CERATOPSID

Sullivan, Corwin, Sissons, Robin, Sharpe, Henry, Nguyen, Khoi & Theurer, Brandon, 2024, Skeletal reconstruction of fossil vertebrates as a process of hypothesis testing and a source of anatomical and palaeobiological inferences, Comptes Rendus Palevol 23 (5), pp. 69-83 : 79-80

HINDLIMB OF AN INDETERMINATE CERATOPSID DINOSAUR

One of us (BT) created a reconstruction of UALVP 42, an indeterminate ceratopsid left partial hindlimb comprising almost all the crural and pedal elements, to explore and illustrate the articular configuration of the lower hindlimb in Ceratopsidae for a future descriptive paper (Theurer et al. work in progress). The reconstruction was informed by published descriptions of ceratopsid hindlimbs (e.g. Brown 1917; Lull 1933; Currie et al. 2016: fig. 15) and a ceratopsid footprint ( Gierlinski & Sabath 2008: fig. 10F), and to a lesser extent by descriptions of hindlimbs of other ornithischians (e.g. Forster 1990: fig. 21; Salgado et al. 1997: fig. 5). A model segmented from a CT scan of the juvenile Chasmosaurus belli Lambe, 1902 skeleton UALVP 52613 was also available for comparison.

UALVP 42 was collected by George F. Sternberg in 1920, from exposures of the Belly River Group (Campanian) on Sand Creek in southern Alberta. Sternberg subsequently created a mount of the specimen that was displayed from 1935 to the late 1950s, reconstructing the distal hindlimb skeleton in a physical sense. As well as positioning the bones, Sternberg restored some of them extensively with plaster to conceal damage ( Fig. 6). The proximal and distal ends of the tibia, in particular, were heavily retouched. Having been mixed with brown paint, the plaster is difficult to distinguish from the original fossil bone. Therefore, the bones were CT scanned using a Siemens Somatom Definition Flash scanner at the University of Alberta Hospital (voltage: 120 kV; current: 300 mA; voxel size: 0.6 mm), and the genuine bone was segmented out using Dragonfly ORS. A defined range of intensities was used to create an initial “point and click” segmentation, which was then refined manually a few slices at a time. The scanned bones were imported into Autodesk Maya, an animation program that can be utilised to position digital models in 3D space and produce 2D orthographic and perspective renderings of them from arbitrary angles.

The only distal hindlimb bone missing from UALVP 42 is the proximal phalanx of digit I. The Sternberg mount included another bone in place of this element, but comparison with the left metacarpal IV of Styracosaurus albertensis CMN 344 ( Holmes et al. 2005: pl. 30G-J), in particular, indicates that this substitute is actually a ceratopsid metacarpal. Sternberg may have borrowed the metacarpal, which has not been retouched with plaster in the same manner as the genuine hindlimb elements of UALVP 42, from another specimen, but this possibility is unsupported by any documentation.To complete our reconstruction of UALVP 42, a 3D model of phalanx I-1 from an associated skeleton of the ceratopsid Centrosaurus apertus (UALVP 16248) was created using photogrammetry and the program Agisoft Metashape, and a small piece missing from the anteromedial corner of the proximal end was reconstructed in Pixologic ZBrush. The resulting model was imported into Maya and scaled to an appropriate size for UALVP 42, based on the median ratio of phalanx I-1 length to metatarsal I length (0.889) in several other ceratopsid specimens in which both elements are preserved ( Table 1 ). This procedure involved a tacit assumption, amenable to testing in future comparative studies, that the morphology of phalanx I-1 was unlikely to vary much across ceratopsid species.

Maya was used to reconstruct the articulation of the bones of UALVP 42, plus the rescaled phalanx I-1, and generate an image of the reconstructed configuration ( Fig. 7A). Sternberg’s restoration of the shape of each individual bone was provisionally accepted as a well-educated guess, with the obvious exception of phalanx I-1, but the digital reconstruction distinguished visually between bone and plaster based on the segmented models. One advantage of this method of reconstruction was that internal consistency among the resulting 2D images was guaranteed, given that they all depicted the same 3D model. Therefore, successive versions of the reconstruction always passed the test of internal consistency provided no two elements overlapped in 3D space. Furthermore, “versions” of the underlying 3D model could be quickly generated by rotating and translating individual bones to experiment with different possible configurations, and quickly evaluated by viewing the model from different angles. Accordingly, the iterative process outlined above, in which visual hypotheses are tested, rejected and refined over successive rounds, gave way to a more freeflowing approach in which generation, testing, rejection and refinement of “micro-hypotheses” pertaining to parts of the model took place more or less continuously.

Subjecting the proximal tarsal elements to this type of manipulation led to an unexpected arrangement of the astragalus relative to the calcaneum and to the outer condyle of the distal end of the tibia, which in ceratopsids combines with the two proximal tarsal elements to form the articular surface for the distal tarsals and the proximal ends of the metatarsals ( Brown & Schlaikjer 1940). Sternberg’s original mount placed the calcaneum lateral to the outer condyle of the tibia and only slightly anteriorly displaced ( Fig. 6). This initially led us to likewise place the astragalus medial and slightly anterior to the outer condyle of the tibia in our digital reconstruction ( Fig. 7B). However, it quickly became apparent that positioning the astragalus in this way, without creating an impossible geometry by impinging on the tibia, introduced a large gap between the lateral articular surface of the astragalus and the outer tibial condyle ( Fig. 7C). Such a large gap seemed unrealistic, so the hypothesis of a near-linear arrangement of the astragalus, calcaneum and outer condyle was rejected and alternatives were investigated. Angling the astragalus so that the medial side was positioned more anteriorly than the lateral side eliminated the gap ( Fig. 7D, E) and left the anterior part of the proximal surface of the astragalus resting against a relatively flat area on the anteromedial portion of the distal end of the tibia, and the lateral articular surface of the astragalus against the outer tibial condyle. The anterior margin of the distal articular surface formed by the astragalus, outer tibial condyle and calcaneum is then distinctly concave. It should be noted that acceptance of Sternberg’s restoration of the missing portions of the tibia influences the exact position, but not the overall orientation, that appears optimal for the astragalus.

Sternberg’s placement of the calcaneum almost directly lateral to the outer condyle of the tibia ( Fig. 7B, C) was evaluated by comparison to UALVP 52613 and published descriptions of ceratopsid hindlimbs (e.g. Lull 1933), which indicated that the calcaneum should instead lie anterior to the outer tibial condyle. Repositioning of the calcaneum in accordance with this evidence further accentuated the anterior concavity of the articular surface for the distal tarsals and metatarsals ( Fig. 7D). The articular relationship between the astragalus and tibia in UALVP 42, and the resulting concavity of the anterior margin of the distal articular surface formed by these elements and the calcaneum, are discoveries arising from the process of reconstruction and supported by comparison with published descriptions and UALVP 52613.