On the Manner of Locomotion of the Dinosaurs Especially Diplodocus, with Remarks on the Origin of the Birds.
By Oliver P. Hay
Continued from Part Three.
The femur of Allosaurus, of the Upper Jurassic, possesses a head that projects strongly inward; and this was provided with a well-defined smooth articular surface, which is elongated transversely to the animal and convex from front backward. The surface of the ilium against which this head fitted is also smooth. Now the conformation of the head of the femur and the ilium is such that the femur must have diverged considerably from its fellow, thus widely separating the knees. The tibia is shorter than the femur, and the inner condyle appears to stand lower than the outer. The metatarsus is relatively short. I see no way, therefore, for the feet to be brought, except with unusual effort, near the line of direction in walking or near each other in standing. The limbs of the Allosaurus may be compared to those of the penguins, although in Allosaurus the femora may not have been directed so strongly forward and the feet may have been more digitigrade. It would probably be very difficult for the penguin to plant its feet one in front of the other in walking. I believe therefore that Allosaurus had a wide trackway and that when it walked and ran it preserved its equilibrium by whisking its tail from side to side.
Examination of a femur, accompanied by the tibia and the fibula, in the U. S. National Museum, apparently that of a Tyrannosaurus, shows the same form of the head of the femur that is found in Allosaurus, thus making it probable that this dinosaur also had a straddling gait. Professor Osborn (Bull. Amer. Mus. Nat. Hist., xxii, p. 293) presents a figure of the femur of Tyrannosaurus. He says that the plane of the head makes an angle of 45° with the axis of the vertebral column, and that therefore the distal ends of the femora are approximated. Whether the angle is in front or behind the head of the femur is not stated. In Allosaurus the head is directed inward and forward. The effect of this would certainly be to throw the knees outwad and to plant the foot farther away from the line of direction. The convergence of the femora is rare even among the mammals. If Professor Osborne is right the hind legs of Tyrannosaurus had attained the human stage in the respect mentioned.
Another potent reason for believing that the dinosaurs just named, together with Iguanodon and Trachodon, walked with a wide tread is found in the form of the body. In mammals the abdomen is usually contracted posteriorly, so that between the thighs it is shallw, permitting the femora to remain parallel with each other or even to converge. Therefore, in walking, the feet are placed near or on the line of direction. In the birds the baggy abdomen descends between the thighs and spreads these, thus requiring the convergence of the long lower segments to bring the feet together. The kangaroos have the abdomen much like that of the birds; and in them the thighs are found to diverge toward the knees, but the long tibiæ permit the feet to be placed close to each other in standing and leaping. In Allosaurus and Iguanodon the belly came down nearly to the knees and passed backward between the thighs into the tremendous tail. It must be that the knees were much farther apart than the upper ends of the femora were and that the tread was wide. The writer is further of the opinion that in the bipedal dinosaurs the femora were directed more strongly forward than they are usually placed in restoraions, although not so much so as in birds. This position would tend to reduce the height of the reptiles and would make the thighs more divergent.
In a paper published by Mr. William H. Ballou (Century Mag., lv, 1897, pp. 15–23), but the facts and suggestions of which were furnished by Professor E. D. Cope, there is a figure representing two individuals of Hadrosaurus (Trachodon) mirabilis. One of these is on the shore, resting on its hind legs and haunches, the other is standing and feeding in the water. By examining these restorations, made by Mr. Charles R. Knight, one may judge regarding the probability that these reptiles could leave a straight row of tracks behind them.
Mr. S. H. Beckles9 has described and figured some series of large footprints found in the Wealden near Hastings, England. These have been identified by Dollo (Bull. Mus. roy. d'Hist. nat. Belgique, ii, 1883, p. 117, pl. iii, fig. 8) as the tracks of Iguanodon mantelli. A study of these footprints shows that in the case of the series designated by cc the length of the step was close to 5 feet while the width of the trackway was about 2 feet 2 inches. The tips of the inner toes came, however, pretty close to the line of direction. It must be observed that in all of these tracks the toes are turned inward so much that the axis of the middle toe prolonged passes through the next imprint in front, made by the opposite foot. Now, I find no reason for supposing that in life the toes were so directed inward. None of the figures of Iguanodon so represent them; nor are the toes thus placed in any of the restorations of Trachodon. The explanation of the matter seems to be that the reptile, if reptile it was, was lounging leisurely along, with short steps, and, to keep its equilibrium, was swinging its body around a perpendicular axis passing through the pelvis, the tail being thrown in one direction, the trunk in the opposite. In this way the feet would be planted not far from the line of direction and pointing toward it. Had the animal been running, the feet would have been planted farther from the line of direction and with the toes directed forward.
Now, if these conclusions regarding the gait of the Upper Jurassic and Upper Cretaceous carnivorous dinosaurs are justified, is it probable that the Triassic Anchisaurus colurus, with an equally heavy abdomen and with less elongated and more primitive limbs, had the ability to walk, just as a bird does, accurately placing one foot directly in front of the other and under the center of gravity? It seems to the writer that we need more proof of it. If it could so walk, one might inquire what was the use of all the modifications undergone by the dinosaurs up to the end of the Cretaceous. It seems most probable that Anchisaurus walked usually in crocodilian or lacertilian style, with, however, the femora drawn somewhat more closely to the sides. Now and then, when in great haste and for short distances, it was probably able to progress bipedally in an awkward fashion. In the same category may be placed some of the European dinosaurs figured by Dr. v. Huene, such as Thecodontosaurus antiquus and the species of Plateosaurus. Others, such as Pachysaurus ajax and Massosaurus carinatus, probably walked more or less habitually on their hinder limbs, but with a wide trackway and with much swinging of the tail from side to side.
Dr. v. Huene's statement of his view of the manner of insertion of the femur has been quoted above. To the writer it seems probable that the whole proximal end of the bone constituted the head and was inserted into the acetabulum, as in lizards and crocodiles, and that the thigh was directed outward still more than Dr. v. Huene has supposed.
What then, made those bird-like tracks that are so abundant in the sandstones of the Connecticut River valley? Why not birds, indeed? Although remains of birds have not yet been found in Triassic rocks there can be little doubt that these animals had already freed themselves from the dinosaurs. Already long before the close of the Jurassic the hinder limbs of birds had, as we learn from Archæopteryx taken on its present form, with doubtless ability to plant its footsteps on the line of direction. This limb was at that early time far in advance of the hind leg of the dinosaurs of even the Upper Cretaceous; and it was doubtless even in the Triassic far in advance of the limb of the dinosaurs of that time. No bird remains have been found where those famous tracks occur, it is true. It is also true that nearly 100 kinds of tracks have been distinguished, while only 8 or 10 species of dinosaurs have been discovered in the North American Triassic; and of these only one has had its tracks identified. Therefore, it seems to the writer entirely reasonable to suppose that these bird-like tracks, even some of them that show the presence of fore feet and tail, were really made by birds. For if the birds divulged from the dinosaurs early in the Triassic their wings were as yet probably unfitted for continuous flight in the air. Many of them were probably running animals and some of them may still have retained a tendency to grow to a large size. Success in flying necessitated in later times a reduction in size of body. In the Trias the hands had not yet become reduced and transformed through the development of great pinion feathers, and they may have been at times applied to the ground in walking and resting. The tail was yet long, little befeathered, and might drag on the ground and leave a trail. And it must not be regarded as wholly certain that the tracks of large bipedal animals of later times were made by dinosaurs. There may have been in the Jurassic and the Cretaceous, as well as in the Tertiary, running birds of even greater size than the largest moa, whose foot was hardly inferior in size to that of many dinosaurs. On the other hand, such dinosaurs as Compsognathus and Hallopus may have walked like birds, but the remains of such are found in the Triassic no more than those of birds.
If now such Theropoda as Anchisaurus colurus, more advanced probably in every respect than the Sauropoda ever were, did not walk habitually erect, like mammals, on either two or four legs, but progressed either in more or less crocodilian manner on all fours or in a straddling way on the hind legs, is it probable that the sauropods ever walked high up on four legs in the jaunty manner in which they have been represented? It is to be considered that these great herbivorous reptiles possessed a huge abdomen, deep and probably broad, which extended backward and merged into the tail, necessitating the divergence of the relatively long femora. The outer surfaces of the pubic and ischiadic bones were clothed with great masses of muscles, as were too the insides of the femora. Assuming that the legs were as straight as they have been represented, the feet could have been hardly closer together than the knees, probably considerably further apart. A bulky animal walking thus could preserve its equilibrium only by either swaying the body from side to side, to throw it over the advanced foot, or throwing the tail toward that side. In the case of the fore foot the long neck might be used to preserve the balance. One might amuse and instruct himself by working out the movements of the animal according as it was walking, trotting, pacing, or perchance galloping.
The writer is not willing to assert that Diplodocus and its relatives never straightened out their legs, thus lifting themselves well above the ground, and never walked thus. Even the crocodiles have been known to do this, as a rare occurence10. In the U. S. National Museum there is a specimen of the Florida crocodile mounted in this position. The femora are directed forward and outward, the tibiæ downward. The feet are widely separated as a mechanical necessity. What is disputed by the present writer, is that this was the customary attitude of the sauropods; and their great bulk makes it doubtful if it was ever assumed.
The writer is of the opinion that the feet of the primitive dinosaurs had the inner digits somewhat more strongly developed than the median and outer ones; that is, they were entaxonic, not mesaxonic, resembling in this respect the feet of the crocodiles. A reason for this conclusion is found in the fact that all the feet of the sauropods are entaxonic and also the fore feet of the earliest known theropods. It is therefore more probable that the hinder feet of the latter reptiles became mesaxonic from an entaxonic condition than that their fore feet and both fore and hind feet of the sauropods should be transformed. That the manus of the theropods was entaxonic may be seen from Marsh's figure of the fore foot of Anchisaurus colurus and A. polyzelus (Dinosaurs N. A., pls. ii, iii) and from Dr. v. Huene's figures. Furthermore, the hinder feet of the early theropods present plain indications of a former entaxonic arrangement. The foot of Ammosaurus11 shows a very stout first digit, not greatly shorter than the others, while the second does not fall behind the third and fourth in diameter of the bones, little in length. The superiority of the second to the third seems to have been retained in Allosaurus. When the hind leg began to be drawn forward against the side and the weight of the body was thrown to a greater extent on the median digits a stimulus appears to have been given to the development of the third digit, while the first, relieved to some extent of its former duty, became reduced and turned backward.
In the later theropods the manus also became mesaxonic. This is seen in Marsh's restoration of the skeleton of Ceratosaurus (op. cit, pl. xiv). Mr. C. W. Gilmore, who has recently mounted this skeleton has shown me the remains of the one hand preserved. Most of the phalanges are missing. There are present four metacarpals, and there are no traces of the fifth in the rock. The first is considerably reduced, the second is largest. Thus, there is evidence that all the feet of the carnivorous dinosaurs became transformed from the entaxonic to the mesaxonic condition. It further appears that the sauropods retained the primitive condition of the feet, fore and hinder, more persistently than did the other groups of the order.
For reptiles that progress by creeping, having the humerus and the femur at right angles with the body in the middle of the step, the entaxonic condition seems most effective. It is found in the crocodiles and the turtles, being especially well displayed in the trionychids and the land tortoises. In reptiles the first digit is usually retained long after the disappearance of the fifth. In the lizards, however, the fifth is often longer than the first, a condition dependent perhaps on their habit of climbing about on rocks and trees. In the mammals, on the other hand, it is the first digit that earliest suffers reduction.
An attempt has already been made on a previous page to account for the origin of the bipedal habit in reptiles. Evidences are present, it is believed, which show that bipedalism in the dinosaurs was not due to specialization of the anterior limbs. If an examination be made as to the relative lengths of the fore and the hinder limbs in the carnivorous dinosaurs it will be found that in Anchisaurus colurus the fore limb is about three-fourths as long as the hinder; in Plateosaurus quenstedti about two-thirds; in Pachysaurus ajax, about one-half. These are Triassic dinosaurs. In Ceratosaurus, of the Upper Jurassic, the fore limb is only about two-fifths as long as the hinder. In Tyrannosaurus, of the Upper Cretaceous, the fore limb is diminutive, in case the humerus found with the specimen really belonged to it.12 As we have seen, the great pollex of the late Triassic forms had become much reduced in the Upper Jurassic species. Therefore, in place of specialization, the whole limb suffered degeneration. If now it be asserted that bipedalism in the theropods was occasioned by specialization of the fore limb for other purposes than locomotion, we shall have the case presented of an organ which, as soon as it was free to continue its specialization, began to degenerate. Without doubt however, the fore limb continued to be used for various purposes, just as the ostrich continues to use its diminutive wings.
9 Quart. Jour. Geol. Soc., x, 1854, 456, pl. xix. Back
10 Hornaday, Two Years in the Jungle, pp. 55, 266. Back
11 Marsh, op. cit. pl. iii, fig. 6. Back
12 Osborn. Bull. Amer. Mus. Nat. Hist., xxii, pl xxxix. Back
Dr. Hay concludes his extensive remarks in Part Five.