What Makes a Dinosaur? - by Ralph Chapman

This document discusses what defines a dinosaur and the key anatomical features that distinguish dinosaurs from other prehistoric animals. It notes that dinosaurs lived during a specific period of time and had three defining anatomical characteristics: 3 or more sacral vertebrae, a glenoid fossa that faces caudally, an acetabulum that is completely open medially, and a femur with a ball-shaped head. The document also discusses that birds are modern dinosaurs based on their descent from feathered theropod dinosaurs like dromaeosaurs.