ArchosaurEdit
Archosaurs are a principal vertebrate group that encompasses the living birds and crocodilians, as well as their extinct kin, most notably the non-avian dinosaurs and the flying pterosaurs. Emerging in the late Permian and diversifying through the Triassic, archosaurs became one of the most successful and enduring reptile lineages, shaping both terrestrial and aerial ecosystems for hundreds of millions of years. The crown group Archosauria includes the two major modern lineages—Birds and Crocodilians—and their numerous extinct relatives, a pattern that reflects both deep evolutionary roots and remarkable adaptive potential. Archosauria Dinosaurs Pterosaur Crocodilians Birds
Archosaurs are distinguished by a combination of skull and postcranial features that reflect their early shift toward erect or semi-erect gait, efficient respiratory and cardiovascular traits, and a tendency toward skull openings that reduce weight while accommodating strong jaw muscles. Among these traits are a pair of openings in the skull in front of the eye socket, the antorbital fenestra, and often additional openings in the jaw, such as the mandibular fenestra. These features, along with a semi-erect stance in many lineages, set archosaurs apart from many other reptiles of their era. Over time, different archosaur subgroups refined these traits in ways that supported fast, energy-efficient locomotion and versatile feeding strategies. Antorbital fenestra Mandibular fenestra Dinosaur Aves Crocodilia
Evolutionary history and diversification
Archosaurs first appear in the fossil record toward the end of the Permian, with early forms that were experimenting with an upright or semi-erect posture and more active lifestyles than many of their contemporaries. The Triassic period marks a pivotal radiation, with the archosaur crown line splitting into two expansive routes: the avemetatarsalian (bird-line) branch and the pseudosuchian (crocodile-line) branch. This divergence laid the groundwork for the later dominance of both groups in different ecological niches. Permian Triassic Avemetatarsalia Pseudosuchia
Avemetatarsalia and the bird-line
Within Avemetatarsalia, the lineage that would lead to Birds underwent a dramatic transformation. Small, feathered theropod dinosaurs became the closest living relatives of modern birds, and through a long history of experimentation with flight and perched life, they culminated in the evolution of powered flight and a suite of avian adaptations. Today, birds are regarded as living dinosaurs, among the most diverse and ecologically successful vertebrates on Earth. The origin of birds remains a central topic of study, with evidence from skeletal fusion, feather evolution, and the unusual limb proportions that characterize many early avian relatives. Origin of birds Theropod Pterosaur
Pseudosuchia and the crocodile-line
On the crocodile line, pseudosuchians diversified into a variety of forms that occupied aquatic, semi-aquatic, and terrestrial habitats. Crocodilians, as the sole surviving representatives of this branch, exemplify the sustained success of archosaurs in freshwater and coastal environments. The long survival of crocodilians through multiple mass extinctions highlights a conservative, effective body plan well suited to ambush predation, versatile snorkeling and aquatic life, and robust parental care in many lineages. Crocodilians Pseudosuchia
The end-Cretaceous and after
The latter part of the Mesozoic era saw the global dominance of non-avian dinosaurs, followed by a mass extinction at the end of the Cretaceous that fundamentally reshaped terrestrial life. Birds, the remaining lineage of archosaurs, persisted and diversified, filling ecological roles left vacant by the dinosaurs. Crocodilians also endured, maintaining a long fossil and living record that testifies to their enduring design. Cretaceous–Paleogene extinction event Dinosaurs Birds
Anatomy, physiology, and ecology
Archosaurs share a number of anatomical themes that facilitated their ecological versatility. The skull openings described above reduce weight and allow for robust jaw musculature, which supports diverse feeding modes—from massive herbivory to agile predation. The postcranial skeleton in many archosaurs shows adaptations for an upright or semi-erect gait, enabling more efficient propulsion on land and, in several lineages, powerful running, soaring, or swimming capabilities. In addition, archosaurs developed a combination of respiratory and cardiovascular features that supported high metabolic rates in some descendants, especially the birds. Archosaur Antorbital fenestra Mandibular fenestra Dinosaur Birds Crocodilians
Pterosauria and the flight problem
Pterosaurs, flying archosaurs, demonstrate one phase of archosaur evolution focused on aerial locomotion. While not dinosaurs, they share the archosaural heritage and illustrate the breadth of ecological experimentation within the group. Their wing structures, feather-like integuments in some forms, and varied life habits highlight the adaptive plasticity of archosaurs in the Mesozoic skies. Pterosaur Flight Avemetatarsalia
Birds as living dinosaurs
The status of birds as living dinosaurs is one of the clearest expressions of archosaur diversification. Birds retain many ancestral archosaur traits in a modern context while evolving distinctive adaptations such as endothermy, alveolar air sacs, and a highly specialized respiratory system that supports sustained flight. The relationship between birds and their dinosaur forebears continues to inform debates about behavior, metabolism, and evolution in deep time. Origin of birds Dinosaur Aves
Controversies and debates
As with many broad and ancient groups, discussions about archosaurs involve ongoing revisions as new fossils, imaging, and analytic methods come online. Some of the notable debates include:
The precise definition of Archosauria and its subgroups, and how best to rank Avemetatarsalia, Ornithodira, and related clades in modern classification systems. Contemporary paleontology frequently revisits cladistic results to reflect new data. Archosauria Avemetatarsalia Ornithodira
The origin of flight in birds and the sister relationships among early avian relatives. The evidence from feathered dinosaurs, trackways, and skeletal morphology continues to shape whether flight evolved primarily through gliding precursors, wing-assisted incline running, or other pathways. Origin of birds Feathered dinosaurs
The ecological and behavioral implications of the end-Cretaceous extinction. While birds and crocodilians survived, the extinction event dramatically altered ecosystems, prompting reinterpretations of Mesozoic food webs and the adaptive radiations that followed. Cretaceous–Paleogene extinction event Dinosaur Birds
The status and interpretation of pterosaurs within Avemetatarsalia and the broader archosaur tree. Ongoing discoveries refine their placement relative to dinosaurs and the evolution of flight within archosaurs. Pterosaur Avemetatarsalia Ornithodira