H adrosauroids are the most morphologically derived ornithopod dinosaurs. The clade consists of all iguanodontians closer to Hadrosaurus foulkii than to Iguanodon bernissartensis (Prieto-Márquez 2010a). These animals have been recorded from the late Early to Late Cretaceous (Barremian–Aptian to late Maastrichtian) of all continents except Africa and Oceania (Horner et al. 2004; Norman 2004). Their fossil record is among the richest and best preserved within the Dinosauria, including dozens of articulated skeletons, multi-individual assemblages, eggs and embryonic materials, soft-tissue impressions and footprints (Lull and Wright 1942; Horner 2000).
Knowledge on how the skeletal morphology of these animals changes through ontogeny is instrumental in understanding their palaeobiology and evolutionary history. On one hand, characters that are ontogenetically variable need to be identified and distinguished from those containing information for phylogenetic inference and taxonomic diagnoses. In addition, palaeobiological studies focusing on areas such as life history or heterochrony rest upon the identification of ontogenetic stages for the taxa of interest, which in turn rely upon proper identification of ontogenetically dependent morphological attributes. In comparison with the vast literature on hadrosauroid anatomy, systematics and palaeobiology, works entirely or mostly devoted to the documentation of skeletal changes during ontogeny are uncommon (Rozhdestvensky 1965; Waldman 1969; Maryanska and Osmólska 1981; Horner and Currie 1994; Weishampel and Horner 1994; Dilkes 2001; Grigorescu and Csiki 2006; Brett-Surman and Wagner 2007; Evans et al. 2005; Guenther 2009).
In the case of most speciose and morphologically derived hadrosauroid subclade, the Hadrosauridae, substantially complete ontogenetic series of skeletal elements are available for many taxa (Horner et al. 2000, 2004; Guenther 2009; Evans 2007, 2010). By contrast, the fossil material available for most non-hadrosaurid hadrosauroids is insufficient for revealing morphological changes through ontogeny. Notably, at least half of the currently recognized species of basal hadrosauroids are known from a single specimen, namely: Claosaurus agilis (Carpenter et al. 1995); Equijubus normani (You et al. 2003a); Glishades ericksoni (Prieto-Márquez 2010b); Jinzhousaurus yangi (Wang and Xu 2001); Nanyangosaurus zhugeii (Xu et al. 2000); Penelopognthus weishampeli (Godefroit et al. 2005); Cedrorestes crichtoni (Gilpin et al. 2007); Shuangmiaosaurus gilmorei (You et al. 2003b); Jintasaurus meniscus (You and Li 2009); and Jeyawati rugoculus (McDonald et al. 2010). For other species, the available material is also meagre: Lophorhothon atopus is known from two subadult specimens of similar developmental stage, one represented by cranial and the other by postcranial material (Langston 1960; Lamb 1998); Tanius sinensis is represented by a partial skull and a few postcranial bones, each assigned to a different specimen (Wiman 1929); Probactrosaurus mazongshanensis is only represented by a partial skull and various postcranial bones (Lü 1997; Norman 2002); Altirhinus kurzanovi is known from adult specimens and a few appendicular and axial elements representing two fragmentary subadult specimens (Norman 1998); and Protohadros byrdi is known from a partial skull specimen and two isolated teeth, each assigned to a different exemplar (Head 1998). In the case of the recently described Tethyshadros insularis, various well-preserved and relatively complete specimens exist but no juvenile individuals are present (Dalla Vecchia 2009). Although the hadrosauroid Gilmoreosaurus mongoliensis is also known from a number of exemplars, the vast majority of these (with the exception of three juvenile tibiae and a few pedal phalanges) correspond to individuals that represent approximately the same ontogenetic stage (Gilmore 1933; Prieto-Márquez 2010c). This leaves only a small fraction of known hadrosauroids with enough material to document ontogenetic changes: Bactrosaurus johnsoni (Gilmore 1933; Godefroit et al. 1998), Telmatosaurus transsylvanicus (Weishampel et al. 1993; Grigorescu and Csiki 2006), P. gobiensis (Rozhdestvensky 1966; Norman 2002), Eolambia caroljonesa (Kirkland 1998; Head 2001; Garrison et al. 2007) and Levnesovia transoxiana (Sues and Averianov 2009). Of these, only T. transsylvanicus has been the focus of a detailed study on growth changes (Grigorescu and Csiki 2006).
Bactrosaurus johnsoni is represented by one of the best-preserved and well-represented ontogenetic series available for basal hadrosauroids. This species was named and described by Gilmore (1933) based on an assemblage of disarticulated elements corresponding to various individuals from different ontogenetic stages. They came from a bonebed (locality number 141) in strata corresponding to the Iren Dabasu Formation, near the Chinese town of Erenhot, along the eastern segment of the border between China and Mongolia (Gilmore 1933; Godefroit et al. 1998). Recent studies disagree on the age of the Iren Dabasu Formation: according to Van Itterbeck et al. (2005), the formation is late Campanian to early Maastrichtian in age, whereas in the view of Averianov (2002) and Sues and Averianov (2009), it is as old as late Turonian to early Coniacian. Gilmore (1933) provided very little information on growth changes in B. johnsoni, consisting of a few brief remarks on the number of teeth, the shape of the scapula blade and the shape of the pedal ungual phalanges. More recently, Godefroit et al. (1998) described new materials referable to B. johnsoni that were collected near Erenhot by members of the Sino-Belgian Dinosaur Expedition, from a quarry (locality SBDE 95E5) located within one kilometre of Gilmore’s locality 141.
This study fills a gap in our understanding of the ontogenetic morphological changes that took place in the cranial and appendicular skeleton of basal hadrosauroids. This is accomplished through detailed documentation of the juvenile anatomy of Bactrosaurus johnsoni, with emphasis on those morphological attributes that are variable through ontogeny. Furthermore, this analysis evaluates the impact of ontogenetic variation on phylogenetic characters that have been proposed in the literature for inferring the evolutionary interrelationships of hadrosauroids.