Dinosaurs were a special group of reptiles that first appeared more than 230 million years ago and became some of the most successful land animals in Earth history. A true dinosaur is not just any ancient reptile, and it is not defined by being large, extinct, or scary-looking. Paleontologists identify dinosaurs by shared anatomical traits, especially features of the hips, legs, and posture.
These clues help scientists separate dinosaurs from pterosaurs, marine reptiles, crocodile relatives, and other prehistoric animals.
Key Facts
- True dinosaurs belong to Dinosauria, a group within Archosauria.
- Dinosaurs had legs held directly under the body, unlike sprawling reptiles.
- A key dinosaur trait is an open hip socket called a perforate acetabulum.
- Dinosaurs lived during the Mesozoic Era, about 252 million to 66 million years ago.
- Age of a fossil layer can be estimated by relative dating: lower rock layers are usually older than upper layers.
- Speed can be estimated from trackways using v = d/t when distance and time are known.
Vocabulary
- Dinosauria
- Dinosauria is the scientific group that includes all true dinosaurs and their descendants, including modern birds.
- Acetabulum
- The acetabulum is the hip socket where the thigh bone fits into the pelvis.
- Fossil
- A fossil is preserved evidence of past life, such as bone, tooth, footprint, shell, or leaf imprint.
- Paleontology
- Paleontology is the scientific study of ancient life using fossils and rock evidence.
- Mesozoic Era
- The Mesozoic Era is the time interval from about 252 million to 66 million years ago when non-bird dinosaurs dominated many land ecosystems.
Common Mistakes to Avoid
- Calling every prehistoric reptile a dinosaur is wrong because pterosaurs, ichthyosaurs, plesiosaurs, and many other ancient reptiles were not members of Dinosauria.
- Using size to identify a dinosaur is wrong because some dinosaurs were small while some non-dinosaurs were enormous.
- Thinking dinosaurs dragged their bodies like lizards is wrong because true dinosaurs had an upright limb posture with legs positioned under the body.
- Assuming all dinosaurs went extinct 66 million years ago is wrong because birds are living dinosaurs that survived the end-Cretaceous extinction.
Practice Questions
- 1 A fossil bed is dated to 150 million years ago. How many million years after the start of the Mesozoic Era, about 252 million years ago, was this animal buried?
- 2 A dinosaur trackway has 12 footprints spaced 1.5 m apart from one footprint to the next. What is the distance from the first footprint to the last footprint?
- 3 A fossil animal has a long tail, sharp teeth, and lived 160 million years ago, but its legs sprawled out to the sides and its hip socket was not open. Explain why paleontologists would not classify it as a true dinosaur.