Archosauria
Archosauria, often referred to as archosaurs, is a significant group of reptiles that includes dinosaurs, pterosaurs (flying reptiles), and crocodylomorphs (alligators and crocodiles), along with their modern descendants—birds and crocodilians. This group emerged during the Late Permian period and thrived throughout the Mesozoic Era, which spanned from approximately 252 million years ago to 66 million years ago when many dinosaurs went extinct. Archosaurs are characterized by distinct anatomical features, such as an ankle structure that allows for an upright posture, and teeth set in sockets rather than grooves, enhancing their feeding efficiency.
The evolutionary history of archosaurs is marked by rapid diversification, especially following the Permian-Triassic extinction event, which created ecological opportunities for their expansion. They are divided into two main clades: Crurotarsi, which includes modern crocodilians, and Avemetatarsalia, which encompasses birds and pterosaurs. This evolutionary lineage showcases adaptations that have allowed archosaurs to occupy various ecological niches, including terrestrial, aquatic, and aerial environments.
Despite the wealth of information available, many aspects of archosaur evolution remain under investigation, particularly concerning their common ancestry and the nature of their diversification during the Jurassic period. Ongoing paleontological research continues to unveil new insights into this dynamic group, reshaping our understanding of their evolutionary history.
Archosauria
Introduction
Archosauria (sometimes referred to as Archosaurs) are a group of reptiles that includes dinosaurs, crocodylomorphs, pterosaurs (flying reptiles), and their living descendents, represented by all extant birds and crocodilians (alligators and crocodiles). The ancient archosaurs were among the most successful and abundant clades of terrestrial vertebrates to exist during the entire Mesozoic Era, spanning from 252 million years ago until the last of the dinosaurs died out some 66 million years ago.
Archosaur ancestry can be traced back to before the Late Triassic (235–202 million years ago) when all continental landmasses were joined together as the supercontinent Pangaea. The Triassic was a significant time period for archosaurian evolution, radiation (diversification), and distribution, as it followed the worst extinction episode ever seen, known as the Permian–Triassic extinction event (P–Tr extinction). This mass extinction opened up many ecological niches and habitats, allowing for the rapid establishment and expansion of the first archosaurs as early as 245 million years ago.
Fast Facts
Pronunciation: Archosaur (pronounced AR-kuh-sawr), meaning “ruling lizard”
Time Period: Late Permian to Current (260 million years ago–today)
Size: 6.2 cm to 60 m (2.5 in–200 ft) in length
Weight: 1.6 g to 120 metric tons (0.06 oz–265,000 lbs)
Diet: Carnivorous, Herbivorous, and/or Omnivorous
Location: Worldwide
Lifespan: Up to a few hundred years
Homologous Traits
All archosaurs are classified as diapsid amniotes. Amniotes are four-limbed vertebrates whose young develop in amniotic sacs (either inside the mother or outside in an egg). Diapsids are reptiles with two holes on either side of the skull, behind the eyes.
As with all living organisms, evolution molded and shaped archosaur development. Basal (early) species changed from lizard-like creatures with sprawling gaits to animals with limbs positioned directly under their bodies. This was caused by changes in the archosaur's hip socket and joint. Fossil evidence shows a change in archosaur's hip joints in the early to middle Triassic which allowed for a more erect gait. The earliest archosaurs appear in the fossil record around 255 million years ago and were characterized by long hind limbs, short forelimbs, and a special type of ankle that helped them develop a more erect gait. Archosaur teeth were different and more efficient than those of earlier reptiles, in that they were fixed in sockets rather than grooves. Archosaurs are thus marked by their continuous improvement in locomotor (moving around) ability, which distinguished them from synapsids, the ancestors of today's mammals.
Evolutionary Divergences
Early in the Mesozoic, as animal life was reestablishing itself following the P–Tr mass extinction, the first known archosaurs were rapidly expanding across the land in early forms, such as the carnivorous Proterosuchus, Vjushkovia and Erythrosuchus, all of which preyed on large herbivorous reptiles. These species lasted for many millions of years, but by the close of the Triassic, almost all early archosaurs had become extinct. However, archosaurs such as dinosaurs and pterosaurs remained until the end of the Cretaceous, when all dinosaurs and about 50 percent of all other life died out. Of course, this was not the end of the archosaur story, with two groups—the crocodilians and the avians (birds)—surviving to this day.
Unusually, the rapid evolution and radiation of archosaur species during the Triassic period occurred in a very short time. Although all resultant groups of archosaurs, specifically the crocodylomorphs, pterosaurs, and dinosaurs, had evolved by the end of the Triassic, the most significant radiations occurred during the ensuing Jurassic period, when the most important lineages of the different groups of dinosaurs were established.
Archosaurs are a true monophyletic group, meaning all are descended from a single common ancestor and thus share certain basic traits. Archosaurs evolved from even more primitive reptiles and are now organized into two clades, the Crurotarsi (including all crocodilians and the extinct Triassic phytosaurs, aetosaurs, prestosuchids, rauisuchids, and poposaurs) and the Avemetatarsalia (including all dinosaurs, pterosaurs, birds, and the earlier Triassic lagosuchids and lagerpetonids). Regardless of which taxonomic branch an archosaur belongs to, it is classified as an archosaur largely based on its ankle structure (the names Crurotarsi and Avemetatarsalia both refer to foot or ankle bones).
Creatures in This Group
Archosauria consists of dinosaurs, crocodylomorphs, and pterosaurs, as well as their living descendents represented by all extant birds and crocodiles. Archosaurs are considered a “crown group,” a phylogenetic classification of all branches possessing a shared derived characteristic that also includes the most recent common ancestor of both living and extinct descendents. Mapping the relationship often resembles a triangle or the “crown” shape from which it gets its name. One of the characteristics shared by Archosauria is teeth set into sockets. The crown group Archosauria contains two clades, Avemetatarsalia and Crurotarsi. Avemetatarsalia encompasses the family Scleromochlidae and unranked clade Ornithodira. Avemetatarsalia contains a wide range of creatures, ranging from pterosaurs with pneumatic bones, no tails, and no teeth to dinosaurs, with any number of features, among which are tails, armour, and shredding, leaf- or peg-shaped, or ripping teeth. Under Avemetatarsalia, the unranked clade Ornithodira branches into the order Pterosauria, with winged genera such as Pteranodon, Pterodactylus, and Quetzalcoatlus, and the unranked clade Dinosauromorpha. Dinosauromorpha include the family Lagerpetonidae and all true dinosaurs in the superorder Dinosauria, encompassing a range from the small Microvenator to the huge Tyrannasaurus. Modern-day birds are included in the Dinosauria superorder, but outside of phylogenetic classification are commonly known as the class Aves.
Crurotarsi, also known as Pseudosuchia, includes the predecessors of the order Crocodylia as well as extant alligators, crocodiles, caimans, and gharial. Crurotarsi are crocodilians and their extinct relatives that appeared in the Early Triassic. Phytosauria are semi-aquatic and predatory archosaurs who met their demise somewhere at the end of the Triassic. Suchia is the only clade from the Crurotarsi group with a living lineage. Among them are primitive Rauisuchia, a clade of meat-eating archosaurs with large skulls that became extinct at the end of the Triassic. While living, they were the planet's reigning predators. Rauisuchia were represented by such creatures as Teratosaurus, found in Germany, and Postosuchus, found in North America. Their disappearance cleared the way for meat-eating theropods.
Aetosauria, another clade of Crurotarsi, have only been found in Late Triassic sediment, leading many to believe that they populated Pangaea. They are represented by Stagonolepis in Scotland and Desmatosuchus in North America. Their snouts have been described as pig like and they were armored; Desmatosuchus had two rows of spikes along its back. The Paracrocodylomorpha clade is the only clade with extant species among Rauisuchia. It includes crocodylomorphs and other Triassic clades. Among the crocodylomorphs are the sphenosuchian Junggarsuchus, which had a greyhound-like posture, some marine creatures like Dakosaurus and living crocodilians such as Alligatoridae, Crocodylidae, and Gavialidae.
Ecology
Although there is some debate due to the poor fossil record, it is believed that not long after they appeared, the archosaurs divided into two separate clades, the Crurotarsi (formerly identified as Pseudosuchia) and the Avemetatarsalia (formerly identified as Ornithosuchia), also known as the crocodile-bird split. Although the two groups diverged before the development of the distinctive ankle structure and erect limb posture of later archosaurs, both groups appear to have developed these characteristics independently of each other, an example of convergent evolution.
Following the mass extinction event at the start of the Mesozoic, basal archosaurs rapidly spread into habitats and niches left unoccupied due to the extinction of many of the world's large herbivores and carnivores. Although frequently thought of as land-based creatures, archosaurs also dispersed successfully into the water and air, with crocodilians and phytosaurs living successfully in aquatic environments, and pterosaurs (and modern-day birds) evolving flight.
With such diverse habitats and morphologies (shapes), archosaur diets were also diverse. Most very early archosaurs have been classified as carnivores; however, many later archosaurs, such as the sauropod dinosaurs and several ancient crocodiles, were herbivorous. Such dietary diversity meant many of these animals could live together, utilizing different resources and filling different ecological niches.
The evolution of the characteristic ankle structure was a significant step forward for the later archosaurs, as erect posture (with the legs under the body, as opposed to sprawling, like a crocodile) was an important adaptation that improved walking stamina and eventual archosaurian diversification. More significantly, perhaps, the evolution of specialized teeth also appears instrumental in much of archosaurian success. Archosaur teeth were different from those of earlier reptiles in that they were fixed in sockets rather than grooves, which made them stronger, more durable, and more suited to forceful feeding strategies.
Paleontology News
Most paleontologists agree that Archosauria is monophyletic and includes the crocodilian and avian lineages, but debate continues regarding their common ancestor, as well as the timing and manner of archosaurian radiation and faunal turnovers. Much remains unknown about the Jurassic archosaurs due to poor fossil records, and much of what is known is limited to the northern hemisphere, as fossil finds in the southern hemisphere are especially scarce. However, renewed interest in the early archosaurs has seen the number of creatures included in this group double over the last two decades.
Recent discoveries of ancient birdlike archosaurs in Asia, Europe, and America have, for example, hinted at the possibility that modern-day birds many not have evolved from the theropod dinosaurs, as has long been the mainstream view. New fossil finds and improved technology means that classifying ancient animals is an ever evolving science.
Bibliography
Allen, V. R., et al. "The Evolution of Pelvic Limb Muscle Moment Arms in Bird-Line Archosaurs." Science Advances, vol. 7, no. 12, 2021, p. eabe2778.
Brusatte, Stephen L., et al. “Macroevolutionary Patterns in the Evolutionary Radiation of Archosaurs (Tetrapoda: Diapsida).” Earth and Environmental Science Transactions of the Royal Society of Edinburgh, vol. 101, 2011, pp. 367–82.
Fastovsky, David E., and David B. Weishampel. Dinosaurs: A Concise Natural History. Cambridge UP, 2009.
---. Evolution and Extinction of the Dinosaurs. 2nd ed., Cambridge UP, 2005.
Lucas, Spencer George. Dinosaurs: The Textbook. 4th ed., McGraw, 2004.
Martin, Anthony J. Introduction to the Study of Dinosaurs. 2nd ed., Blackwell, 2006.
Nesbitt, Sterling J. “Arizonasaurus and Its Implications for Archosaur Divergence.” Proceedings of the Royal Society B: Biological Sciences, vol. 207, 2003, pp. S234–237.
---. “The Early Evolution of Archosaurs: Relationships and the Origin of Major Clades.” Bulletin of the American Museum of Natural History, vol. 352, 2011, pp. 1–292.
Norell, Mark, et al. Discovering Dinosaurs: Evolution, Extinction, and the Lessons of Prehistory. U of California P, 1995.
Parker, Steve. Dinosaurus: The Complete Guide to Dinosaurs. Firefly, 2009.
Sereno, P. C. “Basal Archosaurs: Phylogenetic Relationships and Functional implications.” Journal of Vertebrate Paleontology, vol. 11, no. S4, 1991, pp. 1–53.
Weishampel, David B., et al. Dinosauria. 2nd ed., U of California P, 2007.