The origin of birds has been a topic of debate and research for over 150 years. Birds share many anatomical features with dinosaurs, which suggests they evolved from theropod dinosaurs sometime during the Jurassic or Cretaceous period. Identifying which theropod groups gave rise to birds and deciphering their evolutionary relationships is an ongoing effort by paleontologists.
What are theropod dinosaurs?
Theropod dinosaurs were a diverse group of bipedal saurischian dinosaurs that included all the carnivorous dinosaurs. They first appeared in the Late Triassic period (~230 million years ago) and were the dominant large predators throughout the Jurassic and Cretaceous periods.
Theropods displayed a wide range of sizes, from small meat-eaters the size of chickens to giant apex predators like Tyrannosaurus rex. They walked on two legs, had clawed hands with three fingers, and many later groups had feathers. Theropods were ancestrally carnivorous, although some later groups evolved omnivorous or herbivorous diets.
Well-known theropod groups include the tyrannosaurs, allosaurs, coelurosaurs, spinosaurs, and birds themselves. There is strong evidence that birds evolved from small-bodied coelurosaurian theropods sometime in the Middle-Late Jurassic period.
Evidence that birds evolved from theropod dinosaurs
Birds share over 100 distinct anatomical features with theropod dinosaurs, which indicate they evolved from a dinosaur ancestor, including:
- Bipedal stance. Birds and theropods walked on two hindlimbs.
- Clawed fingers. Three-fingered hands with sharp claws.
- Similar hip structure. Key hip bones are fused together.
- Hollow bones. Many theropods and all birds have pneumatic bones.
- Wishbone. A furcula bone (wishbone) formed by fused clavicles.
- Feathers. Direct evidence of feathers in many theropod fossils.
- Ovary structure. Modern-style ovaries in birds and coelurosaurs.
Fossil specimens like Archaeopteryx and the feathered dinosaurs of China provide irrefutable evidence that birds descended from maniraptoran theropods. No other dinosaur group shows as many bird-like characteristics.
Key theropod groups leading to birds
The transition from theropod dinosaurs to birds occurred over tens of millions of years. Here are some of the key theropod groups involved in this transition:
Coelurosauria
Coelurosauria is the clade containing all theropods more closely related to birds than to carnosaurs. They first appeared in the Middle Jurassic. Key traits include:
- Lightweight build
- Longer arms and hands
- Feather precursors known from skin impressions
- Higher metabolism than other theropods
Major coelurosaurian groups include tyrannosauroids, compsognathids, ornithomimosaurs, therizinosaurs, alvarezsaurs, and maniraptorans.
Maniraptora
Maniraptora is the clade containing all coelurosaurs more closely related to birds than to ornithomimosaurs. This diverse group first appeared in the Middle-Late Jurassic and includes modern birds. Key traits include:
- Feathers, proven by many exceptional fossils (e.g. Microraptor)
- Wishbone (fused clavicles)
- Wings with flight feathers on arms
- Bird-like shoulders and arms
- Reduced fingers (3 or less)
Major maniraptoran clades include alvarezsaurs, therizinosaurs, oviraptorosaurs, troodontids, dromaeosaurs, and early birds.
Paraves
Paraves includes the most bird-like maniraptoran theropods. This group from the Late Jurassic/Early Cretaceous includes true flying birds and other close relatives. Key traits are:
- Large sternal keel (for flight muscle attachment)
- Loss of bony tail
- Enlarged forebrain and optic lobes
- Increased pneumatization of skeleton
Major paravian groups are dromaeosaurids, troodontids, and early birds like Archaeopteryx.
Which theropods were closest to birds?
Determining exactly which theropods were most closely related to modern birds is an active area of research. Some key points:
- Most evidence indicates paravians were the sister group to true birds.
- Dromaeosaurids like Velociraptor and troodontids like Anchiornis seem to form a clade with birds.
- Archaeopteryx and close relatives like Xiaotingia were very early flying birds.
- Feathered dinosaurs like Microraptor may be very close to the first true birds.
Ongoing fossil discoveries continue to clarify relationships. Dromaeosaurs, troodontids, and early birds seem to form a closely interrelated group at the base of the avian radiation.
When did birds evolve from theropods?
The transition from maniraptoran theropods to the first primitive birds is thought to have occurred sometime in the Middle to Late Jurassic period, between 165-150 million years ago. Some key points:
- Feathered non-avian dinosaurs appeared ~165 mya
- Archaeopteryx from ~153-151 mya could fly
- Diversification of major bird groups by the Early Cretaceous
The timing of this transition is supported both by morphological changes and the molecular evidence from living birds, which diverged from other dinosaurs sometime in the Early-Middle Jurassic.
| Time period | Evolutionary events |
|---|---|
| Early Jurassic | Origin of coelurosaurian theropods |
| Mid Jurassic | Origin of maniraptoran coelurosaurs with feathers |
| Late Jurassic | Origin of paravians closely related to early birds |
| Late Jurassic | Earliest known birds like Archaeopteryx |
Conclusion
Birds evolved from small maniraptoran theropods sometime in the Mid-Late Jurassic period. Feathered paravians like dromaeosaurs and troodontids were their closest dinosaur relatives and probably represent a branch very close to true birds. Ongoing fossil discoveries continue to improve our understanding of bird origins, but it is clear they descended from bipedal predatory dinosaurs over 150 million years ago.
