Camptosaurus dispar, Marsh, 1879

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 : 164

CAMPTOSAURUS DISPAR MARSH, 1879 ( GILMORE, 1909)

Camptosaur-grade ornithopods have been systematically reviewed in recent years ( McDonald, 2011) and include Ca. dispar , Cumnoria prestwichi ( Hulke, 1880) ; Owenodon hoggii ( Owen, 1874; Norman & Barrett, 2002; Galton, 2009); Uteodon aphanoecetes ( Carpenter & Wilson, 2008; McDonald, 2011); and Osmakasaurus depressus ( Gilmore, 1909; McDonald, 2011).

Camptosaurus dispar is chosen as a representative medium-sized (5–7 m long) camptosaur-grade iguanodontian ornithopod that has been reasonably welldescribed ( Gilmore, 1909) and one that is close to the size range exhibited by Hypselospinus . Remains attributed to Ca. dispar are stratigraphically distribut- ed between the Kimmeridgian and Tithonian stages and are thus substantially chronostratigraphically older than Hypselospinus .

Teeth and jaws

The maxillary and dentary crowns are similar in general morphology to those seen in dryosaurids and Hypselospinus , but the form of the dentary teeth (in particular) is distinctive. Unlike Hy. fittoni , the marginal denticles on the mesial and distal edges of the crown are not shelf-like and mammillate. The detailed structure of the primary, secondary, and strand-like accessory (subsidiary) ridges of dentary crowns are distinct in comparison with Hy. fittoni . The lingual surface of the dentary crowns in Camptosaurus displays a primary ridge that is offset distally on the crown surface but not strongly differentiated from a secondary ridge; the secondary ridge is not broad and mound-like and the accessory (tertiary) ridges are distributed more regularly across the crown and are straighter (apicobasally). The lower jaw (dentary) ramus is robust, straight, and is both proportionally shorter as well as containing fewer tooth positions than are present in the dentary of Hy. fittoni .

Axial skeleton

The cervical centra are low (dorsoventrally compressed) and lack the strong opisthocoely seen in Hypselospinus . Dorsal vertebrae have low, cylindrical centra and short neural spines. Posterior dorsals and anterior caudals do not exhibit the extreme elongation of the neural spines seen in Hy. fittoni .

Appendicular skeleton

The principal shoulder bones are similar, in general shape, to those seen in Hypselospinus ; however, the sternals are distinctive because they are reniform, rather than being hatchet-shaped (the classic ‘styracosternan’ condition). The forelimb is stout with the individual elements comparatively robust; however, the structure of the radius differs significantly in these two taxa when the proximal and distal condyles are compared (pers. observ., 2011). The carpus and manus show some similarity in overall anatomy and phalangeal count. The carpus is co-ossified and there is a spine-like pollex ungual in both taxa; however, the metacarpals are shorter and more obviously divergent producing a broadly splayed hand in Camptosaurus and the nonpollex unguals of digits 2 and 3 are more pointed and claw-like (this contrasts markedly with the structures seen in Hy. fittoni ). The pelvis exhibits a range of differences from that seen in Hy. fittoni : notably the pubis of Camptosaurus has a pubic shaft that is equal in length to that of the ischial shaft, whereas the prepubic process is laterally compressed, blade-like, and has parallel dorsal and ventral margins, with no distal expansion. The femur in Camptosaurus is curved along its length; its shaft is stout, but its sides are not strongly angular; there is an elongate, finger-like, and genuinely ‘pendant’ fourth trochanter positioned midshaft; and the extensor intercondylar groove is deep, but very broadly open on the extensor surface. The pes in Camptosaurus appears to be functionally tridactyl, but digit 1 has a small, splint-like shaft that adheres to the medial surface of metatarsal II, and has an articular distal condyle, which supports a digit with three small phalanges; the unguals taper and terminate in narrow, but rounded, tips.