Birds are closely related to a group of reptiles known as archosaurs. The two main groups of archosaurs that are closely related to birds are crocodilians (alligators, crocodiles, caimans, and gharials) and dinosaurs.
Evidence for the dinosaur-bird connection
There is an overwhelming amount of evidence that demonstrates the close evolutionary relationship between theropod dinosaurs and birds. Here are some of the key pieces of evidence:
- Feathers – Feathers are a unique feature of birds, and feather-like structures have been found in many theropod dinosaur fossils. These dinosaur “protofeathers” suggest the two groups share a common feathered ancestor.
- Skeletal similarities – Birds and theropod dinosaurs share many skeletal features, including a furcula (wishbone), elongated forelimbs, and a sacrum composed of fused vertebrae.
- Clavicles – The clavicle (collar bone) of theropod dinosaurs is similar to the furcula of birds.
- Lungs – Birds have a unique highly-efficient respiratory system with air sacs and a flow-through lung design. Evidence suggests theropods had a similar respiratory anatomy.
- Development – The embryonic development of birds is very similar to that of theropod dinosaurs but distinct from other living reptiles.
- Nesting – Many theropods built nests, sat on their eggs, and cared for their young much like modern birds.
- Brain similarities – CT scans of fossil skulls show theropods and birds share similarities in brain structure and organization.
This evidence points to theropod dinosaurs being the closest relatives to birds and their most recent common ancestor.
Dinosaur groups related to birds
Birds evolved within a group of theropod dinosaurs called maniraptorans. There are two main maniraptoran groups that are particularly closely related to birds:
- Dromaeosaurids – These include Velociraptor and Deinonychus. They were medium-sized predators covered in feathers. They possessed a sickle-shaped claw on the second toe and stiffened tails.
- Troodontids – These were small to medium-sized omnivorous theropods like Troodon. They had very large brains and forward-facing eyes like owls. Many possessed feathers.
Characteristics that support a close relationship between these groups and birds include:
- Wishbones
- Feathers
- Bird-like lungs
- Egg brooding behavior
- Shared bone structures in the wrists and hands
There is debate over whether dromaeosaurids or troodontids are the closest relatives to birds. Current evidence suggests dromaeosaurids share more derived characteristics with birds, making them a bit closer evolutionarily. But both groups show evidence of how dinosaurs evolved into birds.
Other dinosaur groups related to birds
In addition to dromaeosaurids and troodontids, some other theropod groups have been suggested to have a relatively close relationship to birds:
- Alvarezsaurids – Small insect-eating theropods like Mononykus. They had very short forelimbs with one large claw.
- Therizinosaurs – Herbivorous theropods like Therizinosaurus with large claws and feathers.
- Oviraptorosaurs – Mostly toothless theropods like Oviraptor with bird-like beaks that brooded their eggs.
The exact placement of these groups relative to birds is debated. However, they do show some anatomical features suggestive of bird ancestry or a close relationship.
Crocodilians and the archosaur connection
Crocodilians (crocodiles, alligators, caimans, gharials) are the closest living relatives of birds, but they are not direct ancestors. Rather, birds and crocodilians share a common ancestor within the archosaur group. Archosaurs include:
- Crocodilians
- Dinosaurs (including birds)
- Pterosaurs
Archosaurs first emerged around 250 million years ago during the Triassic period. They dominated ecosystems throughout the Triassic and Jurassic before pterosaurs went extinct and non-avian dinosaurs died out in the Cretaceous mass extinction ~66 million years ago.
Primitive crocodilians and the first dinosaurs evolved from archosaur ancestors in the Early to Middle Triassic. Birds later evolved within the dinosaur lineage during the Jurassic. Key anatomical clues of the archosaur connection include:
- An antorbital fenestra (hole in skull in front of the eye)
- Mandibular fenestrae (holes in the jaw bones)
- An ectopterygoid bone in the palate
- Diapsid skulls with two openings behind the eye
These give archosaurs a highly characteristic skull morphology distinct from other reptiles. The archosaur group connects crocodilians and dinosaurs, demonstrating their shared common ancestry.
Conclusion
In summary, overwhelming evidence demonstrates that birds evolved from small carnivorous theropod dinosaurs during the Jurassic period. Birds still retain many anatomical, developmental, and behavioral connections to their theropod dinosaur ancestors. Within theropods, dromaeosaurids and troodontids appear to be the closest dinosaurian relatives of birds based on shared derived traits. More broadly, crocodilians share a deep common ancestry with dinosaurs and birds within the archosaur group, which dominated Mesozoic era ecosystems.