This increased the importance of active defence by the thagomizer, which could ward off even large predators because the tail was in a higher position, pointing horizontally to the rear from the broad pelvis.

Professor Othniel Charles Marsh that year classified such specimens in the new genus Stegosaurus, from which the group acquired its name, and which is still by far the most famous stegosaurian.

He assumed that the plates formed a flat skin cover — hence the name, meaning "roof saurian" — and that the animal was bipedal with the spikes sticking out sideways from the rear of the skull.

This increased the age range of good fossil stegosaurian material, as they represented the first relatively complete skeletons from the Middle Jurassic and the Early Cretaceous.

Nevertheless, European and North-American sites have become productive again during the 1990s, Miragaia having been found in the Lourinhã Formation in Portugal and a number of relatively complete Hesperosaurus skeletons having been excavated in Wyoming.

Apart from the fossils per se, important new insights have been gained by applying the method of cladistics, allowing for the first time to exactly calculate stegosaurian evolutionary relationships.

[1] Stegosaurians had characteristic small, long, flat, narrow heads and a horn-covered beak or rhamphotheca,[1] which covered the front of the snout (two premaxillaries) and lower jaw (a single predentary) bones.

In stegosaurians, the typical archosaurian skull opening, the antorbital fenestra in front of the eye socket, is small, sometimes reduced to a narrow horizontal slit.

In reality, the hand bones of stegosaurians were arranged into vertical columns, with the main fingers, orientated outwards, forming a tube-like structure.

Later species, however, convergent to the Ankylosauria developed very broad pelves, in which the iliac bones formed wide horizontal plates with flaring front blades to allow for an enormous belly-gut.

[citation needed] Later forms show very variable configurations, combining plates of various shape and size on the neck and front torso with spikes more to the rear of the animal.

They seem to have lost the tail club and the flank rows are apparently absent also, with the exception of the shoulder spine, still shown by Kentrosaurus and extremely developed, as its name indicates, in Gigantspinosaurus.

Discoverer Othniel Charles Marsh suggested a single median row of plates running post-cranially along the longitudinal axis[14] and Lull argued in favour of bilaterally paired arrangement throughout the series.

[15] Current scientific consensus lies in the arrangement proposed by Gilmore - two parasagittal rows of staggered alternates, after the discovery of an almost complete skeleton preserved in this manner in rock.

[18] Stegosaurids also lack lateral scute rows that run longitudinally on either side of the trunk in Huayangosaurus and ankylosaurs, indicating yet another secondary loss of a plesiomorphic characters.

[10] However, the absence of lateral scutes as well as pre-maxillary teeth mentioned above are not specifically diagnostic of stegosaurids, since these features are also present in some other stegosaurians, whose phylogenetic relationships are unclear.

A keratinous covering would also allow greater surface area for the plates to be uses as a mating display structures, which could be potentially coloured like the beaks of modern birds.

[25] Following phylogenetic analyses, Huayangosauridae was expanded to also include the taxon Chungkingosaurus, known from specimens from younger Late Jurassic deposits of the Shaximiao Formation.

[4][9] Huayangosauridae was formally defined in 2021 by Daniel Madzia and colleagues, who used the previous definitions of all taxa closer to Huayangosaurus taibaii than Stegosaurus stenops.

[30] Footprints attributed to the ichnotaxon Deltapodus brodricki from the Middle Jurassic (Aalenian) of England represent the oldest probable record of stegosaurians reported so far.

A few million years later, during the Callovian-Oxfordian, from China much larger species are known, with long, "graviportal" (adapted for moving only in a slow manner on land due to a high body weight) hindlimbs: Chungkingosaurus, Chialingosaurus, Tuojiangosaurus and Gigantspinosaurus.

[35][36][37] It has often been suggested that the decline in stegosaur diversity was part of a Jurassic-Cretaceous transition, where angiosperms become the dominant plants, causing a faunal turnover where new groups of herbivores evolved.

[1] A purported stegosaurian dermal plate was reported from the latest Cretaceous (Maastrichtian) Kallamedu Formation (southern India); however, Galton & Ayyasami (2017) interpreted the specimen as a bone of a sauropod dinosaur.

They found that throughout the ontogeny, the dorsal osteoderms are composed of dense ossified collagen fibres in both the cortical and cancellous sections of the bone, suggesting that plates and spikes are formed from the direct mineralization of already existing fibrous networks in the skin.

[1] This is corroborated by Spassov's (1982) observations that the plates are arranged for maximum visible effect when viewed laterally during non-aggressive agonistic behaviour, as opposed to from a head-on aggressive stance.

[43] Two years later, a new ichnogenus called Stegopodus was erected for another set of stegosaurian tracks which were found near Arches National Park, also in Utah.

While has no body fossil evidence currently known for stegosaurs, handprints from underground coal mines near Oakey, Queensland, resembling Garbina tracks suggests their occurrence in this country from at least the Middle to Upper Jurassic (Callovian–Tithonian).

He also observed that Stegosaurus could have maneuvered its rear easily by keeping its large hindlimbs stationary and pushing off with its very powerfully muscled but short forelimbs, allowing it to swivel deftly to deal with attack.

Kevin Padian, a paleontologist at the University of California, Berkeley, remarked that Saitta had misidentified features in his specimen's bone tissue sections and said "there's no evidence the animal has stopped growing".

The finding suggests that these dinosaurs would be capable of breaking smaller branches and leaves with their teeth, but would not be able to bite through a thick object (12 mm or more in diameter).