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 One.
If the mammal-like gait of Diplodocus be insisted upon on the ground of straightness of the femur it may be pointed out, as I did in the article in the American Naturalist, that the femora of sphenodon and of lizards, animals that creep, are straight. If it be contended that it is in the heavy-bodied animals that a straight femur is correlated with a lifting of the body from the ground during locomotion, it may be permitted to recall that the femora of Allosaurus and Tyrannosaurus, great carnivorous dinosaurs, are distinctly bent. The femora of Trachodon are straight, while those of Camptosaurus and Laosaurus are curved. Curvature of the femur seems, therefore, to have no relation to size of body or erectness of pose. The femora of crocodiles, little and great, are curved; as were too those of their predecessors, Aetosaurus, of the Triassic, and of Alligatorellus, of the Jurassic, the former with femora hardly four inches long, the latter with these bones about an inch in length.
Diplodocus has been erected on column-like legs partly because it has been supposed that the great weight of its body required this. However, the legs of animals are not straight in proportion to the weight of their bodies. The legs of the largest camels seem not to be straighter than the legs of the llamas. Some rhinoceroses and some oxen have very heavy bodies; nevertheless, their femora lack much of being in line with their tibia and these much of being in line with the medapodials. Certainly it is not because of the immense weight of the body that the legs of a man are straight.
There must, of course, be a limit to the size of an animal that can move itself about on land, in whatever position; but it may be suggested that a reptile that could not walk about as crocodiles do, resting at least now and then, its body on the ground, could not well have erected itself when once it had lain down. That the largest crocodiles are far from the limit of active movement on the land may be judged from the following extract taken from W. Saville-Kent4.
It must be further taken into consideration that the weight of a crocodile 25 feet long, with short, thich neck, large head, long body, and heavy tail, would be much greater than that of a sauropod of the same length, in which most of the length is composed of slender neck and comparatively slender tail.
It is generally conceded that such carnivorous dinosaurs as Allosaurus, Dryptosaurus, and Tyrannosaurus, and such herbivorous forms as Trachodon and Camptosaurus walked bipedally erect. If now comparison be made of the femora of any of these with those of the sauropods great differences will be noted. The shaft of the former appears to be more elaborately modeled and to consist of finer and harder bone; all the articular surfaces are smooth and they carry the conviction that the original surfaces, barring a thin layer of cartilage, are preserved; there is a definite head, separated from the shaft by a distinct neck and nearly filling the acetabulum. In its low stage of differentiation the femora of the sauropods resemble greatly those of the crocodiles and are hardly above those of the lizards. They furnish no warrant for the belief that their predecessors walked in mammalian fashion.
The structure of the foot of Diplodocus indicates that this reptile walked in a way very different from that in which the bipedal dinosaurs walked. In the latter the foot had the third toe most strongly developd (mesaxonic); in the sauropods the two inner toes were the strongest, the third somewhat weaker, while the other two were greatly reduced. This difference of structure must have had its history and its meaning. That the feet of Diplodocus were shortened and more or less digitigrade indicates that they were employed for walking, not at all for swimming. The feet of the crocodiles are to be regarded as entaxonic, the inner digits being of stouter build, although slightly shorter than the third; but here the digits are elongated and webbed to assist in swimming. When the animal is walking, the pressure comes against prinicipally the inner side of the foot. The trionychid turtles have the three inner digits most strongly developed and clawed; the others are slender and unarmed. The clawed digits are, of course, the ones employed for excavating hiding places in the sand and mud and getting foothold in walking and running; and these turtles are, for moderate distances, rapid and powerful runners on the land and on the bottoms of streams.
It is true that the foot of man is entaxonic and is directed nearly forward, but its history is wholly different from that of the sauropod foot. It is certain that the ancestors of man were climbing animals, with hallux strongly developed and opposable to the other digits. Being later employed for locomotion on the ground, the foot underwent a transformation to its present form. The form assumed at any time by an organ must depend greatly on the form previously possessed. Doubtless the Sauropoda and the Theropoda started out with the same pedal outfit, and there seems to be no reason for supposing that the former passed through an arboreal stage and back into an ambulatory stage.
The position of the trochanter major of the sauropods is open to question and there are differences of opinion. Marsh5 regards as this trochanter the outer upper angle of the femur, including a part of the rough surface forming the proximal end of the bone. Hatcher's view (Mem. Carnegie Mus., I. p.46) appears to be the same. Osborn6 has identified as the trochanter the rough surface which descends for some distance below the upper end of the femur on the fibular border. Neither of these views seems to the writer satisfactory. If the femora of the Triassic dinosaurs described by v. Huene in his monograph, Die Dinosaurier der europäischen Triasformation, be examined it will be found that the trochanter in question is placed at a considerable distance below the head of the bone, on the dorsal surface, and near the fibular border. In the more highly specialized dinosaurs of the Jurassic the trochanter is a distinct process arising from the position described and ascending nearly to the level of the head. In such dinosaurs as Trachodon and Triceratops the trochanter has reached the outer upper angle of the femur, and is well separated from the head by a distinct neck. The writer believes that in the sauropods the trochanter occupied the same primitive position that it has in the Triassic Theropoda. It is not essential that it should be represented by a process or even by any unusual roughness, as is shown by the femur of the crocodile.
This being the case, what explanation is to be made of the outer portion of the rough surface on the proximal end of the femur? The writer believes that it forms a part of the head of the bone and entered into the acetabulum. The matter will be discussed. In order to ilustrate a possible position of the femur in the acetabulum a figure is here presented (Fig. 2).
FIG 2. LEFT ACETABULUM, CONTAINING SECTION OF PROXIMAL END OF FEMUR; THIS SECTION SHOWN BY HEAVY LINE. X 1/10; il., ILIUM; isch., ISCHIUM; pub., PUBIS.
This has been obtained by placing a section of the proximal end of the femur, taken from Hatcher's figure in Memoirs of the Carnegie Museum, vol. I, p. 46, in the acetabulum as shown in the same writer's figure in the second volume of the same Memoirs, plate IV, fig. 2. The so-called head of the femur is toward the left, against the pubic process. According to this figure, there was room in the acetabulum for the femur, standing at right angles with the pelvis, so that it could rotate on its longer axis and could swing backward and forward. Such movements would be required in case the reptile walked as does the crocodile. In the execution of these movements it would probably happen, as it does in lizards, that some part of the head would at times be outside of the acetabulum. in order to show the resemblance of this joint in the lizards to the one depicted, a drawing (Fig. 3) is shown of the acetabulum and head of the femur of Metapoceros.
FIG 3. ACETABULUM OF LIZARD METAPOCEROS, CONTAINING SECTION OF HEAD OF FEMUR. X 2. SECTION OF FEMUR SHOWN BY HEAVY LINE. ALSO SIDE VIEW OF FEMUR X 2. Fem., FEMUR; il., ILIUM; isch., ISCHIUM; pub., PUBIS.
However, the articulation at the hip was probably not effected in just this way. It appears that in some cases the proximal end of the femur is wider than the acetabulum. Dr. E. S. Riggs informs me that in Apatosaurus (Brontosaurus) and Brachiosaurus the upper end of the femur is about 23 inches wide, exceeding the fore-and-aft diameter of the acetabulum by 3 or 4 inches. I do not regard this fact as wholly irreconcilable with the view illustrated by figure 2, the head of the femur having sometimes a greater diameter than the acetabulum, as in the land tortoises. Nevertheless, I will not argue the matter. A somewhat different arrangement at the articulation is more probable.
4 Living Animals of the World, p. 547. Back
5 Dinosaurs N. A., Pl. XVI, fig. 3, t. Back
6 Mem. Amer. Mus. Nat. Hist., i, p. 211, fig. 14. Back
Dr. Hay continues his explorations into the acetabulum in Part Three.