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 Four.
Various opinions have been expressed regarding the origin of the Sauropoda. Marsh13 expressed the opinion that the group included the most primitive forms of dinosaurs. Baur14 held that the Sauropoda had no close relationships to the other reptiles usually classed with them as dinosaurs. Osborn15 believes that it is possible to derive the sauropod type from a primitive quadrupedal theropod type. In his work already so often quoted, Dr. v. Huene expresses his view that the sauropods were derived from the carnivorous dinosaurs. He sums up his conclusion as follows (p. 351):
Regarding the time of origin of the Sauropoda Dr. v. Huene has the following to say (p. 351):
Dr. v. Huene calls attention to the numerous characters common to the Theropoda and the Sauropoda, and he believes that the latter inherited these common characters from the former suborder. Such a derivation would, the present writer holds, require extremely important modifications in the structure of the early Theropoda. The hind foot had, at the end of the Trias, become decidedly mesaxonic, with the hallux greatly reduced and probably somewhat turned backward. To create the foot of Diplodocus, for example, the hallux and the second digit must have been stimulated to increased growth; that is, the foot must have been made entaxonic; whereas, the upright gait that is usually attributed to Diplodocus ought to have increased the size of the middle digits and further reduced the hallux. The metatarsals that had become lengthened had to be shortened. The fore limb, that in the late Triassic theropods had become reduced in length, sometimes greatly so, must have taken on renewed vigor and increased size. All the modifications that had been attained and all the tendencies established that looked toward making bipeds out of these theropods had to be reversed.
Probably little or no importance can be attached to the fact that no remains of sauropods have yet been encountered in the Triassic deposits. It is certain that but a small proportion of the animals that made those Connecticut Valley tracks have left us other traces of their existence. Then, it is extremely probable that comparatively few of the residents of that region were accustomed to parade on those desolate and dangerous tidal flats. The sauropods especially, being slow-footed plant-eaters, would naturally have sought localities where there were fewer long-legged enemies and where the grazing was more satisfying.
To the writer, therefore, it appears most reasonable to suppose that the Sauropoda were a more primitive stock than the Theropoda and that the latter were derived from the early Triassic representatives of the former. Those primitive sauropods were no doubt far smaller than any of the group that are known to us. They probably had shorter necks, although with no fewer vertebræ; the vertebræ were less complexly constructed than those of their Jurassic descendants, and fewer of these had coösifised to form the sacrum. The digits, too, were probably longer and the outer ones were less reduced. We can hardly doubt they crawled on their bellies.
The conviction has been expressed that bipedalism in the dinosaurs was caused by the relative reduction of the fore limbs. On the other hand, the writer believes that bipedalism among the birds was the result of specialization of the fore limbs. These different tendencies gave the signal for the parting of the dinosaurs and the birds. The birds were the gainers by the separation. They secured all that the dinosaurs got and far more besides. The two groups separated at an early period, early in the Triassic, possibly even in the Permian. It was undoubtedly at a time when the members of neither the one group or the other had begun to walk on the hinder legs only. The feet, fore and hinder, were yet entaxonic. The hinder fifth digit was probably somewhat reduced, while the hallux was large and directed forward. Not until after the divergence of the two groups did the legs of the birds begin to be turned against the flanks and the body to be lifted from the ground. As greater and greater pressure began to be thrown on the middle digits the hallux began to be dwarfed and to be relegated to the hinder part of the foot. Archæopteryx shows that the hand had been entaxonic, for in it the two outer digits had wholly disappeared; while the pollex, though somewhat reduced, was yet large and functional.
It seems quite certain that the differentiation of the fore limb was initiated by the appearance of incipient feathers in the form, perhaps, of enlarged scales, which stood out from the ulnar side of the arms. The presence of these feathers, or scales, led to the flapping of the wings in the air, not conversely. Perhaps the individuals on which these rudimentary feathers first appeared were accustomed to clamber about over rocks and shrubs and the limbs of trees. Possibly the primitive birds, although not more than many lizards, strictly arboreal, often found safety and repose amid the branches and leaves of the Triassic ferns, calamites, and conifers. Possessing a fringe of feathers on their arms, they soon found these of advantage when they were running or making leaps to catch their prey or to escape capture by their enemies. When once they had made this discovery, the race entered on the conquest of the realms of the air.
It will be observed that the writer, in opposition to Dr. Francis Nopcsa16 holds that the primitive birds became bipedal while they were learning to fly and because of it, instead of becoming so long before the flying habit was initiated. It will be observed that the fore limbs of Dr. Nopsca's "Pro-avis" are already greatly reduced, and it might be questioned whether such limbs could be rejuvinated. It is certain that the ostriches have for untold generations been flapping their wings, to aid in running, but these limbs have steadily degenerated.
As believed by Dr. v. Huene, the Orthopoda probably took their origin from the Theropoda. If the views expressed by the present writer are true or approach truth, birds came on the arena before either of the suborders of dinosaurs just named; and hence most of the characters which have suggested relationship between the birds and the dinosaurs, which characters have ben so clearly presented by Dollo and Nopcsa in the papers already quoted, have all arisen independently in the two groups as a result of their starting from the same goal and speeding in nearly the same direction. On the other hand, the sauropods are nearest the stock from which sprang the birds, and it is in their skeletons that we must seek for the primitive common characters.
To the writer it seems probably that the avidinosaurs were not amphibious animals, but dwellers on the land. It is not likely that wings were developed on animals that lived much in the water. The Theropoda and the Orthopoda continued to inhabit the land, although this did not prevent them from seeking their food in swamps or from refreshing themselves in the water. After the sauropods had attained such bulk that locomotion on the land became troublesome they betook themselves to the streams, in order to enjoy the advantages of easier transportation; and then they became still more massive. Had they originally been aquatic and had they continued so, their feet would have remained more like those of crocodiles, less digitigrade and less shortened than they were in Diplodocus.
In his paper on the relationships of the birds and the dinosaurs17 Professor Osborn says:
In formulating this apparently important generalization Professor Osborn did not qualify it with the statement that most of the so-called tridactyl animals are really tetradactyl, the hallux being present and usually functional. Nor could he have had before him the skeleton of any of the sloths, animals that are strictly tridactyl behind, but which are neither bipedal nor endowed with great speed. Tridactylism prevailed among the extinct horse-like perissodactyls and is a characteristic of modern tapirs. On the other hand, there may exist a swift bipedal progression independently of tridactylism. The ostrich makes rapid headway with only two toes, one might almost say, with a toe and a half. The kangaroos are wonderful bipedal leapers, whose functional digits are reduced to two, the fourth and the fifth. Man may be justly counted among the swift runners, trained individuas making their mile in four and a quarter minutes, and he possesses a pentadactyl entaxonic foot. No bipedal artiodactyl is recalled, but, as illustrating a possibility, one must not forget to mention Pan, the shepherd god of old Arcady. From which considerations it may be concluded that the bipedal rapid runners have adopted no standard form of foot.
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Accompanying the present paper is a drawing (Pl. I) which is intended to represent the habits of Diplodocus, especially as regards its habitual pose of body and its manner of locomotion, as conceived by the writer. This drawing was executed by Miss Mary Mason Mitchell, after consultation with the author of the paper. Two individuals are in the foreground. One is collecting food from the surface of the water; the other has the head high in the air and is jealously regarding the approach of another, which is swimming. In the far distance is a fourth specimen lying stretched out at full length on the bank.
In the paper published by Mr. Ballou, referred to on page 15, there is a figure which represents a group of four individuals of Amphicœlias latus, a dinosaur closely related to Brontosaurus and attaining a length of from 60 to 80 feet. These animals are shown as walking about on the bottom of a river, feeding on the vegetation there and rising on their hind legs to reach the air. The idea here suggested is adopted by Professor Osborn18 as correct. Mr. Knight, under Professor Osborn's direction, has made a restoration of Brontosaurus19 in which the same idea regarding the habits of the sauropods is inculcated. In this restoration, a number of individuals, otherwise invisible, are sticking their heads out of the water. The ability of any large animal to walk thus submerged must depend on its having a massive skeleton, as have the hippopotamus and the manatee. In Diplodocus, to the contrary, almost every conceivable device has been employed to reduce the weight of the skeleton. The great vertebræ contain large and small internal cavities, while externally the processes are carved into thin plates and buttresses and the centra are deeply excavated on each side. Moreover, as has been shown by Hatcher,20 the limb bones are hollow. It would seem to have been hardly more possible for Diplodocus to walk about immersed in water than it would be for a man to do the same. Even if the reptile could have remained sunken, any pressure by the feet in the effort to walk would have sent it to the surface.
After the text and the drawings of this paper had been completed the writer received the Scientific American of November 6, 1909, in which is printed a popular article on the attitude of Diplodocus. In this article mention is made of a paper on this subject recently published by Dr. Gustav Tornier of Berlin, a paper not previously seen by the present writer. Unfortunately too, he has not seen the original papers of Messrs. Drevermann and Boule. No numbers of the Umschau, of Frankfort, for the present year are accessible.
13 Dinosaurs N. A., p. 164. Back
14 Amer. Naturalist, xxv, p. 450. Back
15 Nature, vol. 73, 1906, p. 284. Back
16 Proc. Zoöl. Soc. London, 1907, p. 234. Back
17 Amer. Naturalist, xxxiv, 1900, p. 796. Back
18 Bull. Amer. Mus. Nat. Hist., x, p. 220. Back
19 Amer. Mus. Jour., V. p. 68. Back
20 Mem. Carnegie Mus., i, p. 53, fig. 23. Back
Coming soon: Dr. W. J. Holland, director of the Carnegie Museum, takes the proponents of sprawling sauropods to task for their crimes against anatomy. Stay tuned!