Juveniles of the stegosaurian dinosaurStegosaurusfrom the Upper Jurassic of North America

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Abstract
Two partial skeletons of juvenile individuals of Stegosaurus with estimated body lengths of about 1.5 m (5 ft) and 2.6 m (8.5 ft) are described from the Morrison Formation (Upper Jurassic) of Utah and Wyoming. These juveniles differ from adults in the absence of fusion in composite bones (sacrum, scapulocoracoid, tibia–fibula–astragalus–calcaneum), the smoothness of the surface of the bones, the smaller size of ridges for muscular attachment, a proportionally more slender and elongate scapula, the relatively small size of the olecranon process of the ulna and the head of the femur, and the possible absence of dermal plates (but tail spines were present). Apart from the ratios of the length of the femur to those of the humerus and ilium, and the form of the scapula, ilium and fibula, the bones of juvenile individuals of Stegosaurus are very similar to those of juvenile individuals of the stegosaur Kentrosaurus (Upper Jurassic, East Africa). A probable sexual dimorphism in the sacrum with either four (?female) or five (?male) sacral ribs occurs in Kentrosaurus. The additional sacral rib attaches anterior to the other four; a rib in the Utah juvenile has the same distinctive shape, and it thus may have been a male (sacra of Stegosaurus with four sacral ribs have been previously illustrated).