Weight and Size Limits

The body proportions of animals are affected by a number of considerations, probably the most important of which are the ratios of size to mass. If two individuals were to have exactly the same shape, but one was twice as long as the other, all other linear dimensions would be double those of the smaller individual. The surface area would, however, be the square of the surface area of the smaller individual, while its mass would be cubed. Thus, if a reptile 3 m in length with a surface area of 4 m2 weighed 150 kg, a similarly shaped individual 6 m long would have a surface area of 16 m2 and would weigh 3.375 tonnes. This huge increase in mass relative to linear dimensions invokes one of the many problems involved in scaling (Schmidt-Nielsen 1984; Alexander 1997b). Some consequences of size are not closely related to the laws of physics. For instance, the effectiveness of an eye depends upon is absolute size, not on the size of its possessor. Consequently, larger animals tend to have smaller eyes relatively to their body mass. The same applies to brains. There is indeed a rough correlation between the size of its brain and the intelligence of an animal. Intelligence, however, is proportional more to the surface area of the brain than to its weight, so that mere percentage brain-weight is no criterion of intelligence. The dinosaurs may not have been quite so slow, unresponsive and silly as is sometimes thought! (see Buchholtz 1997).

Greater weight involves considerably stronger bones. When an animal stands on land, stresses are set up in the bones and muscles that support its weight. Whereas the cross-sectional areas of bones and muscles in isometric animals are proportional to (body weight)0 67, the stresses due to weight are proportional to (body weight)0 33. Consequently, since large animals are constructed of the same materials as their smaller relatives, their bones and muscles must be disproportionately thick in order to withstand the same stresses,

Dinosaurier Black And White
■ Fig. 66. Diplodocus (Sauropoda; Upper Jurassic; length ca. 25 m). (Cloudsley-Thompson 1994)

or they must stand and walk in such a way as to minimise stresses, or both. This problem does not arise, of course, in the case of aquatic animals whose weight is supported by buoyancy (Alexander 1971). The ability of bone to resist compression, the relationship between mass and surface area, as well as various physiological problems, including thermal ones (Sect. 7.5), also affect larger animals (Cloudsley-Thompson 1977).

Many Mesozoic reptiles were extremely large and heavy. The Diplodocidae ('double beams'), for example, were enormous plant eaters and included some of the longest of all dinosaurs. Among the best-known genera are Apatosaurus (= Brontosaurus), Atlantosaurus, Brachiosaurus, Diplodocus (Fig. 66), and Seis-mosaurus the 'earth shaker' (Gillette 1994). Despite the fact that they were relatively lightly built - their backbones contained deep weight-reducing hollows -they were, nevertheless, extremely heavy. They maintained an erect posture and had weight-bearing or graviportal limbs, as do elephants and rhinoceroses today.

The limb bones of Apatosaurus were massive. Most of the weight would have fallen on the hindlimbs so, for an individual weighing about 30 tonnes, each time one rear leg moved forward, the other must have carried about 20 tonnes (Fig. 67). From this shape, according to McNeill Alexander (1971), they would have been most likely to break at XX and XY in the figure. Alexander calculated

Kontur Kangura Wydruku

■ Fig. 67. a An Apatosaurus as it probably appeared in life, with a large African elephant drawn to the same scale; b the femur; c the tibia and fibula of Apatosaurus showing forces that would just break them. (Cloudsley-Thompson 1978 after Alexander 1971)

from the tensile strength of bone that they would just be broken at these places by forces of 20 tonnes. If the muscles were strong enough, the animal could have taken steps at up to about 45° to the vertical with the femur almost horizontal. So the bones were quite strong enough, provided that excessively long steps were not taken and the dinosaur did not stumble. Fossil footprints show that the steps taken were 2.4 m in length, which is quite modest for an animal with hind legs more than 3 m long. No doubt larger genera would have taken relatively smaller steps. The larger they were, the less agile they must have been,just as elephants are less agile than smaller mammals (Alexander 1971).

Thulborn (1990) cited the following calculated speeds: Brachiosaurus (78,258 kg) 18 km/h; Apatosaurus (32,418 kg) 24 km/h; Diplodocus (10,562 kg) 12 km/h; Tyrannosaurus (6,895 kg) 23 km/h; and Triceratops (847 kg) 26 km/h. His figures and those based on the work of T. Garland were comparable, but those of R.T. Bakker were more than twice as high. A more recent analysis by Hutchinson and Garcia (2002),based on estimates of the extensor muscle mass needed to support its body weight compared with what modern birds and cursorial mammals possess, suggests that Tyrannosaurus (6,000 kg) could not have exceeded speeds of 29-36 km/h (8-10 m/s). (Earlier methods for estimating the speeds of dinosaurs are given below.)

At one time it was thought that the large sauropod dinosaurs had exceeded the weight limit, and must have spent all their lives with their weights supported by water. The discovery of the footprints of adult and baby dinosaurs beside one another along the Paluxy River in Texas has proved that Apato-

Archidiskodon
■ Fig. 68. Brachiosaurus (Sauropoda; Upper Jurassic; length ca. 25 m)

saurus excelsus (ca. 35 tonnes) must have been terrestrial (Bird 1944). Moreover, to have used their long necks as snorkels so that they could breathe while standing in deep water would have been a difficult process. In the case of Diplodocus, the lungs might have been 6 m below the surface. To have expanded its lungs against the pressure of water at this depth would have needed enormous chest muscles. It seems probable, therefore, that the largest of the sauropods would have kept clear of marshy places where they might have got bogged down (Alexander 1985,1989). Another of the reasons why the larger dinosaurs - such as Brachiosaurus brancai (Fig. 68), which may have weighed about 80 tonnes - were thought to have been aquatic is that their external nares were situated high on the skull, extending posteriorly to just in front of the orbit. This would have enabled their possessors to breathe with the body entirely submerged, as crocodiles do. However, the living reptiles with a narial structure most closely resembling that of the apatosaurs are terrestrial - ground iguanas (Cyclura spp.) and desert monitors (Varanus griseus). Again, in most truly aquatic reptiles - crocodilians, phytosaurs (Fig. 29), Champsosaurus (Fig. 30), and so on - the tail is a powerful propulsive organ. In contrast, the tail of the apatosaurs was slender, ending in a whiplash like that of many terrestrial lizards, past and present.

The vertebral column of the apatosaurs had several features associated with the support of the body on dry ground, and its rib cage was very deep, resembling that of elephants rather than of hippos. The limbs, too, were elephant-like in their proportions as we have seen, while the diversity of the sauropods gen-

Apatosaurus Elephant Size Comparison

■ Fig. 69a-l. Possible postures of some dinosaurs when browsing on foliage. Above Upper Jurassic. a Haplocanthosaurus, b Brachiosaurus (not a large one), c Camarasaurus, d Barosaurus, e Diplodocus, f Apatosaurus, g Stegosaurus, h Camptosaurus. Below Upper Cretaceous. i Alamosaurus, j Parasaurolophus, k Triceratops, l Ankylosaurus. The Jurassic apatosaur and stegosaur are feeding on cycads and conifers; the Cretaceous apatosaur and low-feeding beaked dinosaurs are browsing on angiosperms. (Adapted from Bakker 1987)

■ Fig. 69a-l. Possible postures of some dinosaurs when browsing on foliage. Above Upper Jurassic. a Haplocanthosaurus, b Brachiosaurus (not a large one), c Camarasaurus, d Barosaurus, e Diplodocus, f Apatosaurus, g Stegosaurus, h Camptosaurus. Below Upper Cretaceous. i Alamosaurus, j Parasaurolophus, k Triceratops, l Ankylosaurus. The Jurassic apatosaur and stegosaur are feeding on cycads and conifers; the Cretaceous apatosaur and low-feeding beaked dinosaurs are browsing on angiosperms. (Adapted from Bakker 1987)

erally also argues for terrestrial habits (Fig. 69). For these and other reasons, it is now accepted that the deep chest, long limbs, and long necks of all the sauropod dinosaurs were adaptations for browsing on land, and were not related to an amphibious or aquatic existence (Fig. 70; Bakker 1971b).

For estimating sizes and size limits, the weights of dinosaurs are more important than are their heights or lengths. Unfortunately, there is considerable uncertainty about calculating the masses of dinosaurs. For example, E.H. Colbert (1962a) estimated the weight of B. brancai as 78 tonnes - using the model method. In 1989, Alexander's estimate was 47 tonnes, but the previous year G.S. Paul had calculated that it was only 32 tonnes. The reason for these differences is that the various authors based their calculations on different fossil individuals. Unmounted limb bones of an Apatosaurus from Tanzania are about 30% larger than those on which Colbert based his calculations! Furthermore, some authors used models in which the dinosaur represented was a bulky animal; others used skinny models. To add to the uncertainty, the densities of dinosaurs are not known in the first place. It also has to be assumed that dinosaur bone was about as strong as the bones of modern birds and mammals. This is probably the case, but we cannot know for certain (Alexander 1989,1997a).

Bakker (1978, 1987) claimed that not only did the sauropod dinosaurs use their long necks to feed from the tops of trees as giraffes do, but that some of them also reared up on their hind legs propped by their tails (Fig. 68). As

Diplodocus Skeletons Drawing

■ Fig. 70A-D. Comparison of hippo (left) apatosaur (centre) and elephant (right). A Cross section of thorax of hippo, apatosaur (Diplodocus) and elephant (Loxodonta). B Outlines of right side of skeletons drawn to same acetabulum-to-shoulder length; r hippo-like rhino (Teleoceras); h hippo (Hippopotamus); c apatosaur (Camarasaurus); b apatosaur; m elephant (Mastodon); a elephant (Archidiskodon). Full length of apatosaur (Brachiosaurus) necks not shown. C Section through elbow to show orientation of articular surfaces. Left Hippo (Hippopotamus); centre apatosaur (Camarasaurus); right elephant (Elephas). A Axis of elbow facets on humerus. U axis of facets on the radium and ulna. D right forefeet of hippo (Hippopotamus), apatosaur (Diplodocus), elephant (Mastodon); M metacarpals; P phalanges. (Cloudsley-Thompson 1978 after Bakker1971b)

■ Fig. 71. Stegosaurus (Ornithischia; Upper Jurassic; length ca. 6 m). (Cloudsley-Thompson 1994)

Beverly Halstead pointed out in a review of Bakker's, The Dinosaur Heresies (1987), the evolution of dinosaur stance and gait was worked out by Alan Charig as long ago as 1965, but is presented without acknowledgement. In order to test the validity of Bakker's claims, Alexander (1985) tried to estimate the positions of the centres of gravity of sauropods and some other dinosaurs to see if it would have been possible for them to have reared up on their hind legs. He argued that the densities of dinosaurs were probably nearer to 1,000 kg m-3 than to 900 kg m-3, as Colbert (1962a) had assumed, and he concluded that both Diplodocus (Fig. 66) and Stegosaurus (Fig. 71) supported about 80% of their weight on their hind feet, and only about 20% on their forefeet. They would therefore both have been capable of rearing up on their hind legs. The trunk of Triceratops (Fig. 72) probably tended to sag and would have required tension in the hypaxial muscles to prevent this. Diplodocus, on the other hand, had no tendency to sag because of the counterbalancing weight of the neck and tail. The neck was presumably supported by a strong nuchal ligament running through the forked neural spines of the cervical and dorsal vertebrae. The leg bones of Apatosaurus were probably strong enough to have permitted the degree of agility shown by modern elephants. Those of Diplodocus were too weak for running while those of Triceratops were strong enough for even greater agility than that of elephants. The pressures on the soles of the feet of large biped dinosaurs were about the same as those of cattle, but higher pressures acted on the feet of sauropods.

Dodson (1990) agreed that sauropods may have reared up on their hind legs on special occasions such as when fighting or reaching for the highest leaves,but he thought it unlikely that they would have reared up very often because, if the head was held high above the heart, an extremely high blood pressure would

Lizard Mesozoic

■ Fig. 71. Stegosaurus (Ornithischia; Upper Jurassic; length ca. 6 m). (Cloudsley-Thompson 1994)

Lizard Mesozoic
■ Fig. 72. Triceratops (Ornithischia; Upper Cretaceous; length ca. 9 m). (Drawing by Anne Cloudsley)

have been necessary to maintain an adequate supply of blood to the brain. Clearly, the cardiovascular pressures in sauropods would have been enormous. Hohnke (1973) pointed out that a specimen of Brachiosaurus (Fig. 68) in the Berlin Museum showed a vertical blood column of about 6.5 m above the heart. This is about twice that of a giraffe, and would have required a ventricular pressure of over 500 mm of mercury (Hg) just to support the column! (Alexander 1997a). Alternatively, the apatosaurs might not have raised their heads high or, if they did, they might have endured a temporary cessation of blood flow to the brain (Seymour 1976).

Pregnancy And Childbirth

Pregnancy And Childbirth

If Pregnancy Is Something That Frightens You, It's Time To Convert Your Fear Into Joy. Ready To Give Birth To A Child? Is The New Status Hitting Your State Of Mind? Are You Still Scared To Undergo All The Pain That Your Best Friend Underwent Just A Few Days Back? Not Convinced With The Answers Given By The Experts?

Get My Free Ebook


Responses

  • lukas
    How to calculate weight of a dinosaurs from its footprint?
    8 years ago

Post a comment