CA-LSE and AST-3D: two new digital tools for reproducing the inner cavities of skeletal elements

The study of human evolution is currently benefiting of the availability of 3D models of unique specimens from the fossil record, as well of skeletal specimens coming from modern collections; such technologies are among the aspects on which the cutting-edge field of Virtual Anthropology relies on.The possibility to work on digital specimens, actually surface meshes defined by a system of coordinates, brings along several well-known advantages, in terms of avoidance of the risks of damages, availability of the specimens themselves and ease in collecting and processing greater datasets. In addition, the amount of information extractable from a single specimen is increased, thanks to an easier magnification and the access to inner structures unavailable to the observation on the physical object. In the context of paleoanthropology the attention to such details in fossils dates back to Taung child’s natural endocast described by Dart. The inner structures and cavities of a skeletal element can often have a crucial diagnostic value. Thanks to the tools of Virtual Anthropology, in the last few years there has been an increase in the number of studies and works concerning the inner cavities: aside from the endocast, paranasal sinuses, trabecular bone and inner ear are some of the structures that are being investigated. For the study of the inner structures is usually applied a protocol which envisages the segmentation, a manual ‘filling’ by using masks of the cavities in sequential CT scans, to eventually obtain, through triangulation, a 3D mesh of the segmented area. Such protocol is time-consumingand requires an arbitrary closure of holes or gaps by the operator, thus determining the generation of some topological artefact; moreover, to be carried out, the segmentation needs expensive software (e.g. Amira, Mimics, Geomagic). In this communication, we present two new methods,Computer-Aided Laser Scanner Emulator (CA-LSE) and Automatic Segmentation Tool for 3D objects (AST-3D) developed in the open source statistical environment Rand hence available for free. The two methods can overcome the issues related to manual segmentation, as time consumption and replicability, by respectively carrying out the reconstruction of the external surface and, by subtraction, of the inner ones (CA-LSE) and vice versa (AST-3D). The amount of time needed to obtain a 3D mesh is significantly reduced and the same structure can be reconstructed by different operators with irrelevant or no differences at all. We applied the methods to different specimens as example of their potentialities: a modern human skull from the Natural History Museum collections of the University of Florence (Italy), a malleus from a Middle Ages individual from Portico D’Ottavia (Rome, Italy), and a Neanderthal deciduous tooth from the Krapina sample (Croatia). Both the methods CA-LSE and AST-3D are embedded in the packages “Arothron” and “Morpho”.