Thyreophorans, which were herbivorous ornithischian dinosaurs that lived from the Early Jurassic through to the Late Cretaceous, are generally known as the "armored dinosaurs" because of their well-developed osteoderms that formed parasagittal rows along their bodies. Members of two clades, the Stegosauria and Ankylosauria, are most representative of thyreophorans. Ankylosauria is further divided into the Ankylosauridae and Nodosauridae. Stegosaurs are distinguished by their:

1 small heads;

2 parascapular spikes;

3 armor just adjacent to their spines; and

4 posterior tail spikes.

In contrast, ankylosaurs had:

1 armor all over their dorsal (and in some cases, ventral) surfaces;

2 skull sutures covered by dermal plates;

3 a synsacrum; and

4 numerous other features.

Ankylosaurids are mainly distinguished from nodosaurids on the basis of skull profiles and the presence or absence of a tail club.

Little definitive information is known with regard to thyreophoran behavior and evolution because the majority of data come from body fossils that are, in many cases, incomplete and fragmentary. Nevertheless, sufficient skeletal material for some genera and complete specimens of others, such as those of the Late Jurassic Stegosaurus and Cretaceous ankylosaurs, have provided windows into thyreophoran behavior and evolution. Primitive forms of thyreophorans that do not fit into either Stegosauria or Ankylosauria, such as Scutellosaurus and Scelidosaurus, demonstrate a beginning of their clade at least in the Early Jurassic in Europe and North America. Ankylosaurs, which lived from the Middle Jurassic to Late Cretaceous, were a successful thyreophoran group that diversified throughout the Cretaceous and had a near worldwide distribution. In contrast, stegosaurs were limited to possibly the Early Jurassic through to the Early Cretaceous (having reached their peak in the Late Jurassic) of mostly northern continents.

Thyreophorans were apparently healthy animals and probably had complex behaviors that challenge perceptions of them based on their relatively small brains. Osteoderms of various sorts (plates, spikes, and clubs)

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