Dermal armor of plated ornithischian dinosaur Stegosaurus from Morrison Formation (Upper Jurassic) of Colorado and Wyoming (based mostly on bones collected in 1877-1889 for O. C. Marsh), and Utah

Most of the important specimens with dermal armor are illustrated for the plated dinosaur Stegosaurus from the Morrison Formation (Upper Jurassic). These include specimens from Garden Park, Colorado and Como Bluff, Wyoming, collected in 1877-89 for O. C. Marsh, and from Utah. The holotype articulated skeleton from Garden Park of the neotype species of Stegosaurus Marsh, 1877, S. stenops Marsh, 1887, is a mostly complete articulated skeleton with all 17 dermal plates preserved. A set of 17 dermal plates (missing plates 3 and 5) and 2 pairs of tail spines from one individual of S. stenops from Como Bluff, is reassembled from three sets of adjacent osteoderms catalogued as separate specimens. An associated specimen from near Shell, Wyoming had 18 dermal plates, not 19 as reconstructed. Other non-Marsh specimens showing overlapping plates include another almost complete articulated specimen from Garden Park and partial skeletons from near Jensen, Utah and Grand Junction, Colorado. The form of the four pairs of terminal tail spines of the syntype of S. ungulatus Marsh, 1879 from Como Bluff represent two individuals, the larger spine pairs 1 and 3 and the smaller pairs 2 and 4. However, the flat distal tail spines, which were probably arranged dorsally as four pairs, are diagnostic. The anterior pair of tail spines are extremely massive in S. sulcatus Marsh, 1887 from Como Bluff. The spines with four subequal bases are extremely slender and elongate in S. longispinus Gilmore, 1914 from near Alcova, Wyoming, the type species of Alcovasaurus Galton & Carpenter, 2016 as A. longispinus (Gilmore, 1914), that was recently referred to Miragaia Mateus et al., 2009 from western Europe as M. longispinus (Gilmore, 1914). The different form of nuchal, dorsal and distal caudal plates from Utah indicate a possible new species of Stegosaurus. A cuirass of small throat ossicles is known only for S. stenops and one contains an embedded carnivorous theropod tooth. An isolated and dorsally incomplete "shoulder plate" from Como Bluff, which supposedly resembled some of the scutes of basal thyreophorans, is reoriented as an anterior dorsal plate with an extremely large base. The first published restoration showing the paired and alternating arrangement of the plates was by C. R. Knight in Lucas (1901a, b; four pairs of tail spines so referable to S. ungulatus). However, Lucas/Knight statuettes with this arrangement and two pairs of tail spines, so referable to S. stenops, still exist from 1899, 1903 and 1904. Lucas (1910a) was the first to publish evidence for such an arrangement. A detailed summary is provided of the subsequent discussion supporting a single median row versus two rows in pairs or as alternating plates. Chirality or external mirror asymmetry occurred in S. stenops with plate 14, the largest, directed towards the right or to the left, but the dermal plates do not exhibit the sexual dimorphism present in Hesperosaurus, another Morrison stegosaur. The proposed functions for the dermal osteoderms of stegosaurs are discussed. These include lateral display for species recognition and sexual interactions, with the plates also involved in different degrees of thermoregulation. In Stegosaurus the tail was held high, so parallel to the ground, and it bore tail spines that were directed laterally and only slightly dorsally so they could function better as defensive and offensive weapons. Spines of juvenile to adult individuals of Stegosaurus retain the plesiomorphic histology with a thin cortex and thick cancellous bone, which was suitable for display but not useful as a weapon. However, the spines of old adult individuals, with a thick cortex and a large central canal, made excellent weapons. Isolated stegosaur tail spines are from old adult individuals, the thick cortex of which favoured preservation.