The hominin inside: an automatic tool to reproduce the internal and external anatomy of bony structures

In recent years, the use of CT-scans in physical anthropology yielded huge improvements of the analysis of skeletal remains. Such process made the “virtual anthropology” one of the most important approaches in the study of human evolution and bioarchaeology. The virtual reproduction of inner volumes has become a frequent procedure for studying the internal anatomy of fossil hominins. Such procedure has usually been carried out by filling manually (segmentation) the regions of interest (ROI), which is a time consuming task. At the same time, isolating the outer shell of a skeletal element is very useful in Geometrics Morphometric applications, for example when using semilandmarks. During “projection” and “sliding”, the semilandmarks can intercept the inner surface failing the superimposition of the reference model on the target sample. Here we describe the application of two methods aiming to automatically generate inner and outer surfaces of the 3D model of a skeletal element. The methods have been implemented in the statistical environment R. A development version of the R-package “Morpho” [2] contains the function virtualMeshScan, that allows to select all vertices of a given triangular mesh that are visible from at least one of several specified points of view (POVs). Visibility is determined by evaluating whether a straight line between vertices and the POV intersects the mesh. To obtain the part of the visible mesh from the POVs, all vertices not tagged as “visible” are removed from the mesh itself.There are no constraints to where the POVs are placed. The R-package “Arothron” [3] provides functions useful for the extraction, from a point of view (POV), of the visible vertices belonging to the point cloud of a mesh.This method, developed by Katz [1], is based on the definition of a given number of POVs placed on a sphere that surrounds the object. In this way, the procedure simulates a scan of a 3D mesh, in which the vertices forming the external surface are marked as visible points. The vertices of the mesh that define the internal structure (not marked as visible points) are subtracted from the whole vertices matrix. Here we present the application of these two methods on three examples: a modern human skull (VA023) from Germany repository, the d1 tooth (Homo neanderthalensis) from Krapina and a malleus from a Middle Ages individual (Portico D’Ottavia, Roma). In conclusion, we introduce two procedures capable of isolating the inner volume of a skeletal element and useful to separate both inner (no visible) and outer (visible) components of a 3D model. In this communication, we tested and showed the performance applying these methods on anatomical area of different complex morphology and size. Potentially the massive application on huge samples could lead to an increased production of ROIs and thus of their availability to the researchers via online sharing.