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135 The Osteology of the Dinosaurs Thomas R. Holtz Jr. and M. K. Brett-Surman With rare but spectacular exceptions, dinosaur body fossils consist almost entirely of bones and teeth. The soft parts of the body–skin, muscles, and other organs–were destroyed by decay processes fairly quickly after death. Only bones and teeth, the hard mineralized parts of a dinosaur, are durable enough to be preserved over tens of millions of years. Except for footprints and much rarer traces such as eggs and skin impressions, fossilized skeletal material represents the only physical remains of the ancient dinosaurs. Thus the osteology (the study of bones) of dinosaurs is our main source of knowledge about these extinct animals. This chapter is intended to give a brief overview of some of the general aspects of the dinosaurian osteology; later chapters will give specifics concerning the details of particular groups of dinosaurs or aspects of their biology revealed by their bony remains. Dinosaurs are tetrapod vertebrates–in other words, animals with bony skeletons and four limbs. All tetrapods, including amphibians, mammals, turtles, lizards, and birds, are built along the same general body plan. For example, the forelimb, or arm, of all tetrapods has one upper arm bone closest to the body, two bones below the elbow, several small bones in the wrists, and then a series of longer finger bones. (In some animals, such as snakes, the forelimbs have disappeared, but the ancestors of these animals had arms of the basic structure.) The reason all these animals share this common body plan is that all are descended from the same ancestral stock with that plan. The differences between the particular shapes of the bones arise from the same body plan having been modified, or adapted, to different uses–for example, the wings in birds or bats, the digging claws of moles, the grasping hand of a Velociraptor, or the pillarlike forefoot of a Brachiosaurus. Because of this common descent, we can recognize bones that are homologous; that is, they represent bones descended from the same original structure. Thus the upper arm bone of any tetrapod is homologous to the upper arm bone of any other tetrapod. For an example of homology, compare the right hand of a human and the right forepaw of the plant-eating dinosaur Dollodon (formerly a species of Iguanodon; see Paul 2008) (Fig. 8.1). These hands are oriented in the same direction, with the back of the hand facing us and the palm facing away. In the dinosaur, the homologue to the thumb has been fused into a large pike. The last digit, homologous to the pinkie of a human, has evolved into an opposable finger. Opposability (the ability to place the digit on the palm) is characteristic of the thumbs of humans. Two different anatomical features in two different animals that have the same function but that are evolved from different parts of the body are called analogous. Thus the opposable 8 Holtz and Brett-Surman 136 digit of Dollodon is analogous to the human thumb, but homologous to the human pinkie. It must be noted that in comparative anatomy, the term homologous was not originally used in an evolutionary context. Sir Richard Owen, who coined the term Dinosauria in 1842 (see Torrens, Chapter 2 of this volume), is also responsible for the use of homology in an anatomical context. Owen (1846, 1849) believed that for each major group of organisms, there was a single basic body plan, or blueprint, of which all species in that group are variations. This body plan, the archetype, was not considered to have ever existed in the physical universe; rather, it was a mental construct representing the simplified anatomical organization of each major group of organisms , such as vertebrates, mollusks, or insects. In this context, the pectoral fin of a trout, the wings of birds, the forelimbs of horses, and the arms of humans were considered homologous because each was a variation of the same structure in the vertebrate archetype. Sir Charles Darwin and his primary advocate, Thomas Henry Huxley, co-opted the concept of homology into the new theory of evolution by means of natural selection. In their view, where all animals of the same body plan have a common origin, a homologous structure in two or more organisms represents variations of the same structure that was present in a real common ancestor. (See Desmond 1982 for a detailed discussion of the social and political as well as the scientific...

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