Dinosaurs are defined primarily by their evolutionary lineage and specific anatomical features inherited from a common ancestor. Here are the key points that make a dinosaur a dinosaur:
Evolutionary Definition
- Dinosaurs belong to the clade Dinosauria, which includes the most recent common ancestor of birds (like pigeons) and non-avian dinosaurs (like Triceratops), plus all descendants of that ancestor
- They are a subgroup of archosaurs, a larger group of reptiles that also includes crocodiles and pterosaurs, but dinosaurs form a distinct branch within this group
Anatomical Characteristics
- Upright stance: Unlike sprawling reptiles such as lizards and crocodiles, dinosaurs have legs positioned directly under their bodies, allowing more efficient movement and better support of their weight
- Open acetabulum: Dinosaurs have a hole in the hip socket (acetabulum), which permits this upright leg posture
- Skull features: Dinosaurs share skull openings, including a hole between the eye socket and nostril, and two holes behind the eye socket, traits shared with other archosaurs
- Other skeletal traits: Dinosaurs have prominent muscle attachment sites on the thigh and upper arm bones, a ridge on the shin bones, and three or more fused sacral vertebrae near the hips
- Three-toed foot and S-shaped neck: These features are common in dinosaurs and are seen in modern birds, which are classified as living dinosaurs
Other Important Points
- Dinosaurs lived on land; they are not marine reptiles like plesiosaurs or mosasaurs, nor are they flying reptiles like pterosaurs, which are close relatives but not dinosaurs
- Birds are considered dinosaurs because they evolved from theropod dinosaurs and share the defining traits of the group
- Classification is based on evolutionary history and inherited anatomical features, not on size or whether the animal is extinct
In summary, a dinosaur is a land-dwelling archosaur with an upright stance supported by a distinctive hip structure, specific skull openings, and other skeletal features, all tracing back to a common ancestor shared by both extinct non-avian dinosaurs and modern birds
