Archosaur

Archosaurs are a fascinating group of reptiles that includes some of the most famous animals in history. The two living members of this group today are birds and crocodilians. However, archosaurs also include many extinct creatures like non-avian dinosaurs, pterosaurs, phytosaurs, aetosaurs, and rauisuchians, as well as many Mesozoic marine 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. Older definitions of archosaurs focused on physical features like a special opening in the skull called an antorbital fenestra, serrated teeth, and an upright stance. Some older reptiles had these features too but lived before the split between the crocodile and bird lineages.

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 huge mass extinction event called the Permian-Triassic extinction about 252 million years ago, archosaurs began to thrive. They became the dominant land animals from the Middle Triassic period until the end of the Cretaceous period about 66 million years ago, when an asteroid impact caused a major extinction event. Birds and some crocodilian relatives survived this event 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, which include birds and crocodilians, have special features that set them apart from other four-legged animals. One key trait is their teeth, which sit in deep sockets in their jaws. This helped keep their teeth secure while they ate. They also have openings in their skulls called antorbital fenestrae and in their lower jaws called mandibular fenestrae. These openings helped make 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, by lacking a special sense organ called the vomeronasal organ. These unique traits helped archosaurs thrive, even surviving major events in Earth's history.

Origins

Archosaurs are a special group of animals that come from a larger group called archosauriforms, which are themselves 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, including some large meat-eating ones that looked like crocodiles, found in places like Russia and China. The earliest fossils related to birds were found in Tanzania and 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 event, archosaurs and related animals quickly became the dominant land animals in the early Triassic period.

One reason archosaurs became so successful might be that they could move more efficiently. Another reason could be that archosaurs had better respiratory systems, allowing them to breathe more effectively, especially in low-oxygen conditions. The early Triassic was also a dry time, and archosaurs may have been better at saving water than synapsids, partly because they could excrete waste as a paste rather than dilute urine.

Main forms

Scientists classify archosaurs mostly by how their ankles are built. Early archosaurs had ankles where certain bones were fixed together. Later, some groups developed different ankle structures that allowed them to walk more efficiently.

One group, the Pseudosuchia, developed ankles that could rotate, which helped them move in different ways. Another group, the Avemetatarsalia, had a special kind of ankle that made running easier. These animals also had other unique features, like long necks and lightweight bodies. Some of them, such as dinosaurs and pterosaurs, became very well-known from fossils found from the Middle Triassic period onward.

Classification

Archosauria is a group of reptiles that includes birds and crocodilians as the only living members today. This group also includes many extinct relatives such as dinosaurs, pterosaurs, and other ancient reptiles. Scientists define Archosauria as a crown group, meaning it includes only the descendants of the most recent common ancestor of birds and crocodilians.

The term "Archosauria" was first used in 1869 by paleontologist Edward Drinker Cope. Over time, scientists have refined how they classify these reptiles. Today, they understand that archosaurs split into two main lines: one leading to crocodilians and the other to birds and their extinct relatives like dinosaurs and pterosaurs. This classification helps scientists study the evolution and relationships among these fascinating creatures.

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 called the Cretaceous–Paleogene extinction event happened. 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 actually 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 allowed them to stand and run more efficiently. This upright stance needed more energy, suggesting these animals 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 took to the skies. Some dinosaurs, like Spinosaurus, might have spent time in water.

The metabolism of archosaurs remains debated. 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 would explain their active lifestyles and erect postures. Studies on alligators show they have a unique way of breathing that allows air to flow in one direction, a feature also found in birds and many dinosaurs, which may have helped them thrive in low-oxygen conditions.

Main article: Metabolism in archosaurs

Reproduction

Most archosaurs lay eggs. Birds and crocodilians lay hard-shelled eggs, just like extinct dinosaurs and some other ancient reptiles. However, some pterosaurs and certain ancient reptiles had soft-shelled eggs, showing that hard shells were not always the norm.

Archosaurs were originally able to leave their young to fend for themselves, as seen in many dinosaurs, pterosaurs, and early crocodilian-like reptiles. Later, crocodilians, some dinosaurs, and other reptiles began to take care of their young. Most of these animals bury their eggs, and the sex of the babies depends on the temperature of the nest. Birds, however, sit on their eggs and determine sex genetically, which may have helped them survive better than other dinosaurs.

Main article: temperature-dependent sex determination