The Stegosaurs

The stegosaurs were bird-hipped (ornithischian) dinosaurs closely related to the ankylosaurs and distantly related to the ceratopsians, pachycephalosaurs, and ornithopods. Stegosaurs belonged to the ornithischian sub-group Thyreophora. This group's name comes from a combination of two Greek words that can be read "shield bearers"; they are distinguished by body armor lined up in longitudinal rows along their bodies. Early thyreophorans had simple body armor, while the later members--such as the stegosaurs and ankylosaurs--developed full-fledged spikes and plates. As a general rule, thyreophorans had a small brain-to-body-size ratio, implying that their intelligence may have been on par with (if not lower than) modern-day cows. Also, thyreophorans had smaller forelimbs than they did hind-limbs.

Before thyreophora split into stegosauria and ankylosauria, early thyreophorans looked nothing like the armored dinosaurs most of us know. Emausaurus, a forerunner of the large thyreophorans, was around two meters long and had three conical scutes and one spine. 

Emausaurus, a forerunner of the larger thyreophorans

Cladograms for stegosauria differ depending on the scientist presenting them; some cladograms divide stegosauria into huayangosauridae and stegosauridae. Others subdivide into kentrosauridae and stegosauridae. This website follows the second categorization, simply because it's easier to remember. The kentrosaurs were the stegosaurs marked by spikes; the stegosaurids were those stegosaurs marked by plates. The most famous stegosaur of them all, Stegosaurus, falls into the latter category. Kentrosaurus, whose name fits the stereotype of kentrosauridae, is marked by spikes, rather than plates, protruding from its back. Some stegosaurs have both spikes and plates, and they're categorized in kentrosauridae or stegosauridae depending on the cladogram writer's whim.

examples of kentrosauridae

examples of stegosauridae

Paleontologists speculate that the plates of the stegosauridae may have served a variety of functions: armor, sexual recognition, or even thermoregulation (functioning in such a way as to help regulate body temperature) have been suggested. An early evolutionary innovation within stegosauria was the development of thagomizers, or tail spikes, as a weapon; whereas ankylosauridae embraced tail clubs, stegosauria went the route of spikes. As stegosaurs evolved, so did their skulls and beaks; their skulls became narrower, and their beaks adapted in a variety of ways so that they could eat the most nutritious parts of the Mesozoic cycads. As plant diversity dwindled into the Cretaceous, the stegosaurs found themselves adapted to a dying world; by the middle of the Cretaceous, stegosauria had gone extinct. Their ankylosaur cousins, however, would last until the Great Extinction.