Box

Stance: it's both who you are and what you do

Tetrapods that are most highly adapted for land locomotion tend to have an erect stance.This clearly maximizes the efficiency of the animal's movements on land, and it is not surprising that, for example, all mammals are characterized by an erect stance.Tetrapods such as salamanders (which are adapted for aquatic life) display a sprawling stance, in which the legs splay out from the body nearly horizontally.The sprawling stance seems to have been inherited from the original position of the limbs in early tetrapods, whose sinuous trunk movements (presumably inherited from swimming locomotion) aided the limbs in land locomotion.

Some tetrapods, such as crocodiles, have a semi-erect stance, in which the legs are directed at something like 45° downwards from horizontal (Figure B4.4.1). Does this mean that the semi-erect stance is an adaptation for a combined aquatic and terrestrial existence? Clearly not, because a semi-erect stance is present in the large, fully terrestrial monitor lizards of Australia (goanna) and Indonesia (Komodo dragon). If adaptation is the only factor driving the evolution of features, why don't completely terrestrial lizards have a fully erect stance, and why don't aquatic crocodiles have a fully sprawling stance?The issue is more complex and is best understood through adaptation to a particular environment or behavior; as well as through inheritance.

If we consider stance simply in terms of ancestral and derived characters, the ancestral

Figure B4.4.1. Stance in four vertebrates.To the left, the primitive amphibian and crocodile (behind) have sprawling and semi-erect stances, respectively.To the right, the human and dinosaur (behind) both have fully erect stances.

condition in tetrapods is sprawling. An erect stance represents the most highly derived state of this character; but are animals with sprawling stances not as well designed as those with erect stances? In 1987, D. R. Carrier of Brown University hypothesized that the adoption of an erect stance represents the commitment to an entirely different mode of respiration (breathing) as well as locomotion (see Chapter 15 on "warm-bloodedness" in dinosaurs). Those organisms that possess a semi-erect stance may reflect the modification of a primitive character (sprawling) for greater efficiency on land, but they may also retain the less-derived type of respiration. Dinosaurs (see Figure 4.15) and mammals both have fully erect stances, which represent a full commitment to a terrestrial existence as well as to a more derived type of respiration.The designs of all these organisms are thus compromises among inheritance, habits, and mode of respiration. Who can say what other influences are controlling morphology?

Interestingly, the cladogram (see Figure 4.5) shows that the most recent common ancestor of dinosaurs and mammals - some primitive amniote -was itself an organism with a sprawling stance. Because dinosaurs and mammals (or their precursors) have been evolving independently since their most recent common ancestor; an erect stance must have evolved twice in Amniota: once among the synapsids and once in dinosaurs.

Figure B4.4.1. Stance in four vertebrates.To the left, the primitive amphibian and crocodile (behind) have sprawling and semi-erect stances, respectively.To the right, the human and dinosaur (behind) both have fully erect stances.

Figure 4.16. The distribution of warm-bloodedness and flight inTetrapoda, presented on a cladogram constructed from Figures 4,5 and 4.12. Filled stars denote known instances of warm-bloodedness and flight; unfilled stars and italics denote the possibility that warm-bloodedness and/or flight characterized a group.The cladogram shows that warm-bloodedness and flight evolved at least three times inTetrapoda. Within Synapsida, warm-bloodedness occurs in all mammals, and flight occurs in bats. Within Archosauria, warm-bloodedness and flight occurred in pterosaurs (which are now known to have been insulated), and in birds (Saurischia). Warm-bloodedness has been proposed for Ornithischia, suggesting to some that all Dinosauria might be characterized by warm-bloodedness (see Chapter 15). Warm-bloodedness and flight, however are not fundamental characteristics of Archosauria; Figure 4.12 shows that archosaurs (e.g., crocodiles) were certainly primitively non-flying, and almost certainly cold-blooded animals.The cladogram thus shows that warm-bloodedness and flight evolved independently three times: once in bats, once in pterosaurs, and once in birds (or their dinosaurian near-relatives).

Figure 4.16. The distribution of warm-bloodedness and flight inTetrapoda, presented on a cladogram constructed from Figures 4,5 and 4.12. Filled stars denote known instances of warm-bloodedness and flight; unfilled stars and italics denote the possibility that warm-bloodedness and/or flight characterized a group.The cladogram shows that warm-bloodedness and flight evolved at least three times inTetrapoda. Within Synapsida, warm-bloodedness occurs in all mammals, and flight occurs in bats. Within Archosauria, warm-bloodedness and flight occurred in pterosaurs (which are now known to have been insulated), and in birds (Saurischia). Warm-bloodedness has been proposed for Ornithischia, suggesting to some that all Dinosauria might be characterized by warm-bloodedness (see Chapter 15). Warm-bloodedness and flight, however are not fundamental characteristics of Archosauria; Figure 4.12 shows that archosaurs (e.g., crocodiles) were certainly primitively non-flying, and almost certainly cold-blooded animals.The cladogram thus shows that warm-bloodedness and flight evolved independently three times: once in bats, once in pterosaurs, and once in birds (or their dinosaurian near-relatives).

In dinosaurs, an erect stance consists of a suite of anatomical features with important behavioral implications. In particular, the head of the femur (thigh bone) is distinctly offset from the shaft. The head of the femur itself is barrel shaped (unlike the familiar ball seen in a human femur), so that motion in the thigh is largely restricted to forward and back, within a plane parallel to that of the body. The ankle joint is modified to become a single, linear articulation. This type of joint, termed a modified mesotarsal joint, allows movement of the foot only in a plane parallel to that of the body (forward and back). Note that again this situation differs from that in humans, in which the foot is capable of rotating movement. The upshot of these adaptations of stance is that all dinosaurs are highly specialized for cursorial (i.e., running, as in the "cursor" on a computer screen) locomotion. Dinosaurs are quintes-sentially terrestrial beasts (see Box 4.4).

This, then, completes our long trek through Vertebrata to find Dinosauria. We can now answer more completely the questions set forth at the beginning of this chapter:

• "How many times has warm-bloodedness evolved in the vertebrates?" It would appear that "warm-bloodedness" (an unfortunate term that we gladly abandon in Chapter 15) has evolved as many as three times: once in synapsids (in mammals or their near ancestors) and twice in ornithodi-rans (in birds and in pterosaurs). It is possible, however, that it evolved only twice and that the basal ornithodiran - that is, the organism that was the ancestor of all ornithodirans - was itself "warm-blooded." If so, then all ornithodirans were primitively "warm-blooded," which means that all dinosaurs must have been "warm-blooded," too. The evidence for and against this possibility will be explored in Chapter 15.

• "How many times has powered flight been invented by vertebrates?" We know that flight occurred once in the synapsids (in bats) and twice in Ornithodira (in birds and in pterosaurs). If flight had evolved in the ancestor of all ornithodirans, then flight would be primitive among the ornithodirans, and all ornithodirans should be flying creatures (at least primitively). Obviously, many among Dinosauria were not (a flying Stegosaurus or T. rex strains credulity), and so we can be relatively certain that powered flight evolved independently in three lineages of vertebrates. These relationships are shown in Figure 4.16.

• "Is a cow a fish?" This is discussed in Box 4.3, and the bottom line is that many of the features that we might intuitively use to group organisms are primitive characters and not suitable for recognizing the pattern of evolution.

• "Did all dinosaurs become extinct?" The negative answer to this question requires the fuller elaboration provided in Chapters 13, 14, 17 and 18. Finally, it should now be clear why a bird is closer to a crocodile than to a lizard. Birds and crocodiles are archosaurs, whereas lizards are not. The common ancestor of birds and crocodiles was some early archosaur, and thus it is only at the level of Diapsida that lizards, crocodiles, and birds are related. This, as noted previously, bodes ill for the traditional Reptilia: crocodiles, snakes, lizards, turtles, and the tuatara do not form a monophyletic group, unless birds are also included.

I m po rtant read i ngs Benton, M. J. (ed.) 1988. The Phylogeny and Classification of the Tetrapods. Vol. I.

Systematic Association Special Volume 35A. Oxford, 377pp. Butterfield, N. J. 1990. Organic preservation of non-mineralizing organisms and the taphonomy of the Burgess Shale. Paleobiology 16, 272-286.

Carpenter, K. and Currie, P.J. (eds.) 1990. DinosaurSystematics. Cambridge University Press, New York, 318pp.

Carrier, D. R. 1987. The evolution of locomotor stamina in tetrapods: circumventing a mechanical constraint. Paleobiology 13,326-341.

Gauthier, J. A. 1986. Saurischian monophyly and the origin of birds in Padian, K. (ed.) The Origin of Birds and the Evolution of Flight. Memoirs of the California Academy of Sciences no. 8, pp. 1-56.

Gauthier, J. A., Kluge, A. G. and Rowe, T. 1988. Amniote phylogeny and the importance of fossils. Cladistics 4,105-209.

Gould, S.J. 1989. Wonderful Life: The Burgess Shale and the Nature of History. W. W. Norton, New York, 347pp.

Gould, S.J. (ed.) 1993. The Book of Life. Ebury-Hutchinson, London, 256pp.

LaPorte, L. F. (ed.) 1974. Evolution and the Fossil Record. Scientific American Offprint Series. W. H. Freeman and Company, San Francisco, 222pp.

Moy-Thomas, J. A. and Miles, R. S. 1971. Palaeozoic Fishes. W. B. Saunders Company, Philadelphia, 259pp.

Padian, K. and Chure, D.J. (eds.) 1989. The Age of Dinosaurs. Short Courses in Paleontology no. 2. The Paleontological Society, University of Tennessee Press, Knoxville, TN, 210pp.

Prothero, D. R. and Schoch, R. M. (eds.) 1994. Major Features of Vertebrate Evolution. Short Courses in Paleontology no. 7. The Paleontological Society, University ofTennessee Press, Knoxville, TN, 270pp.

Schopf, J. W. 1983. The Earth's Earliest Biosphere: Its Origin and Evolution. Princeton University Press, Princeton, NJ, 543pp.

Stahl, B.J. 1985. Vertebrate History: Problems in Evolution. Dover Publications, New York, 604pp.

Wellenhfer, P. 1996. The Illustrated Encyclopedia of Prehistoric Flying Reptiles. Salamander Books, London, 192pp.

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