Basal archosaurs were the stock from which all later archosaurs arose. These early archosaurs were once known by the scientific term Thecodontia, a name given to them by British paleontologist Richard Owen (1804-1892) in 1859. This group originally included any and all archosaurs that dated from the Triassic Period but were not clearly dinosaurs or crocodiles. Although it was assumed that "thecodonts" had a common tetrapod ancestor, the evolutionary relationships among the descendants of this ancestor, called Tetrapoda, were not clear. Although "thecodonts" shared a few rudimentary traits that united all archosaurs, paleontologists found many differences in "thecodonts" that prevented their being joined into neatly defined groups. This made "thecodonts" a paraphyletic assemblage—an unnatural taxon of organisms that did not include all of the descendants of their common ancestor.
In the more than 148 years since Owen named the Thecodontia, paleontology has been kind to its practitioners when it comes to the discovery of Triassic reptiles. Much more is known now than in 1859 about the kinds and varieties of archosaurs that led up to the appearance of dinosaurs, pterosaurs, and crocodylians. While some significant information gaps remain, particularly with regard to painting a complete picture of the earliest of the archosaurs, scientific knowledge about the rise and diversification of archosaurs has made possible a more robust assessment of which archosaurs were related to each other in the Triassic Period. Because they now have more abundant data about these fossils, paleontologists have been able to use cladistic analysis to rethink the classification of archosaurs.
Cladistic analysis, or cladistics, is an analytical technique for comparing the morphological features, DNA sequences, and behavior of taxa. The primary focus of this analysis is the features of the organisms' skeletons. Organisms are said to be members of the same clade if they are all the descendants of a single common ancestor, even if the common ancestor is yet unknown.
Using cladistics, organisms are classified by their shared characteristics. These shared characteristics confirm the evolutionary links that bind different species into related groups and may also reveal subtle aspects of the skeleton, such as a slight bony ridge on a hip bone or the shape of a joint that connects two limb bones. Given enough well-preserved fossil specimens of specific animals, paleontologists can analyze morphological features statistically to unite species that share the most traits.
In the past 20 years, much work has been done in cladistics to improve the understanding of the evolutionary connections among early archosaur groups. One landmark study was that of paleontologist Jacques A. Gauthier (b. 1951), now of Yale University. In 1986,
Gauthier published a paper in which he used cladistic analysis to explain the origin of birds from theropod dinosaurs. In the course of his analysis, Gauthier also suggested that some of the archosaurs that once were placed within the category of "thecodonts" be united into a group of related animals that he called Ornithodira: the dinosaurs, pterosaurs, birds, and proto-dinosaurs discussed below. Gauthier's important research jump-started an effort that continues to this day to apply cladistic analysis to an understanding of dinosaur evolution.
Several years after Gauthier's landmark work, a paleontologist named Paul Sereno (b. 1957), from the University of Chicago, applied similar cladistic techniques to clarify the origin of cru-rotarsan archosaurs and early dinosaurs. Sereno is one of today's best-known fossil explorers and has made field discoveries on five continents. Much of his early work involved the exploration of Tri-assic fossil beds in South America. This gave him an opportunity to study and unearth new specimens of archosaurs that spanned the time associated with the origin of dinosaurs. In 1991, Sereno published a study that united several taxa of archosaurs into the Crurotarsi, the crocodylians and their relatives.
Gauthier and Sereno could not fit all of the known archosaurs into the ornithodirans and crurotarsans. This left a veritable waste bin of remaining taxa that were difficult to classify. Richard Owen's term Thecodontia was abandoned in favor of simply calling the remaining known animals basal, or early, archosaurs.
Basal archosaurs include several intriguing kinds of reptiles that thrived from the Late Permian to the end of the Triassic. Many were generally lizardlike in appearance, yet unrelated to the lineage of true lizards, which belong to the other branch of the Diapsida, the Lepi-dosauromorpha. These early archosaurs had the antorbital fenestra found in the skulls of all later archosaurs. Basal archosaurs also had teeth that were embedded in sockets in the jaw, a trait that would be passed along to dinosaurs and other archosaurs. The waste bin of basal archosaurs also contains several distinctive groups of reptiles whose family lines were well established before some members of the
Archosauria split into the major surviving lineages of the Crurotarsi and Ornithodira.
The end-Permian extinction wiped out many of the terrestrial plants and animals that dominated the end of the Paleozoic Era. As the Triassic got under way, the drier, more temperate climate brought about turnover in both flora and fauna. Spore-bearing tropical plants gave way to seed ferns and conifers. This change may have devastated many lines of amniote herbivores that did not adapt rapidly enough to the changing food supply. Along with those plant-eaters went many of the large-bodied carnivores, thus clearing the way for a new cast of players in the Early Triassic.
Among the plant eaters that survived the end-Permian extinction was Lystrosaurus, a bulky beast that can be likened to a "pig-reptile." This plodding synapsid measured about 3.3 feet (1 m) long. It used a pair of tusklike teeth and a beaklike jaw to snip away the tough-bodied vegetation that earlier herbivores probably could not eat easily. Because fossil remains of Lystrosaurus are found in such widespread locations as China, Russia, India, South Africa, and Antarctica, they provide evidence that these continents were once connected, at the beginning of the Mesozoic Era. Lystrosaurus was a dicynodont, not an archosaur, but it may have been one of the creatures hunted as prey by the first carnivorous members of the ruling reptiles.
One early carnivorous archosaur was Proterosuchus (South Africa). It measured about 5 feet (1.5 m) long, including its long tail, and had a skull that was vaguely lizardlike, a crocodile-l ike body with sprawling posture, and a long neck unlike that seen in either lizards or crocodylians. With its relatively short legs, Proterosuchus was probably not a fast runner. It has been found in South African fossil habitats that once were part of a wet floodplain. This suggests that Proterosuchus may have lived a life similar to that of modern crocodylians, dwelling near the shore and scrambling through shallow waters to catch trapped fish and slow-moving amphibians.
A close relative of Proterosuchus was Erythrosuchus, a much larger predator that was better adapted for hunting on land and that also was found in Early Triassic deposits of South Africa. Erythrosuchus and its Russian relative Vjushkovia developed changes to hips, limbs, and digits that improved these animals' speed and agility on land. Each of these archosaurian predators was the largest carnivore in its habitat; both approached the enormous bulk that medium-sized dinosaurs would enjoy later in the Mesozoic. Erythrosuchus measured about 6.6 feet (2 m) long, and Vjushkovia measured an astounding 17 feet (5 m). Their skulls were long and narrow, and their jaws were filled with sharp, conical teeth for grasping and tearing prey. They probably made quick business of relatively defenseless dicynodonts such as Lystrosaurus.
Another enigmatic basal archosaur from the Early Triassic was Euparkeria, a lwo-foot (60 cm) creature found in Early Triassic rocks of South Africa. With long and powerful hind legs, a strong neck, and a mouth full of recurved, biting teeth, Euparkeria had many affinities with the first dinosaurs that arose in the Late Tri-assic. This slender-bodied reptile also had bony armored scales on its back—a trait seen, often in spectacular extremes, in some later archosaurs, including dinosaurs. Euparkeria had hind legs that were one-third longer than its front legs. This suggests that Euparkeria was a tiny powerhouse that sometimes could scamper on two feet— an early appearance of bipedalism, another trait later perfected by many kinds of dinosaurs.
By the Middle Triassic, after the establishment of basal archo-saurs, two general trends took shape in the continued evolution of the archosaurs. The first trend was represented by the crurotarsans and included all archosaurs whose skeletal features were more like crocodylians than birds. In contrast, the creatures of the second t rend—t he ornithodirans—i ncluded all archosaurs whose skeletal features were more like birds than crocodylians. This seemingly simple distinction between the crurotarsans and the ornithodirans is based on the comparison of hundreds of data points related to the skeletal features of fossil specimens. A foundation of research
such as this makes it easier for paleontologists to mark the evolutionary position of any newly discovered fossil reptile from the Mesozoic that might fall into one of these two important groups of archosaurs.
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