Thyreophorans were small- to medium-sized quadrupedal ornithischians with rows of scutes down the back. Aside from some primitive forms, the two great groups of thyreophorans were Stegosauria and Ankylosauria, linked together within a monophyletic Eurypoda. Eurypodans, as a group, were not renowned for their intellects; some of the lowest brain : weight ratios known for dinosaurs come from Eurypoda.
Stegosaurs are relatively poorly known eurypodans with hind limbs significantly longer than the forelimbs, and paired rows of plates or spines down the back, terminating in a tail with elongate spines: likely a defensive weapon. Their fossil record extends from the Middle Jurassic to the Early Cretaceous.
Stegosaurs are rare finds, and the suggestion is that they functioned in isolation rather than gregariously. The plates in Stegosaurus may have been involved in thermoregulation. Stegosaurs - like all eurypodans - had cheeks, which suggest chewing. The teeth, however, occluded relatively poorly, suggesting somewhat inefficient grinding. Due in part to their rarity, the reproductive strategies and behavior of stegosaurs are still largely unknown.
Ankylosaurs were armored tank-like quadrupeds, coated with a pavement of osteoderms. They lived from the mid-Jurassic to latest Cretaceous. Two groups are known: Nodosauridae and Ankylosauridae. Nodosaurids were slightly more lightly built, with tall parascapular spines, while ankylosaurids were more tank-like, and equipped with a large tail-club.
Ankylosaurs were low browsing animals. Their teeth and cheeks were rather like those of stegosaurs, but ankylosaurs had secondary palates, which may have aided in the efficiency of mastication. Unquestionably, though, much of their energy was obtained through gut fermentation, as suggested by the striking breadth of their girths. The presence of bonebeds suggests that, unlike stegosaurs, ankylosaurs may have been gregarious animals. Little is known of their reproductive strategies, and, by morphology, they were evidently animals that relied heavily upon defense, either by simply hunkering down or, in the case of ankylosaurids, by wielding their tail-clubs.
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