The relationship between birds and dinosaurs is a topic of great interest and debate among paleontologists. Many lines of evidence suggest that birds evolved from theropod dinosaurs during the Jurassic period, and that primitive birds coexisted with non-avian dinosaurs for millions of years. However, some scientists argue that the evidence is inconclusive and that birds may have evolved earlier, before the rise of the dinosaurs. Here we review the evidence surrounding this question and examine the arguments on both sides of the debate.
Evidence that birds evolved from dinosaurs
There is a substantial body of evidence indicating that birds evolved from maniraptoran theropod dinosaurs during the Jurassic:
Feathered dinosaur fossils
Numerous non-avian dinosaur fossils showing clear impressions of feathers and plumage have been discovered over the past 25 years. These include multiple specimens of dromaeosaurs, troodontids, oviraptorosaurs and primitive tyrannosauroids. The feathers display similarities to modern bird plumage. This suggests that feathers evolved first for insulation and display in primitive theropods before being adapted for flight.
Skeletal similarities
Birds share multiple unique skeletal features with theropod dinosaurs that are not found in other animals. This includes a furcula (wishbone), elongated forelimbs, fused clavicles, and a semilunate carpal bone. Analyses of skeletal morphology place birds nested deep within the theropod evolutionary tree.
Lung structure
The unique avian lung design, in which air flows in a one-way loop, is also found in maniraptoran dinosaurs. This provides evidence that the increased respiratory efficiency of the avian lung first evolved in theropods rather than early birds.
Nesting behavior
Many deinonychosaurs and oviraptorosaurs are known to have brooded their nests. Several spectacular fossils preserve an adult dinosaur sitting on top of a nest in a bird-like brooding posture. This indicates that nesting and parental care behaviors pre-dated the origin of birds.
Rapid growth rates
Studies of bone tissue show that primitive theropods had very rapid growth rates, comparable to modern birds. Rapid growth is characteristic of warm-blooded animals, suggesting dinosaurs were endothermic (as are modern birds).
Feathered dinosaurs as transitional forms
The discovery of feathered non-avian dinosaurs has provided paleontologists with a succession of transitional forms illustrating the dinosaur-to-bird transition:
Sinosauropteryx
This small compsognathid dinosaur from Early Cretaceous China preserves clear impressions of downy proto-feathers covering its body. It was not capable of flight but the coat of feathers suggests they already served an insulatory function.
Protarchaeopteryx
Slightly younger Jurassic fossils from China display longer, symmetrical feathers on the arms and tail. This demonstrates an intermediary stage between Sinosauropteryx-like proto-feathers and modern flight feathers.
Microraptor
Multiple exquisitely-preserved Microraptor specimens show extensive feathering on both the forelimbs and hindlimbs. The hindwing feathers demonstrate how flight evolved in stages through exapted gliding and parachuting behaviors.
Anchiornis
This late Jurassic dinosaur had advanced asymmetrical flight feathers on the arms and tail, a toothed beak, and leg scales – a nearly perfect morphological intermediary between non-avian dinosaurs and archaeopteryx-like birds.
Evidence that birds existed alongside dinosaurs
While the consensus view is that birds evolved from dinosaurs in the Jurassic, some paleontologists argue that certain lines of evidence suggest modern birds arose earlier, during the Triassic:
Disputed bird fossils
Some Triassic archosaur fossils like Protoavis have been proposed as possible birds, but these claims are controversial and most experts consider these non-avian dinosaurs. However, a few possibly bird-like footprints are known from Triassic sediments.
Molecular clock estimates
Some analyses that use DNA sequence data to estimate divergence times place the split between birds and other dinosaurs deep in the Early Triassic, implying that primitive birds existed before true dinosaurs originated. However, molecular clocks are highly variable depending on calibration methods.
Anatomical analyses
A few studies have argued that Archaeopteryx and Confuciusornis show more similarities to non-theropod dinosaurs like Herrerasaurus than to dromaeosaurs. This implies birds split off before the theropod lineage bifurcated. But other detailed investigations of anatomy robustly place birds within Maniraptora.
Arguments against Triassic birds
The majority of paleontologists remain unconvinced by the evidence for Triassic birds:
Lack of fossils
There is currently no unambiguous fossil evidence of birds until the Late Jurassic, over 50 million years after the first dinosaurs appeared. This huge gap implies that birds originated in the Jurassic, even if they did descend from Triassic archosaurs.
No bird-like dinos in Triassic
There are no dinosaurs with bird-like features like feathers and wishbones until 30 million years into the dinosaur radiation. If birds split off first, we would expect bird-like traits to show up much earlier in basal dinosaur groups. Their absence suggests theropods became progressively more bird-like over time.
Biogeography
The earliest Jurassic birds like Archaeopteryx are found in the same deposits as feathered dinosaurs, implying they shared habitats and were descendants. There is no geographic separation between Triassic “birds” and Jurassic/Cretaceous theropods that would suggest an early evolutionary divergence.
Lung structure
Since the unidirectional airflow lung seems to have evolved first in maniraptorans, it follows birds inherited this characteristic from dinosaurian ancestors, rather than developing it independently long before.
Conclusion
The vast majority of evidence indicates that birds evolved from dinosaurs during the Middle-Late Jurassic period. There is simply no convincing fossil or anatomical evidence to contradict the well-supported dinosaur-bird link. While a smattering of Triassic fossils have been proposed as early birds, these claims remain speculative.
By the late Jurassic, a succession of non-avian dinosaurs with progressively more bird-like characteristics provide ironclad transitional forms illustrating the evolution of flight feathers, skeletal adaptations for flight, and other avian traits that were inherited by birds. While a few paleontologists still argue for “birds before dinosaurs”, this hypothesis does not adequately explain the fossil record, biomechanical evidence and evolutionary timeline.
In short, all known evidence indicates that birds descended from maniraptoran theropod dinosaurs by the Middle Jurassic, and continued to diversify alongside non-avian dinosaurs until the end-Cretaceous mass extinction. There is no persuasive support for the idea that modern birds existed in the Triassic before the origin of dinosaurs. The fossil record clearly shows that birds and dinosaurs coexisted for over 100 million years.