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Which anatomical features distinguish dinosaurs from their closest archosaur relatives?

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Illustration showing various dinosaur species drawn to scale and grouped by geologic era: Late Cretaceous, Early Cretaceous, Jurassic, and Triassic. Dinosaurs range from large species like Brachiosaurus and Tyrannosaurus to smaller ones like Velociraptor and Compsognathus, with a human silhouette included for size comparison. Measurement bars at the bottom indicate 1 meter and 3 feet for scale reference. — dinosaur A selection of dinosaurs grouped by the geologic interval in which they lived.

Dinosaurs are distinguished from their closest archosaur relatives by a specific suite of anatomical features inherited from their most recent common ancestor. These defining characteristics include:

The loss of the prefrontal bone located above the eye.1
A long deltopectoral crest on the humerus.1
Three or fewer joints on the fourth finger of the hand.1
Three or more hip vertebrae.1
A fully open hip socket (acetabulum).1
A cnemial crest on the shin bone (tibia).1

While dinosaurs share broader archosaurian traits—such as socketed teeth and a diapsid skull with two large holes behind the eye sockets—these specific skeletal modifications set them apart from other archosaurs like crocodilians and pterosaurs. Compared to their contemporaries, these features contributed to an improved stance and posture, which facilitated more efficient locomotion and supported the evolutionary success of the group.12