The question of which evolved first, dinosaurs or birds, has been debated by paleontologists for over a century. Both dinosaurs and birds are archosaurs, meaning they descended from a common ancestor. The traditional view was that dinosaurs evolved first, during the Triassic period over 230 million years ago, and later gave rise to the first primitive birds during the Jurassic period around 150 million years ago. However, discoveries over the past few decades have challenged this narrative, suggesting that many bird traits evolved much earlier in small feathered dinosaurs. So did dinosaurs come first, or did bird-like dinosaurs exist alongside other dinosaurs? Let’s take a detailed look at the evidence.
When did the first dinosaurs appear?
The earliest undisputed dinosaur fossils date to around 230 million years ago, during the latter part of the Triassic period. Some of these early dinosaurs include Eoraptor, Herrerasaurus, and Plateosaurus. They were small to medium-sized bipedal predators and herbivores that lived during the Carnian and Norian ages of the Triassic.
Some possible older dinosaur fossils are known, but they are more controversial. Tracks from Poland date to around 250 million years ago and may be dinosaurian. Primitive dinosauromorphs like Marasuchus, Lagerpeton, and Pseudolagosuchus appeared between 245-230 million years ago and may have been very close to true dinosaurs. But most paleontologists consider the well-studied Carnian-Norian dinosaurs to be the first definitive dinosaurs.
When did the first birds appear?
The origin of birds has proven more mysterious. Archaeopteryx is often considered the first true bird, appearing around 150 million years ago during the Jurassic period. But recent discoveries have revealed many more feathered dinosaur fossils that display remarkably bird-like traits earlier in the Mesozoic Era.
In the 1990s and 2000s, spectacular feathered dinosaur fossils emerged from the Liaoning province of China. These included Sinosauropteryx, Protarchaeopteryx, Caudipteryx, and Microraptor. Instead of evolving after classic dinosaurs, these small feathered dinosaurs lived alongside giants like Stegosaurus and Allosaurus during the Jurassic period.
Even older feathered dinosaurs have since been discovered. In 2012, scientists described Daohugou fossils from Inner Mongolia dating to the Middle Jurassic, some 165 million years ago. Tiny feathered Anchiornis huxleyi fossils were dated to 160 million years ago, also from Liaoning. More recently in 2018, exquisitely preserved fossils of the pigeon-sized Juravenator starki were described from Germany, dating to around 160-165 million years ago. It appears to be one of the most bird-like dinosaurs yet known, strengthening the idea that bird traits evolved very deep in the dinosaur family tree.
How bird-like were early feathered dinosaurs?
Let’s take a closer look at how bird-like some of these early feathered dinosaurs were:
Sinosauropteryx
– Lived 125 million years ago during early Cretaceous period
– Small bipedal theropod dinosaur, about 1.5 m (5 ft) long
– Entire body covered in simple filament-like feathers, including tail
– First feathered non-avian dinosaur ever discovered
Protarchaeopteryx
– Lived 125 million years ago during early Cretaceous period
– Turkey-sized feathered theropod dinosaur
– Wing feathers more asymmetrical than Archaeopteryx
– Capable of short flights between trees according to some studies
Microraptor
– Lived 125-120 million years ago during early Cretaceous
– One of the smallest known dinosaurs, about 75 cm long
– Entire body covered in long flight feathers, including legs
– Initial studies suggested it was a glider, but likely capable of powered flight
Anchiornis
– Small feathered troodontid dinosaur that lived 160 million years ago
– Weighed just 110 grams and 34 cm long from head to tail
– Covered in simple body feathers, with large wing feathers on arms
– Shape of wings and legs suggest it may have been capable of powered flight
Were early birds just feathered dinosaurs?
Based on these sensational Chinese fossils, it appears that many traits we associate with birds like feathers, wings, and flight evolved very early in feathered theropod dinosaurs, tens of millions of years before Archaeopteryx. This fuzzes the line between dinosaurs and birds. Rather than a linear progression from dinosaurs to birds, it seems dinosaurs were already experimenting with bird-like anatomical features deep in the dinosaur family tree during the Middle-Late Jurassic.
This has led some scientists to argue that birds aren’t so much descended from dinosaurs, but actually are a type of dinosaur that evolved flight. Under this view, the classic bird-like dinosaurs like Velociraptor and Microraptor were actually primitive birds, and Archaeopteryx represents a more advanced bird. We could think of early birds and feathered dinosaurs as all different types of “avian dinosaurs”.
However, others argue there are still important differences between primitive feathered dinosaurs and Archaeopteryx that mark the evolution of true birds, such as:
Key differences between feathered dinosaurs and Archaeopteryx:
Feature | Feathered dinosaurs | Archaeopteryx |
---|---|---|
Wishbone | Absent | Present |
Wing-folding mechanisms | Missing | Present – allowed wings to fold neatly against body |
Perching foot | Non-perching foot | Hallux reversed allowing grasp of branches |
So while feathered dinosaurs had many bird-like characteristics, Archaeopteryx possessed subtle additional skeletal features that represented a more advanced aerodynamic design. Most paleontologists still consider Archaeopteryx the first indisputable bird, while feathered dinosaurs were bird-like dinosaur ancestors of modern birds.
Did birds descend from theropod dinosaurs?
While the origins of birds were mysterious for a long time, evidence has mounted that birds did indeed descend from theropod dinosaurs. Feathered dinosaursfilling the gap between classic dinosaurs and Archaeopteryx lend strength to this idea.
Several anatomical similarities link birds to theropod dinosaurs:
Skeletal similarities between birds and theropod dinosaurs:
Feature | Theropod dinosaurs | Birds |
---|---|---|
Hip structure | Pubis points backwards | Pubis points backwards |
Wrist bones | Half-moon shaped | Half-moon shaped |
Shoulder girdle | Coracoid and scapula fused | Coracoid and scapula fused |
Arm/forelimb bones | Long three-fingered hands | Long three-fingered hands |
Lungs | Unidirectional airflow | Unidirectional airflow |
Molecular evidence also links birds and dinosaurs. Extracting proteins from Brachylophosaurus fossils, scientists matched them to ostrich and chicken proteins, showing an evolutionary relationship. Additionally, microscopic examination of dinosaur bone cells and blood vessels reveal strong similarities to birds as well.
Did birds descend directly from feathered dinosaurs?
While the dinosaurian ancestry of birds is now well-established, the exact dinosaur ancestors of modern birds is less clear. Archaeopteryx was long considered a direct ancestor, but it is too primitive in some respects. Feathered dinosaurs found in China show more similarities to today’s birds.
Several small feathered dinosaurs have been proposed as potential direct ancestors:
Candidates for direct bird ancestors:
Species | Age | Evidence as bird ancestor |
---|---|---|
Anchiornis | Late Jurassic, 160 million years ago | – Large wing feathers on arms – Feet resemble modern perching birds |
Xiaotingia | Late Jurassic, 155 million years ago | – Very bird-like in skull anatomy – Small body with long legs and arms |
Microraptor | Early Cretaceous, 125 million years ago | – Wings on both arms and legs for four-winged gliding – Skull and hip bones very bird-like |
However, the exact dinosaur that gave rise to the first birds remains controversial. The evolutionary lineage from feathered dinosaurs to modern birds likely includes many species in a branching tree rather than a straight line. There was a gradual transition from non-avian dinosaurs to primitive birds over tens of millions of years.
How do we resolve whether birds or dinosaurs came first?
The discovery of feathered dinosaur fossils in the 1990s and 2000s blurred the lines between dinosaurs and birds. Rather than a linear progression from dinosaurs to birds, it now appears small bird-like dinosaurs were already experimenting with feathers, wings, and gliding flight alongside the first true dinosaurs over 200 million years ago.
Primitive bird-like dinosaurs evolved very early in the evolution of dinosaurs. But true birds like Archaeopteryx with advanced aerodynamic features did not appear until closer to 150-160 million years ago.
So in summary:
Dinosaurs came first, but soon evolved bird-like characteristics
– The first undisputed dinosaurs are known from 230 million years ago
– Feathered dinosaurs emerged soon after dinosaurs, as early as 200 million years ago
– These small feathered dinosaurs were experimenting with bird-like traits deep in the dinosaur family tree
– Archaeopteryx at 150 million years ago is considered the first true bird
Rather than a linear progression from dinosaurs to birds, it’s now clear dinosaurs were beginning to acquire bird-like adaptations like feathers very early on, even as they diversified into classic dinosaurs like Stegosaurus and Tyrannosaurus that ruled the Jurassic and Cretaceous. Dinosaurs and birds have a very blurred and intertwined evolutionary history over tens of millions of years. But dinosaurs as a broader group still appeared first before the subset of feathered dinosaurs that gave rise to birds evolved their full suite of aerodynamic capabilities. The first dinosaur fossils pre-date the first bird fossils by over 80 million years.
Conclusions
The evolution of birds from dinosaurs has been one of the great detective stories of paleontology over the past few decades. While birds were long mysterious in their evolutionary origins, we now have conclusive evidence that birds descended from small feathered theropod dinosaurs over hundreds of millions of years:
– Feathered dinosaurs filling the gap between dinosaurs and birds demonstrate an evolutionary continuum between the two groups. Birds inherited traits like feathers and wishbones from early feathered dinosaurs.
– Anatomical similarities in the skeletons, soft tissues, and molecular makeup link birds and theropod dinosaurs.
– Small agile feathered dinosaurs like Anchiornis and Microraptor show the gradual acquisition of bird-like characteristics deep in the dinosaur family tree.
However, while feathered dinosaurs had bird-like traits, the first undisputed birds like Archaeopteryx had evolved subtle additional skeletal adaptations that allowed for powered flight. So while the first dinosaurs predate Archaeopteryx by tens of millions of years, birds evolved their unique aerodynamic skills gradually over long spans of time by building upon frameworks that first evolved in their close dinosaur relatives. The intertwined evolutionary history of dinosaurs and birds played out over hundreds of millions of years, as bird-like dinosaurs experimented with the transition to true flapping flight.