1 Trigonid crests at the enamel-dentine surface of Atapuerca-Sima de los Huesos lower molars by means of microtomographic (mCT) techniques. Morphological expression and evolutionary inferences. Martínez de Pinillos González, Marina Martinón-Torres, María. Dental Anthropology Group, CENIEH, Burgos. Keywords: permanent lower molars, Homo heidelbergensis, Middle Pleistocene, Sima de los Huesos, microtomographic techniques, trigonid crest patterns. Abstract Hitherto, the analysis of lower molars dentine surface, within de genus Homo, has been basically limited to Homo neanderthalensis and Homo sapiens. In this study, we present the analysis of the Middle Pleistocene permanent lower molar sample (n=62) from the Sima de los Huesos (SH) site, assigned to the Homo heidelbergensis species. The aim of our analysis is to characterize the pattern of trigonid crests expression at the outer enamel and the enamel-dentine junction surfaces (OES and EDJ) of the SH sample, to evaluate the concordance of expression between both surfaces and to explore possible evolutionary meanings of the expression of this feature as it was previously done based on the external morphology of the teeth. The sample has been scanned with medium and high resolution of the nondestructively technique of Micro-Computed Tomography and the final 3D models were reconstructed and analyzed using the AMIRA 5.3.3 software. Our results reveal a greater variability of the trigonid crest pattern at the dentine compared to the enamel, and in M 3 s compared to M 1 s and M 2 s. Our analysis also confirms a good correspondence in the expression of trigonid crest patterns between the outer and inner surface of a molar that suggests the EDJ is mostly responsible for the morphology of the OES. This aspect is particularly useful as it may imply the possibility of predicting the type of trigonid crest pattern at the OES and thus, it will allow us to enlarge our study sample by including teeth that were too worn to be analyzed based on the outer surface. Moreover, our study reveals that the SH sample coincides with H. neanderthalensis in the almost constant expression of a continuous middle trigonid crest. Other aspects of the variability of the trigonid crest expression at the dentine are presented and discussed. Introduction The variation in dental form, as well as the frequency and degree of expression of many dental traits are highly heritable, and that makes teeth more useful than other skeletal remains to explore