Bactrosaurus

Norman, David B., 2015, On the history, osteology, and systematic position of the Wealden (Hastings group) dinosaur Hypselospinus fittoni (Iguanodontia: Styracosterna), Zoological Journal of the Linnean Society 173 (1), pp. 92-189 : 160-161

BACTROSAURUS JOHNSONI GILMORE, 1933

(GODEFROIT ET AL., 1998)

Bactrosaurus is represented by abundant skeletal remains of a medium-sized (6–7 m long) ornithopod collected from the Iren Dabasu Formation, Erenhot, China: Turonian−Coniacian ( Sues & Averianov, 2009). However, it should be noted that estimates of the age of these beds have ranged from Albian to Maastrichtian ( Prieto-Márquez, 2011a).

Teeth and jaws

The dentary teeth are described as being leaf-shaped and ‘distinctly... wider’ ( Godefroit et al., 1998: 27) than those of the maxillary dentition. The dentary teeth are slightly recurved distally (as in Telmatosaurus ) and the primary ridge is less prominent than that seen on the maxillary crowns. The primary ridge is displaced slightly distally. A secondary ridge is present on the mesial sector of the crown, and some of the posterior teeth in the dentition are described as bearing a third longitudinal ridge. The dentary is robust, straight, and has a deep alveolar trough to accommodate the dentition. The coronoid process is tall and perpendicular to the long axis of the dentary and has an expanded apex. The dentition appears to extend posteriorly as far as the posterior edge of the base of the coronoid process.

Axial skeleton

The vertebral column conforms to that seen in medium−large-bodied iguanodontians. The cervicals are strongly opisthocoelous and have short, neural spines. Dorsals have spool-shaped centra that retain shallow opisthocoely throughout, and whose articular margins are not very thickened, in contrast to those of Hy. fittoni . The neural spines are elongate, but are thickened axially, and notably transversely toward the apex; they do not exhibit the extreme slenderness and elongation seen in Hy. fittoni .

Appendicular skeleton

The scapular blade flares distally, and proximally the acromial process is developed into a promontory that is in line with the axis of the scapular blade (rather than being J-shaped as in Hy. fittoni ). The sternal bones are hatchet-shaped and have an extremely elongate ‘handle’ and a comparatively short ‘blade’ (differing in proportion from those of Hy. fittoni ). The humerus is strongly sigmoid and ‘stocky’, with a prominent deltopectoral crest. The ulna is subequal in length to the humerus and is comparatively slender and tapers distally before thickening slightly. The radius is comparatively slender and bowed along its length and again thickens where it articulates with the distal end of the ulna and carpal region. The manus elements (metacarpals) have been described briefly, but remain largely unillustrated ( Godefroit et al., 1998) and have been described as resembling, in proportions, those of Mantellisaurus . Prieto-Márquez (2011a: pl. 4) provid- ed photographs of juvenile metacarpals that confirm Godefroit’s description (these are more slender than those of Hy. fittoni ).

The ilium has an elongate, untwisted preacetabular process that terminates in a modest flange and there is a prominent medial ridge near its origin on the main blade of the ilium. The ilium illustrated by Godefroit et al. (1998: fig. 30) is clearly a left ilium (rather than a right as stated) and all of the surface-related annotations are incorrect. There is a lateral expansion of the dorsal margin of the iliac blade posterodorsal to the ischiadic peduncle (‘supraacetabular process’ of Prieto-Márquez, 2011a) and the postacetabular process tapers to a blunt terminus and appears to lack a brevis fossa. The pubis has a thin, dorsoventrally flared prepubic process (cf. Gilmore, 1933: fig. 37 and Godefroit et al., 1998: fig. 32, pl. 12) and the ischium has a robust, thick, and straight shaft with a distal, anteriorly expanded, ‘boot’. The femur has a straight shaft, a triangular, crested fourth trochanter positioned at midshaft, and the extensor intercondylar groove is tunnel-like. The distal hindlimb elements do not show any unusual characters, being typical of medium−largebodied ornithopods generally, and the pedal unguals are arrow-head shaped in plan view, but have broadly rounded (rather than narrow and bluntly truncated) distal tips and weak development of the lateral claw grooves.