Imaging the inner ear in fossil mammals: High-resolution CT scanning and 3-D virtual reconstructions

The bony labyrinth of mammals, a delicate and complex cavity within the petrosal, houses the organs of hearing and equilibrium of the inner ear. Because this region is typically lodged deep within the skull, there have been few morphological studies of the bony labyrinth in fossils-where it frequently is completely enveloped by surrounding bone and sediment matrix. The recent development of high-resolution X-ray Computed Tomography (CT) scanning provides a powerful new tool for investigating such tiny, often inaccessible structures. Here we introduce a protocol for producing three-dimensional (3-D) virtual visualizations of the bony labyrinth from high-resolution CT images. As a case study, we scanned the skull of a basal platyrrhine primate, Chilecebus carrascoensis, using the high-resolution CT facility at Pennsylvania State University, reconstructing the endocast of the bony labyrinth from the resulting data. Segmenting the original CT images is a vital step in producing accurate 3-D virtual reconstructions. We failed in efforts to isolate the bony labyrinth endocast through automated means, owing to the similar densities of the matrix filling the sinuses and spongy bone cavities of the specimen, and the bony labyrinth itself. Differing density contrasts across the fossil/matrix interface and overlapping grayscales of the fossil and matrix, required Half Maximum Height thresholds to be measured dynamically. Multiple thresholds are advantageous for processing the CT data of fossils that are inherently heterogeneous in material properties and densities. The iterative interaction between an operator and a computer offers the only means presently available for reliably discriminating the endocast of the bony labyrinth from the remainder of the specimen.