Non-Invasive Imaging Methods Applied to Neo- and Paleo-Ontological Cephalopod Research
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Biogeosciences, 11, 2721–2739, 2014 www.biogeosciences.net/11/2721/2014/ doi:10.5194/bg-11-2721-2014 © Author(s) 2014. CC Attribution 3.0 License. Non-invasive imaging methods applied to neo- and paleo-ontological cephalopod research R. Hoffmann1, J. A. Schultz2, R. Schellhorn2, E. Rybacki3, H. Keupp4, S. R. Gerden1, R. Lemanis1, and S. Zachow5 1Institut für Geologie, Mineralogie und Geophysik, Ruhr Universität Bochum, Universitätsstrasse 150, 44801 Bochum, Germany 2Steinmann-Institut für Geologie, Mineralogie und Paläontologie, Rheinische Friedrich-Wilhelms-Universität Bonn, Nussallee 8, 53115 Bonn, Germany 3Helmholtz-Zentrum Potsdam, Deutsches GeoForschungsZentrum GFZ Sektion 3.2, Geomechanik und Rheologie, Telegrafenberg, D 429, 14473 Potsdam, Germany 4Institut für Geologische Wissenschaften, Fachrichtung Paläontologie, Freie Universität Berlin, Malteserstrasse 74–100, 12249 Berlin, Germany 5Zuse Institut Berlin, Takustrasse 7, 14195 Berlin, Germany Correspondence to: R. Hoffmann ([email protected]) Received: 28 October 2013 – Published in Biogeosciences Discuss.: 29 November 2013 Revised: 14 March 2014 – Accepted: 29 March 2014 – Published: 22 May 2014 Abstract. Several non-invasive methods are common prac- tially preserved within the surrounding rocks, requires imag- tice in natural sciences today. Here we present how they ing methods that are primarily used in non-destructive test- can be applied and contribute to current topics in cephalo- ing. The conservation of the specimen is of main importance pod (paleo-) biology. Different methods will be compared in using these methods since former techniques used destruc- terms of time necessary to acquire the data, amount of data, tive methods leading to the loss of the specimen or parts accuracy/resolution, minimum/maximum size of objects that of the specimen. Enhanced tomographic imaging techniques can be studied, the degree of post-processing needed and enable a view inside solid objects with astonishing spatial availability. The main application of the methods is seen in resolution. Typical tasks for the reconstruction of structures morphometry and volumetry of cephalopod shells. In partic- from tomographic image data are digital image processing ular we present a method for precise buoyancy calculation. (like filtering and enhancement), segmentation (e.g., classifi- Therefore, cephalopod shells were scanned together with dif- cation), and geometry processing. ferent reference bodies, an approach developed in medical The oldest record of the application of non-invasive meth- sciences. It is necessary to know the volume of the reference ods in the field of cephalopod research dates back to the bodies, which should have similar absorption properties like publication of Lehmann (1932). By using x-rays Lehmann the object of interest. Exact volumes can be obtained from (1932) produced stereo images of a 3-D impression of the surface scanning. Depending on the dimensions of the study internal structures of a Lower Jurassic ammonite. A few object different computed tomography techniques were ap- years earlier Sollas (1904) invented a method using serial plied. sections that allowed the visualization of internal structures of fossils. This approach has formed the basis of modern grinding tomography (e.g., Pascual-Cebrian et al., 2013). Much has been improved since the earliest attempts, in- 1 Introduction vasive or not, for 3-D reconstructions of fossils and their internal structures (Sollas and Sollas, 1914; Tipper, 1976; During the last decade non-invasive diagnostics has be- Luo and Ketten, 1991; Herbert et al., 1995; Hurum, 1996; come increasingly important for paleontological research. Stock, 2006; Tafforeau et al., 2006; Sutton et al., 2001, The analysis of shapes of fossil hard parts, either fully or par- Published by Copernicus Publications on behalf of the European Geosciences Union. 2722 R. Hoffmann et al.: Non-invasive imaging methods Hoffmann and Zachow, 2011; Falkingham, 2012). Specifi- cally for cephalopods non-invasive methods were used lately for reconstructing internal soft parts, mainly buccal masses of ammonoids, e.g., Klug et al. (2012), Kruta et al. (2013, 2014), and Tanabe et al. (2013), and were also used on ex- tant cephalopods, e.g., Westermann et al. (2002) and Kerbl et al. (2013). Recently, Sutton et al. (2014) published an exten- sive overview of techniques for virtual paleontology, includ- ing destructive and non-destructive approaches. Advantages and disadvantages of non-invasive methods (surface scan- Figure 1. Multiplanar reconstructions (MPR). (A) Tomographic ning, computed tomography, magnetic resonance tomogra- data set of a hollow Lower Jurassic (Toarcian) Eleganticeras (about phy) tested by the authors are discussed below. 1 cm in diameter) with a size of 9 GB, acquired with a v|tome|x s Advantages of the non-invasive approach are: external (180 kV tube current, res. 6 µm). MPR and visualization obtained and/or internal structures of specimens can be studied in with the ZIBAmira 3-D software. (B) Digitally merged data set of great detail in 2-D and 3-D while the specimen itself remains two separately scanned (res. 45 µm) parts of a broken Upper Creta- untouched (Fig. 1), scan data can be shared within the sci- ceous concretion of about 13 cm in diameter that contains one spec- imen of Gaudryceras sp. (about 8 cm in diameter, coll. Y. Shigeta, entific community, used for a virtual visit of museum collec- Tokyo). tions (e.g., http://www.3d-fossils.ac.uk/home.html), to main- tain a record of type material that may later be destroyed or lost in the future, or can be stored for later studies to solve new questions. A clear advantage for future work is the pos- tion of whether ammonoids were benthic crawlers or could sibility to store and share the CT data and make them avail- freely swim in the water column, with wider implications for able for subsequent studies of other researchers. Rare fossils predator–prey relationships. It is therefore of critical impor- (e.g., holotype material) can be duplicated as 3-D prints and tance for those interested in the interpretation of δ18O signa- sent to scientists all over the world or used for lectures (Rah- tures derived from ammonoid shells, as well as for the recon- man et al., 2012), exhibitions and outreach programs. Sin- struction of the evolution of paleoenvironments. For recent gle pieces of a broken specimen can be scanned separately, constraints on ammonoid paleobiology and life history the merged digitally – as shown by Hoffmann (2010) for an Up- reader is referred to Ritterbush et al. (2014). per Cretaceous hollow preserved ammonite – and printed in In order to answer this question we performed the first one piece afterwards (Fig. 1b). Multiplanar reconstructions test with the shells of the recent Nautilus and Spirula and (MPR, Fig. 1a) and surface and volume renderings (Fig. 3b) calculated the buoyancy values for each. For both species can also be applied to the data sets. Presentation animations it is well known that they were able to swim freely in the and instructive movies can be created. Finally, accurate sur- water column. Furthermore, non-invasive methods can be face models can be derived from these methods and used for applied not only for volumetry but also for 2-D measure- advanced functional analyses such as finite element analysis ments of cephalopod shells and will contribute to the quanti- (FEA), computational fluid dynamics (CFD, e.g., Shiino et tative morphological approach taking intraspecific variability al. 2012) and multibody dynamic analysis (MBD, Anderson of species into account (Hoffmann et al. 2012). et al., 2012; Bates and Falkingham, 2012). Because FEA is Due to their accretionary growth, molluscs record their beyond the scope of this article the reader is referred to Ray- whole ontogeny with their shells. 2-D measurements of dis- filed (2007) for a review of FEA in palaeontology. One main tances and their ontogenetic changes (ontogenetic trajecto- goal for future research should be the establishment of a 3-D- ries) with a higher resolution (from 45◦ down to 1◦) com- shaped database for ammonoids, comparable to the GenBank pared to the cutting approach (180 or 90◦) increase the pre- of biologists. We chose cephalopods as ideal candidates for cision of morphological description. First 2-D measurements this non-invasive method study because of their longevity, were performed on the micro-CT images of the extant Nau- high abundance and wide distribution during Earth’s his- tilus and Spirula and afterwards on the micro-CT images of tory, their meaning for biostratigraphy due to high evolution- fossil ammonites following the key of description for am- ary rates, importance for tracing evolutionary mechanisms monoid conchs of Korn (2010). Due to the advantages of and their complex internal structure and its possible func- some of the non-invasive methods we were able to add in- tion, which was under debate for centuries. Besides mea- ternal characters to the classical conch parameter (e.g., am- suring distances the main challenge here is to test whether monitella length, septal angle, septal thickness profiles, sep- the ammonoids – a group that appeared during the Devonian tal spacing, diameter of the siphuncle at a certain conch di- and went extinct at the Cretaceous–Paleogene event – could ameter, relative position of the siphuncle, and conch wall swim or not. We emphasize here that this question is more- thickness at a certain conch diameter). Only after segmenta- over important