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 Two.
Certain principles must be regarded as indisputable. One of these is that primitively, in the common ancestor of the dinosaurs, the crocodiles, and the lizards, probably in the early dinosaurs themselves, the whole proximal end of the femur constituted the anatomical head. Another is that before there could be any such structures and conformations of these as we find at the hip joint of Allosaurus, for instance, or of Trachodon, every possible stage from the one just described must have been past through. Through countless generations the thigh must gradually have assumed a more and more forward position in habitual locomotion. While muscles and nerves were being trained to this end the femur must have been developing a projecting head, that part of the proximal end on the fibular side was being excluded from the acetabulum, and the rotation of the proximal end of the femur around a perpendicular axis was being changed to rotation around a horizontal axis, which in mammals would pass through both femoral heads. Now, as regards the hinder leg and the hip joint, at what stage in the long journey indicated above, do we find Diplodocus? Obviously those who believe that this animal ought to be set up on its legs in the way seen in drawings, plaster restorations, mounts of the actual bones, and the plaster facsimilies of the skeleton that are being distributed over the world, must hold that Diplodocus had reached practically the ultimate, or mammalian stage. The writer believes that it had attained only the first station in this journey.
A study of the femora of the sauropods shows that the proximal end varies somewhat in shape. Usually it is more or less truncated or it is slightly concave toward the fibular side and convex toward the tibial side. Figure 4 represents in outline a side view of the proximal half of the bone, as represented by Hatcher.
FIG 4 OUTLINE OF SIDE VIEW OF PROXIMAL END OF FEMUR OF DIPLODOCUS X 1/16.
As already stated, the proximal border is very rough, as shown by figure (Fig. 5) also taken from Hatcher.
FIG 5 PROXIMAL END OF FEMUR OF DIPLODOCUS. h, THE SO-CALLED HEAD
Undoubtedly this was covered by a thick layer of cartilage. Cope (Amer. Naturalist, xii, 1878, p. 84) says that if the layer of cartilage were ossified it would be an epiphysis, like that of the mammals. Figure 6 presents the same outline as does figure 4, but to it there has been added a dotted line which is intended to indicate the writer's view of the form of the upper end of the femur when the cap of cartilage was present.
FIG 6 PROXIMAL END OF FEMUR OF DIPLODOCUS X 1/16. THE DOTTED LINE SHOW THE LIMITS OF THE CARTILAGE.
The stage of development reached by the animal was that at which a femoral head was being developed on the tibial side of the bone and the fibular border was being freed from the articular cup. Although the whole proximal end may, in some genera, have been too broad to enter the cavity the greater part did so enter. Doubtless, when the leg was extended forward, a considerable part of the cartilage-covered surface on the fibular border was out of the cup, and when the leg was directed backward the rounded anterior part of the head was out. This is exactly what happens in the lizard and, for that matter, in most animals. The head of the femur of Diplodocus, compared with that of the crocodile, differed in having its long axis coincident with the plane through both condyles; while in the crocodile the head is twisted from the plane mentioned about 75°. Figure 7 represents the same humerus as figure 6, but lines are drawn across the head to show varying relations of the bone to the acetabulum. The line aa may be regarded as a section through the acetabulum when the leg is thrown far forward; bb, when the leg is at right angles with the body; cc, when the leg is thrown well backward. Of course, as the leg is swung from front to rear, the femur will turn also on its long axis.
FIG 7 PROXIMAL END OF RIGHT FEMUR, WITH ITS CAP OF CARTILAGE, AND HOIZONTAL SECTION THROUGH ACETABULUM. dd, SECTION OF ACETABULUM; aa, LINE CORRESPONDING TO dd WHEN LEG IS THROWN FORWARD; bb, LINE CORRESPONDING TO dd WHEN LEG IS AT RIGHT ANGLES WITH BODY; cc, LINE CORRESPONDING TO dd WHEN LEG IS THROWN BACKWARD.
As is well known, the acetabulum of the Sauropoda is widely open in the skeleton. I am not aware that any on has discussed the way in which in life this opening was filled. It seems improbable that it was shut simply by membrane, for this would have been too yielding to the pressure of the head of the femur, if inserted as generally supposed. It seems most probable that the opening was occupied by a mass of cartilage, an unossified portion of that common cartilage from which were developed the ilium, the pubis, and the ischium. This would have formed a firm concave bed on which the convex head of the femur could rotate. If the femur was inserted as the writer supposes it was, its pressure would have been exerted mostly against the bony side-walls of the acetabulum and but little against the tissue filling the inner opening.
In his splendid monograph on Die Dinosaurier der europäischen Triasformation Dr. v. Huene has presented numerous restorations of the Triassic carnivorous dinosaurs (Pls. IC-CX). In order to show the author's conception of their modes of progression, three species, Plateosaurus reinigeri, Thecodontosaurus antiquus and Anchisaurus colurus are restored each in two positions, walking on all fours and on their hinder extremities only. Dr. v. Huene has the following to say (p. 291) regarding the position of the hinder limbs:
Notwithstanding this explanation, one is struck by the very mammal-like position of the body and the limbs of these reptiles in the quadrupedal pose. Elbows and knees are drawn well towards the sides and the digits are directed straight forward. At least, the pose of these restorations is quite different from that of any living reptiles.
One of these species, Anchisaurus colurus was described by Marsh from the Triassic of the Connecticut Valley, and he publihed a restoration of the skeleton in his work The Dinosaurs of North America, Pl. IV. Dr. R. S. Lull7 has identified this dinosaur as the maker of the tracks known as Anchisauripus dananus (Hitch.) This identification is extremely interesting, in case it can be substantiated. The bones of the hind foot of Anchisaurus colurus fit accurately on the tracks named. These tracks are placed close to or on the line along which the animal was moving, the "line of direction" (Beckles), and there are, in several specimens known, no indication of either the fore feet or the tail.
A study of the various printed restorations of this species reveals an animal of elongated body, with limbs not greatly unlike those of a crocodile, the hinder legs being a little longer in proportion to the length of the body than in the crocodile, while the fore legs are about three-fourths the length of the hinder ones. In the crocodile the fore limb is little more than two-thirds as long as the hinder.8 As compared with the hind foot of the crocodile that of Anchisaurus is a little longer. Now, with this view of the creature, what is there in it to lead one to suppose that it erected itself on its hinder limbs, unless it were on rare occasions; and on such occasions would it not have borne itself as did the running lizard figured by Saville-Kent? What one is asked to believe is that it bore itself so loftily that it is never found to have put its hands on the ground or to have dragged its tail in the mud. Furthermore, this reptile walked with all the skill and the circumspection of the heron and the barn-yard fowl; for each foot was brought forward and placed very near or on the line of direction and at the same time the tail was jerked violently toward the same side, in order to bring the center of gravity over the advanced foot. The dinosaur in question seems to have had no other use for its tail than to serve as a counterpoise to the weight of the head and trunk.
Omitting the feet, the legs of most birds consist of three long segments, viz: the femur, the tibia, and the tarsometatarsus. The relatively short femora diverge downward so that the knees are almost always farther apart than are the great trochanters, sometimes much farther. Nevertheless the feet in walking are generally placed on the line of direction, a result brought about through the convergence of the elongated middle and lower segments of the two legs. If they are not brought close to this line, as in the short-legged ducks and geese, the walk becomes a waddle.
7 Mem. Bost. Soc., Nat. Hist., v. p. 487. Back
8 Dollo, Bull. Mus. roy. d'Hist. nat. Belgique, ii, 1883, p. 107. Back
Dr. Hay's argument waddles further along in Part Four.