There is strong evidence that theropod dinosaurs are the ancestors of modern birds. Theropods were a group of bipedal, mostly carnivorous dinosaurs that included well-known dinosaurs like Tyrannosaurus rex and Velociraptor. Birds share many anatomical similarities with theropods that indicate they evolved from a theropod ancestor sometime during the Jurassic or Cretaceous period.
Similarities Between Theropods and Birds
Birds share dozens of distinct skeletal features with theropod dinosaurs that are not found in other dinosaur groups. Here are some of the most important:
- Bipedal stance. Like birds, theropods walked on two legs. This is in contrast to other dinosaur groups like sauropods and ceratopsians that walked on four legs.
- Hollow bones. The bones of theropods and birds are hollow rather than solid. This is an adaptation that allows for lighter weight.
- Three-toed feet. Theropod dinosaurs and birds both have three main forward-facing toes on their feet.
- Wishbone. Theropods and birds have a furcula, or wishbone. This V-shaped bone is formed by the fusion of the collarbones.
- Feathers. Some non-avian theropod fossils preserve primitive feather-like structures, supporting the link between dinosaurs and birds.
In addition to skeletal features, theropods share other attributes with birds that point to an evolutionary relationship:
- Nesting behavior. Some theropods built nests and brooded eggs, just like modern birds.
- parental care. Fossil evidence shows that some theropod parents cared for their young after hatching.
Evidence from Cladistics
Cladistic analysis provides compelling genetic evidence that birds evolved from maniraptoran theropods during the Jurassic period. In cladistics, organisms are grouped based on shared derived characteristics. Birds and theropods share numerous derived traits that are not found in earlier dinosaur groups. This points to a close evolutionary relationship.
Specifically, birds belong to a group called Paraves, which includes the theropod subgroups Troodontidae, Dromaeosauridae, and Oviraptorosauria. All of these theropod groups show bird-like traits, and they share a more recent common ancestor with birds than with other dinosaur lineages.
Transitional Fossils
Several important transitional fossils have been discovered that bridge the gap between theropod dinosaurs and ancient birds:
- Archaeopteryx – Lived 150 million years ago during the Jurassic Period. Had both dinosaur and bird characteristics like feathers and wings as well as teeth and a long bony tail.
- Microraptor – Lived 125 million years ago. Had wings but retained many dinosaur features. Provides evidence that wings evolved before flight.
- Sinornithosaurus – Lived 125 million years ago. One of the first dinosaurs found with primitive feathers covering its body.
These remarkable fossil discoveries demonstrate the evolutionary transition from theropods to birds during the Mesozoic Era.
Evidence from Developmental Biology
Evidence from developmental biology also supports the link between theropods and birds. As embryos develop, birds go through stages that resemble the development of dinosaur snouts and tails. This suggests they share a deep evolutionary history.
In birds, the gene called Sonic hedgehog is responsible for growth of the lower leg bones. In theropod dinosaurs, the same gene controls growth of the fingers. Fossil evidence confirms that theropod hands evolved into the wings of birds. These developmental similarities provide genetic evidence birds arose from theropod ancestors.
Skeptics of the Dinosaur-Bird Link
While the vast majority of scientists agree that birds descended from theropod dinosaurs, a handful of skeptics remain unconvinced. These scientists point to a few supposed differences between birds and dinosaurs that they claim call the dinosaur-bird link into question.
One example is the knees of birds, which bend backwards rather than forwards as in dinosaurs. However, there are good explanations for this adapted knee structure linked to the evolution of flight. It does not negate the overwhelming similarities that point to theropod ancestry.
A few researchers also argue that birds may have evolved from earlier archosaur ancestors, a group that includes crocodilians as well as dinosaurs. But genetic studies firmly place birds within the theropod dinosaur family tree.
Conclusion
In summary, there is overwhelming evidence from many fields of biology that theropod dinosaurs gave rise to birds during the Mesozoic Era. The evolutionary transition from dinosaurs to birds is one of the best documented examples of macroevolution in the fossil record. While a minority of ornithologists remain skeptical, the vast majority of experts fully accept theropods as the direct ancestors of birds.