BRIEF COMMUNICATION UTILITY OF HIGH RESOLUTION X-RAY COMPUTED TOMOGRAPHY (HRXCT) FOR PALEOBOTANICAL STUDIES: AN EXAMPLE USING LONDON CLAY FRUITS AND SEEDS 1 MELANIE L. DEVORE, 2,6 PAUL KENRICK, 3 KATHLEEN B. PIGG, 4 AND RICHARD A. KETCHAM 5 2 Department of Biological and Environmental Sciences, Georgia College & State University, Milledgeville, Georgia 31062-0001 USA; 3 Department of Palaeontology, The Natural History Museum, Cromwell Road, London SW7 5BD, UK; 4 School of Life Sciences Faculty & Administration, Arizona State University, Box 85287-4501, Tempe, Arizona 85287-4501 USA; and 5 University of Texas X-ray High-Resolution CT Facility (UTCT), Department of Geological Sciences, University of Texas at Austin, Austin, Texas 78712-1100 USA High resolution x-ray computed tomography (HRXCT) was used to image pyritized fossil fruits from the Lower Eocene London Clay flora to test the utility of this technique for paleobotanical application. The combination of carbon-pyrite preservation and void spaces between fruit and seed layers within fossils provides differences in density and composition that enable excellent imaging. Fossil fruits of Palaeorhodomyrtus subangulata (Bowerbank) Reid & Chandler (Myrtaceae) were investigated in situ within their silicone fluid conservation medium, which protects these unstable fossils from oxygen and humidity. HRXCT recovers taxonomically informative anatomical and morphological detail and provides a means of nondestructive examination of delicate type materials and other important specimens. These results suggest that HRXCT will be applicable to a broad spectrum of pyritized fossils to record structural details in inherently unstable materials. Key words: fossil fruits and seeds; high resolution x-ray computed tomography (HRXCT); London Clay; paleobotanical techniques; pyrite permineralization. Pyrite (FeS 2 ) is a common permineralizing agent of fossil plants and preserves the detailed structure to the cellular and sometimes subcellular level (Grimes et al., 2002). Pyritized remains present challenges in both preparing material for study and conservation. Anatomical details are normally observed in this opaque material by reflected light or scanning electron microscopy (Collinson, 1999; Kenrick, 1999). However, both of these methods, which involve fracturing or wafering and polishing specimens are inherently destructive and cause loss of material and damage to specimens (Hass and Rowe, 1999; Jones, 1999). Pyrite is unstable in museum collections; even under ambient temperature and humidity, it presents a challenge for long-term conservation. The mineral is prone to oxidation, which leads to deterioration and ultimately to complete loss of specimens (Newman, 1998). This is particularly problematic for type and figured material. Many approaches to conservation have been tried, with the most successful of these involving the reduction or exclusion of oxygen and the maintenance of low relative humidity (Newman, 1998; Shute and Foster, 1999). Inert barriers such as silicone fluid successfully reduce deterioration, but they also pose problems, because removal of specimens from these media for study can result in damage or deterioration. High resolution x-ray computed tomography (HRXCT) has the potential to solve both problems simulta- neously by providing a method to image the internal anatomy of pyritized fossils nondestructively without removing them from their conservation medium. X-ray computed tomography (CT) was developed originally by Cormack and Hounsfield as a medical diagnostic tool (Hounsfield, 1980). Since then, the method has been extended to additional industrial and scientific applications, as reviewed by Ketcham and Carlson (2001). CT creates two-dimensional digital slices based on the attenuation of x-rays as they pass through an object, which is related to density and chemical composition. A three-dimensional digital map is then assem- bled from the multiple slices. Because x-ray dose is not an issue for fossils, a specialized technique, high resolution x-ray computed tomography (HRXCT), can be used. HRXCT can take advantage of a variety of optimizations such as higher energy x-rays, smaller detectors, and longer exposure times than is practical in conventional medical CT devices. Generally, HRXCT has higher resolving power than CT systems, with a typical scale of resolution of approximately 100 lm and is capable of up to 10 lm or better (Ketcham and Carlson, 2001). This greater resolution power is clearly within a range that would permit critical studies of plant tissues and, to a lesser degree, cell types. Both medical CT and industrial HRXCT systems are used increasingly by vertebrate and invertebrate paleontologists in place of destructively sectioning rare and valuable specimens (e.g., Conroy and Vannier, 1984; Haubitz et al., 1988; Rowe et al., 1993; Zinsmeister and DeNooyer, 1996; Alonso et al., 2004; Clarke et al., 2005). Botanists have used the method to measure density of extant woods (Taylor et al., 1984; Funt and Bryant, 1987; Lindgren, 1991; Fromm et al., 2001) and to quantify spatial distribution of rooting systems in trees 1 Manuscript received 4 May 2006; revision accepted 6 October 2006. This research was funded by National Science Foundation Grant EAR- 0345569 and a Faculty Research Grant, Georgia College & State University (M.L.D.), and National Science Foundation Grant EAR- 0345838 (K.B.P). Operation of the University of Texas High-Resolution X-ray CT Facility is supported by National Science Foundation Grant EAR-0345710. The authors thank N. D. Wilkens and B. H. Tiffney for their comments on a draft of the manuscript. 6 Author for correspondence (e-mail: melanie.devore@gcsu.edu) 1848 American Journal of Botany 93(12): 1848–1851. 2006.