In the early days of geology the formations of the entire geological record, so far as it was then known, were divided into three groups,--the PRIMARY, the SECONDARY (now known as the Mesozoic), and the TERTIARY, When the third group was subdivided into two systems, the term Tertiary was retained for the first system of the two, while the term QUATERNARY was used to designate the second.

DIVISIONS OF THE TERTIARY. The formations of the Tertiary are grouped in three divisions,--the PLIOCENE (more recent), the MIOCENE (less recent), and the EOCENE (the dawn of the recent).

Each of these epochs is long and complex. Their various sub- divisions are distinguished each by its own peculiar organisms, and the changes of physical geography recorded in their strata. In the rapid view which we are compelled to take we can note only a few of the most conspicuous events of the period.