The ontogeny and taxonomy of the dome-headed pachycephalosaurs are topics of continued debate. Pachycephalosaurid diversity in the Maastrichtian of North America is particularly controversial, and the validity and composition within the genus Sphaerotholus remains unresolved. While the type species, S. goodwini, is generally considered valid, debate has centred around the validity and taxonomy of S. buchholtzae and S. edmontonensis. Here we employ morphometrics, histology and phylogenetic analysis to resolve these issues. An ontogenetic assessment of S. buchholtzae (N > 20) confirms previously observed ontogenetic morphologies: inflation of the frontoparietal dome, obliteration of tesserate surface texture, blunting of the peripheral nodes and decreasing void space within the dome. While linear bivariate analysis finds S. edmontonensis nested within S. buchholtzae, three-dimensional geometric morphometrics supports S. edmontonensis and S. buchholtzae as distinct species. Phylogenetic analysis recovers a Sphaerotholus lineage with S. goodwini as sister-taxon to a clade formed by S. edmontonensis and S. buchholtzae. The stratigraphic, phylogenetic, morphometric and ontogenetic data support the validity of both S. edmontonensis and S. buchholtzae, and their placement within the genus Sphaerotholus. The morphological similarities of S. edmontonensis to immature S. buchholtzae, and the slightly older geological age of S. edmontonensis, suggest that S. edmontonensis and S. buchholtzae may be part of an anagenetic lineage.

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A partial skeleton from the Little Snowy Mountains of central Montana is the first referable specimen of the Morrison Formation macronarian sauropod Camarasaurus. This specimen also represents the northernmost occurrence of a sauropod in the Morrison. Histological study indicates that, although the specimen is relatively small statured, it is skeletally mature; this further emphasizes that size is not a undeviating proxy to maturity in dinosaurs, and that morphologies associated with an individual’s age and stature may be more nebulous in sauropods.

Within Diplodocoidea (Dinosauria: Sauropoda), phylogenetic position of the three subclades Rebbachisauridae, Dicraeosauridae, and Diplodocidae is strongly influenced by a relatively small number of characters. Neural spine bifurcation, especially within the cervical vertebrae, is considered to be a derived character, with taxa that lack this feature regarded as relatively basal. Our analysis of dorsal and cervical vertebrae from small-sized diplodocoids (representing at least 18 individuals) reveals that neural spine bifurcation is less well developed or absent in smaller specimens. New preparation of the roughly 200-cm long diplodocid juvenile Sauriermuseum Aathal 0009 reveals simple nonbifurcated cervical neural spines, strongly reminiscent of more basal sauropods such as Omeisaurus. An identical pattern of ontogenetically linked bifurcation has also been observed in several specimens of the basal macronarian Camarasaurus, suggesting that this is characteristic of several clades of Sauropoda. We suggest that neural spine bifurcation performs a biomechanical function related to horizontal positioning of the neck that may become significant only at the onset of a larger body size, hence, its apparent absence or weaker development in smaller specimens. These results have significant implications for the taxonomy and phylogenetic position of taxa described from specimens of small body size. On the basis of shallow bifurcation of its cervical and dorsal neural spines, the small diplodocid Suuwassea is more parsimoniously interpreted as an immature specimen of an already recognized diplodocid taxon. Our findings emphasize the view that nonmature dinosaurs often exhibit morphologies more similar to their ancestral state and may therefore occupy a more basal position in phylogenetic analyses than would mature specimens of the same species. In light of this, we stress the need for phylogenetic reanalysis of sauropod clades where vital characters may be ontogenetically variable, particularly when data is derived from small individuals.

Sauropod dinosaurs were the largest terrestrial vertebrates; yet despite a robust global fossil record, the paucity of cranial remains complicates attempts to understand their paleobiology. An assemblage of small diplodocid sauropods from the Upper Jurassic Morrison Formation of Montana, USA, has produced the smallest diplodocid skull yet discovered. The ~24 cm long skull is referred to cf. Diplodocus based on the presence of several cranial and vertebral characters. This specimen enhances known features of early diplodocid ontogeny including a short snout with narrow-crowned teeth limited to the anterior portion of the jaws and more spatulate teeth posteriorly. The combination of size plus basal and derived character expression seen here further emphasizes caution when naming new taxa displaying the same, as these may be indicative of immaturity. This young diplodocid reveals that cranial modifications occurred throughout growth, providing evidence for ontogenetic dietary partitioning and recapitulation of ancestral morphologies.

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Stegosauria is a clade of ornithischian dinosaurs characterized by a bizarre array of dermal armor that extends from the neck to the end of the tail. Two genera of stegosaur are currently recognised from North America: the well-known Stegosaurus stenops, and the much rarer Hesperosaurus mjosi. A new specimen of Hesperosaurus mjosi was discovered in some of the most northerly outcrops of the Upper Jurassic Morrison Formation near Livingston, Montana. The new specimen comprises cranial, vertebral and appendicular material as well as a dermal plate, and the excellent state of preservation of the palate reveals new anatomical information about this region in stegosaurs. Histological examination of the tibia indicates that the individual was not skeletally mature at time of death. Comparison with previously studied Stegosaurus and Hesperosaurus individuals indicates that Hesperosaurus mjosi may have been a smaller genus than Stegosaurus stenops. Physiological processes scale with body mass, M, according to the relationship M 0.75 in extant megaherbivores, thus larger animals are better able to cope with more arid environments where forage is less abundant. Under this scenario, it is possible that Stegosaurus stenops and Hesperosaurus mjosi were environmentally partitioned, with the larger S. stenops occupying more arid environments. Analyses of the temporal overlap and latitudinal range of Morrison stegosaurs would allow this hypothesis to be investigated.