Archosauria

Edward Drinker Cope first erected the name Archosauria in 1869 in his monograph 'Synopsis of the Extinct Batrachia, Reptilia and Aves of North America', published in the Transactions of the American Philosophical Society. Cope's original concept was broad and heterogeneous, encompassing dinosaurs, crocodilians, thecodonts, sauropterygians, rhynchocephalians, and even anomodonts (now recognized as synapsids). For much of the 20th century, the term was used morphologically to encompass any reptile possessing an antorbital fenestra, thecodont dentition, and related features — a concept that roughly corresponds to what is now called Archosauriformes.

The modern, phylogenetically precise definition of Archosauria was established by Jacques Gauthier in 1986, who redefined it as a crown group: the clade containing the most recent common ancestor of living birds and living crocodilians, plus all descendants of that ancestor. This crown-group definition excludes more basal forms such as proterosuchids, erythrosuchids, and euparkeriids, which share many archosaurian morphological features but fall outside the bird–crocodilian crown node. These excluded taxa are now classified within the broader group Archosauriformes. Michael Benton proposed the alternative name Avesuchia in 1999 for the crown group, but Archosauria remains the predominant term in current usage.

Archosauria is divided at its base into two great lineages that diverged in the Early Triassic. These have been named and defined in various ways as phylogenetic understanding has evolved.

Pseudosuchia (the crocodilian-line archosaurs) includes all archosaurs more closely related to Crocodylus niloticus (the Nile crocodile) than to Passer domesticus (the house sparrow). In addition to the living crocodilians (Crocodylia, approximately 27 species), this lineage historically included a spectacular diversity of Triassic forms: phytosaurs (superficially crocodile-like aquatic predators), aetosaurs (armored herbivores), ornithosuchids (carnivorous quadrupeds with reversed ankle joints), rauisuchians (large apex predators, some fully erect), and the ancestors of the crocodylomorph lineage that gave rise to modern crocodilians. During the Triassic, pseudosuchians were the ecologically dominant archosaur lineage, occupying most large-bodied terrestrial predator and herbivore niches. Their diversity collapsed at the end-Triassic extinction (~201 Ma), after which only crocodylomorphs persisted.

Avemetatarsalia (the bird-line archosaurs, also historically called Ornithodira for the more derived members) includes all archosaurs more closely related to Passer domesticus than to Crocodylus niloticus. This lineage encompasses the pterosaurs (Pterosauria), the non-avian dinosaurs (Dinosauria excluding Aves), birds (Aves, approximately 10,000 living species), and a series of basal forms including aphanosaurs (such as Teleocrater), silesaurids (such as Asilisaurus), and lagerpetids (such as Lagerpeton). The earliest avemetatarsalians appeared in the Anisian stage of the Middle Triassic (~247–242 Ma) and were generally smaller, more lightly built, and more frequently bipedal than their pseudosuchian contemporaries. The ornithosuchian lineage (in the broad phylogenetic sense) diversified dramatically following the end-Triassic extinction, with dinosaurs becoming the dominant large terrestrial vertebrates throughout the Jurassic and Cretaceous periods.

Archosaurs are diagnosed by a suite of skeletal features shared across the clade, many of which evolved in the lineage leading to the crown group and are thus also present in basal archosauriforms. The most significant include the antorbital fenestra, an opening between the nostril (naris) and the orbit (eye socket) on each side of the skull, which reduces skull weight and may have housed pneumatic sinuses or glandular tissue; the mandibular fenestra, a window in the lower jaw (mandible) that further lightened the skull; thecodont tooth implantation, in which teeth are set into deep bony sockets (alveoli) rather than being fused to the jaw surface, making them more resistant to dislodging during feeding; the fourth trochanter, a prominent ridge or process on the femur that served as the attachment point for powerful caudofemoralis muscles, enabling a more upright gait; reduction of the fifth pedal digit (the equivalent of the human 'little toe'); and loss of palatal teeth (teeth on the roof of the mouth).

Additionally, archosauriform and archosaur lineages developed dorsal osteoderms (bony armor plates embedded in the skin along the back), though these were subsequently reduced or lost in many derived groups including most dinosaurs and all birds.

Because birds and crocodilians are the only living archosaurs, comparative physiology of these two groups allows reconstruction of features likely present in their common ancestor — and by extension, in many extinct archosaurs including dinosaurs.

Both birds and crocodilians possess a fully divided four-chambered heart, in contrast to the three-chambered hearts of most other living reptiles (lizards, snakes, turtles). This feature was likely ancestral for Archosauria and may have supported higher sustained metabolic activity.

Both groups exhibit unidirectional pulmonary airflow — air moves through the lungs in a single direction rather than in the tidal (back-and-forth) pattern seen in mammals and most other tetrapods. In birds, this is achieved through an elaborate system of air sacs, while in crocodilians the mechanism involves aerodynamic valving within the bronchial tree. Research published since 2010 (Farmer & Sanders, 2010; Schachner et al., 2013; Farmer, 2015) has demonstrated that unidirectional flow is present in both lineages, strongly suggesting it was ancestral for Archosauria. This efficient respiratory system may have been a key advantage allowing archosaurs to thrive in the low-oxygen conditions of the Early Triassic.

Pneumaticity — the invasion of bones by extensions of the respiratory system (air sacs) — is extensively developed in birds and was also present in many non-avian dinosaurs and pterosaurs. Postcranial pneumaticity is not observed in crocodilians, but craniofacial pneumatic sinuses are shared by both lineages, suggesting at least some degree of skeletal pneumaticity was ancestral for the clade.

The evolutionary lineage leading to archosaurs extends back to the late Permian period (~260–252 Ma), when the earliest archosauromorphs (such as Protorosaurus) and archosauriforms (such as Archosaurus rossicus) appeared. These early forms lived alongside the then-dominant synapsids (the lineage including mammals and their ancestors).

The Permian–Triassic mass extinction (~252 Ma), the most severe in Earth's history, eliminated approximately 90–96% of marine species and approximately 70% of terrestrial vertebrate species. Most of the previously dominant synapsid groups (gorgonopsians, anomodonts other than Lystrosaurus) were devastated. In the aftermath, archosaurs and other archosauriforms diversified rapidly during the Early and Middle Triassic (~252–237 Ma), filling ecological niches vacated by the extinction. Several factors have been proposed for the archosaur advantage in post-extinction environments: their uric acid-based excretory system (shared with all living diapsids), which conserves water more efficiently than the urea-based system of mammals, would have been advantageous in the predominantly arid Early Triassic climate of the supercontinent Pangaea; their efficient unidirectional respiratory system may have been beneficial in the suspected low-oxygen atmosphere following the extinction; and the development of progressively more erect limb postures may have provided greater stamina.

The oldest true (crown-group) archosaur fossils are known from the Olenekian stage of the Early Triassic (~247–251 Ma), including fragmentary remains of pseudosuchians such as Xilousuchus from China and Scythosuchus from Russia. By the Middle Triassic, both major lineages were well established. The Late Triassic (~237–201 Ma) saw the appearance and early diversification of dinosaurs (by approximately 235–230 Ma), pterosaurs, and crocodylomorphs.

The end-Triassic extinction (~201 Ma) eliminated most pseudosuchian diversity, and the subsequent Jurassic and Cretaceous periods (201–66 Ma) were dominated by the avemetatarsalian lineage — particularly dinosaurs on land and pterosaurs in the air — while crocodylomorphs diversified in aquatic and semi-aquatic niches. The Cretaceous–Paleogene extinction event (~66 Ma) eliminated all non-avian dinosaurs and all pterosaurs, but birds and several crocodyliform lineages survived, continuing the archosaur legacy into the Cenozoic Era. Today, with approximately 10,000 bird species and 27 crocodilian species, archosaurs remain a significant component of global vertebrate diversity.

One of the most important anatomical distinctions between the two archosaurian lineages involves ankle structure, which directly relates to locomotory capability. Pseudosuchians possess a 'crurotarsal' or 'crocodile-normal' ankle, in which a peg on the astragalus (the medial proximal ankle bone) fits into a socket on the calcaneum (the lateral proximal ankle bone). This flexible arrangement permits both a sprawling posture and a semi-erect 'high walk', giving pseudosuchians a versatile range of gaits. Avemetatarsalians evolved an 'advanced mesotarsal' ankle, in which the astragalus is greatly enlarged and the calcaneum is reduced, creating a simple hinge joint that restricts movement to a single parasagittal plane. This arrangement is suited to an erect, bipedal posture and was inherited by dinosaurs and subsequently by birds. The two ankle types were among the first features used to recognize the fundamental division within Archosauria, beginning with the work of Sankar Chatterjee in 1978 and A.R.I. Cruickshank in 1979.

The concept of Archosauria provides the essential phylogenetic framework for understanding what a dinosaur is. Dinosauria is a clade nested within Avemetatarsalia, which is itself nested within Archosauria. Understanding archosaurs is therefore prerequisite for understanding dinosaurs in their evolutionary context: dinosaurs inherited their erect bipedal posture, four-chambered heart, efficient respiratory system, and many other features from their archosaurian ancestors. Recognizing that birds are living archosaurs — and specifically, living dinosaurs — is one of the most important conceptual advances in modern paleontology, and it rests on the phylogenetic framework established by the definition of Archosauria as a crown group encompassing both avian and crocodilian lineages.