Sauropsida

Sauropsida (Greek for "lizard faces") is a clade of amniotes, which are animals that can give birth on land or lay eggs with protective shells. It is often thought of as being similar to the class Reptilia, but it includes much more. Sauropsida includes all animals more closely related to modern reptiles than to mammals. This large group has many members, both living and extinct.

One of the most exciting parts of Sauropsida is that it includes birds. Birds are actually a special kind of theropod dinosaurs, which means they are part of the sauropsid family tree. Even though we often think of birds and reptiles as very different, they share a common ancestor. This shows how all living things are connected through evolution.

The group Sauropsida is divided into different parts. The main living groups are called Eureptilia, which includes all modern reptiles and birds, along with some extinct groups. There is also a group called Parareptilia, which is mostly made up of extinct animals. Scientists are still learning more about how these groups are related to each other.

The idea of Sauropsida started in 1864 when a scientist named Thomas Henry Huxley noticed that birds and reptiles had similarities in their fossils. He grouped them together based on this evidence. Since then, scientists have kept studying and learning more about these fascinating animals and their history on Earth.

History of classification

The term Sauropsida ("lizard faces") has a long history, dating back to Thomas Henry Huxley in 1863. Huxley used this term to describe animals he believed were closely related to birds and dinosaurs, based on fossils like Hesperornis and Archaeopteryx. He grouped these animals together, noticing how similar they were to reptiles.

Later, in 1916, E. S. Goodrich redefined Sauropsida to include lizards, birds, and their relatives, separating them from mammals. This helped scientists better understand how these animals evolved differently from mammals. Over time, scientists have continued to study and refine how we classify these animals, especially with new fossil discoveries and better tools for understanding family trees. Today, Sauropsida is often used to include all modern reptiles and birds, helping us see how these groups are connected through evolution.

Evolutionary history

Main article: Evolution of reptiles

Sauropsids, which include reptiles and birds, evolved from early land animals about 320 million years ago during the Carboniferous Period. The Mesozoic Era, lasting from about 250 million to 66 million years ago, is famous for its huge sauropsids, including many dinosaurs that ruled the land, sea, and sky—this time is often called the Age of Reptiles. At the end of the Mesozoic, a major event caused most large sauropsids, including many dinosaurs, to disappear. Birds, however, survived and thrived, becoming one of the most common groups of land animals today.

Phylogeny

The cladogram here shows the family tree of sauropsids, based on a study by M.S. Lee in 2013. It combines genetic and fossil data to show how different groups are related. Turtles, even though they look different, are considered diapsid reptiles by many scientists, though some studies place them among archosaurs.

More recent studies, like those by Laurin & Piñeiro in 2017 and Modesto in 2019, suggest different family trees for early sauropsids. Some scientists also think that groups traditionally called "microsaurs" might actually be early sauropsids, though this is still debated. Other research, such as Simoes et al in 2022, found that certain ancient groups like Captorhinidae are closely related to the common ancestors of all sauropsids and synapsids.

Structure difference with synapsids

The last common ancestor of synapsids and Sauropsida lived around 320 million years ago during the Carboniferous period.

Sauropsids, which include modern reptiles and birds, have a different way of handling waste compared to synapsids (which include mammals). Synapsids produce urea, which needs water to be removed from the body, making it hard for them to live in dry areas. Sauropsids, however, produce uric acid, which can be expelled with feces and does not need much water. This allowed sauropsids to live in many different environments, including very dry ones.

The brains of sauropsids also work differently from those of synapsids. While synapsids have a layered brain structure, sauropsids have a more flexible arrangement. This may help explain why some animals like birds, which are sauropsids, can be very intelligent even with relatively small brains. Some scientists think that even large dinosaurs with small brains, like Tyrannosaurus, might have been smarter than we previously believed.

Main article: Reptiliomorpha