Birds are a group of warm-blooded vertebrates that constitute the class Aves. They are endothermic, have feathers, wings, lay eggs, and are toothless. Birds evolved from theropod dinosaurs during the Jurassic period, around 150 million years ago. As living dinosaurs, birds are most closely related to reptiles, specifically crocodilians and turtles.
Birds are reptiles
Birds share many anatomical and physiological traits with reptiles that point to a close evolutionary relationship. Some of the key similarities between birds and reptiles include:
- Endothermy – Birds and some reptiles can regulate their own body temperature.
- Scaly legs – Bird legs are covered in scales like reptiles.
- Clawed digits – Birds and many reptiles have claws on their fingers and toes.
- Protective integument – Feathers in birds and scales in reptiles protect the body.
- Amniotic egg – Birds and reptiles both lay amniotic eggs with protective shells.
- Nucleic genomes – Birds have similar chromosome numbers to reptiles.
- Skull anatomy – The skull structure of birds resembles that of reptiles.
- Single occipital condyle – Birds and reptiles have one point of articulation between the skull and vertebrae.
- Similar lung structure – Birds and reptiles have rigid lungs.
- Urinary system – Birds and reptiles excrete uric acid, not urea.
- Sexual reproduction – Fertilization is internal in birds and reptiles.
These shared characteristics point to a common ancestry between birds and reptiles. Birds split from reptiles and eventually evolved into the 10,000+ species we see today. But they retain many of the primitive features of their reptilian forebears.
Birds are archosaurs
Within reptiles, birds are most closely related to a group known as archosaurs. Archosaurs include crocodilians, pterosaurs (extinct flying reptiles), and dinosaurs. Genetic and anatomical research has shown conclusively that birds descended directly from small carnivorous dinosaurs called theropods during the Jurassic period.
Archosaurs share unique features including:
- An antorbital fenestra – An extra opening in the skull in front of the eyes.
- A mandibular fenestra – An extra opening on the lower jawbone.
- Teeth set in sockets – Unlike other reptiles where teeth are fused to the jawbone.
- Diapsid skull – Two holes behind each eye orbit.
- An erect stance – Legs directly under the body rather than sprawled out to the sides.
These specialized anatomical traits unite archosaurs as a clade. Birds inherited these features from dinosaur ancestors within this archosaur group.
Birds are theropod dinosaurs
Theropods represent a sub-group of dinosaurs from the clade Saurischia. They were exclusively carnivorous and bipedal with key features like:
- Hollow bones – Also seen in birds.
- Three-toed feet – Also retained in bird species today.
- S-shaped neck – Allows greater flexibility as in birds.
- Forelimbs shorter than hindlimbs – As in modern birds.
- feathers -primitive ‘proto feathers’ found on many theropod fossils.
Well-known theropods include Tyrannosaurus rex, Velociraptor, and Deinonychus. But one sub-group known as maniraptorans, which includes Oviraptor and Troodon, show the most bird-like traits leading to birds’ direct descent.
Evidence birds descended from theropods
Many lines of evidence support birds evolving from theropod dinosaurs:
- Feathered dinosaur fossils – Numerous theropod fossils show clear impressions of feather-like structures.
- Similar bone structure – Bone scans can’t distinguish bird and theropod fossils.
- Three-fingered hands – Theropod arms and bird wings have the same basic structure.
- Egg brooding – Some small theropods likely brooded eggs just as birds do today.
- Shared features – Birds and advanced theropods share over 100 anatomical traits.
- Transitional fossils – Many dinosaurs display a mix of bird and reptile features.
This evidence paints a very clear picture of birds descending from maniraptoran theropods towards the end of the Jurassic period around 150 million years ago. They evolved from small, feathered, bird-like dinosaurs closely related to Archaeopteryx and Microraptor.
Crocodilians are birds’ closest living relatives
Birds and crocodilians are the last two surviving lineages of archosaurs. As birds’ closest living non-avian relatives, crocodilians and birds display many common specialized traits including:
- Four-chambered heart
- Unidirectional breathing
- Coproxysis – Slow digestion
- Cementum -Tissue covering teeth
- Thick horny epidermis
- Similar immune system
- Pedal salivary glands
- Nictitating membrane – Transparent third eyelid
Genetic studies estimate crocodilians and birds diverged 240-250 million years ago. But their shared archosaur anatomy reveals their close evolutionary relationship despite the vast differences between modern bird and crocodilian body forms.
Turtles are another close relative
Turtles are interesting because some scientists also consider them a type of ‘living fossil’ archosaur. Turtles show anatomical parallels with birds including:
- Skull anatomy – Jaw muscles and sense organs similarly placed.
- Neck retraction – Ability to withdraw head and neck into shell.
- Limb gait – Elbows and knees move parallel to the body.
- Rib fusion – For stability and muscle attachment.
- Cloacal respiration – Oxygen absorption through the cloaca.
The turtle shell may have even evolved from fused ribs and vertebrae as an extension of the rib cage. This supports turtles potentially being derived archosaurs. But debate continues around turtle origins and ancestry.
Conclusion
Birds share a unique evolutionary heritage with reptiles and are direct descendants of small feathered theropod dinosaurs. Their closest living relatives are fellow archosaurs, the crocodilians and possibly turtles. But birds display such unique adaptations for flight that they have become an extremely diverse and successful vertebrate class quite distinct from their dinosaur ancestors.