Birds are warm-blooded animals characterized by toothless beaked jaws, feathers, high metabolic rates, and four-chambered hearts. They can be found worldwide and are considered the most numerically-successful class of tetrapods. There are approximately 10,000 living species of birds, more than 50% of which are passerines, or perching birds. Most birds have wings which evolved from forelimbs, giving them the ability to fly. However, due to evolution, there are several flightless birds, such as ratites and penguins. Are birds reptiles or do they fall into a different classification? According to modern classification, birds are part of the group Aves, however, under certain classification structures, birds can be classified as reptiles.
There are two types of classification systems used by biologists: Linnaean taxonomy and Phylogenetic taxonomy. The Linnaean taxonomy system was created in 1730 by Swedish scientist Carolus Linnaeus and classifies organism by their characteristics rather than their origin or ancestry. According to Linnaean classification, a reptile is any animal that is covered with scales, and therefore birds do not qualify as reptiles under this system. However, the Phylogenetic taxonomy system, created in 1940 by German biologist Willi Henning in 1940, classifies animals based on their ancestry and assumes unique characteristics of animals are considered evidence of their ancestry. According to the phylogenetic classification system, a reptile is any animal which originated from the original group of reptiles, technically including birds and mammals. Therefore, birds are considered reptiles under Phylogenetic taxonomy, just as other animals that descended from vertebrates are called vertebrates.
Birds Are Dinosaurs?
When people refer to birds as reptiles, they tend to mean that birds are more closely related to reptiles than mammals. In particular, birds are more closely related to crocodiles and lizards than other reptiles. The first reptiles split approximately 300 million years ago, and 40 million years later another group called therapsids (modern mammals) branched off. About 120 million years later, several other groups split, including dinosaurs, which were reptiles and distantly related to snakes, lizards, and turtles. However, approximately 65 million years ago dinosaurs were killed during an extinction event, except the feathered dinosaurs. Feathered dinosaurs, also known as avian dinosaurs, had long arms, elongated beaks, and were covered in feathers. Avian dinosaurs evolved over the last 65 million years to become modern birds. Therefore, birds are not just closely related to dinosaurs, they actually are dinosaurs. Additionally, it is this connection that tends to cause birds to be referred to as reptiles. However, the modern definition of reptiles generally refers to tetrapods such as crocodiles, lizards, turtles, and tuataras.
Modern Classification of Birds
According to modern classification, birds are part of the group Aves. This group is subdivided into two subgroups: Palaeognathae and Neognathae. Palaeognathae primarily include flightless and weak-flying birds, while Neognathae contains all other birds. The two subgroups are often classified as a superorder. Birds also have similar characteristic as reptiles. For example, some reptiles have scales, as do some birds, especially on their feet. Birds also lay hard-shelled eggs, like most reptiles. Early birds had non-differentiated teeth, while reptiles also have teeth. However, reptiles have been described as cold-blooded animals, but birds are warm-blooded. Overall, this remains a topic of debate among scientists.