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Relationship of Reptiles and Birds
Traditional Classification of Birds and Reptiles
Birds and reptiles have traditionally been classified into separate classes within the phylum Chordata or vertebrates. Birds were grouped into the class Aves while reptiles were placed in the class Reptilia. New fossil evidence as well as genetic studies have caused a shift in the traditional classification system. Birds and reptiles have many of their own unique characteristics but they also share many similar features. These new studies have led some to call birds avian dinosaurs and yet others to classify birds within the group Reptilia. Read on and you be the judge!
- have scales not feathers
- usually lay eggs but sometimes bare live young
- most lay leathery-shelled eggs on land
- most reptiles do not care for eggs once laid
- young hatched in adult form
- ear holes instead of ears
- most are quadrapeds with four legs or no legs
- haave a heavier, more solid bone structure
- lower cardiac output than birds but higher than amphibians due to incomplete double circulation of blood
- cold-blooded (ectothermic)
- have a small brain size compared to body size
- have jaws with teeth
- lungs have subchambers called flaveoli
Amniotes are a terrestrial vertebrates hhaving eggs characterized by several layers of protective membranes (the amnion, chorion and allantois). Reptiles, mammals, and birds are all amniotes.
- warm-blooded (homeothermic)
- lay hard-shelled eggs
- eggs require incubation by the bird to hatch
- birds hatch featherless and helpless; require nurturing by one or both parents
- they have no teeth but instead have a bill which is a bony, keratin-covered projection forming their mouth
- have wings (even flightless birds have at least vestigial wings)
- most of their bones are pneumatic, meaning they are hollow and filled with air spaces connected to the respiratory system.
- four-chambered heart with high cardiac output due to complete double circulation of blood
- have a high efficient metabolism
- they are bipedal
- highly developed communication skills
- have a furcula (wishbone) that protects the chest cavity during wing beats
- highly developed navigational skills for migration and returning to food sources
- lungs have air sacs
Anapsids - characterized by skulls without openings (fenestra) near the temples (turtles).
Synapsids - characterized by skulls with one opening in their skull (temporal fenestra) behind each eye (mammals).
Diapsids - characterized by two holes (temporal fenestra) in each side their skulls (Snakes, lizards, crocodilians, birds, and dinosaurs)
Characteristics Common To Both Birds And Reptiles
- both are vertebrates
- both lay shelled amniotic eggs
- birds are diapsids; most reptiles are diapsids
- both have scales although in birds scales are limited to the leg region; feathers in birds are modified scales
- both have nucleated red blood cells
- feathers and scales both made from protein keratin
- both have hearts with a right systemic aortic arch
- both have the same jaw structure made up of 5 fused bones
- neither have sweat glands
- both have one occipital condyle as compared to mammals which have two
- both have only one middle ear bone
- both have sclerotic rings, which are rings that go around their eyes and support them.
- both baby birds and reptiles start out with an egg tooth designed to help them break out of their shell
New Classification Of Birds
New genetic studies by a team of Boston researchers have confirmed that turtles are more closely related to crocodilians and birds (archosaurs) than to lizards, snakes and tuatara (lepidosaurs).
Evolutionary Origin Of Birds From Dinosaurs
New studies have provided strong evidence that birds are reptiles which evolved from a particular group of dinosaurs—the Theropods, making them close relatives of T. Rex. Birds are avian dinosaurs and technically considered reptiles. Birds have scales on their legs and feathers are produced by tissues similar to that producing scales in reptiles. Birds also lay eggs like most reptiles. As well they have similar soft anatomy including their musculature, brain, heart and other organs.
Birds are thought to be most closely related to coelurosaurian dinosaurs so much so that birds are considered to be coelurosaurs. They share the following skeletal characteristics:
- the pubis shifted in both from an anterior to a more posterior position.
- elongated arms and forelimbs and clawed hands
- flexible wrist
- large eye orbits
- hollow, thin-walled bones
- reduced tail
- neck curved in a S-shape
- similar egg-shell microstructure
- presence of a furcula
- possibly feathers
- feet positioned directly below the body
Next time you watch a falcon diving for a pigeon or blue jays darting for food at your backyard feeder understand that you are gaining some insight into what extinct dinosaurs were like. Although they have had millions of years to evolve many differences, they are still closely linked by their genetics to the T.Rex, Velociraptor and other coelurosaurian dinosaurs. Dinosaurs never really went extinct. They are still with us in a variety of forms.. Think about that the next time you watch the sparrows and chickadees at your back yard feeder. They come from impressive and often fearsome stock!
Hutchinson, John. DinoBuzz. Are Birds Really Dinosaurs? January 22, 1998.