You are in the floodplains of China, Asia, 165 million years ago.

The humid air of the Middle Jurassic was thick with the scent of cycads and damp earth. You pressed yourself against the thick bark of a conifer, holding your breath as the bushes ahead parted.

It wasn't the roaring, large Stegosaurus you expected, but something more intimate and armored—a Huayangosaurus. It was barely 15 feet long, a dainty, quadrupedal herbivore compared to its later relatives. Its small skull was busy plucking soft ferns with premaxillary teeth that no later stegosaur would ever have.

The most striking feature was the double row of armor running down its spine; they weren't the giant plates of a museum exhibit, but rather spiked, jagged plates that shivered in the heat. As it turned, you saw the thick shoulder spines—stark warnings against predators. Just as it seemed to notice you, it let out a low rumble, dismissed your presence, and swung its tail—laden with two pairs of long thagomizer spikes—lazily toward a nearby tree. It was a masterpiece of early armored evolution.