Popular Science Monthly/Volume 73/December 1908/A Great Permian Delta and its Vertebrate Life, with Restorations by the Author
|A GREAT PERMIAN DELTA AND ITS VERTEBRATE LIFE, WITH RESTORATIONS BY THE AUTHOR|
ASSISTANT PROFESSOR OF HISTORICAL GEOLOGY AND PALEONTOLOGY, UNIVERSITY OF MICHIGAN
AS early as 1878 it was known that the remains of a wonderful group of animals lay hid in the rocks of north central Texas, that had lived their appointed time and passed away before the earth history was half completed; since then collectors have gone into this region more or less regularly, contending in the early days with hostile Indians and later with bad water and difficult transportation. Since 1895 the author has made several trips, gathering vertebrate fossils for the University of Chicago and the American Museum of Natural History in New York. The descriptions below are based on these collections and those of earlier workers.
Perhaps one can get the best idea of the age of the rocks and the fossils by remembering that they were laid down in the portion of geological time called the Permian age, just after the period of the coal deposition. Reckoning the completed history of the earth as about one hundred millions of years, these rocks and fossils are from thirty to forty millions of years old. At the beginning of the Carboniferous age, when the coal was laid down, the part of the continent that is now called the Mississippi-Valley was covered by a wide sea, but during this age there was a progressive shallowing which culminated in the elevation of the Appalachian mountains in the east and the appearance of dry land from the new mountains on the east to the forebears of the Rockies on the west.
The appearance of dry land was at once the cause of the development of the wonderful group of Permian animals and the reason that so few are preserved to us, for it is only when the hard parts of animals or plants are buried in some water-soaked layer of the earth or are covered by water that they can be petrified and preserved. If they remain exposed to the air they are soon destroyed; so, of the skeletons of the thousands of buffalo left lying on the plains but a few years ago, there remain to-day but a few rotten and frost-split horns and bones. Undoubtedly in the muddy banks and bars of the rivers there are skeletons undergoing the slow process of petrifaction which will preserve them to be the chief treasure of some future museum. And so because of the land conditions which prevailed over so much of the continent the record of the great land fauna of the Permian age is very faulty and imperfect. It is only in some exceptional place where large quantities of bones were swept together under favorable conditions that they have been preserved and such an exceptional depository occurs 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 in western Oklahoma. 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 the 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 and Willbarger 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 bank and been torn by predatory animals, but in their course to the sea they were farther disintegrated, so that, rolled by the rivers and beaten by the waves, they sank to their burial as little more than water-worn fragments; ends of limb bones, isolated vertebræ and broken skulls which do little more than tantalize the student with the hints of new forms and new relationships which can not be verified. In some places these water-worn fragments are so thick upon the ground that they can literally be shoveled up by the wagon load. In some rare instances the bodies of animals found their way unharmed into the water and, distended by 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 and in a most wonderful state of preservation, but they are exceedingly rare, not more than one turning up in a season's search.
Of all the wonderful animals revealed by their petrified remains, perhaps the most striking are the reptiles. The reptiles made their first appearance in the Permian or in the latter portion of the Carboniferous preceding, but here at the very inception of their line they developed a great diversity of form and habit. There were aquatic and terrestrial forms, carnivorous, herbivorous and omnivorous forms, forms simple and closely resembling their amphibian ancestors and forms so bizarre in their structure that the world has produced nothing more strange. The reptiles descended from the amphibians and it is natural to look in these beds, where the lowest of the reptiles are found, for the connecting link between the two, but as yet this form has not been discovered; the approach from both sides, however, is so close that it is frequently impossible to determine the nature of a specimen from a single bone or small portion of the skeleton.
The simplest of the early carnivorous reptiles were aquatic, living in the waters of the great rivers or perhaps even in the ocean. The body was long and slender and" the tail was exceptionally so, in correlation with the swimming habit. Aside from the more technical points, the interest in the development of the primitive reptiles centers in certain changes of the teeth and the dorsal spines of the vertebra?. In one of the simplest forms, Poliosaurus, the teeth have the form of simple cones of nearly equal size in all parts of the jaw; such a dentition indicates that it preyed upon small animals which it seized and swallowed whole after the manner of snakes and many lizards. The dorsal spines are low and do not project beyond the skin. The animal probably resembled very closely the living monitor of the Nile.In another and closely related reptile, Theropleura, the teeth have become differentiated, those at the anterior end of both the upper and the lower jaws, the incisors, are enlarged and have taken on the appearance of tusks; posterior to the incisors there is a slight notch in the edge of the upper jaw, caused by the growth of the lower incisors, and posterior to this there are tusk-like teeth in the upper jaw, the canines. The growth of the incisors and canines increased the power of grasping and holding prey. Moreover, in the posterior teeth of the
jaws the teeth are somewhat flattened, so that cutting edges are developed which would lacerate the prey in capture or before swallowing and minimize the chances of escape. In Clepsydrops the dentition is still more specialized; the incisors and canines are longer and the notch between them has developed into a toothless diastema; the skull is rather thin and high and the eyes are located far back in the skull. The spines of the vertebrae are very high and project from the back like the fin of a fish, in this matter contrasting strongly with the simpler forms in which there is no such fin or frill. It must be remembered that the spines are connected directly with the vertebral column and are not simply developed in the skin as is the case with the fishes. In the modern Basiliscus and Iguana there are frills on the back, but the strengthening spines are dermal like those of the fishes.
In Dimetrodon, the last of the series, these characters reached their culmination. The incisor and canine tusks have attained a relatively enormous length and strength, and projected from the jaws as much as three inches; the diastemal notch is larger and deeper and the posterior teeth of the jaws are recurved and have sharp serrate edges so they had all the cutting power of a Malay kris. Could a more effective arrangement be imagined for the cruel business of capturing and holding living prey despite its desperate struggles? The spines on the back developed to enormous length and in some forms tapered to the slenderness of a whip lash. The tail was short and the feet strong and with well-developed claws, all going to show that the animal was terrestrial in habit. The largest species of Dimetrodon reached a length of about eight feet and was easily the largest, strongest animal of its time. We can imagine this fiercely carnivorous form crouching in the bushes or tall grass on the side of some stream and making a
fierce scuttling rush out upon its prey, perhaps some slow-moving reptile or amphibian, perhaps even some smaller individual of its own kind, for there is ample evidence that these animals waged fierce battles among themselves. It is not uncommon to find bones which have been broken during life and healed again, telling of furious reptilian contests in the struggle for mates or for territory, or perhaps with the single idea of a cannibalistic meal.
But a more wonderful animal still has left its remains in the rocks. In this form there were high spines on the back, but instead of the spines being simple they were furnished with projecting processes on the sides, not unlike the yard-arms of the old-fashioned sailing ships. This resemblance led Cope to call the animal Naosaurus, ship-lizard. In a recent restoration of Naosaurus it has been given the skull of the fiercely carnivorous Dimetrodon, the general similarity of the forms
seemed to warrant this, but recent discoveries have made it probable that Naosaurus was not an eater of flesh, but a peaceful, sluggish eater of shell fish and perhaps of vegetation. This animal has perhaps the most wonderful dentition of any known animal the incisor teeth are sharp and chisel-shaped, such as might be useful in cutting strong vegetation; behind these are five sharp triangular cutting teeth, not unlike the sectorial teeth of such flesh eaters as the tiger and lion; behind these are simple cones, such as would be useful in holding a struggling victim. But most wonderful of all, on the palate and in a corresponding position on the lower jaw were heavy plates of bone, covered by short stumpy teeth, such as occur in the jaws of fish which live upon molluscs. The animal was seemingly omnivorous, but instead of having a dentition of a generalized pattern like that of the pig or the human being, there was a set for each kind of diet. The Fig. 5. Portion of the Vertebral Column of Dimetrodon, showing the enormous development of the spines. discovery that Naosaurus was an eater of 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? It is as if the tiger and the badger should meet on common ground and develop highly specialized, unwieldy and seemingly useless structures of close similarity.
Of what use were the spines on the backs of these animals? The structure shows that they were not covered with flesh, but were united by a thin membrane through which the spines showed as plainly as the fin rays in the fin of a fish. It is hard to conceive of this great
Fig. 8. Side View of Two Dorsal Vertebræ of Naosaurus. dorsal frill as anything but a hindrance to any quick movement. Cope suggested, in a spirit of fun, that these animals were the 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 suggested equally abused uses in a similar spirit, but there are very few that can be considered with any degree of seriousness. The obvious suggestion is that the spines served as some form of protective mimicry, perhaps helping the animal to remain concealed among the reeds which bordered the lakes or streams, but this seems hardly necessary when we reflect that the animal was the dominant form of its time and needed no concealment unless it was to aid in lying in wait or in making an unseen approach until sufficiently near for the final rush upon Fig. 7. Front View of One of the Dorsal Spines of Naosaurus. its unsuspecting prey. This last is perhaps a fair suggestion, but it seems that the physiological burden of maintaining such an essentially weak structure must have far outweighed any conceivable advantage of concealment. The spines were slender and were constantly subject to fracture in battle or by accident and the animal must have expended no inconsiderable portion of its energies in repairing the broken structure.
There remains the suggestion that the spines are remnants of a formerly useful structure and their present condition is purely a physiological one due to overgrowth. It seems certain that when a structure has developed so far as to give an animal a great advantage it may continue to grow until it is rather a burden than a help. The structure starting as a protective feature may give the animal such an advantage that it is practically free from all its enemies and in this dominant or protected condition it may become over nourished and the originally useful structures may t continue to grow by a kind of inertia or momentum until they become greatly exaggerated masses of flesh or inordinately developed spines, horns, feathers, etc. Such a development seems to have occurred once and again in the history of the world, and the most bizarre types of life owe something of their condition, at least, to tins principle. The late Professor Beecher, of Yale, has shown that there is a decided tendency, both in plants and in animals, for a species that is nearing the point of its extinction to develop a spiny or horny habit, covering itself with all sorts of excrescences, seemingly in an unregulated effort to find some condition which will prolong its existence. It is certain that these reptiles, dominant as they were, were rapidly completing their allotted span and as the end approached the spines grew ever heavier and heavier, until it seems plausible to suggest that they became at last a great drain on the animal's powers of nutrition and hastened in no slight degree the end.
Fig. 9. Palatal View of the Skull of Naosaurus, showing the peculiar dentition.
The family of the high-spined Pelycosaurs did not monopolize the wonders of the reptilian horde; however the borders of the streams, the swamps and the uplands, harbored other forms whose structure is just beginning to be understood. One group of these, called by Cope the Cotylosauria, is the most primitive of the reptiles and the nearest to the amphibians. These animals were low-bodied and sprawling, with a head completely roofed over by bone and showing between the orbits the third, or pineal, eye. Rudiments of this third eye are still present in the human brain, the pineal gland between the cerebrum and cerebellum, and in many of the modern lizards it is still so far functional as to have a rudimentary retina perhaps capable of perceiving light.Another group of reptiles closely resembling the Cotylosauria in outward appearance, but differing in many details, notably the development of a body armor, were placed by Cope in a separate order, the Chelydosauria. As indicated by the name, the Chelydosauria are probably the ancestors of the turtles. Aside from more technical points this relationship is shown by the development of a more or less complete carapace. The skeleton of the larger forms is pretty well known and the general shape is shown in the restoration of the skeleton of Diadectes, Fig. 11. The teeth are elongated transversely and flattened, telling of a purely vegetable diet; the dermal armor consisted of five overlapping plates on each side lying on the first five dorsal ribs; the anterior one is small, the second much larger and the last three smaller again. In certain small forms, Otocæeus and Conodectes, the armor was much more perfect, consisting of strong dermal plates corresponding
to the ribs in number and extending down over the sides in broad curves so that the animal was completely closed in a bony cuirass. Add to this that the top of the head was heavy and solid, and we can imagine that the animal when it crouched close to the ground, with its head drawn down and in, resembled very closely the modern Armadillo in its attitude of defense and was able to resist the attack of even the long tusked Dimetrodon.
This donning of armor is one of the striking things about the animals of the Permian age; it occurred among the amphibians as well as among the reptiles and is closely correlated with the development of great tusks in the predatory forms. As the armor-piercing weapons grew ever stronger the armor grew ever heavier and more completely adapted to the body. The same thing has happened once and again in the world's history; much later, in Tertiary time, when the world was thirty millions of years older, we have a repetition of the same thing. The great saber-toothed tiger developed canine tusks six inches in length, and the small edentates, the natural prey of the tiger, developed first small isolated bones in the skin, but ever as the tusks grew the bones in the skin became larger and better arranged, until the almost perfect protection of the Armadillo appeared. It was the prophecy of modern warfare between armor-piercing shells and armor plate, we have not seen the end in human history, but in the old days it continued to the practical extinction of both parties to the contest. Perhaps there is a neglected object-lesson here.
Turning from the reptiles to the amphibians, we find a no less wonderful group of animals. During the preceding age, the Carboniferous, the amphibians had been masters of the world; by the Permian, their time of dominance was past and they were already on the downward path that was to end in the obscure toads, frogs and salamanders of our meadows. But they were far from yielding tamely to their fate; they developed in all? possible directions in a seemingly frantic effort to regain their lost dominance. During the Permian and the succeeding Triassic ages, there lived some of the largest amphibians the world has ever seen; they betook themselves to the water and developed eel-like bodies; they lived in hollow trees, as witness the discoveries of Sir Wm. Dawson in the stumps of trees
|Fig. 12. Skull of Diplocaulus, showing the upper and lower surfaces.||Fig. 13. Restoration of Diplocaulus.|
in the coal beds of South Joggins in Nova Scotia; they wrapped themselves in armor and they hid themselves in the ground.
It is possible to describe but a very few of the forms that are known. Eryops was the largest of the amphibian tribe, with a length of about eight feet; it was not unlike a great overgrown newt or salamander with weak sprawling limbs that could not raise the body from the ground, except by a great effort. The skull had a length of two feet and a half in the largest specimens and the lower jaw was hinged at the posterior end of the skull, so that the animal had a most tremendous gape. The jaws were armed-with sharp conical teeth, which in the anterior portions of the jaws were developed into powerful tusks. Probably the animal played a somewhat similar part in the Permian waters to that of the modern alligator, lying nearly covered in the water with only the eyes and nostrils exposed, which were placed on the top of the skull for the purpose, and gliding slowly upon its prey until within a distance that made possible a sudden fierce rush which ended with the passage of the victim down the capacious maw of the Eryops.
Contrasted with Eryops, and probably frequently its victim, was the small Diplocaulus. Though still imperfectly known, enough has been made out about this animal to show that it possessed a form even more grotesque than that of the high-spined reptiles. The head, as shown in Fig. 12, was extremely flat and shaped like an exaggerated crescent with strong horns or spines projecting to the rear from the posterior corners. The eyes and nostrils were located far forward toward the anterior end and were directed straight upward. The lower jaw was very short and articulated with the skull far in front of the ends of the horns. No trace has been found of limbs, and the vertebræ interlock like those of the modern snakes; from this it is probable that these animals were limbless and snake-like in form, but they must have presented a most peculiar appearance with the slender body and enormous head. It is hardly possible that the animal could have raised its head from the ground except by an occasional and violent effort, for the skull was solid and relatively very heavy. Probably the animal was purely aquatic in habit and, lying in the mud of the bottom, wriggled forward, pushing its great head through the slime, from which it gathered the vegetation and small shell fish which formed its food. The position of the eyes and nostrils on the top of the skull renders this position the more certain. On the banks of the streams the amphibians took yet another form, for here they donned a complete coat of mail similar to that of the small reptile.
Imperfect as our knowledge still is of this wonderful group of animals, enough is already known to show how fully strife and warfare filled the world's history even at the beginning, and how every possible advantage of tooth or limb or armor was necessary for success.