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Relationship of Reptiles and Birds

Updated on September 7, 2017
Archaeopteryx lithographica
Archaeopteryx lithographica | Source

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!

Reptile Characteristics

  • 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.

Bird Characteristics

  • 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 classification of birds.
New classification of birds. | Source

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).

Click thumbnail to view full-size
Digital illustration of the coelurosaurian dinosaur Sinosauropteryx prima, based on the holotype specimen. Coloration and pattern follows Zhang et al 2010.
Digital illustration of the coelurosaurian dinosaur Sinosauropteryx prima, based on the holotype specimen. Coloration and pattern follows Zhang et al 2010.
Digital illustration of the coelurosaurian dinosaur Sinosauropteryx prima, based on the holotype specimen. Coloration and pattern follows Zhang et al 2010. | Source

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!

Resources Used

Hutchinson, John. DinoBuzz. Are Birds Really Dinosaurs? January 22, 1998.


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    • Teresa Coppens profile imageAUTHOR

      Teresa Coppens 

      6 years ago from Ontario, Canada

      Thanks aviannovice. So happy too hear from you again. Your approval means a lot being the 'queen of biirds' in my eyes!!!! Glad you enjoyed it!

    • aviannovice profile image

      Deb Hirt 

      6 years ago from Stillwater, OK

      Wonderful! I love it. I haven't seen anything on birds relating them to dinosaurs for quite a nymber of years. Awesome, up, and RIGHT ON TARGET!

    • Teresa Coppens profile imageAUTHOR

      Teresa Coppens 

      6 years ago from Ontario, Canada

      Thanks so much for the reference Krist. I will check it out in more detail. Glad you found the hub interesting and thanks so much for the share!

    • Teresa Coppens profile imageAUTHOR

      Teresa Coppens 

      6 years ago from Ontario, Canada

      crystolite, so happy you enjoyed the hub. Glad I could be of assistance!

    • Teresa Coppens profile imageAUTHOR

      Teresa Coppens 

      6 years ago from Ontario, Canada

      Nate, glad you enjoyed the hub. I have always loved prehistory and the fact that dinosaurs still do walk the earth I find somehow comforting. Life continues no matter what the challenges!

    • KrisL profile image


      6 years ago from S. Florida

      Enjoyed this hub.

      I knew a bit about the origins of birds, but this filled in some of the blanks.

      You might also want to include a bit on the feathered dinosaurs: here's a wikipedia article that even has a reconstructed picture you can use:

      Voted "interesting" and shared.

    • crystolite profile image


      6 years ago from Houston TX

      I have always taught that birds evolved from reptile genera. Thanks for satisfying my conscience. With this, am convinced life has always developed from a general source. Thanks for sharing.

    • NateB11 profile image

      Nathan Bernardo 

      6 years ago from California, United States of America

      Very interesting to read about the similarities and differences between birds and reptiles and also the relationship between dinosaurs and birds. I like the idea of witnessing birds' behaviors and relating it to dinosaurs of pre-history. Thanks for sharing this fascinating information!

    • Teresa Coppens profile imageAUTHOR

      Teresa Coppens 

      6 years ago from Ontario, Canada

      Thanks boys for you positive feedback. I too love watching the birds and imagine my own backyard Jurassic Park. Creatures never really die out. Something of them survives and as in this case transforms into another form of beauty and innovation. Life is beautiful and amazingly versatile. I love it that others appreciate that too. Thank you gentlemen!

    • JKenny profile image

      James Kenny 

      6 years ago from Birmingham, England

      I loved this Teresa, reading this reminded me of a hub I wrote about what would it be like if dinosaurs had survived into the present era. Of course the ultimate irony is that they did, and in fact there's more species of them than mammals. So, in a way you could say that we're still living in the age of the dinosaurs. I also remember reading somewhere that T-Rex is more closely related to modern birds than even crocodiles.

      I love being able to go outside and watch little feathered dinosaurs flying around. Who needs Jurassic Park eh?

      Well done and thank you for sharing.

    • billybuc profile image

      Bill Holland 

      6 years ago from Olympia, WA

      Sometimes I think I must be the biggest nerd in the world. I love stuff like this! Great hub Teresa! I hope there are other Hubbers out there who will find this as fascinating as I do.


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