Archosaur Evolution and Diversification

The Archosauria is defined as having, at minimum, the following characteristics:

■ Openings anterior to the orbits (antorbital fenestrae).

■ Teeth with serrations compressed laterally and none on the palate.

■ Dentary fenestrae.

■ Differently shaped calcaneum.

■ Elongated ilium and pubis.

Some paleontologists place Archosauria within the clades Archosauromorpha and Archosauriformes, the latter originating from the former (Chapter 5). The majority of paleontologists agree upon the designation of Archosauria as a clade that had arrived by the Early Triassic, with members that evolved into lineages, both dinosaurian and otherwise. A group of fossil reptiles, known previously by paleontologists as "thecodonts," was once considered as synonymous with the archosaur group that gave rise to the dinosaurs, crocodilians, and birds. However, cladistic analyses show that thecodonts make up a paraphyletic grouping (such as Reptilia), hence its use as a term is now discouraged in phylogenetic classifications. However, it is commonly mentioned in older literature and represents changing ideas in science.

A likely representative fossil for a common ancestor of the archosaurs is the Early Triassic Euparkeria of South Africa (Fig. 6.8). Euparkeria was a small (about 1 meter long) but relatively long-limbed reptile that possessed antorbital fenestrae, a key feature of all archosaurs. Clades within the Archosauria, which seemingly descended from ancestors like Euparkeria, are the Crurotarsi and Ornithodira. Crurotarsi includes living crocodilians (alligators and crocodiles), but it encompasses many diverse fossil forms as well. An ankle where the astragalus and calcaneum form a joint between the tarsals and lower part of the limb bones characterizes this clade. Crurotarsans were well-represented during the Middle and Late Triassic by large, crocodile-like carnivorous parasuchids (also known as phytosaurs) and rauisuchians (Fig. 6.9), as well as the armored and herbivorous aetosaurs. Rauisuchians were unique among large archosaurs at the time because their fore-limbs were considerably shorter than their hind limbs, which suggests that they were capable of walking on two legs. They may have been among the first such archosaurs to evolve this mode of locomotion.

Despite their abundance and success, all species of phytosaurs, rauisuchians, and aetosaurs became extinct by the end of the Triassic. However, by the Late Triassic, ornithodirans had diverged into two clades: Pterosauria and Dinosauria. Pterosaurs, the so-called "flying reptiles," were among the most famous of the

Archosauria is the clade often associated with the origin of the dinosaurs.

FIGURE 6.8 Skull of Euparkeria, a basal archosaur from the Early Triassic of South Africa, which was not a dinosaur. From Cowen (2000), History of Life, 3e, Blackwell Science, Inc., Malden, MA, p. 182, fig. 11.13.

FIGURE 6.9 Cast of Postosuchus, a large rauisuchian from the Late Triassic of the southwestern USA: Mesalands Dinosaur Museum, Tucumcari, New Mexico. Despite its very fierce appearance, Postosuchus was not a dinosaur.

terrestrial creatures during the Mesozoic, a notoriety related to the interpretation that they were the first vertebrates known to have achieved self-powered flight. Pterosaurs developed a remarkable adaptation whereby digit IV of each forelimb extended far beyond the other digits and had a membrane attached from its distal end to the torso to form a wing (Fig. 6.10). During their time on Earth, which correlated with and was nearly as long as the geologic range of the dinosaurs, pterosaurs evolved into forms as small as a sparrow to the largest animals that ever flew. For example, the Late Cretaceous pterosaur Quetzalcoatlus had a wingspan of about 12 meters as it soared over what is now the state of Texas. But in spite of their repute, grandeur, and chronological association with dinosaurs, the pterosaurs, like many of the other amniotic vertebrates mentioned in this chapter, are still not defined as dinosaurs, although they may have been the closest to having a common ancestor.

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