INTRODUCTION

In recent year, digital files have increasingly been used for scientific research instead of real bones or casts. Currently, the most common way of obtaining a digital representation of a specimen is computer assisted tomography (CT) (see e.g., Zuo and Jing 1995; Gould et al. 1996; Knoll et al. 1999; Stokstad 2000; Golder and Christian 2002; Ridgely and Witmer 2004, 2006; Sereno et al. 2007; Witmer and Ridgely 2008). These digital images can consist of cross sections, but usually are three dimensional models of internal shapes of an object, e.g., in order to assess as yet unprepared specimen or depict internal structures without damaging the object (e.g., Witmer and Ridgely 2008). Models of external shapes can be used to rapid prototype (RP) scaled models or exhibition copies, because the high accuracy of CT scans justifies the high costs of CT scanning and RP. This technique also allows mirroring of specimen or combining several partial specimens into one complete individual or bone. Neutron tomography (NT) has also been tested (Schwarz et al. 2005), with mixed results.

Another method to obtain 3D files is laser scanning, either from three perpendicular views or with a surround scan. Alternatively, repeated scans can be taken at many angles and combined in the computer. An extensive project at the Technische Universität Berlin used laser scanners to digitize complete mounted skeletons and skin mounts (see also Gunga et al. 1995; Gunga et al. 1999, Bellmann et al. 2005; Suthau et al. 2005; Gunga et al. 2007; Gunga et al. 2008). Bates et al. 2009 also employ such laser scans, albeit apparently at a lower accuracy. Also, some of the dinosaur skeletons mounted in the MFN exhibition were high resolution laser scanned as separate elements by Research Casting International during the museum renovation in 2006/2007.

All three methods produce vast amounts of data, depicting the object in very high detail. When such high resolution is not needed, the large file size becomes cumbersome. As long as only external surfaces are of concern, mechanical digitizing provides a cheap and fast alternative (Wilhite 2003a, 2003b), delivering small files of sufficient accuracy for most applications. Mechanical digitizing means creating a computer representation of a physical object by means of using an apparatus that samples 3D landmarks on the object's surface through touching it. Other techniques involving digitizing were used by Goswami (2004) and Bonnan (2004), who focused on specific bone landmarks. In contrast to our methods, these do not produce complete 3D images of bones and will not be addressed here. Similar in handling and data output to the methods described here is the sonic digitizer used by Hutchinson et al. (2005). It is limited to collecting point data, but provides a large range of up to 14 feet, albeit at slightly lower accuracy.

Here we detail improvements for digitizing techniques for dinosaur bones as described by Wilhite (2003a, 2003b), expanding the size range of suitable bones for the method significantly, both for larger and smaller specimens. New methods also allow complex shapes to be digitized with relative ease, and remove the need to edit the digitizing data in other programs before use. Also, the extraction of surface data from CT data in AMIRA 3.11® and the subsequent editing is described briefly. This CT based data is used to evaluate the accuracy of mechanical digitizing data.

Fossils (vertebrate or invertebrate) digitized with the methods described here can easily be added to online databases, instead of or alongside with photographic images. Most databases, such as the database of the New Mexico Museum of Natural History (Hester et al. 2004) or the American Museum of Natural History can easily accommodate small-scale previews as well as complete files, since the file formats are common and the file sizes relatively small in comparison to CT or laser scan data. Stevens and Parrish (2005a, 2005b, Kaibridge Incorporated) used several files created during this project for modeling Brachiosaurus in Dinomorph™. The University of Texas runs another digital library (Digital Morphology) based on high-resolution CT scans. Objects digitized via dense point clouds as described herein could conceivably be added to this database as stereolithographies (*.stl files), provided sufficient resolution is obtained. For most applications, pointcloud files created with the Microscribe® can be used interchangeably with laser scan files of (or reduced to) similar resolution. The digital files can also be used to rapidly test possible skeletal assemblages, joint mobility ranges (Wilhite 2003a, 2003b), inter- and intraspecific variation (e.g., Wilhite 2005). Virtual skeletons created from them in CAD softwares such as Rhinoceros® can be an aid in planning museum mounts.

Another possible application is rapid prototyping. Scale models of bones can be produced at almost any scale, as well as molds for casting, or negatives of the bones that can serve as storage casts or as mounting racks for museum exhibition. High resolution rapid prototyping or 3D printing (600dpi) calls for CT or laser scan data, due to the ability to exactly create surface textures, but at lower resolutions (300dpi), accurate NURBS or STL objects from mechanical digitizing are of sufficient quality to create exhibition copies of fragile specimens or mirror images to replace missing elements in skeletal mounts. Research Casting International used full scale 3D prints of the exhibition skeleton of the MFN Kentrosaurus to construct the armature that was used for the new mounting of the skeleton in 2007.

Our methods probably work well for a plethora of disciplines aside from vertebrate paleontology, such as archeology. However, aside from a single trial using a fossil vertebrate footprint, we developed them solely on vertebrate body fossils. Researchers from other fields are encouraged to experiment with mechanical digitizing, and to adapt and improve upon the methods described here.