The term “archosaur” translates to “ruling reptiles,” a name hinting at their impressive evolutionary history. This group of reptiles first appeared around 250 million years ago and includes a diversity of animals, from extinct giants to familiar creatures still with us today. Scientifically, Archosauria is a crown group that includes the most recent common ancestor of all living birds and crocodilians, along with all of its descendants. This lineage has shaped terrestrial life for hundreds of millions of years.
Defining Archosaur Traits
A set of shared anatomical features, or synapomorphies, distinguishes archosaurs from all other reptiles. One of the most significant is the antorbital fenestra, an opening in the skull located between the eye socket and the nostril. This feature is thought to have lightened the skull without compromising its structural integrity, an important adaptation for these active animals.
Another defining characteristic is the mandibular fenestra, an opening in the lower jawbone that likely helped to reduce its overall weight. Many archosaurs also possessed teeth set in deep sockets, a condition known as thecodonty. This made their teeth less likely to break off during feeding, a distinct difference from many other reptiles whose teeth are in a shallow groove.
A third unifying trait is found on the femur, or thigh bone. Archosaurs possess a prominent ridge called the fourth trochanter, which served as a strong attachment point for powerful leg muscles. This feature is linked to a more erect posture, with limbs held more directly under the body rather than sprawling out to the sides. This stance allowed for more efficient and sustained movement.
The Two Major Branches
Early in their evolutionary history, archosaurs split into two distinct lineages that persist to this day. This division separates the group into what paleontologists informally call the “crocodile-line” and the “bird-line” archosaurs. Each branch embarked on a unique evolutionary path, giving rise to forms that occupied different ecological niches.
The first of these branches is Pseudosuchia, which translates to “false crocodiles.” This name is somewhat misleading, as this group includes all modern crocodilians—crocodiles, alligators, caimans, and gharials—and their extinct relatives more closely related to them than to birds. During the Triassic Period, this lineage was diverse and included armored, herbivorous aetosaurs and large-bodied terrestrial predators like rauisuchids. While modern pseudosuchians are semi-aquatic ambush predators, their ancestors exhibited a wider range of lifestyles, including fully terrestrial forms that walked with an upright gait.
The second major branch is Avemetatarsalia, meaning “bird ankles,” which encompasses all archosaurs more closely related to birds than to crocodiles. This lineage includes well-known extinct animals like non-avian dinosaurs and the flying pterosaurs. It is from this branch that modern birds evolved, making them living dinosaurs. A feature of this group is a specialized ankle structure that moves like a simple hinge, which provided stability for running on two legs.
Rise to Dominance
The archosaurs originated in the late Permian period but were relatively obscure components of the global fauna. Their ascent to dominance was a direct consequence of the Permian-Triassic extinction event around 252 million years ago. This event was the most severe mass extinction in Earth’s history, wiping out approximately 70% of terrestrial vertebrate species and creating an ecological vacuum.
Before the extinction, terrestrial ecosystems were dominated by synapsids, the group that includes the ancestors of mammals. With the demise of most of these competitors, archosaurs were positioned to take over. The arid climate of the subsequent Triassic period may have favored the more drought-resilient archosaurs. They rapidly diversified, spreading across the supercontinent of Pangea.
This diversification was not instantaneous but occurred in phases. Following an initial survival period, archosaurs underwent a major evolutionary radiation as global ecosystems began to stabilize. Their numbers and diversity increased, establishing them as the dominant land vertebrates for the remainder of the Mesozoic Era. This interval is often called the “Age of Reptiles” but is more accurately described as the “Age of Archosaurs.”