Archosaurs are a group of diapsid reptiles that first appeared during the late Permian period and became the dominant terrestrial vertebrates during the Mesozoic era. They include living birds and crocodilians as well as extinct groups like dinosaurs, pterosaurs and many others. But what exactly defines an archosaur and sets them apart from other reptiles?
Distinctive anatomical features
Archosaurs can be distinguished from other reptiles by several key anatomical features:
- Skull has two pairs of temporal fenestrae – holes in the sides of the skull behind the eyes.
- Teeth are set in sockets rather than fused to the jawbone.
- Fourth trochanter on the femur – bony projection used for muscle attachment.
- Ankle joint hinges between the astragalus and calcaneum bones.
The two pairs of temporal fenestrae (openings) in the skull allowed for larger jaw muscles and stronger bites. The hinged ankle joint gave archosaurs more upright posture and agile movement compared to sprawling reptiles. These key adaptations paved the way for archosaurs to flourish and diversify.
Archosauria clade
Archosaurs comprise a monophyletic clade called Archosauria. This means all archosaurs descended from a single common ancestor and share these derived features. Archosauria is divided into two major lineages:
- Crurotarsi – Includes crocodilians and extinct relatives.
- Ornithodira – Includes dinosaurs, pterosaurs, birds and extinct relatives.
Phylogenetics studies based on morphological and genetic evidence consistently recover Archosauria as a natural group. Key synapomorphies (shared derived traits) unite the various archosaurs even when comparing ancient forms to modern birds and crocodilians.
Evolutionary history
The earliest archosaur fossils come from the Permian period about 250 million years ago. These were small generalized forms not belonging to the major lineages. Archosaur diversity exploded in the Triassic period after a mass extinction event wiped out their main competitors, the synapsids (“mammal-like reptiles”).
Major archosaur groups radiated and dominated terrestrial ecosystems throughout the Triassic and Jurassic. Dinosaurs were the most diverse and abundant archosaurs during the later Mesozoic. After the Cretaceous-Paleogene extinction 66 million years ago wiped out dinosaurs (except birds), crocodilians became the only surviving archosaur lineage.
Today there are around 25 crocodilian species and 10,000 bird species representing a small remnant of archosaur diversity. But they remain highly successful organisms in their respective environments.
Distinguishing archosaur groups
While united by common anatomical features, the major archosaur lineages also have unique specializations that distinguish them:
Crocodilians
- Four chambered heart (also shared with birds)
- Powerful jaws and teeth for grabbing prey
- Thick scutes covering body
- Semiaquatic ambush predators
Dinosaurs
- Upright gait with legs under body
- Hands and feet adapted for walking and grasping
- No scales – skin covered in filamentous feathers or keratin
- Dominantly terrestrial herbivores and carnivores
Pterosaurs
- Bones reinforced for flight
- Wings formed by skin and muscle connecting limbs
- No feathers – covered in fur or bare skin
- Toothless beaks in some species
Birds
- Feathers, beaks, light skeletons adapted for flight
- High metabolism, four chambered heart
- Endothermy – able to regulate body temperature
- Small body size, powered flight, song vocalizations
These are just a few of the traits that adapted each archosaur group to different lifestyles and ecosystems.
Archosaur success
Archosaurs rose to dominance because of evolutionary innovations in their anatomy, physiology and behavior. Key factors in their success include:
- Efficient respiratory and circulatory systems
- Agile movement – upright stance, versatile limbs
- Strong bites – massive jaw muscles, teeth in sockets
- Enlarged brains and sense organs – advanced cognition
- Insulatory coverings – scales, feathers, warm-bloodedness
- Rapid growth rates – attaining large size quickly
These adaptations allowed archosaurs to outcompete rival tetrapod groups and exploit new ecological niches. They gave rise to giant long-necked sauropods, swift raptors, armored tanks like ankylosaurs, gigantic aerial pterosaurs and more.
Conclusion
In summary, archosaurs are defined by a suite of anatomical features like fenestrated skulls, ankle joints and teeth in sockets. They comprise a monophyletic group that has existed for over 250 million years. Archosaurs dominated Mesozoic ecosystems and persist today as crocodilians and birds. They became so successful by evolving key physiological, behavioral and biomechanical innovations that allowed them to thrive in diverse environments.