JPT Nº21

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INTRODUCTION

 

Tyrannosaurus rex, the embodiment of a terres-trial top-predator, is probably the most stunning dinosaur, recognized by every child and adult, and still stimulates the imagination of the public. After more than 100 years of research, Tyranno-saurus rex is still a welcome subject of research for paleontologists. The earliest documented re-mains of the theropod dinosaur Tyrannosaurus rex are teeth from the Denver Formation near Golden, Colorado, found in 1874 (Breithaupt et al., 2005). Since then, around fifty skeletons of Tyrannosaurus rex have been found since Bar-num Brown excavated the first partially preserved specimen in Wyoming in 1900 (Lar-son, 2008), today stored in the Natural History Museum in London. The North American dinosaur species is considered to have been one of the largest theropod dinosaurs – only the North Afri-can Spinosaurus and the Argentinian carcharodontosaurid Giganotosaurus, both from the Early Cretaceous period, were bigger (Coria and Salgado, 1995; Calvo and Coria, 2000; Dal Sasso et al., 2005; Ibrahim et al., 2014). The here investigated Tyrannosaurus rex was found in 2012, a skeleton unearthed from the Hell Creek Formation in Carter County, the southeast corner of Montana. This individual is well pre-served, with 170 fossilized bones present, including a nearly complete skull. That skull therefore ranks as one of the top three, following “Stan” (BHI-3033) from Harding County, South Dakota, which has the so far most complete skull ever found, only lacking right articular and left coronoid of the lower jaw (Larson, 2008), and “Sue” (FMNH PR2081) from Ziebach County, also South Dakota, with a damaged left temporal re-gion and broken-off left postorbital (Brochu, 2003). Since the new Tyrannosaurus rex is a subject of great interest for research, imaging is a crucial tool for acquiring scientific data.

Computed tomography (CT) is of irreplaceable value in clinical medicine, and continued re-search is still leading to increasing imaging capabilities for nearly all organs and medical con-ditions. The steadily improving CT technique is also useful in the field of paleontology. In recent years several computed-tomographic analyses of the braincases of fossil tetrapods and other fossil specimens have been performed with the aim of investigating anatomical structures and evolu-tionary development (Cruzado-Caballero et al., 2015; Knoll et al., 2015; Benoit et al., 2016; Paulina-Carabajal et al., 2016). CT scans provide insight into the internal 3D anatomy of the investigated specimen, which can be used as a basis for the reconstruction of otherwise ob-scured anatomical structures. Additionally, the information gathered from CT scans provides a basis for higher-level scientific questions such as simulations of biomechanics, animal behavior and physiology (Snively and Theodor, 2011; Cuff and Rayfield, 2013; Bourke et al., 2014; Racicot et al., 2014; Sharp, 2014). Although CT is com-monly used in paleontology, only little technical information is available about the CT scan proce-dures and technical parameters (Cox, 2015). During the last few decades, the spatial resolu-tion of clinical CT scanners has improved from 3 mm (1990) to 1 mm (2000) and 0.35 mm (2010) edge length of isotropic voxels. The wide availa-bility of clinical CT scanners and their high spatial resolution, allowing surface rendering in addition to obtaining information from the inside of the fossil specimens, makes this technique particu-larly interesting for paleontological research (Schilling et al., 2014).

The best-established 3D imaging technique in paleontology is photogrammetry, since it can be applied to specimens of any size, provides the highest spatial resolution of all imaging modali-ties and can illustrate the specimen's actual color. Within paleontological research, there is a vast range of applications for photogrammetry (Mallison and Wings, 2014). Being the primary method of choice for all surface-only 3D visuali-zations (Sutton et al., 2014), photogrammetry facilitates studies that rely on the 3D surface ar-chitecture and 3D modeling of specimens, as for example body mass estimations or studies using Finite-Elements-Analysis (Rayfield, 2004). Its combination of versatility, movability and rela-tive ease of use makes this high resolution method a universal tool for digitization of pale-ontological specimens.

The fundamental difference between CT and pho-togrammetry is the CT’s ability to depict internal structures, while photogrammetry is a purely line-of-sight method for capturing outside shape, with a highly variable resolution from meters to microns (satellite images to electron microscope images) for one uniform workflow.

The purpose of this study was to analyze the technical efforts, workflow, and image quality of computed tomography using a clinical CT scan-ner for a large fossil specimen, the skull of a Tyrannosaurus rex, and to compare clinical CT with 3D digitizing by photogrammetry.

 

Institutional abbreviations

 

BHI, Black Hills Institute of Geological Research, Hill City, SD, USA
FMNH, Field Museum of Natural History, Chicago, IL, USA
MB, Museum für Naturkunde Berlin, Germany

 

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