-
32. Engineering a Dinosaur
- Indiana University Press
- Chapter
- Additional Information
637 Engineering a Dinosaur Donald Henderson 32 Although they may sometimes appear to be creatures of fantasy, dinosaurs existed as physical entities in a physical world. They were subject to external factors such as gravity, air and water pressures, and temperature changes, as well as internal factors such as blood and gas pressures, muscle tensions, and the stresses and strains that occur in bones, tendons, and ligaments. The material properties of an animal’s tissues, and the internal and external physical forces that they are subjected to, determine the limits for what is possible for the organism in terms of growth, body support, movement, feeding methods, and life history. The study of organisms as physical systems, subject to mechanical constraints, is the field of biomechanics. Many of the factors that zoologists consider when studying the biomechanics of living animals are based on living soft tissues, for example, the sizes and relative proportions of the muscles, the proportions of the different muscle fiber types in different muscles, and forces that these muscles can produce. The fossil record has left us, with rare exceptions, just the bones of dinosaurs, but it is from these bones that we must make inferences as to what the dinosaurs were like as living, functioning organisms. Fortunately, potential sources of information on the now-vanished soft tissues of dinosaurs do exist. Dinosaurs, birds, and crocodilians are all members of a larger group of animals, the Archosauria, and with what we know of the skeletons and anatomy of crocodiles and birds we can make plausible inferences about the anatomy of extinct dinosaurs. Another important source of information comes from studying the biomechanics and ecology of large, extant, terrestrial mammals. The skeletons of elephants, giraffes, and rhinos show many features in common with those of dinosaurs, and these similarities are interpreted to be examples of convergent evolution–the biomechanical challenges of being a 5-tonne animal are similar for both mammals and dinosaurs. Despite the long period of independent evolution of the mammal and dinosaurs, they must both respond to the same physical constraints. This chapter presents an overview of various aspects of the biomechanics of dinosaurs. After a brief discussion of the importance of body mass as it relates to dinosaurs, the chapter follows a tour of the skeletons and bodies of various dinosaurs by progressing from the tail to the head. The immense size of many dinosaurs, as the largest land animals of all time, is a distinguishing feature to the group. Size can be measured in several ways: height, length, width, and weight. As linear measures, the first three of these quantities are statements about the dimensions of the animal in a single direction and ignore the other two dimensions. Quoting just a single Dinosaurian Body Mass Donald Henderson 638 linear dimension has the potential to be vague or misleading when describing animals as unusual as dinosaurs. The height of dinosaur–the distance measured from the top of the head from the ground–is a subjective quantity , as it would depend on how the animal was restored. As an example, the Late Jurassic sauropods Diplodocus carnegii and Brachiosaurus brancai both had total lengths of about 24m (Fig. 32.1). However, the neck of Diplodocus is traditionally restored with a very gentle upward arc, which results in the head being at the same level as tips of the sacral neural spines, giving the animal a “height” of about 4 m. In contrast, the neck of Brachiosaurus is usually restored as being subvertical, which results in the animal having a “height” of 13 m. Similarly, although Diplodocus and Brachiosaurus have equal lengths, the former is estimated to have weighed about 11 tonnes, while the latter is estimated to have weighed almost 26 tonnes (Henderson 2006). Just quoting the length of an animal says nothing about how wide or deep it is, and ignores differences in shape. Given these objections to using linear dimensions to describe the sizes of dinosaurs, the preferred way of indicating the size of a dinosaur, or any other animal, is to use body mass. An animal’s weight provides a measure of how much force the body exerts on the ground, and is independent of body shape. The weight of any object, animate or not, is the product of its mass multiplied by the acceleration that it experiences. The mass of any object is a measure of the amount of material used in its construction, and is distinct from its weight. At the surface...