INTRODUCTION

The examination of internal structures of three-dimensional megafossils such as caprinid bivalves and brachiopods without destruction of specimens has been a challenge. Serial sections spaced at intervals as close as 0.1 mm provide a succession of two-dimensional cross sections that reveal the internal morphology (Ager 1965), but result in the destruction of specimens. X-radiography also has been used to reveal internal structures, and by shooting stereoscopic pairs, a 3-D image can be obtained (Zangerl 1965). X-ray computed tomography (CT) scans of limestone cores show the general outlines of rudists and successive slices can be stacked by computer to form 3-D images (Hughes et al. 2004). The attenuation of X-rays through carbonates presented as colored images reveals density patterns that relate to bulk density and lithology. This technique has also been applied to many studies of vertebrates (e.g., Rowe et al. 2001), and to examination of other organisms such as calcareous algae (Torres et al. 2003), and calcareous fossils (Dominguez et al. 2002).

Here we report on the application of X-ray CT scanning to the study of caprinid bivalves from the Lower Cretaceous (Albian Stage) Edwards Formation in Travis County, Texas. This technique enables the specific identification of silicified caprinid rudists that otherwise could only be identified by sectioning the specimen. Furthermore, this technique provides a full 3-D representation that can be inspected from many positions so that a variety of internal features can be seen.