Archosaur

Archosaurs are a fascinating group of reptiles. The two living members of this group today are birds and crocodilians. Archosaurs also include many extinct creatures like non-avian dinosaurs, pterosaurs, and other ancient reptiles. Scientists define archosaurs as all the descendants of the most recent common ancestor of birds and crocodilians.

Archosaurs split into two main branches. One branch, called Pseudosuchia, includes crocodilians and their extinct relatives. The other branch, Avemetatarsalia, includes birds and their extinct relatives such as non-avian dinosaurs and pterosaurs.

The oldest known true archosaur fossils come from the Early Triassic period. However, animals very close to archosaurs appeared even earlier in the Permian period. After a large event that changed Earth long ago, archosaurs began to thrive. They became the dominant land animals from the Middle Triassic period until the end of the Cretaceous period. Birds and some crocodilian relatives survived and went on to diversify in the following Cenozoic era. Today, birds are one of the most successful groups of land animals on Earth.

Distinguishing characteristics

Archosaurs, like birds and crocodiles, have special features that make them different from other four-legged animals. One important trait is their teeth, which fit into deep sockets in their jaws. This helped keep their teeth safe while they ate. They also have openings in their skulls called antorbital fenestrae and in their lower jaws called mandibular fenestrae. These openings made their skulls lighter. Another feature is a ridge on their thigh bones called the fourth trochanter, which gave them stronger leg muscles.

Archosaurs also differ from their close relatives, the lepidosaurs, because they do not have a special sense organ called the vomeronasal organ. These unique traits helped archosaurs survive through many changes on Earth.

Origins

Archosaurs are a special group of animals. They come from a larger group called archosauriforms, which are part of an even larger group called archosauromorphs. The oldest members of these groups lived long ago in the late Permian period. The first true archosaurs appeared during the Early Triassic period, about 247 to 251 million years ago. Scientists have found fossils of early archosaurs from this time. Some looked like crocodiles and lived in places like Russia and China. The earliest fossils related to birds were found in Tanzania. They date from a bit later in the Triassic period.

Archosaurian domination in the Triassic

Synapsids, which include mammals and their extinct ancestors, were the main land animals during the Permian period. However, most of them disappeared in the Permian–Triassic extinction event. After this, archosaurs and related animals quickly became the main land animals in the early Triassic period.

Archosaurs may have become successful because they could move more efficiently. They also had better respiratory systems, which helped them breathe well, especially when there was less oxygen. The early Triassic was a dry time, and archosaurs may have been better at saving water. This was partly because they could excrete waste as a paste instead of dilute urine.

Main forms

Scientists group archosaurs mainly by how their ankles are built. Early archosaurs had ankles where some bones were fixed together.

Later, some groups changed their ankle structures. This helped them walk better.

One group, the Pseudosuchia, developed ankles that could turn. This helped them move in many directions. Another group, the Avemetatarsalia, had ankles that made running easier. These animals also had special traits, like long necks and light bodies. Some, such as dinosaurs and pterosaurs, are famous from fossils found since the Middle Triassic period.

Classification

Archosauria is a group of reptiles. Today, the only living members are birds and crocodilians. This group also had many extinct relatives, such as dinosaurs and pterosaurs.

Scientists call Archosauria a crown group. This means it includes only the descendants of the most recent common ancestor of birds and crocodilians.

The word "Archosauria" was first used in 1869 by a scientist named Edward Drinker Cope. Over time, scientists have learned more about how to group these reptiles. Now, we know that archosaurs split into two main lines. One line led to crocodilians. The other led to birds and their extinct relatives, like dinosaurs and pterosaurs. This helps scientists understand how these animals evolved and are related.

Extinction and survival

Some ancient archosaurs, like crocodylomorphs, pterosaurs, and dinosaurs, survived a big change in Earth’s history called the Triassic–Jurassic extinction event about 200 million years ago. But many other archosaurs disappeared around that time.

Later, around 66 million years ago, another big change happened called the Cretaceous–Paleogene extinction event. This event ended the time of non-avian dinosaurs and pterosaurs. However, birds, which are the only dinosaurs left, and many crocodile-like animals survived. Today, crocodilians — including crocodiles, alligators, and gharials — and birds are thriving. Birds have more species than any other land animals.

Main article: Cretaceous–Paleogene extinction event

Archosaur lifestyle

Archosaurs had different ways of moving. Early ones walked with a sideways gait, but later groups developed hips that let them stand and run better. This upright stance used more energy, meaning these animals likely had higher metabolisms.

Many archosaurs were meat-eaters, but some, like aetosaurs and certain crocodyliforms, ate plants. Others, such as phytosaurs and some crocodiles, lived in rivers and swamps, while pterosaurs and birds flew in the skies. Some dinosaurs, like Spinosaurus, might have spent time in water.

The metabolism of archosaurs is still being studied. While modern crocodiles are cold-blooded, they have features like four-chambered hearts that are usually seen in warm-blooded animals. Some scientists think their ancestors were warm-blooded, which might explain their active lifestyles and upright postures. Studies on alligators show they breathe in a special way that lets air flow in one direction, a feature also found in birds and many dinosaurs, which may have helped them live in low-oxygen conditions.

Main article: Metabolism in archosaurs

Reproduction

Most archosaurs lay eggs. Birds and crocodiles lay hard-shelled eggs, like some ancient reptiles. Some pterosaurs and other old reptiles had soft-shelled eggs.

Many archosaurs left their young to grow alone. Later, some crocodiles, dinosaurs, and other reptiles started to care for their young. Most of these animals bury their eggs. The sex of the babies can depend on the temperature of the nest. Birds sit on their eggs and decide sex differently, which may have helped them survive.

Main article: temperature-dependent sex determination