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Article (PDF, 444 Foss. Rec., 17, 33–39, 2014 www.foss-rec.net/17/33/2014/ Fossil Record doi:10.5194/fr-17-33-2014 Open Access © Author(s) 2014. CC Attribution 3.0 License. Osteoderm distribution has low impact on the centre of mass of stegosaurs H. Mallison Museum für Naturkunde – Leibniz-Institut für Evolutions- und Biodiversitätsforschung an der Humboldt-Universität zu Berlin, Berlin, Germany Correspondence to: H. Mallison ([email protected]) Received: 7 November 2013 – Revised: 13 February 2014 – Accepted: 14 February 2014 – Published: 7 March 2014 Abstract. It has been hypothesized that the pronounced have possessed parascapular spines [Galton (1982)]. Despite differences of stegosaur humeral shapes, with large forms the large number of individuals of Stegosaurus known, no having more slender and small forms having more robust parascapular spine belonging to the genus has ever been dis- humeri, may be explained by a difference in relative centre covered [Galton and Upchurch (2004)]. The additional mass of mass (COM) placement caused by differing distributions of these large dermal spines in the shoulder region might of osteoderms. To test this hypothesis, digital 3-D models of have caused the center of mass to be located further ante- the bones and osteoderms of the Tanzanian stegosaur Ken- riorly in Kentrosaurus than in Stegosaurus” (Maidment et trosaurus aethiopicus and of the North American stegosaur al., 2012). Stegosaurus armatus were used to create a 3-D computer- In this context it is important to note that Hennig (1925) aided design life reconstruction. On these models osteoderm and Janensch (1925) as well as Galton (1982) listed the os- placement was varied drastically, recreating both existing teoderms in question as parasacral spikes, not parascapular. and hypothetical forms. These models show that COM posi- Mallison (2011c) agreed with this assessment. The bases of tion varies somewhat with realistic osteoderm distributions, all osteoderms show a rugose texture that apparently relates but insufficiently to explain major differences in humeral to attachment of tough connective tissues, and a similar mark shape. The uniform weight distribution between forelimbs of identical size to the base of the spikes in question can and hindlimbs found between the two taxa also casts doubt be found on the dorsal surface of the pelvic shield in all on the hypothesis that differences in relative COM position Kentrosaurus hips. Additionally, there is a good match in caused by other factors than osteoderm distribution can ex- shape between spike and hip. Disagreement with a placement plain differences in humeral robustness. on the hip is based on the presence of somewhat similarly shaped scapular spikes in Gigantspinosaurus, which were found in articulation with the scapulae, proving beyond any doubt that at least one species of stegosaur had parascapular 1 Introduction points (Ouyang, 1992). However, the spike shape seems to differ quite significantly, based on evaluation of photographs Maidment et al. (2012) hypothesized that osteoderm place- provided by S. Hayashi. In any case, Hypothesis 1 of Maid- ment may have had a significant influence on centre of mass ment et al. (2012) is worth exploring in detail, and is here (COM) position, enough so to explain the proportional dif- tested using CAD methods. ferences in thyreophoran humeri they found. They found that Maidment et al. (2012) suggest a further cause of the ap- “the largest stegosaurs have relatively slender humeri in com- parent overall robustness of stegosaur humeri in general: parison with the smallest members of the clade” (Maidment “The robustness of thyreophoran and particularly stegosaur et al., 2012), and proceed as follows (here treated as Hypoth- humeri could be interpreted to indicate that the center of esis 1): “This could be due to changes in the center of mass mass of stegosaurs was located more anteriorly than in other related to distribution of dermal armor; for example, Ken- trosaurus, the smallest stegosaur in the sample, is known to Published by Copernicus Publications on behalf of the Museum für Naturkunde Berlin. 34 H. Mallison: Osteoderm distribution has low impact on the centre of mass of stegosaurs ornithischians, so that the humerus was required to support from the left side, due to the placement of the mount in the proportionally more of the body mass.” museum exhibition. Maidment et al. (2012), however, point out that “Hender- In total 80 photos were used for a photogrammetric re- son [(1999)] modelled the center of mass of several dinosaurs construction (see Falkingham, 2011, for a description of the and showed that the center of mass in Stegosaurus was lo- method) in PhotoScan Professional by Agisoft, pre-release cated as far posteriorly as it was in bipedal taxa such as of Version 1.0.0. Alignment was performed using “high” Tyrannosaurus.” They then proceed to suggest an alternative accuracy and “disabled” pair selection, with a point limit of suggestion, here treated as Hypothesis 2: “Alternatively, the 10 000. The alignment of 76 photos (4 were not aligned) was robustness of stegosaur humeri could be related to a specific visually inspected in-program for plausibility, and found to behaviour. For example, it has been suggested that stegosaurs be acceptable. A dense point cloud was calculated using the utilized a tripodal stance [...]; perhaps increased stress on the settings quality “high” and depth filtering “moderate”. Erro- humerus was generated during rearing as a result of pushing neous points and model parts not pertaining to the mounted off from the ground.” skeleton were manually selected and deleted in PhotoScan. This hypothesis will also be discussed below. The left half of the skeleton was found to be badly modelled Abbreviations: CAD – computer-aided design; COM – in parts (skull, ribcage), but of good quality in the tail. The centre of mass; MB.R.#### – MfN fossil reptile collection right half and the plates had sufficient points for wrapping numbers; MfN – Museum für Naturkunde, Leibniz Institute a useable surface. Therefore, the left side of the trunk was for Research on Evolution and Biodiversity at the Humboldt deleted. The point cloud was then meshed in Geomagic 10 to University Berlin, Berlin, Germany; RBINS – Royal Belgian create a mesh with ∼ 1 million polygons. Institute of Sciences, Brussels, Belgium; SMA – Sauriermu- For creation of the digital 3-D model of the living an- seum Aathal, Aathal-Seegräben, Switzerland. imal, the mesh was then reduced to 25 % polygon count (∼ 260.000 polygons, using “fix boundaries” option), sec- tioned (i.e. limbs and osteoderms were made separate enti- 2 Material and methods ties), imported into Rhinoceros 5.0, and straightened into a standing pose with vertical limbs and no lateral flexion of the 2.1 Bone and skeletal mount 3-D scans vertebral column. The MfN’s Dinosaur Hall displays a mounted skeleton of 2.2 Life reconstruction Kentrosaurus aethiopicus Hennig, 1915, which consists of the lectotype of K. aethiopicus (Hennig, 1925; Janensch, The CAD 3-D life reconstruction of Kentrosaurus used here 1925; Mallison, 2011c) and other material. The skeletal el- is based on an unfinished version of that used in Malli- ements were described in detail by Hennig (1925), whereas son (2011a). The underlying skeletal data (high-resolution the process of bone selection and mounting was described laser scans of the mounted skeleton in the MfN) are figured in Janensch (1925). The mount was dismantled in 2005, the in Mallison (2011c, Fig. 1). It was created in Rhinoceros bones re-conserved and the skeleton remounted in a different, NURBS Modelling for Windows 4.0 and 5.0 (McNeel & more lively pose in 2007 by Research Casting International. Associates). For this study, slight changes were made to a The composition of elements remained unchanged. Those el- version of the model that had not undergone sectioning into ements of the mount that are real fossil bone and the plas- functional units. ter skull were scanned with a high-resolution Minolta laser Soft tissue extents were based on own dissection data, scanner during the re-conversation effort. Previously, these cross-section photographs of Alligator mississippiensis pro- scans were used for a digital skeletal mount and a study of vided by R. Wilhite, CT scans and X-ray images of extant the motion range of the limbs and vertebral column (Malli- sauropsids, and data from Allen et al. (2009). The Supple- son, 2010), and a CAD life reconstruction based on the digi- ment contains the files necessary to reconstruct the entire tal skeletal mount for an assessment of the defence behaviour model. (Mallison, 2011a), and the volumetric model is re-used here. The same model was used to create a model of Other research using the 3-D files is currently being pub- Stegosaurus. Superposition of the 3-D model of the mounted lished (Maidment et al., 2013, 2014). cast and the finished life reconstruction model of Ken- The Stegosaurus armatus model is based on a mounted trosaurus showed that only modest modifications were re- cast of a Stegosaurus in the RBINS. The skeleton is a com- quired to fit the life model to the skeleton, namely a length- posite made from casts of bones in the collection of the ening of the trunk section and the limbs (affected using the SMA, which houses several partial individuals. The mount Scale1D command in Rhinoceros 5.0, which stretches a part was photographed with a Canon EOS 400D digital SLR along one axis only), as well as a ventral expansion of the camera, using a SIMGA 28–80 mm lens. Photographs were trunk segment and tail base segment caused by the longer taken from all around the front, right side and back of the ischia of Stegosaurus. The latter point was not altered, re- mount, but only a handful of photographs could be taken sulting in an undersized trunk for the Stegosaurus model, Foss.
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