Digital alignment: an automatized protocol for virtual reconstruction of incomplete fossil specimens

Museal collections include a large amount of cranial specimens of living and fossil primates. This material served as the basis for detailed morphological studies also relatively to their internal anatomy. Unfortunately, until recently, the study of the cranial cavity (e.g. the endocranial cavity) was often possible only by removing mechanically extensive portions of the cranial vault and of other skeletal portions, in fact compromising the physical integrity of the specimens. Many specimens have gone through fractures and unintentional damage, which may also have caused the loss of original morphological information. Thanks to increasing advances in both computer technology and 3D imaging software, it is possible to virtually acquire the morphology of a physical specimen [1,2]. Consequently, paleoanthropological studies often focus on anatomy, virtual reconstruction, and on the development of algorithms to improve the digital acquisition [3]. In addition to CT scan other techniques have been introduced, such as laser scanner and photogrammetry. Virtual procedures, on the basis of digital reconstruction, are frequently applied to restore human fossil specimens. A digital operation on a 3D specimen is appropriate and/or necessary when the object is fragmented/damaged and/or deformed by taphonomical pressures. Here we present a protocol, developed in R environment, able to align automatically two portions belonging to the same 3D model. In this communication, we introduce the method and we report the results of the application of this tool applied on a skull of Homo sapiens (target model). The target model was divided in two halves and each portion was shifted in the xyz reference Cartesian system. The aim was to compare the efficacy of this protocol (full computer assisted alignment) with a manual alignment. The digital alignment consists of the extrapolation of the rotation matrix to translate, rotate, and scale a fragment of a target model using a reference model. In literature, almost all digital reconstructions of fossil specimens start from an arbitrary model chosen as reference [4]. The first part of the tool is dedicated to the detection of the reference model using a landmark configuration as guide. Once chosen, the reference model was symmetrized using a bilateral configuration landmark. The two halves of the target model were aligned on the symmetrized version of the reference model, and optionally the alignment could be corrected on the basis of an external ratios or angles (in this case the angle-glabella-inion). Finally, the alignment performed using this protocol was compared with 10 manually-performed alignments carried out by 10 anonymous researchers expert in the field. In this case the target model aligned automatically results to be the closer to the starting model; in fact the mesh distance values is minor than alignments manually performed. In sum, the digital alignment of a fragmentary fossil specimen is the first stage of a reconstruction procedure and the correct choice of the reference model is the crucial point of the digital reconstruction of a damaged specimen. The application of a computer-assisted reconstruction implies that the efficiency of the reconstruction depends on the comparative sample (3D models and landmark/semi-landmark sets) and not on the skills of the operator. This first release of the tool will be fully open-access and available to the scientific community for application and methodological improvements. A first application of this protocol is represented by the work done on the Altamura Neanderthal [5].