Dinosaur Necks and Tails

Almost all dinosaurs had long tails and some also had long necks.

In an extreme case, Diplodocus had a neck 7 meters (23 ft) long (including the small head), a tail about 14 meters (46 ft) long and body only 5 meters (16 ft) long. This chapter asks how dinosaurs used long necks and tails.

I will start with necks. The longest are those of sauropods, which include the largest of all dinosaurs. Before the calculations about bone strength had been done, many people doubted whether the biggest sau-ropods could have supported their weight on land. They supposed that they must have lived in lakes, buoyed up by the water (figure 5.1). Their long necks might have served as snorkels, enabling them to breathe while standing on the bottom in deep water.

The sauropods in figure 5.1 look like Diplodocus. If they are, their lungs are 6 meters (20 ft) or more below the surface of the water. Their necks are long enough to reach the surface but it would be very hard for them to breathe air in, because the lungs would have to be expanded against a pressure difference of 6 meters of water. The snorkels used by human divers are only about 30 centimeters (1 ft) long. The dinosaurs in figure 5.1 would have needed enormous chest muscles, to breathe in.

It now seems clear that even the largest sauropods had legs strong enough for walking on land, so there is no need to imagine them living submerged in lakes. Indeed, it has been argued that they probably lived on dry land, keeping clear of marshy places where they could easily have got bogged down because of the enormous pressure on their feet (chapter 3). If they lived on land, as is now generally believed, the long neck cannot have been a snorkel. We must look for some other function.

The shape of Brachiosaurus suggests that it lived like a giraffe, eat-

Water Sauropod Reconstruction
FIGURE 5.1. Snorkcling sauropods, from Gregory 1951.

ing leaves from tall trees. Not only was its neck very long, but the fore legs were longer than the hind (a difference from other sauropods), so its head could apparently be raised to the remarkable height of 13 meters (43 ft). It was much taller than any giraffe (figure 1.1).

If Brachiosaurus carried its head so high, its brain would be about 8 meters (26 ft) above its heart. The heart would have to pump blood at very high pressure, to get it to the brain. If blood failed to get to the brain even for a short time, the animal would collapse unconscious. A fainting Brachiosaurus would come down with a tremendous crash.

Blood pressure is conventionally measured in millimetres of mercury because doctors used to use mercury manometers to measure human blood pressure. It will be more convenient here to use meters of water: a pressure of one meter of water equals 74 millimeters of mercury. Modern reptiles pump blood out of their hearts at pressures that are 0.5 to 1.0 meters of water above the pressure of the surrounding tissues. This pressure difference is needed mainly to drive blood through the capillaries, the fine blood vessels that permeate every tissue of the body. Brachiosaurus would have needed an additional pressure of 8 meters of water to raise the blood to the brain, making a total of 8.5 or more meters of water. (Strictly speaking, the additional pressure would be 8 meters of blood, not water, but this makes little difference because the densities of blood and water are only slightly different.)

The calculated total blood pressure, 8.5 meters of water, is much larger than the blood pressure of any modern animal. Most mammals, including ourselves, pump blood from their hearts with pressures of 1.5 to 2.0 meters of water, and even giraffes do it with pressures no higher than 4.3 meters of water. Giraffes need much less blood pressure than Brachiosaurus because their necks are so much shorter, carrying the brain only 3 meters above the heart.

You might think that the blood systems of giraffe and dinosaur necks could work like siphons, which do not need high pressures to drive liquids through them. However, you need a rigid tube to make a siphon because when it is working, the pressure inside it, near the top, is less than the pressure outside. Veins have flexible walls that would collapse if the pressure inside fell, so they cannot work as siphons.

It would be a mistake to argue that a blood pressure of 8.5 meters of water would be impossible, because it is so much higher than the blood pressures of modern animals. If giraffes were unknown, that kind of argument would lead to the conclusion that giraffes were impossible. However, the blood pressure calculated for Brachiosaurus does seem remarkable, and a very muscular heart would have been needed to produce it.

Brachiosaurus had long fore legs but Diplodocus and Apatosaurus had relatively short ones, and look much less like giraffes. In 1978 Dr. Robert Bakker suggested that they also may have fed from tall trees, rearing up on their hind legs to gain extra height (figure 5.2e,f). It may seem ludicrous to suggest that these enormous animals could manage such gymnastics, but the idea deserves careful consideration. Remem

Reared Barosaurus

FIGURE 5.2. Possible postures of some dinosaurs when browsing on foliage: (a) Haplocanthosaurus; (b) Brachiosaurus (not a very large one); (c) Camara-saurus; (d)Barosaurus: (e) Diplodocus-, (f) Apatosaurus-, (g) Stegosaurus: and (h) (feeding from the ground) Camptosaurus. From Bakker 1978. Reprinted by permission. Copyright © 1978 Macmillan Magazines Ltd.

FIGURE 5.2. Possible postures of some dinosaurs when browsing on foliage: (a) Haplocanthosaurus; (b) Brachiosaurus (not a very large one); (c) Camara-saurus; (d)Barosaurus: (e) Diplodocus-, (f) Apatosaurus-, (g) Stegosaurus: and (h) (feeding from the ground) Camptosaurus. From Bakker 1978. Reprinted by permission. Copyright © 1978 Macmillan Magazines Ltd.

ber that circus elephants can be trained to balance on their hind legs, and that the calculations in chapter 4 suggest that Apatosaurus may have been as athletic as elephants. The stresses in the bones, while standing on the hind legs, would probably be less for Apatosaurus than for elephants. This is because in its normal quadrupedal position Apatosaurus, unlike elephants, carried most of its weight on its hind legs (table 4.1). Standing on the hind legs alone would not increase the load on them very much.

It might be easy for Apatosaurus to support itself, once it was standing on its hind legs, but could it get itself into that position? It would have to get its center of gravity over its hind feet, to take the load off its fore feet. To discover whether that would be difficult we need to know where the center of gravity was. We already know that Apato-saurus and Diplodocus carried most of their weight on the hind legs, which implies that the center of gravity was nearer the hind legs than the fore. We could calculate its position from the data in table 4.1 but we can discover it more directly, by a simple experiment.

For this I used the same models as in chapter 4, the solid plastic ones with holes bored to represent the air-filled lungs. Their centers of gravity should be in approximately the same positions as in the living dinosaurs. I suspended each model by a thread tied to its head and photographed it in side view (figure 5.3a). The model was hanging motionless, in equilibrium, so its center of gravity must have been in line with the thread, somewhere on the line AB. Then I suspended the model from its back and took another photograph (figure 5.3b). The center of gravity must again have been in line with the thread, on the line CD. Finally, I superimposed the two photographs, making the outlines of the model coincide (figure 5.3c). The center of gravity was both on AB and on CD: it must have been at the intersection of the two lines.

Figure 5.4 shows centers of gravity located in this way. Diplodocus standing in the position shown had its center of gravity over the left hind foot. If the animal had moved its right hind foot forward and set it down beside the left one, both hind feet would have been under the center of gravity and it would have been easy for it to rear up on its hind legs.

Diplodocus could apparently have reared up easily because its long, heavy tail counterbalanced the front part of the body and brought the center of gravity well back. Brachiosaurus had a shorter tail and heavier fore quarters, so its center of gravity was further forward. It would have been harder for it to rear up and, as far as I know, neither Bakker nor anyone else has suggested that it did. With its long front legs and neck it could feed from great heights without rearing up.

Bakker suggested that Stegosaurus reared up on its hind legs to feed, and it seems likely that it could have done: its center of gravity was well back, near the hind legs. Triceratops had its center of gravity much further forward (figure 5.4) and might have found it difficult to rear up. However, elephants have their centers of gravity well forward, and can rear up. (Table 4.1 shows that the centers of gravity of elephants are far enough forward to put most of the weight on the front feet in normal standing.)

It is generally assumed that long-tailed sauropods such as Diplodocus and Apatosaurus walked around with their necks nearly horizontal. Most drawings show them in that position, as do the mounted skeletons in museums. We will think about the problem of supporting a long, heavy, horizontal neck.

I am going to suggest that these necks were supported in the same way as the necks of horses, cattle, and their relatives. These animals have a thick ligament called the ligamentum nuchae running along the backs of their necks (figure 5.5). Unlike most other ligaments it consists mainly of the protein elastin, which has properties very like rubber. It can be stretched to double its initial length without breaking and snaps back to its initial length when released.

The ligamentum nuchae is stretched when the animal lowers its head

Centre Gravity Dinosaur Models

FIGURE They are

5.3. Diagrams showing how to locate the center of gravity of a model explained in the text.

Bakker Centre Gravitation Dinosaurs
FIGURE 5.4. Outlines of dinosaurs, showing the positions of their centers of gravity.

to drink or graze, and shortens again when the head is raised. In experiments with deer carcasses, my colleagues and I found that the ligament was 1.4 times its slack length-when the head was raised to the position of figure 5.5a, and almost twice its slack length when it was lowered to the position of figure 5.5b. Notice that the ligament was stretched even when the head was high: I doubt whether a deer can get into a position that allows the ligament to shorten to the point of going slack. If you cut the ligament in a dissection the cut ends spring apart, as if you had cut a stretched rubber band.

Thus the ligamentum nuchae is taut in all normal positions of the neck. Its tension helps to support the weight of the head and neck. The tension and the supporting effect are greatest when the head is lowered (as in figure 5.5b) but even in this position the tension is not by itself enough to give all the necessary support: some tension in neck muscles is needed as well.

The ligamentum nuchae is a feature of hoofed mammals but something rather similar is found in birds, which are much more closely related to dinosaurs. Instead of a continuous ligament running the whole length of the neck they have a series of short ligaments connecting each neck vertebra to the next (figure 5.6a). These ligaments consist largely of elastin, like the ligamentum nuchae. They are stretched when the head is lowered, and shorten again when it is raised.

Figure 5.6c shows a bird vertebra in front view. The centrum is the main body of the vertebra, connected to the vertebrae in front and behind by intervertebral discs. Above it is a hole for the nerve cord and above that again is the neural spine. The elastic ligaments connect the front of one neural spine to the back of the next.

Diplodocus and Apatosaurus have neck vertebrae with V-shaped neural spines (figure 5.6d). I suggest that the V was filled by an elastin ligament that ran the whole length of the neck and back into the trunk. This ligament would have helped to support the neck while allowing the dinosaur to raise and lower its head.

I made some experiments to find out whether the idea was feasible. I cut off the head and neck of the Diplodocus model and measured their volume (as I had done with complete models in chapter 2), and calculated that the mass of the real head and neck would have been 1,340 kilograms. Their weight was this mass multiplied by the acceleration of gravity, 1,340 x 10 = 13,400 newtons. I suspended the amputated head and neck from threads to find their center of gravity, and have shown the weight acting at the center of gravity in figure 5.6b.

We can only guess how thick the ligament was, but it seems likely that it projected above the tops of the neural spines, as shown in figure 5.6d. If so its center line, at the base of the neck, was about 0.42 meter above the center of the centrum.

In figure 5.6b the weight is pulling the neck counterclockwise about the joint at its base and the tension in the ligament is pulling it clockwise. The weight acts 2.2 meters from the joint and the ligament tension 0.42 meters from it. By the principle of levers, the tension that would be needed to balance the weight is 2.2 x 13,400/0.42 = 70,000

FIGURE 5.5. The neck of a Roe deer in the alert position and lowered for feeding. The skeleton and the ligamentum nuchae (stippled) are shown.
FIGURE 5.6. Necks of (a) a turkey and (b) Diplodocus, showing the vertebrae and the elastic ligaments. The forces that acted on the neck of Diplodocus are also shown, (c) and (d) are front views of neck vertebrae of an ostrich and of Diplodocus.

newtons (7 tonnes force). The third force shown in the diagram is the force in the joint itself, where one centrum presses on the next.

The calculated tension may seen enormous, but the ligament was very thick. If it was as thick as in the diagram, its cross-sectional area was 40,000 square millimeters and the stress in it, for a force of 70,000 newtons, would be 1.8 newtons per square millimeter. This is more than the stress in the ligamentum nuchae of a deer with its head down (about 0.6 newtons per square millimeter), and would be enough, or nearly enough, to break ligamentum nuchae. However, this stress would act only if the ligament supported the neck without any help from muscles. If neck muscles took some of the load (as they do in birds) the stress in the ligament would be less. The suggestion of an elastin ligament seems feasible.

Remember that the ligament is only a guess, based on the structure of sauropod vertebrae and comparisons with modern animals. We do not know whether it existed, but the calculations seem to show that if it did it could have done a useful job. Only some sauropods had V-shaped neural spines that could have housed a continuous ligament but other dinosaurs may have had separate ligaments connecting each neck vertebra to the next, as in birds.

Now we will think about tails. We have already seen how a long tail may balance the front part of the body, enabling some dinosaurs to rear up on their hind legs. That function depends on the animal being able to lift its tail off the ground. If the tail lay limp on the ground, much of its weight would be supported directly on the ground and it would be little use as a counterpoise. Many drawings of dinosaurs show the tail trailing on the ground but few sets of footprints show the mark that would be made by a trailing tail (chapter 3). It seems possible that the thin end of the very long tail of Diplodocus trailed on the ground, but this would have little effect on the counterpoise function if this part of the tail was as thin as the slender vertebrae suggest.

Though Diplodocus would have to be able to stiffen its tail to raise it for use as a counterpoise, it would also have to be able to bend its tail to get into the position shown in figure 5.2. The tails of ornitho-pods may have been much stiffer. They have a crisscross arrangement of rods, on either side of the neural spines of their vertebrae, that seem to be ligaments or tendons turned to bone (figure 5.7). (You will find tendons that have turned to bony material whenever you eat turkey, as hard strips embedded in the meat of the lower leg.) Whenever complete skeletons of ornithopods are found, the tail is fairly straight (figure 5.7), suggesting that it was indeed stiff.

How stiff the tail was would depend on whether the rods were ligaments or tendons. Ligaments connect bone to bone, and if the tail vertebrae were connected by rigid bony ligaments the tail would be very stiff indeed. Tendons connect muscles to bones, and if the rods were tendons the tail could have bent up and down a little as its muscles shortened and lengthened. The rods look like tendons to me. Modern mammals have similarly arranged (but non-bony) tendons in their tail muscles.

Bony ligaments or tendons are most prominent in the duck-billed

FIGURE 5.7. Skeleton of Iguanodon, in the position in which it was found Notice the crisscross tendons in the back and tail. From Norman 1 980.
FIGURE 5.8. An ostrich, a man, and a small bipedal dinosaur [Hypsilophodon] running, showing the positions of their hips and their centers of gravity: O hip. • center of gravity.

dinosaurs but have also been found over the hips of Triceratops, in the tail of the theropod Deinonychus and in several other dinosaurs.

The tail must have had a major effect on the running movements of bipedal dinosaurs. Figure 5.8 compares a dinosaur with two modern bipeds, a bird and a human. The dinosaur, with its long heavy tail, has the center of gravity close to the hips, but the bird, with only a tuft of feathers for a tail, has its center of gravity well in front of the hips. The human has no tail but the center of gravity is close to the hips because the trunk is erect. If these bipeds are not to fall over, the average positions of their feet, while on the ground, must in each case be under the center of gravity. Each animal sets the foot down in front of the center of gravity and does not lift it until it is behind the center of gravity. The bird manages by holding its thigh almost horizontal and swinging the leg from the knee. It seems likely that bipedal dinosaurs moved their legs more like people than like birds, because of the position of the center of gravity.

The tails of kangaroos are thick and heavy, but they are flexible, and they swing up and down as the animal hops. If flexible tails are suitable for kangaroos, why should stiff ones have evolved in bipedal dinosaurs? The answer may depend on the difference between running and hopping and on a basic principle of mechanics.

I will use an example from gymnastics to explain the principle of conservation of angular momentum. A gymnast on a trampoline can set his body spinning, while in mid air, by moving his arms. By swinging his arms to the left he makes his body spin to the right. A rotating object has a property called angular momentum that depends on the masses of its parts and on its rate of rotation. The principle says that the gymnast cannot change his total angular momentum without pushing on something, but he can set his arms rotating in one direction and his trunk in the other so that the angular momentums in the two directions cancel out.

As a kangaroo flies through the air in a hop, it swings its legs forward ready for the next landing. In figure 5.9 its legs have to be given counterclockwise angular momentum so its trunk and tail must get matching clockwise momentum. If the tail were rigid the body would rock up and down quite a lot (through 13-18°, according to my calculations]. Actually the tail bends so that it swings up and down through a large angle and the trunk through a much smaller one. These tail movements probably cost the animal very little energy, because they do not have to be powered by muscles. The tail vibrates passively because its long fnon-bony) tendons make it springy.

If the trunk rocked a lot, with the head rocking with it, it might be difficult for the animal to keep watch for danger as it hopped. The springy tail may give the kangaroo an advantage, by reducing the rocking movements of the trunk. A dinosaur that ran rather than hopped would not need such a mechanism because, in running, one leg swings back while the other swings forward and there is little tendency for leg movements to rock the body. A springy tail may be best for a hopper but a stiff one seems fine for a runner.

The tails of some dinosaurs may have served as weapons, as well as for balance. It has often been suggested that the long tapering tails of Diplodocus and Apatosaurus would have made formidable whips. They could have been used to strike a predator, and I wonder whether they could also have been used to make a terrifying noise. When a circus ringmaster cracks his whip, he flicks it so as to make its tip move supersonically. Is it too wild a speculation to wonder whether Diplodocus could crack its tail?

FIGURE 5.9. Diagram of a kangaroo hopping

Dyoplosaurus

FIGURE 5.10. Euoplocephalus or Dyoplosaurus, an ankylosaur with a club on its tail (a) in side view and (b) in top view. Length six meters. From Carpenter (1982).

FIGURE 5.10. Euoplocephalus or Dyoplosaurus, an ankylosaur with a club on its tail (a) in side view and (b) in top view. Length six meters. From Carpenter (1982).

There are other dinosaur tails that seem more obviously to have been weapons. Stegosaurus had sharp spikes on its tail up to half a meter (20 in) long. Some of the ankylosaurs had big lumps of bone at the ends of their tails and presumably used them as clubs (figure 5.10).

This chapter has told a series of short stories. Sauropods are unlikely to have used their long necks as snorkels. Very large breathing muscles would be needed for snorkeling at any substantial depth, and in any case it seems unlikely that they lived in water. It seems more likely that they lived on land, raising their necks to feed from high branches. Their hearts would then have had to pump blood out at high pressure, to get it to the brain. Brachiosaurus probably fed like a giraffe but Diplodocus and similar sauropods may have reared up on their hind legs to get their heads high. Their long, heavy tails would probably have made it easy for these sauropods to get their hind feet under their center of gravity. The long necks of Diplodocus and Apatosaurus may have been supported by an elastin ligament running through the V-shaped notches in their neural spines.

The tails of duck-billed dinosaurs had bony tendons or ligaments alongside the neural spines and seem to have been stiff. The effect of the tail on the position of the center of gravity probably made bipedal dinosaurs move their legs more like people than like birds. The springy tails of kangaroos may help to prevent the body from rocking too much during hopping but stiff tails seem suitable for running bipeds. The tails of some dinosaurs seem to have served as weapons.

Principal Sources

Chapter 5, like chapter 4, is based largely on Alexander (1985), with some of the experiments modified. The data about blood pressure are from Hohnke (1973). The suggestion that some sauropods may have reared up on their hind legs was made by Bakker (1978). The elastin ligaments in the necks of birds were investigated by Bennett and Alexander (1987). Galton (1970) described the stiff tails of duck-billed dinosaurs. Alexander and Vernon (1975) discussed the oscillations of the tails of kangaroos.

Alexander, R. McN. 1985. Mechanics of posture and gait of some large dinosaurs.

Zoological journal of the Linnean Society 8 3:1 -25. Alexander, R. McN. and A. Vernon, 1975. The mechanics of hopping by kangaroos

(Macropodidae). journal of Zoology 1 77:265-303. Bakker, R. L. 1978. Dinosaur feeding behavior and the origin of flowering plants.

Nature 274:661-663. Bennett, M. B. and R. McN. Alexander, 1987. Properties and function of extensible ligaments in the necks of turkeys [Meleagris gallopavo) and other birds, journal of Zoology 21 2:275-28 1. Carpenter, D. 1982. Skeletal and dermal armor reconstruction of Euoplocephalus lulus. Canadian journal of Earth Sciences 19:689-697. Galton, P. M. 1970. The posture of hadrosaurian dinosaurs, journal of Paleontology 44:464-473.

Gregory, W. K. 1951. Evolution Emerging. New York: Macmillan. Hohnke, L. A. 1973. Haemodynamics in the sauropods. Nature 244:309-310. Norman, D. B. 1980. On the ornithischian dinosaur Iguanodon bernissartensis of Bernissart (Belgium). Memoires de l'Institut Royal des Sciences Naturelles de Belgique 178:1-1 03.

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Responses

  • Safa
    How long tails help dinosaur move?
    8 years ago
  • sandra
    When the long necks were extinct?
    7 years ago
  • belinda
    How does a diplocous look?
    7 years ago
  • Aija
    What places had a lot of dinosaurs with long necks?
    7 years ago
  • tom
    Which dinasaur looks like a diplodocus but has a shorter tail?
    2 years ago
  • frans
    How giraffe keep itself from falling in centre of gravity and equilibrium?
    2 years ago

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