Virtual reconstruction of the mandible ATD6-96 (Gran Dolina-TD6, Sierra de Atapuerca, Spain) and the reassessment of the taxonomical signal of Homo antecessor mandibles

Virtual reconstruction of the mandible ATD6-96 (Gran Dolina-TD6, Sierra de Atapuerca, Spain) and the reassessment of the taxonomical signal of Homo antecessor mandibles
Journal 5th Annual ESHE Meeting
Authors Bermúdez de Castro, José María; Martín-Francés, Laura; Martinón-Torres, María; Modesto Mata, Mario; Martínez de Pinillos, Marina; García, Cecilia
Volume PESHE 4
Year 2015
DOI 2195-0776
ATD6-96 is the left half of a gracile mandible of an adult individual with premolars and molars in place. It was found in 2003 in the TD6 level (OIS 21, 0.85 Ka) of the Gran Dolina cave site of the Sierra de Atapuerca, northern Spain. The specimen is broken at the level of the lateral incisor-canine septum. A first study of this specimen was published two years later. In order to obtain additional information, we have made a virtual reconstruction of the mandible. THe specimen ATD6-96 was scanned with a Tomographic system (Tomograph YXILON Compact Yxlon International, X-Ray Gmbh) housed at the University of Burgos, using the following settings: voltage 130 kV, amperage 4 mA and resultant voxel size of 0.109mm x 0.109mm x 0.20mm. The resulting images were imported into Amira (Visage Imaging) software to obtain the 3D volume of the mandibular left side. Following, a second reconstruction was made by mirror-imagining the original stack of images (left hemimandible) to obtain the right hemi-mandible and create the 3D volume. As the canine alveolus is not complete (due to a post-depositional breakage) it was necessary to mirror the remaining half. The reconstructed 3D volume was then aligned and placed resulting in an entire alveolar rim; this was also performed on the right side. Despite the 3D reconstruction of the mandibular fragments, the symphyseal region is still missing. To reconstruct the symphysis, we defined two landmarks placed at the interdental septum, between the canine and the lateral incisor. In order to complete our reconstruction, we have obtained information taking as a reference: 1) a modern human sample, 2) a fossil hominin sample, and 3) the size of the root of the TD6 permanent lower incisors. We rotated the created volumes (right and left hemi-mandibles) on the “x, y and z” planes until the anatomical position was obtained, trying that the wear plane of all teeth is positioned approximately in the same plane, perpendicular to a vertical line. Finally, all the measurements were taken on the 3D volume models for comparative purposes. In our first study, it was concluded that ATD6-96 shows a primitive structural pattern shared with all African and Asian specimens. Furthermore, ATD6-96 is very small and exhibits a remarkably gracility, like the other TD6 mandibles. In this aspect, H. antecessor has clearly diverged from the African hominins, and ATD6-96 shares his/her gracility particularly with the Early and Middle Pleistocene Chinese specimens. Moreover, ATD6-96 shows a remarkable medial pterygoid tubercle, which is very frequent in Neandertals and in the Middle Pleistocene Sima de los Huesos hominins. Concerning the digital reconstruction, the main finding is that ATD6-96 had a derived parabolic dental arcade, measured by the index of the alveolar arcade. Interestingly, the value obtained for this index in ATD6-96 is similar to the mean values of Neandertals and the Sima de los Huesos hominins, and higher that that obtained in other Middle Pleistocene African, Asian and European specimens. Thus, the value of the alveolar arcade index adds to the list of the “Neandertal” features observed in the ATD6 hypodigm so far. As we have concluded in previous studies, all these features cannot not be considered as Neandertal autopomorphies, but traits which may have appeared much earlier than we had previously considered, in an Early Pleistocene hominin population. Summarizing and in general terms, the TD6 mandibles clearly diverged from the Pleistocene African pattern and show more affinities with the Eurasian specimens.

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