By John Guyer & Dean Falk
University at Albany, SUNY & Florida State University
Endocranial casts (endocasts) are sometimes formed naturally as skulls fossilize. During this process, fine sediment fills the braincase, replacing the deteriorating brain tissue. This sediment ultimately fossilizes into a solid mineral substance that reproduces the external morphology of the brain that was imprinted on the endocranium during life. With luck, the result is a "natural endocast" that reveals details of the cerebral cortex (e.g., convolutions reflected by sulcal and gyral patterns), meningeal vascular patterns, and the sutures that delimit the individual bones of the skull.
When fate does not provide us with natural endocasts, we are challenged to create our own. One method is fairly straightforward. When an essentially complete cranium such as that for Sts 5 (Mrs. Ples) is available, silicone latex can be poured into the skull and left to cure. As is the case for natural endocasts, the interior morphology is reflected on the resulting endocast. For the most part, however, we only have cranial fragments. The process then becomes a bit more laborious. The fragments must first be articulated in order for the skull to become a viable mold into which silicone may be poured. Because the resulting endocast provides clues that suggest a particular pattern of reconstruction for the cranial fragments, it can be used to inform further reconstruction of the cranial fragments. In such cases, the product of an initial cranial reconstruction (the endocast) may be used to improve the reconstruction of the skull.
Occasionally, a fossilized skull is found that is solid or filled with matrix. In such cases, the endocranium is hidden and a specialized approach must be taken in order to reveal the features imprinted on its inner surface. One method for doing this is to use 3D-CT technology to create virtual images of skulls and their endocasts (Conroy et al., 1998; 2000). This approach permits separation (virtually speaking) of the fossil from the fill. Hard copies of the skulls or their endocasts may also be produced from the 3D-CT data using laser technology and a process known as stereolithography (Seidler et al., 1997). In this case, the image of the skull is translated from a computer image to a polymer cast. This process is proving to be highly effective through comparative studies using traditional endocasts, virtual endocasts, and their "sisters" prepared from stereolithographic models (Weber et al., 1998).
References
- Conroy, G. C., Falk, D., Guyer, J., Weber, G. W., Seidler, H. & Recheis, W. (2000) Endocranial capacity in Sts 71 (Australopithecus africanus) by three-dimensional computed tomography. Anat. Rec., in press.
- Conroy, G. C., Weber, G. W., Seidler, H., Tobias, P. V., Kane, A. & Brunsden, B. (1998) Endocranial capacity in an early hominid cranium from Sterkfontein, South Africa. Science 280, 1730-1731.
- Seidler, H., Falk, D., Stringer, C., Wilfing, H., Muller, G., zur Nedden, D., Weber, G., Recheis, W. & Arsuaga, J. L. (1997) A comparative study of stereolithographically modelled skulls of Petralona and Broken Hill: Implications for future studies of middle Pleistocene hominid evolution, J. Hum. Evol. 33, 691-703.
- Weber, G. W., Recheis, W., Scholze, T. & Seidler, H. (1998) Virtual anthropology (VA): methodological aspects of linear and volume measurements - first results. Coll. Antropol 22, 575-583.
By John Guyer
University at Albany, SUNY
3-D geometric morphometric analyses utilize three-dimensional landmark coordinate data sets in order to carry out comparative anatomical studies. This methodology is particularly useful for comparing forms in studies of phylogenetic transformation. Richtsmeier, Cheverud & Lele (1992) describe 3D geometric morphometrics as "a true merger of geometry and biology, and one that "requires that the biological form be unambiguously reconstructed from the data collected to represent that form. One of the advantages of this technique is that the resulting geometric form adequately represents the homologous structure from which the coordinate landmarks are taken.
In order for the primary benefit of 3-D morphometric analysis to be realized, the particular statistical model used for analyses of data must be scrutinized. Richtsmeier et al. (1992) contend that the model must have the capacity for translation effect (moving the object within a given coordinate system) and rotation (spinning the object on an axis). These qualities represent a class of distributions for matrix-valued random variables. The superiority of using a coordinate system as compared to singular landmark assignments rests on the coordinate system being free of deterministic assignment by virtue of proximity to other biological structures (see Richtsmeier et al., 1992, for details).
Coordinate sets are referred to as finite elements that are themselves described as spatial planes, or geometric figures, that are composed of several planes and finite landmarks. It is recommended that biologically homologous landmarks be chosen. Analysis entails comparison of two forms -- one assigned as the reference object and the other as the target object. Comparison proceeds not with the measurement of the object, but with the transformation (assessed by measuring deformation) of the reference object into the target object. Thus, what is actually being measured and what provides the basis for comparative analysis, is the contrast that differentiates the two forms (see Cheverud & Richtsmeier, 1986, for further details).
References
- Richtsmeier, J.T., Cheverud, J.M., & Lele, S. (1992) Advances in anthropological morphometirics. Annu. Rev. Anthropol. 21:283-305.
- Cheverud, J.M., Hartman, S.E., Richtsmeier, J.T., Atchley, W.R. (1991) A quantitative genetic analysis of localized morphology in mandible of inbred mice using finite element scaling analysis. J. Craniofac. Genet. Dev.Biol. 11:122-137.
- Cheverud, J.M., Richtsmeier, J.T. (1986) Finite-element scaling applied to sexual dimorphism in rhesus macaque (Macca mulatta) facial growth. Syst. Zool. 35(3):381-399.
By John C. Redmond
University of Louisiana at Lafayette