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Reconstruction of the atlas (C1) of the La Chapelle- aux-Saints Neanderthal through geometric morphometric techniques

The almost complete Neanderthal skeleton of La Chapelle-aux-Saints (LC) was discovered in 1908 by Amédée and Jean Bouyssonie and Louis Bardon in the homonymous French town [1]. Marcellin Boule provided the first anatomical description of the fossil, where he considered that LC had a less curved cervical spine than the modern human, based on the orientation of the articular facets and the spinous processes [2]. The atlas of LC was recovered in four fragments: right and left lateral masses, right transverse process, and a fragment of the anterior tubercle [3]. The aim of the present project is to reconstruct the atlas of LC using virtual morphological and geometric morphometric (GM) techniques to include this fossil in the study of the comparative anatomy of the cervical spine and head posture of Neanderthals. The comparative sample consists of 27 3D atlas models of different individuals of the genus Homo ; (H. neanderthalensis N=5, H. antecessor N=1, and H. sapiens N=21). Atlas shape is quantified through 119 3D (semi)landmarks [4, 5]. Previous analyses [4] showed that a Form Space PCA ordered atlas variation along an allometric component, which was common to both Neanderthals and modern humans (PC1), and an interspecific component, which separated modern humans and Neanderthals (PC2). This study is based on these variables to provide a quantitative reconstruction of the LC atlas. Thus, the GM reconstruction of LC is carried out following a sequence of different steps. First, the target shape coordinates are calculated. Procrustes form space PCA is used to identify shape variables associated with size (allometry) and with inter-specific differences. In parallel, two linear regression models are applied: Regression 1 for predicting the size (centroid size, CS) of the complete vertebra of LC based on the CS of its preserved left lateral mass, and Regression 2 for predicting the allometric form (target PC1 score) that corresponds to a generalized atlas morphology with the given size of LC as obtained by regression 1. Using an interspecific deformation vector (target PC2 score, Neanderthal mean), the generalized allometric form is transformed into a mean Neanderthal allometric form (an allometric form with characteristics of a mean Neanderthal atlas). This target configuration is warped by Procrustes-registrated and TPS- estimated missing semilandmarks towards a new shape that contains the geometry of the original LC lateral masses. Finally, the original lateral masses are virtually registered at their corresponding positions defined by the homologous landmarks of the reconstructed vertebra. PC1 (47.4% of total variance) ordered the sample along a common allometric gradient and PC2 (11.4% of total variance) separated Neanderthals and H. sapiens. Regression 1 (r2=0, 73 ; p= 0.001) yielded a total CS of 234.92 that corresponds to the CS of the lateral mass of LC (56.37). Regression 2 (r2=0, 98 ; p=0.001) predicted a target PC1 score of 0.026. The Neanderthal mean PC2 score was 0.724, leading thus to the target coordinates of PC1 and PC2. Next, this allometric and Neanderthal-like 3D model, is warped into the configuration of the original lateral masses, and the original fragments are added by Procrustes registration.This reconstruction of the atlas of La Chapelle-aux-Saints shows the potential of the virtual morphology combined with 3D-GM technics and represents a hybrid between statistical methods and geometric morphometric techniques. This atlas model has Neanderthal features in anterior and posterior arch and the specific morphology of LC in lateral masses. It shows the characteristic dorsoventrally elongated neural canal of Neanderthals, although the validity of this method should be tested by checking its anatomical relationship with other fossils recovered from La Chapelle-aux-Saints considering also possible differences in orientation of the articular facets [2].

cervical vertebra ; atlas ; Neandertal ; geometric morphometrics

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