Tomographic techniques for the study of exceptionally preserved fossils

Abstract

Three-dimensional fossils, especially those preserving soft-part anatomy, are a rich source of palaeontological information; they can, however, be difficult to work with. Imaging of serial planes through an object (tomography) allows study of both the inside and outside of three-dimensional fossils. Tomography may be performed using physical grinding or sawing coupled with photography, through optical techniques of serial focusing, or using a variety of scanning technologies such as neutron tomography, magnetic resonance imaging and most usefully X-ray computed tomography. This latter technique is applicable at a variety of scales, and when combined with a synchrotron X-ray source can produce very high-quality data that may be augmented by phase-contrast information to enhance contrast. Tomographic data can be visualized in several ways, the most effective of which is the production of isosurface-based 'virtual fossils' that can be manipulated and dissected interactively.

Figure 1

Tomography. (a) Three parallel and evenly spaced serial tomograms (1–3) through an idealized gastropod fossil. (b) Resultant tomographic dataset.

Figure 2

Axial computed tomography (CT). The X-ray source is directed through the sample onto the detector; the single tomogram shown is computed from data within the tomographic plane in all radiographic images of the rotational series. Note that CT tomograms are conventionally inverted so that high-attenuating regions are light rather than dark.

Figure 3

(a–d) Tomograms generated using different techniques. See original publications for specimen and repository information, (e–g) virtual fossil reconstructions. (a) Physical–optical tomogram (digital microscope photography following serial grinding) of ‘marrelomorph’ arthropod X. chledophilia (Siveter et al. 2007), Herefordshire Lagerstätte (Silurian, England), ×2.5. (b) Synchrotron radiation X-ray tomographic microscopy (SRXTM) tomogram of the scalidophoran Markuelia hunanensis (Cambrian, China), from dataset of Donoghue et al. (2006), ×80. (c) Magnetic resonance imaging tomograms of guard of the belemnite Belemnopsis sp., Tendaguru Formation (Jurassic, Tanzania), ×2 (reproduced from Mietchen et al. 2008). (d) Confocal laser scanning microscopy tomogram of a permineralized spirally coiled cyanobacterial trichome (Heliconema funiculum), Bitter Springs Formation (approx. 850 Ma, Australia), ×940 (reproduced from Schopf et al. 2006). (e) Hardware-rendered single-isosurface reconstruction of curculionid insect (weevil), Bouldnor Formation (‘Bembridge Marl’; Eocene, England) based on X-ray micro-tomography data edited to remove objects in matrix not relevant to fossil, ×6. Maidstone Museum and Bentlif Art Gallery, MNEMG 2008. 11. (f) Hardware-rendered multiple isosurface reconstruction of the scalidophoran M. hunanensis (Cambrian, China) partially dissected to reveal scalid structure, based on unedited SRXTM data, ×82 (reproduced from Donoghue et al. 2006). (g) Ray-traced multiple isosurface reconstruction of marrelomorph arthropod X. chledophilia, Herefordshire Lagerstätte (Silurian, England), based on physical–optical data edited to remove unwanted objects and identify fossil structures, ×1.2 (reproduced from Siveter et al. 2007).