Dinosaur Paleobiology
Defining Dinosaurs
To paleontologists, a dinosaur isn't just any large, extinct reptile. Under the rules of , a dinosaur is defined as the group containing the most recent common ancestor of Triceratops and modern birds, and all of its descendants. This definition reveals a startling truth: dinosaurs never actually went entirely extinct, as modern birds are direct descendants of those ancient survivors.
Click any node on the phylogenetic tree to reveal what makes a valid clade, or toggle the traditional view to see why grouping reptiles without birds is scientifically invalid.
What physically unites this massive, diverse family tree? The ultimate anatomical clue lies in the hip. Dinosaurs possess a unique, fully open hole in their hip socket called the acetabulum.
The main feature dinosaurs have in common? A completely open hip socket, allowing for limbs to be placed directly underneath the body, making dinosaurs very different in locomotion and posture than living nonavian reptiles, and very similar to their living members the birds.
This hip design allowed dinosaurs to walk with an erect, upright posture directly beneath their bodies, rather than sprawling outwards like modern lizards. Because of this strict anatomical requirement, famous prehistoric creatures like flying pterosaurs and ocean-dwelling plesiosaurs are not actually dinosaurs at all, as they completely lacked this specialized hip structure.