Pachyrhinosaurus lakustai Currie, Langston & Tanke 2008

CASE STUDY II: FORELIMB OF PACHYRHINOSAURUS LAKUSTAI This case study focusses on a reconstruction of the shoulder girdle and forelimb of an adult Pachyrhinosaurus lakustai, a centrosaurine ceratopsid from the Upper Cretaceous Wapiti Formation of northern Alberta (Fig. 5A, B), that was produced mainly by one of us (KN) for a forthcoming descriptive paper (Vice et al. work in progress). The reconstruction was partly based on a previously published illustration by Thompson & Holmes (2007), depicting the shoulder girdle and forelimb of the chasmosaurine Vagaceratops irvinensis (Holmes, Forster, Ryan & Shepherd, 2001) in what they considered to be a “neutral” semi-erect standing position (Fig. 5D). The P. lakustai reconstruction followed that of Thompson & Holmes (2007) in showing a right forelimb in a semi-erect standing pose, in anterior and lateral views, but differed from their reconstruction in being orthographic in nature. One obstacle encountered during the reconstruction process was the limited availability of reference material. All known P. lakustai specimens are from the Pipestone Creek Bonebed south of Wembley, Alberta, in which juvenile to adult bones are preserved in a disarticulated condition with varying degrees of distortion (Ralrick & Tanke 2008). The consequent lack of a complete, undistorted, and articulated adult P. lakustai forelimb led to heavy reliance on other ceratopsids, including Centrosaurus, Styracosaurus, Vagaceratops, Triceratops, and Pachyrhinosaurus sp. from the Wapiti River Bonebed (WRB) locality (Fanti et al. 2015), to fill the gaps in the P. lakustai hypodigm. For example, several well preserved centrosaurine specimens of varying completeness, such as the Centrosaurus humerus UALVP 55164 and the Styracosaurus partial skeleton UALVP 55900, provided information on osteological details such as tubercles and ridges. Given the need to rely on observations from other ceratopsid taxa, even the final version of the reconstruction represents a more or less provisional hypothesis that could readily be tested further should an articulated pectoral girdle and forelimb of P. lakustai be found in the future. The reconstruction used reference photos of P. lakustai bones as a starting point. The orthographic approach made it necessary to “flatten” many elements captured in the reference photos, removing the effects of perspective to keep the positions of key landmarks consistent across both views. To minimise the amount of perspective distortion in the original reference photos, a telephoto camera lens with a focal length from 50-75 mm was used whenever this was logistically feasible. Photos of complete and articulated ceratopsid forelimbs, primarily an articulated right Centrosaurus apertus forelimb (UALVP 55261) that was available for direct examination, and Triceratops (NSM PV 20379) and Vagaceratops irvinensis (CMN 41357) specimens described in the literature (Thompson & Holmes 2007; Fujiwara 2009), were used as a basis for scaling and orienting the elements in both views. Because most of the bones were steeply inclined either anteroventrally or posteroventrally, perspective had a strong impact on the anterior view, which was corrected by projecting multiple landmarks from the lateral view into the transverse plane as a guide to the proper proportions of each element. Perspective effects on the lateral view were less pronounced, because the limb segments were not strongly angled relative to the sagittal plane, and were considered to lie within the margin of acceptability given the use of the telephoto lens. The reconstruction process led to novel insights into the forelimb anatomy of P. lakustai, particularly with respect to the structure of the metacarpus. The metacarpal configuration of centrosaurines is poorly known, so the reconstruction of the P. lakustai manus was informed by previous work on associated, and in some cases articulated, specimens of other neoceratopsians, mostly chasmosaurines (Thompson & Holmes 2007; Fujiwara 2009; Mallon & Holmes 2010). Thompson & Holmes (2007) reconstructed the manus of V. irvinensis with only slight contact between the proximal heads of the metacarpals, which were all depicted as being in approximately the same plane (Fig. 5D). The metacarpus of P. lakustai was initially reconstructed in a similar configuration, but this resulted in a proximal articular surface that was substantially wider than the opposing articular surface formed by the distal ends of the ulna and radius, a clearly implausible arrangement even allowing for the likely presence of both ossified and cartilaginous carpal elements. Additionally, Holmes (2022 pers. comm.) indicated that Thompson & Holmes (2007) had deliberately flattened and spread out the metacarpus of V. irvinensis to better show the morphology of each element. IV III II I Accordingly, the reconstruction of P. lakustai was revised to show a more transversely arched metacarpus with the proximal heads of the metacarpals in closer contact (Fig. 5A, B). This resulted in a proximal articular surface that better fitted the distal articular surface of the antebrachium, and the arched metacarpus also resembled those of Triceratops (Fujiwara 2009) and the well articulated but indeterminate chasmosaurine CMN 8547 (Mallon & Holmes 2010). Moreover, transverse arching of the metacarpus is consistent with the fact that the proximal surface of metacarpal II strongly tapers ventrally (Fig. 5C), and the presence of rugosities indicates possible close intermetacarpal contacts near the proximal ends of some of the metacarpals known from the Pipestone Creek Bonebed. However, the proximal end of metacarpal III appears more medially expanded relative to the shaft in P. lakustai than in chasmosaurines, implying correspondingly greater separation between the shafts of MC II and MC III (Thompson & Holmes 2007; Fujiwara 2009; Mallon & Holmes 2010). For this reason, the shafts of these metacarpals are separated by a distinct gap in our reconstruction of the manus of P. lakustai (Fig. 5B), rather than closely adjacent as in Fujiwara’s (2009: fig. 8B) reconstruction of the manus of Triceratops Marsh, 1889 and Thompson & Holmes’ (2007) reconstruction of V. irvinensis (Fig. 5D). Verification of the arched configuration and the degree of separation between the shafts of MC II and MC III awaits a detailed description of the articulated manus of a centrosaurine, and ideally of P.lakustai specifically. However, the chasmosaurine condition, the morphology of the proximal ends of the metacarpals of P. lakustai, and the geometric fit between the articular surfaces of the antebrachium and metacarpus all weigh against the hypothesis that the metacarpals were widely spaced and in a single plane, suggesting that the metacarpus of P. lakustai was indeed transversely arched.

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