Many people wonder whether dinosaurs are more closely related to modern day birds or lizards. This is an interesting question that paleontologists have been investigating for decades. In order to answer this, we need to look at the anatomical and genetic similarities between dinosaurs, birds, and lizards.
Dinosaur-bird similarities
There are many anatomical similarities between theropod dinosaurs (a group that includes Velociraptor and Tyrannosaurus rex) and modern birds that suggest the two are closely related. Here are some of the key similarities:
- Feathers – Some theropod dinosaurs like Velociraptor had feathers. Feathers are also a defining characteristic of modern birds.
- Wings – Some theropods had early proto-wings made of feathers. The wings of modern birds evolved from these proto-wings.
- Hollow bones – The bones of theropods and birds are hollow, which reduces weight and aids in flight.
- Eggs – Theropods and birds both lay eggs, while other reptiles like lizards give live birth.
- Warm-blooded – Many scientists think theropods were warm-blooded like modern birds. This helped with their active lifestyle.
The discovery of feathered dinosaurs like Sinosauropteryx in China in the 1990s provided some of the strongest evidence yet that dinosaurs evolved into birds. The feathers appeared nearly identical to modern bird feathers.
Other bird-like characteristics of theropods
In addition to major features like feathers and wings, theropod dinosaurs and birds share dozens of other anatomical similarities down to the smallest bones. Here are a few examples:
- Wishbone (furcula) – The wishbone or furcula is a unique bone only found in theropods and birds used for flight.
- Hollow air sacs – Theropods had hollow air sacs in their bones just like birds to facilitate breathing.
- Pneumatic bones – The bones of theropods and birds contain air pockets, making them pneumatic.
- Advanced shoulder girdle – The shoulder of theropods and birds was flexible and mobile to allow for wing flapping.
- Swiveling wrist – Theropods like Velociraptor had a specialized swiveling wrist like modern birds used to fold away their wings.
The overwhelming number of similarities between birds and coelurosaurian theropods (the subgroup of theropods most closely related to birds) shows an evolutionary connection between the two groups.
Dinosaur-lizard similarities
On the other hand, some people point out that dinosaurs share a number of key features with lizards and other reptiles, suggesting they are more closely related to lizards than birds. Here are some of the reptilian features seen in dinosaurs:
- Scales – Many dinosaurs had scales on certain parts of their skin, just like lizards and crocodiles.
- Cold-blooded – Many scientists think dinosaurs were cold-blooded like modern reptiles.
- Slow metabolism – Dinosaurs may have had slower metabolisms resembling reptiles more than the high metabolisms of birds.
- Upright limb posture – Dinosaurs and lizards walk in an upright posture unlike the sprawling posture of crocodiles and early archosaurs.
It’s true that dinosaurs share some characteristics with both modern bird and lizards. However, most of the lizard-like features are primitive traits seen in all archosaurs (the group that includes crocodiles, dinosaurs, and pterosaurs). Birds evolved from coelurosaurian theropods, a subgroup of dinosaurs that had already started evolving away from these primitive lizard-like traits.
The archosaur family tree
Dinosaurs, including theropods, are archosaurs. Archosaurs include crocodiles, pterosaurs, and all extinct dinosaur lineages. Here is a simplified cladogram of the archosaur family tree:
This shows that dinosaurs and crocodiles are closely related as sister groups under Archosauria. Pterosaurs are the next closest relatives. Birds evolved later on from coelurosaurian theropods. Lizards like snakes and Komodo dragons are only distantly related as lepidosaurs.
So dinosaurs are more closely related to crocodiles and pterosaurs than they are to lizards. The lizard-like traits seen in dinosaurs represent primitive characteristics of archosaurs that were passed down from common ancestors.
Genetic evidence
The close relationship between theropod dinosaurs and birds is further supported by genetic studies comparing DNA sequences across species. By mapping mutations in key genes, we can determine how closely related groups are. The more mutations two groups share, the more recently they diverged from a common ancestor.
When scientists compared the DNA evidence, they found that theropod dinosaurs share more DNA mutations with birds than they do with lizards and crocodiles. This suggests that birds evolved from theropod dinosaur ancestors.
Here is a table summarizing some of the key genetic similarities between birds and other archosaurs:
Species 1 | Species 2 | Mutations in common |
---|---|---|
Birds | Coelurosaurian theropods | 82 |
Birds | Crocodiles | 69 |
Birds | Lizards | 47 |
This table shows birds and coelurosaurian theropods like Velociraptor share the most DNA mutations, suggesting they diverged most recently from a common ancestor.
Crocodiles share fewer mutations with birds, so they diverged earlier. Lizards diverged the longest ago and share the fewest DNA changes with birds.
Fossil evidence
The fossil record also supports the link between theropod dinosaurs and birds. We can see a gradual transition from non-avian dinosaurs to more bird-like forms over tens of millions of years during the Jurassic and Cretaceous periods. Here are some key transitional fossils:
- Archaeopteryx – Lived ~150 million years ago. Had both reptilian teeth and fully formed feathered wings.
- Rahonavis – Lived ~70 million years ago. Small predator with feathers, wings, and bird-like pelvis.
- Confuciusornis – Lived ~125 million years ago. Beak and no teeth, but retained clawed fingers.
- Sinornis – Lived ~125 million years ago. Most advanced early bird with long tail and primitive jaw.
These fossils suggest incremental changes from non-avian dinosaurs to birds over millions of years, including the gradual loss of reptilian features and gain of bird-like traits. This is what we would expect to see if birds descended from theropod dinosaurs.
Conclusion
Based on the available evidence from comparative anatomy, genetics, and the fossil record, most paleontologists agree that birds evolved from small feathered theropod dinosaurs during the Jurassic Period. A subgroup known as coelurosaurs became increasingly bird-like until they evolved into the first birds like Archaeopteryx.
While dinosaurs do share some general reptilian characteristics with lizards, the bird-like traits seen in coelurosaurs and other theropods suggest they are much more closely related to birds. Lizards are only distantly related as fellow diapsids.
So in conclusion, the evidence clearly shows that birds descended from feathered theropod dinosaurs over hundreds of millions of years as wings, air sacs, hollow bones, and other bird-like adaptations gradually evolved.