Evolution of reptiles

Reptiles are a diverse group of animals with a history spanning hundreds of millions of years. In traditional terms, reptiles are defined by having scales or scutes, laying hard-shelled eggs on land, and possessing an ectothermic metabolism. Ectothermic means their body temperature is controlled by the environment around them.

Modern science uses a different system called phylogenetic nomenclature. This system groups animals based on their shared ancestry. Under this view, the group Reptilia is identical to Sauropsida. This definition includes birds but excludes mammals and their ancestors.

The origin of reptiles began about 320 to 310 million years ago. They likely evolved from advanced labyrinthodont amphibians in the swamps of the late Carboniferous period. One of the oldest possible candidates for an early reptile is Casineria. Another unquestionable early reptile is Hylonomus, which lived about 315 million years ago. Hylonomus was a small, lizard-like creature measuring only 20 to 30 cm in length. It had many sharp teeth and likely ate insects. While these early reptiles were small, they paved the way for much larger animals later in history.

As reptiles evolved, their skull structures changed in significant ways. Scientists study these changes by looking at the openings in the skull roof. One group, the Synapsida, developed one opening behind each eye. This lineage eventually led to modern mammals. Another major branch is the Sauropsida. Within Sauropsida, the Diapsida group developed two holes in the skull behind each eye. These holes served important biological functions. They helped lighten the weight of the skull and provided more space for jaw muscles. This allowed for a much more powerful bite.

The Permian period saw further specialization among these groups. Synapsids like the pelycosaurs became the first large, fully terrestrial vertebrates. As the climate grew drier in the mid-Permian, these were replaced by more advanced therapsids. Meanwhile, diapsid reptiles split into two main lineages. One group was the Lepidosauromorpha, which includes modern lizards and snakes. The other group was the Archosauromorpha, the ancestors of crocodiles and dinosaurs. These early diapsids remained relatively small and inconspicuous during the Permian period.

The Mesozoic era is often called the "Age of Reptiles." This name was coined by the 19th-century paleontologist Gideon Mantell. During this time, archosaurs became the dominant land animals. They were characterized by elongated hind legs and an upright, erect pose. This group produced the dinosaurs and the pterosaurs. Some dinosaurs grew to be the largest land animals to ever exist. Other lineages, like the mosasaurs, became major predators in the sea.

Evolutionary changes also led to the rise of birds. Birds belong to the biological class Aves and are part of the dinosaur clade Theropoda. The transition from reptiles to birds involved several key developments. These included the evolution of feathers, the development of a beak, and the creation of hollow bones. They also developed warm-bloodedness and the ability to fly. Archaeopteryx lithographica is one of the earliest known bird species from the Late Jurassic.

Today, birds and crocodiles are the only living members of the Archosauria group.

The dominance of large reptiles ended with a mass extinction at the close of the Cretaceous period. Scientists believe this was caused by massive volcanic activity and a meteor impact. This event wiped out the non-avian dinosaurs and many large marine reptiles. Only small, feathered theropods survived to become modern birds. This transition marked the end of the Mesozoic and the beginning of the "Age of Mammals." However, reptiles did not disappear. They continued to diversify throughout the Cenozoic era.

Today, reptiles remain a highly successful group of animals. There are approximately 9,766 extant species of reptiles currently living on Earth. This is nearly twice the number of the 5,400 known mammal species. Most living reptiles belong to the order Squamata. Other groups, like the Testudines, include turtles. Turtles evolved unique shells from their ribs and backbone to protect against predators. While they are slower on land, they remain a vital part of the world's biodiversity.