A three-dimensional CT (CAT) scan through a rock with Permian alga Ivanovia tebagaensis

Journal of Paleontology, Jan 1999 by Torres, Andrew M

ABSTRACT-Embedded thalli of Permian Ivanovia tebagaensis, a calcified, cyathiform, codiacean, Chlorophyta, were X-rayed with a medical CT (computed tomography) scanner. The two-dimensional digital X-ray image files were copied to floppy disks and transferred to an IBM-compatible personal computer. The images were cropped, and built into a data volume with a commercial three-dimensional software program. The data were color edited so that only the algae were visible. The data volume was then systematically manipulated to visualize some hitherto unknown details of the structure of the utricles, and of a vegetative propagule, a developing bud.

INTRODUCTION

A PERMIAN codiacean green alga from Tunisia, Ivanovia te. bagaensis, was cyathiform with the cups some 2 cm wide and 3 cm tall. The form was determined from numerous beautifully preserved sections of thalli seen on the surfaces of a cubical polished limestone hand specimen, about 75 mm on a side, and a single section (Torres, 1995). Additional sectioning was not undertaken because of the rarity of the material. Examination of polished surfaces under high magnification revealed that the coenocytic thalli consisted of a membrane, or wall, made up of intertwined elongated tubes forming a central medulla. The medullary tubes developed dichotomous branches laterally to form a series of utricles (Torres and Baars, 1992) comprising cortices, one on the outside and one on the inside of the cup. The spaces between the utricles were apparently calcified as is the case with the related living Halimeda. Of special interest was the finding that the inner and outer cortices were dimorphic; that is, the inner cortex of the cup consisted of densely packed utricles quite different from the loose, spongy utricles of the outer cortex of the cup. The possible physiological significance of such dimorphism remains a mystery and a similar structure is unknown in any other alga, fossil or living. Evidence was also presented that the alga reproduced asexually by budding (Torres, 1995).

Upon the death of the thallus, it is thought that the calcifying aragonite that formed between and on the surface of the utricles became utricular molds that filled with fine calcite crystals thus preserving much of the structure in the form of inner and outer utricular casts (Mu, 1991; Torres, 1997). The larger internal spaces occupied by the medullary strands were filled with mosaic sparry calcite plates and only rarely is any structure seen in the medulla.

A problem commonly encountered in paleontology is to determine the three-dimensional external form of embedded fossils. A second related problem is to reveal some of the details of the internal structure of a given organism. Traditionally, reconstructions of three-dimensional objects have been made from physical serial sections which are analyzed and rendered by hand into three-dimensional objects (Spencer and Spencer, 1995; Torres et al., 1992). More recently serial sections have been analyzed with computer imaging programs (Herbert et al., 1995). However, a serious drawback to these approaches is that in preparing the reconstruction, the fossils themselves are destroyed.

Medical or industrial CT (computed tomography) or CAT (computerized axial tomography) scans provide a powerful and exciting nondestructive technique for studying embedded fossils and fossil interiors. In addition, reconstructions from the X-ray sections can be made with computer based three-dimensional imaging programs associated with the CT machine or with desktop computers (Conroy and Vannier, 1984; Zinsmeister and De Nooyer, 1996). Obvious difficulties with using.a CT include the relative unavailability of both the CT instrument itself, and the CT's software for the analyses; that is, it is uncommon to have access to a CT, and in those cases where access is available, one cannot expect to use the CT's imaging programs for detailed analyses that may require considerable time. Ideally, the CT scan images could be transferred to a personal computer in the researcher's laboratory and analyzed at leisure with commonly available software. Some studies have used three-dimensional computer graphics software programs written in-house (Zinsmeister and De Nooyer, 1996), or commercial programs (Gould et al., 1996), or programs associated with the CT machines themselves (Hamada et al., 1991; Marx and D'Auria, 1988; Rowe et al., 1997).

The primary purpose of this paper is to present some aspects of three-dimensional imaging of the alga achieved with nondestructive medical CT scanning, using a desktop personal computer, and commercially available software to construct and to examine the rendered data volume. Examples used to demonstrate the power of the technique include the fine structure of utricular casts, and the demonstration of a bud deep within the hand specimen. In addition, a scanning electron micrograph is presented as evidence to explain why the CT scans imaged the algae rather than other objects in the hand specimen. And, it is shown that what was color-rendered with the three-dimensional imaging program were indeed the algal membranes.