Sauria

Saurian diversity

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British reptiles, amphibians, and fresh-water fishes (1920) (Lacerta agilis)

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Sauria is a clade of reptiles that includes all living diapsids, with the exception of birds and crocodilians. This group is distinguished by a number of morphological and physiological features that have allowed its members to adapt to a wide range of environments on Earth. The Sauria clade is divided into two main subgroups: Lepidosauria, which includes lizards, snakes, and tuatara; and Archosauria, which encompasses dinosaurs (and consequently birds) and crocodilians.

Classification

The classification of Sauria has undergone significant changes over the years as new fossil discoveries and advances in DNA sequencing technologies have provided a clearer picture of the evolutionary relationships among reptiles. Sauria is now recognized as a monophyletic group, meaning that it includes all descendants of a common ancestor.

Lepidosauria

Lepidosauria is one of the two major groups within Sauria, characterized by the presence of overlapping scales and a unique jaw articulation that allows for a wide range of motion. This group includes:

• Squamata: The largest order of reptiles, which includes lizards, snakes, and amphisbaenians.
• Rhynchocephalia: A once-diverse group that is now represented by a single living species, the tuatara (Sphenodon punctatus), found only in New Zealand.

Archosauria

Archosauria is the other major subgroup of Sauria and is distinguished by several skeletal features, including the presence of an antorbital fenestra, a hole in the skull in front of the eyes. This group includes:

• Dinosaurs: A diverse group of reptiles that dominated terrestrial ecosystems for over 160 million years. Although most dinosaur species became extinct during the Cretaceous–Paleogene extinction event, their descendants, the birds, continue to thrive today.
• Crocodilia: This order includes crocodiles, alligators, caimans, and the gharial. They are closely related to birds and are the only other surviving archosaurs.

Evolution

The earliest known members of the Sauria clade appeared during the late Permian period, over 250 million years ago. The split between Lepidosauria and Archosauria is thought to have occurred in the late Permian or early Triassic period, setting the stage for the diverse evolution of reptiles. Throughout the Mesozoic era, also known as the "Age of Reptiles," dinosaurs became the dominant terrestrial vertebrates, while lepidosaurs diversified into a wide array of ecological niches.

Physiology and Adaptations

Members of the Sauria clade exhibit a wide range of physiological adaptations that have enabled them to colonize diverse environments. These adaptations include variations in metabolism, reproductive strategies (such as oviparity and viviparity), and thermoregulation methods. For example, while most reptiles are ectothermic (relying on external sources of heat to regulate their body temperature), birds, which are part of the Sauria clade, are endothermic and capable of maintaining a constant body temperature through metabolic processes.

Conservation

Many species within the Sauria clade are currently facing threats from habitat destruction, climate change, and pollution. Conservation efforts are crucial to protect these species and preserve biodiversity. Efforts include habitat restoration, legal protection, and research into the biology and ecology of threatened species.