Popular Science Monthly/Volume 36/January 1890/Birds with Teeth
|BIRDS WITH TEETH.|
THE birds of our present world, however different they may be from each other in size, shape, color, etc., are remarkably uniform in their anatomical construction. Adapted to a life in the air, they all possess bones which are more or less pneumatic that is, contain air-cavities, to lessen the weight of the skeleton. Altogether the reduction of weight has been brought down to perfection, and a flying bird carries very little, if any, unnecessary substance. Locomotion in the air requires, further, a vigorous action of the wings, and such a motion could hardly be executed in presence of a loose and shaky body. But the skeleton of the body of a bird is not loose; on the contrary, it is very solid. The vertebrae of the backbone are grown together and form a firm column. With this column the broad ribs unite in the back, while in front they are held together by the sternum; and to make this cradle of bones more compact and fit to resist the action of the wings, and to protect the interior organs, the ribs touch each other with what the anatomists call "uncinate processes." Whoever is called upon to carve at the table a chicken or a turkey will experience how solid and protective the construction of the skeleton of the body is.
If the main body is in this way compact and immovable, some other part must be so much the more flexible, and this is the neck. If we watch a swan oiling its feathers with its bill, see the cervical evolutions of a flamingo, or an owl sitting with its head reversed, we are apt to experience a painful sensation in our necks and may wonder whether the bird will assume its normal position without breaking something. But all these motions are executed with the most perfect ease and security, and the construction of the vertebræ, which enables the birds to perform them, is simple and effective. These vertebræ do not articulate with each other by plain faces as the vertebræ in our bodies, but the articulations are saddle-shaped, so that the prominence of one vertebra fits into the excavation of the next one, and vice versa. If we take two adjoining vertebræ of the neck of a bird and try their motion, we shall find that this articulation admits two ways of turning—from Fig. 1.—Vertebra of a Bird with Saddle-shaped Articular Face. one side to the other as well as up and down. This saddle shape of the articular face of the cervical vertebræ is found without exception in all existing birds and in no other animal.
There are other features which are met with in all birds and exclusively in them for instance, the plumage. The horny bill is without teeth. The vertebræ of the tail are grown together and form a plowshare-shaped bone. This bone supports the tail feathers, which can be opened and closed like a fan, and which serve as a rudder and a parachute. The bones of the anterior extremities are transformed in such a way that they form an excellent framework for the wings; but, although the anatomists
distinguish easily radius, ulna, digits, etc., one would hardly suspect that these wing bones are perfectly analogous to those in the fore-feet of quadrupeds, or in our arms and hands.
Now let us turn our attention to the reptiles which exist at the present time to the lizards, crocodiles, snakes, and turtles. These cold-blooded, scaly animals seem to have nothing at all in common with the warm-blooded, feathered inhabitants of the air; and yet our first scientists, for instance, Prof. Huxley, unite reptiles Fig. 2.—Section of the Earth's Crust. and birds as one class of animals. However closely we may study the anatomy of a chicken and of a crocodile, and search for points which are common to both, we shall be only moderately inclined to follow the example of these naturalists and consider birds and reptiles as near relatives. But we shall become convinced of their close connection if we study also their geological history.
In the first instance, we then see that the reptiles of the present time are only small remnants of a once numerous and powerful tribe. In certain former geological periods we find reptiles to be the kings of the earth; we discover reptiles of enormous sizes; reptiles of all kinds of shape and anatomical construction; and among them there are some which resemble birds much more than does a crocodile or a lizard of the present day. In the second instance, if we look at the birds of former periods we find that, in the same way as we go backward in the history of our earth, these former inhabitants of the air differ more and more from the specialized pattern after which all our present birds are built, that they become more and more reptile-like, and that there can hardly be any doubt that the birds are indeed branched off from the great reptilian trunk of the animal kingdom.
There are three fossil birds of pretertiary age known almost completely; two of them were found in the Cretaceous formation of North America, and one in the Jurassic formation of Germany.
During the Cretaceous period the Rocky Mountains were not yet the high elevation which they are now, but were existing probably only as a succession of low islands. The land of several States of the Union was deposited at that time at the bottom of the ocean, east of this line of islands. One of these States is Kansas, in which, therefore, the cold water was then still more supreme than it is now. In the Cretaceous deposits of Kansas the remains of the bird Hesperornis have been found. It was a
large animal, about five feet long, with well-developed, strong legs, but perfectly obsolete wings. The anatomical construction indicates the way of living. It was entirely without the power of flight, living in the way of penguins, mostly on the water, diving for fish, which it was admirably adapted to catch, and with which the Cretaceous ocean here was teeming. The shores were visited probably only for the purpose of breeding. There is nothing very peculiar in it's anatomy, with the exception of one fact, which, however, distinguishes it at once thoroughly from all birds living now: its long bill was provided with teeth, teeth which resemble those of reptiles. Few discoveries have at their time caused so much interest among geologists as when through Kansas fossils the fact was established that birds with teeth formerly existed.
The deposits of Kansas in which the Hesperornis has been found have yielded also the remains of another Cretaceous bird,
Fig. 4.—Restoration of Ichthyornis victor (after Marsh). One half natural size.
the Ichthyornis. This animal was small, of about the size of the tern, and probably also resembled it in its habits and mode of aliving; at any rate, it was a good flier, with powerful wings and small legs. The Ichthyornis also had teeth in both jaws. But in still another respect it was different from all our present birds. The articular faces of its vertebræ were not saddle-shaped, but simply excavated on both sides; they were biconcave. This form of vertebrae is met with in a few recent and many extinct reptiles, and in the amphibians. But it is especially characteristic of fishes, and the name has been chosen on account of it, for Ichthyornis means "fish-bird." The fossil remains from which Hesperornis and Ichthyornis have been described are exhibited in the Museum of Yale College, in New Haven.
A long time, a whole geological period, before Hesperornis and Ichthyornis were enjoying life and eating fish on the vast Cretaceous Fig. 5.—Cervical Vertebra of Ichthyornis dispar, x 2. ocean of the western United States, there was on the other side of the Atlantic, at about the site of the town of Solenhofen, in Bavaria, a lake or marine gulf in a protected situation, with very quiet waters. The lime carried by the rivers into this gulf was deposited at the bottom, in an exceedingly uniform and undisturbed manner, as a very fine sediment. At present, after millions of years, the gulf is dried up and the sediment has been hardened to a limestone, the grain of which is unusually small and regular. In fact, no limestone in any country
In this lithographic stone of Bavaria there was found, in the
year 1860, the impression of a feather. This proved the existence of a bird during the Jurassic period—that is, of a feathered animal much older than all other known fossil birds. It was therefore named Archæopteryx, which means "old wing." The feather was there; a sharp lookout was kept now for the bird itself, and, indeed, one year later, a nearly complete skeleton of it was found. It was bought by the British Museum, in London, for fourteen thousand marks, and has been described by Prof. Owen, Sixteen years later, in 1877, a second specimen was discovered in Solenhofen. The electrician, Dr. Siemens, in Berlin, did not wish that this fossil should also go out of Germany; therefore he bought it for twenty thousand marks, and sold it afterward, at the same price, to the Prussian Government. It is now in the Berlin Museum.
Both specimens together furnish us with an almost complete picture of the Jurassic bird. The Archæopteryx was about the size of a pigeon. The Berlin specimen proves that its bill was also provided with teeth. Furthermore, the vertebrae show also that biconcave form, as in Ichthyornis and in lower animals. But there are a number of other features in the Archæopteryx which remove it still further from the birds of the present time, and make it resemble a reptile: 1. Its wing bones were not grown together in the way of all the other birds, but they were partially separate, as in the fore-foot of a quadruped, and there were even three claws, so that the animal could also grasp with its wings. 2. Its ribs were small, thin, without uncinate processes, and therefore formed no compact and solid cradle. 3. Its pelvic bones were almost like those of certain extinct reptiles. 4. Its tail was not short, with a plowshare-shaped bone and a fan-like arrangement of the tail feathers, as in all other birds, but it was long, composed of not less than twenty vertebræ, and the feathers were fixed along the whole length of this tail. In other words, it was a regular lizard tail, but covered with feathers. It is no wonder that some scientists did not consider the Archæopteryx as belonging to the birds, but thought it was a reptile. Apart from its plumage, however, there is too much in its anatomy that is avian. At any rate, it was a very reptile-like bird, and its power of flight was certainly not great. It probably fluttered more than flew, and occasionally used its claws to support itself. The formation of its eye bones seems to indicate that it was of nocturnal habits, like our owls.
Other fossils, lying as yet unknown in the strata of our earth and waiting for the ardent scientific digger, will teach us considerably more about the evolution of the birds, but the outlines of it are already mapped out by what we possess at present; and it must be said, especially about the Archæopteryx, that it sheds more light upon the development of the animal kingdom during former periods than perhaps any other known fossil.