Stance its 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 ofthe 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 ofthe 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° downward from horizontal (Figure B4.3.1). 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.

Monitor Lizards Bipedal Stance

Figure B4.3.1. 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 the dinosaur (behind) both have fully erect stances.

If we consider stance simply in terms of ancestral and derived characters, the ancestral 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 ofBrown University, Rhode Island, USA, 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 12 on "warm bloodedness" in dinosaurs). Those organisms that possess a semi-erect stance may reflect the modification ofa primitive character (sprawling) for greater efficiency on land, but they may also retain the less-derived type of respiration. Dinosaurs (see Figure 4.11) 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 4.7) shows that the most recent common ancestor of dinosaurs and mammals - some primitive amniote - was itselfan 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 at least twice in Amniota: once among the synapsids and once in dinosaurs.

Figure B4.3.1. 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 the dinosaur (behind) both have fully erect stances.

dinosaurs" then shares a close relationship with a slender, long-limbed animal from the Middle Triassic of Argentina called Lagosuchus (Figure 4.18). Lagosuchus was a small (less than 1 m long), bipedal carnivore or insectivore, with long legs reminiscent of a rabbit's. Few paleontologists would disagree that this tiny creature embodies many of the features

Figure 4.17. Hypotheses for the relationships of dinosaurs and their closest known relatives: (a) that of J. A. Gauthier and K. Padian; (b) that of P.C. Sereno.

Dinosauria Ornithodira

Dinosauria

Dinosauria

Dinosauromorpha

Ornithodira

Dinosauromorpha

Ornithodira

Figure 4.17. Hypotheses for the relationships of dinosaurs and their closest known relatives: (a) that of J. A. Gauthier and K. Padian; (b) that of P.C. Sereno.

that were ancestral for all Dinosauria; the diminutive Lagosuchus is probably close to the ancestry of all the spectacular vertebrates encompassed within Dinosauria.

Hypothesis no. 2. University of Chicago dinosaur specialist P. C. Sereno, in contrast, places Lagosuchus, as well as several other small, contemporary archosaurs (Lagerpeton, Pseudolagosuchus, and Marasuchus) as the closest dinosaurian relatives (Figure 4.17b). More far-flung relationships of these dinosauromorphs are with pterosaurs.

There is an interesting and perhaps surprising consequence of this phylogeny. With archosaurs like Lagosuchus closest to dinosaurian ancestry, apparently dinosaurs were primitively obligate bipeds. This means that the earliest dinosaurs were creatures that were completely and irrevocably bipedal. Because the primitive stance for archosaurs is quadrupedal, and because Dinosauria is monophyletic, it follows that creatures like Triceratops, Ankylosaurus, and Stegosaurus, in fact, all quadrupedal dinosaurs, must have secondarily

Figure 4.19. The pelvis of the hadro-saurid Prosaurolophus. A splint of the pubis points posterior, along the base of the ischium (see arrow), exemplifying the ornithischian condition.

evolved (or re-evolved) their quadrupedal stance. They must have (phylogenetically) got back down on four legs, as it were, after having been up on two. In fact, you can see the remnant of bipedal ancestry when you look at a stegosaur or a ceratopsian, in which the back legs are quite a bit longer than those at the front.

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