Stegosaurs

Stegosaurs flourished during the Jurassic — their heyday perfectly aligned with the age of the great sauropods, and the two groups are often found together in the same formations. Huayangosaurus in China is the earliest well-known member, retaining teeth at the front of its jaw that later stegosaurs would lose. By the Late Jurassic, Stegosaurus in North America and Kentrosaurus in Africa had perfected the body plan: a broad body, tiny head, and a spectacular array of back structures.

The plates of Stegosaurus have generated more scientific debate than almost any other dinosaur feature. CT scans reveal extensive blood vessel channels inside the plates, supporting a thermoregulatory or display function rather than defense. Kentrosaurus took a different approach — trading broad plates for narrow paired spines, culminating in shoulder spines that biomechanical studies suggest could inflict serious damage when swung into an attacker.

Stegosaurs are notably absent from the Late Cretaceous fossil record of the Northern Hemisphere — the group seems to have declined after the Jurassic-Cretaceous transition, surviving only in isolated Gondwanan populations into the Early Cretaceous. Why they disappeared while ankylosaurs thrived remains one of paleontology's open questions.