Popular Science Monthly/Volume 20/January 1882/Jurassic Birds and their Allies
|JURASSIC BIRDS AND THEIR ALLIES.|
By Professor O. C. MARSH.
ABOUT twenty years ago, two fossil animals of great interest were found in the lithographic slates of Bavaria. One was the skeleton of Archæopteryx, now in the British Museum, and the other was the Compsognathus preserved in the Royal Museum at Munich. A single feather, to which the name Archæopteryx was first applied by Von Meyer, had previously been discovered at the same locality. More recently, another skeleton has been brought to light in the same beds, and is now in the Museum of Berlin. These three specimens of Archæopteryx are the only remains of this genus known, while of Compsognathus the original skeleton is, up to the present time, the only representative.
When these two animals were first discovered, they were both considered to be reptiles by Wagner, who described Compsognathus, and this view has been held by various authors down to the present time. The best authorities, however, now agree with Owen that Archæopteryx is a bird, and that Compsognathus, as Gegenbaur and Huxley have shown, is a Dinosaurian reptile.
Having been engaged for several years in the investigation of American Mesozoic birds, it became important for me to study the European forms, and I have recently examined with some care the three known specimens of Archæopteryx. I have also studied in the Continental museums various fossil reptiles, including Compsognathus, which promised to throw light on the early forms of birds.
During my investigation of Archæopteryx, I observed several characters of importance not previously determined, and I have thought it might be appropriate to present them here. The more important of these characters are as follows:
1. The presence of true teeth, in position, in the skull.
2. Vertebræ biconcave.
3. A well-ossified, broad sternum.
4. Three digits only in the manus, all with claws.
5. Pelvic bones separate.
6. The distal end of fibula in front of tibia.
7. Metatarsals separate, or imperfectly united.
These characters, taken in connection with the free metacarpals, and long tail, previously described, show clearly that we have in Archæopteryx a most remarkable form, which, if a bird, as I believe, is certainly the most reptilian of birds.
If now we examine these various characters in detail, their importance will be apparent.
The teeth actually in position in the skull appear to be in the pre-maxillary, as they are below or in front of the nasal aperture. The form of the teeth, both crown and root, is very similar to the teeth of Hesperornis. The fact that some teeth are scattered about near the jaw would suggest that they were implanted in a groove. No teeth are known from the lower jaw, but they were probably present.
The presacral vertebræ are all, or nearly all, biconcave, resembling those of Ichthyornis in general form, but without the large lateral foramina. There appear to be twenty-one presacral vertebræ, and the same, or nearly the same, number of caudals. The sacral vertebræ are fewer in number than in any known bird, those united together not exceeding five, and probably less.
The scapular arch strongly resembles that of modern birds. The articulation of the scapula and coracoid, and the latter with the sternum is characteristic; and the furculum is distinctly avian. The sternum is a single broad plate, well ossified. It probably supported a keel, but this is not exposed in the known specimens.
In the wing itself the main interest centers in the manus and its free metacarpals. In form and position these three bones are just what may be seen in some young birds of to-day. This is an important point, as it has been claimed that the hand of Archæopteryx is not at all avian, but reptilian. The bones of the reptile are indeed there, but they have already received the stamp of the bird.
One of the most interesting points determined during my investigation of Archæopteryx was the separate condition of the pelvic bones. In all other known adult birds, recent and extinct, the three pelvic elements, ilium, ischium, and pubis, are firmly anchylosed. In young birds these bones are separate, and in all known Dinosaurian reptiles they are also distinct. This point may, perhaps, be made clearer by referring to the two diagrams before you, which I owe to the kindness of my friend Dr. Woodward, of the British Museum, who also gave me excellent facilities for examining the Archæopteryx under his care. In the first diagram, we have represented the pelvis of an American Jurassic Dinosaur allied to Iguanodon, and here the pelvic bones are distinct. The second diagram is an enlarged view of the pelvis of the Archæopteryx in the British Museum, and here too the ilium is seen separate from the ischium and pubis.
In birds the fibula is usually incomplete below, but it may be co-ossified with the side of the tibia. In the typical Dinosaurs, Iguanodon, for example, the fibula at its distal end stands in front of the tibia, and this is exactly its position in Archæopteryx, an interesting point not before seen in birds.
The metatarsal bones of Archæopteryx show, on the outer face at least, deep grooves between the three elements, which imply that the latter are distinct, or unite late together. The free metacarpal and separate pelvic bones would also suggest distinct metatarsals, although they naturally would be placed closely together, so as to appear connate.
Among other points of interest in Archæopteryx may be mentioned the brain-cast, which shows that the brain, although comparatively small, was like that of a bird, and not that of a Dinosaurian reptile. It resembles in form the brain-cast of Laopteryx, an American Jurassic bird, which I have recently described. The brain of both these birds appears to have been of a somewhat higher grade than that of Hesperornis, but this may have been due to the fact that the latter was an aquatic form, while the Jurassic species were land birds.
As the Dinosauria are now generally considered the nearest allies to birds, it was interesting to find, in those investigated, many points of resemblance to the latter class. Compsognathus, for example, shows in its extremities a striking similarity to Archæopteryx. The three clawed digits of the manus correspond closely with those of that genus; although the bones are of different proportions. The hind-feet also have essentially the same structure in both. The vertebræ, however, and the pelvic bones of Compsognathus differ materially from those of Archæopteryx, and the two forms are in reality widely separated. While examining the Compsognathus skeleton, I detected in the abdominal cavity the remains of a small reptile which had not been previously observed. The size and position of this inclosed skeleton would imply that it was a fœtus; but it may possibly have been the young of the same species, or an allied form, that had been swallowed. No similar instance is known among the Dinosaurs.
A point of resemblance of some importance between birds and Dinosaurs is the clavicle. All birds have those bones, but they have been considered wanting in Dinosaurs. Two specimens of Iguanodon, in the British Museum, however, show that these elements of the pectoral arch were present in that genus, and in a diagram before you one of these bones is represented. Some other Dinosauria possess clavicles, but in several families of this sub-class, as I regard it, they appear to be wanting.
The nearest approach to birds now known would seem to be in the very small Dinosaurs from the American Jurassic. In some of these, the separate bones of the skeleton can not be distinguished with certainty from those of Jurassic birds, if the skull is wanting, and even in this part the resemblance is striking. Some of these diminutive Dinosaurs were perhaps arboreal in habit, and the difference between them and the birds that lived with them may have been at first mainly one of feathers, as I have shown in my memoir on the Odontornithes, published during the past year.
It is an interesting fact that all the Jurassic birds known, both from Europe and America, are land-birds, while all from the Cretaceous are aquatic forms. The four oldest known birds, moreover, differ more widely from each other than do any two recent birds. These facts show that we may hope for most important discoveries in the future, especially from the Triassic, which has as yet furnished no authentic trace of birds. For the primitive forms of this class we must evidently look to the Palæozoic.
- Read before Section D, British Association for the Advancement of Science, at York, September 2, 1881.