CHAPTER I.. The First Land Dwellers.

THE FIRST LAND DWELLERS.

In the first book of this series, “Animals of Ancient Seas,” (No. 47) we followed the evolution of animal life from the most primitive one-celled creatures to the fishes, and from the most primitive fishes to those which possessed lung-like organs that enabled them to breathe from air as well as water. The next stage in evolution was the development of feet and legs from the fish fin, and this was accomplished before the end of that geologic period known as the Devonian, or more commonly, as the “Age of Fishes.” True, no actual remains have been found, but one fortunate collector succeeded in getting out a fossil footprint, which is all that is needed to prove the case. This footprint was made by an animal that looked much like the mud puppy, or salamander, of modern streams and lakes, and was related also to the frogs and toads. Like these creatures, he spent the early part of his life in water, breathing with gills, and possessing a more or less fish-like form, while in his maturity he lived at least partly on land, and breathed by means of lungs. This type of development, almost universal among his class, has gained for them the name of Amphibians, or “double livers.”

During those periods which are correctly known as the Mississippian and Pennsylvanian, but which are more commonly grouped under the one name Carboniferous, the amphibians increased in variety, and gave rise to a new and immensely important group of animals, the Reptiles. Since everyone knows the general characters of this group, we shall not take the trouble to define them. The important thing about them, so far as evolution is concerned is that they are capable of living on land throughout their whole lives, not even the eggs having to be deposited in water. This, as can readily be seen, is a very fundamental character, for as long as animals were confined to water during their earliest stages, great progress was quite impossible. Imagine what a state humanity would be in if every child had to begin life in some brook, lake, or pond, and could not emerge from the water until it was fifteen to twenty years old!

Unfortunately for our knowledge of life in the past, fossils of both amphibians and reptiles are by no means common in the Carboniferous systems. It was not until the times of coal deposition were almost past, and great mountains and broad plains were taking the place of shallow seas and sombre swamps, that land dwelling animals became really abundant. This appearance of dry land was not only the cause of the development of the wonderful array of animals that inhabited the Permian world, but also the reason why so few of them are preserved as fossils, for it is only when the bones of animals become buried in water-soaked layers of sediment, or covered by water, that they are readily petrified. An animal that dies and remains exposed to the open air stands very little chance of becoming a fossil; its bones are gnawed by carrion feeders, worn by the wind, and broken up by decay. Thus of the hundreds of thousands, even millions, of buffalo, cattle, antelope, and elk whose carcasses have dotted the plains of this continent in the past half century there remain but a few rotten and frost-split horns and bones. Without question there are, in the muddy banks and bars of rivers, skeletons and parts of skeletons that are undergoing the slow process of petrification, and that will be dug up and treasured in the museums of some future civilization. And just as the record of animal life of today will be but a fragment of that which actually exists, so the record of the great land fauna of the Permian period is but the merest trace of the myriad creatures of that time. It is only in the most exceptional places that great quantities of bones were brought together under conditions favorable to fossilization, and such a district is found in northern Texas.

“When the Appalachian Mountains were raised, an extension of their southern end reached across what are now Arkansas and Oklahoma, terminating in the Wichita Mountains.... North of this range rose a broad upland, reaching from the Rockies to the Appalachians and to the Canadian line on the north; south of the mountains a shallow sea reached nearly to their base, and some great rivers from the mountains and uplands poured their flood waters into the sea and built up a great delta. The remains of animals which haunted the banks of the rivers were swept into them in time of flood and carried out to be deposited in the delta, which covered most of what is now Wichita, Archer, Wilbarger counties in the State of Texas. Naturally, most of the remains which found their way into the streams were already fragmentary, as they had rotted on the banks, and had been torn by predatory animals, but in their course to the sea they were still further disintegrated so that, rolled by the waters and beaten by the waves, they sank to their burial as little more than water-worn fragments; ends of limb bones, isolated vertebrae, and broken skulls, which do little more than tantalize the student with the hints of new forms and new relationships that can not be verified. In some places these water-worn fragments are so thick upon the ground that they literally can be shoveled up by the wagon load. In some instances the bodies of animals found their way unharmed into the water and, distended by the gases of decomposition, floated far and uninjured until they came to rest on some mud flat beyond the reach of sharks or other predatory animals. Such skeletons are preserved entire with most remarkable perfection, but they are exceedingly rare, and it is a fortunate collector who turns up even one in the course of a whole season of search.”

Cacops, a remarkable Permian amphibian.]

Among the most remarkable of the animals found in the stratified rocks of this ancient delta are the amphibians. During the preceding ages, as we have seen, they were the masters of the continental world--the largest and most progressive of land dwellers. But by the time of the Permian they were on the downward road which was to lead them, not to extinction, but to the obscure positions of toads, frogs and salamanders. But this decline did not come at once, and with overwhelming completeness; the early amphibians did not yield tamely to the fate of old age. During the Permian, and the succeeding age, the Triassic, there lived some of the largest and strangest amphibians that the world has known. Some of them returned to the water and developed eel-like bodies with enormous, arrow-like heads; some lived in trees, where their remains have been found by geologists studying coal deposits; others protected themselves by armor and crawled about more or less unmolested, while not a few even went so far as to take refuge in the ground.

The giant among all amphibians was Eryops, of the Permian, with a length of about eight feet. He looked somewhat like an immensely overgrown salamander with flabby sprawling legs that were incapable of lifting his body from the ground except by the greatest effort. His skull was from two to two and a half feet in length, and looked like that of a gigantic frog, the jaws were equipped with sharp, cone-shaped teeth and sharp, powerful tusks. Evidently Eryops held about the same position in Permian swamps of Texas as the alligator does in the Georgia bayous of today, lying near the surface of the water with only its eyes and nostrils exposed until the approach of a victim stirred it to movement.

Sharply contrasted with the giant Eryops, and doubtless often its victim, was the unique Diplocaulus. The head of this amphibian was the form of a rather angular crescent, with strong spines directed to the rear, and protected by heavy armor. The eyes and nostrils were directed straight upward, and situated at the forward end of the skull. The body, remarkably enough, was long, thin, and eel-like, with only rudimentary limb bones, and delicate ribs. Probably the animal spent its entire life in the water, wriggling along and pushing its enormous head before it along the slimy bottom. Perhaps, when attacked from above, Diplocaulus was thrown into a frenzy of energy that lifted his heavy, armored head above the bottom, and thrashed it about in the water, but anything like swimming must have been quite beyond his power.

Diplocaulus, the most bizarre amphibian known.]

But, bizarre though the amphibians of this Permian deposit are, they are exceeded by the reptiles. These creatures, as we have seen, made their appearance in the Pennsylvanian, or late “Carboniferous,” and already, in the very beginning of their history, they showed an astonishing diversity of form and habit. There were reptiles that lived in the water, and reptiles that lived on dry land; reptiles that ate flesh, reptiles that ate plants, and reptiles that ate both. Some of them were extremely primitive, closely resembling their amphibian ancestors, while others were like nothing that has lived either before or since.

The simplest of the flesh-eating reptiles were aquatic, living in the waters of swamps and rivers, and perhaps even in the sea. Their bodies and tails were long and slender; their teeth were simple, cone-shaped affairs of almost equal size throughout the entire length of the jaw. Such a dentition indicates that the reptiles possessing it preyed upon small animals which they did not have to tear to pieces; they merely seized their food and swallowed it, leaving the entire process of digestion to the internal organs, just as do the snakes of today. The upper portions--called dorsal spines--of the backbones in these forms were short, as they are in most lizards, and did not project through the skin.

In other, and somewhat higher reptiles, the teeth appear to be differentiated, those at the forward ends of both jaws being enlarged to form tusks, while the posterior ones are flattened, so that cutting edges are developed that would cut the prey and assist materially in its capture. In some of these same forms the dorsal spines of the backbones are very high, and project above the back as does the fin of a fish.

In Dimetrodon, the last of the series of spiny backed, carnivorous reptiles, these characters reached their culmination. The incisor and canine tusks attained an enormous length and strength, projecting from the jaws as much as three inches. The notch back of the incisors is large and deep, and the posterior teeth of both jaws are recurved and have sharp, serrated edges so that they must have had all of the cutting power of a Malay knife or kris. A more effective set of weapons for the cruel business of capturing prey and holding it despite its desperate, agonized struggles hardly could be conceived. At the same time, the spines on the back developed to an enormous length, and in some species tapered with the delicacy of a whip lash. The tail was short and strong, and the feet armed with well-developed claws, all of which go to show that the animal was purely land dwelling in habit. Dimetrodon reached lengths of more than eight feet, and was by all odds the largest and most powerful creature of his day. We can imagine him crouching in the bushes or tall grasses by the side of some stream, watching for other reptiles that come to quench their thirst, and making a fierce, scuttling rush upon his prey. Most often, perhaps, this was another reptile, but the ponderous, slow-moving amphibians also must have suffered from the attack of the powerful Dimetrodon. There is ample evidence that these animals often waged fierce battles among themselves, for it is common to find bones which have been broken during life and healed again, telling of furious reptilian contests for mates or territory, or perhaps with the single brute idea of a cannibalistic meal.

Restoration of Dimetrodon, the largest animal of the Permian lands.]

Naosaurus (Edaphosaurus), the profusely spined mud-grubber. (After Case.)]

But a more wonderful animal still has left its remains among the rocks of this delta. Like Dimetrodon, it possessed high spines on the back, but instead of being simple and tapering, they were furnished with projecting processes on either side, much like the yard arms of old fashioned sailing ships. This resemblance led Cope, a pioneer American paleontologist, to call the animal Naosaurus, or “Ship Lizard.” But in spite of his fierce appearance, Naosaurus was totally unlike Dimetrodon in habit, being a peaceful, sluggish eater of shellfish, and perhaps of plants. This animal probably has the most remarkable dentition of any creature known. The incisor teeth are chisel-like, as though useful in cutting strong, coarse vegetation; behind them are five triangular teeth not unlike those of such flesh-eaters as the lion and tiger; behind these are simple cones, like those of the primitive lizards. But, most wonderful of all, on both the palate and the corresponding portion of the lower jaw are heavy plates of bone covered by short, stumpy teeth, such as occur in the jaws of those fish which live upon shellfish. Seemingly, therefore, the animal ate all sorts of food, but instead of having a generalized pattern of teeth like the pig or man, it possessed a separate set for each type of diet.

The discovery that Naosaurus was an eater of plants and molluscs, and not a predatory form, makes more perplexing than ever the question as to the use of the spines on the back. On such a thick-bodied, sluggish mud-grubber the cross-barred spines must have had about the same value as an ornamental frieze on a canal boat. What conditions of environment could have produced similar structures on creatures of such dissimilar habits as Dimetrodon and Naosaurus?

The structure of the animal shows that these spines were not covered by flesh, but by a thin membrane of skin, through which the spines showed as plainly as the rays in the fin of a fish. It is hard to conceive of this great dorsal frill as being anything but a hindrance to both pursuer and pursued. Cope suggested, in a spirit of fun, that these animals were the natural precursors of the modern fin-keeled yacht, and that when they wished to navigate the Permian waters they swam upon their backs. Other authors have made other suggestions of about the same note, but few have been able to make any serious contribution on the subject. The obvious suggestion is that the spines served as some form of protective mimicry, helping the animal to remain concealed among the reeds which bordered the lakes or streams. But this seems hardly necessary in an animal that was the dominant form of its time and needed no concealment unless as an aid to lying in wait or making an unseen approach until sufficiently near for its final rush upon its unsuspecting prey. This last, is perhaps, a fair suggestion, but it seems that the burden of maintaining such a weak structure must have far outweighed any advantage of concealment. The spines were slender and were constantly being broken in battle and by accident, and the animal must have expended no inconsiderable amount of energy in repairing the damaged structure.

There remains the suggestion that the spines were remnants of a formerly useful structure, and that their mature condition was purely due to overgrowth. It seems certain that when a structure has developed so far as to give an animal a distinct advantage it may continue to grow until it is a burden rather than a help. Thus the spines, originating as a protective feature, may have given their possessors such a decided advantage that they were almost free from attack. In this stage they may have become over-nourished, and have continued growing by a sort of inertia or momentum until they reached their greatly exaggerated development. Furthermore, there is a tendency, both in plants and in animals, for a species nearing its extinction to develop a spiny or horny habit. This we saw in the invertebrates, among the aging Cephalopods, and it is quite conceivable that a similar state of affair prevails among these reptiles. For Dimetrodon and Naosaurus, dominant though they were, were nearing the end of their life cycle. In their senile aberrations they carried the development of spines to greater and greater lengths, until, in their declining strength they were unable to maintain their species, and so disappeared from the earth.

Permian vertebrates, restored by Williston and others. 1-3 are reptiles, while 4 is an amphibian.]

FOOTNOTES:

Case, E. C., “A Great Permian Delta and its Vertebrate Life,” Pop. Sci. Monthly, Dec. 1908. This quotation is somewhat paraphrased, while the remainder of this chapter is based very largely on the article.

Paraphrased from Case, Loc. Cit.

A relatively recent discovery, made by Dr. Case, shows that the name Naosaurus must be dropped, being replaced by the earlier name Edaphosaurus. The former name, however, is the commoner, as well as the more descriptive.