Efficacy of diffeomorphic surface matching and 3D geometric morphometrics for taxonomic discrimination of Early Pleistocene hominin mandibular molars

Abstract

Morphometric assessments of the dentition have played significant roles in hypotheses relating to taxonomic diversity among extinct hominins. In this regard, emphasis has been placed on the statistical appraisal of intraspecific variation to identify morphological criteria that convey maximum discriminatory power. Three-dimensional geometric morphometric (3D GM) approaches that utilize landmarks and semi-landmarks to quantify shape variation have enjoyed increasingly popular use over the past twenty-five years in assessments of the outer enamel surface (OES) and enamel-dentine junction (EDJ) of fossil molars. Recently developed diffeomorphic surface matching (DSM) methods that model the deformation between shapes have drastically reduced if not altogether eliminated potential methodological inconsistencies associated with the a priori identification of landmarks and delineation of semi-landmarks. As such, DSM has the potential to better capture the geometric details that describe tooth shape by accounting for both homologous and non-homologous (i.e., discrete) features, and permitting the statistical determination of geometric correspondence. We compare the discriminatory power of 3D GM and DSM in the evaluation of the OES and EDJ of mandibular permanent molars attributed to Australopithecus africanus, Paranthropus robustus and early Homo sp. from the sites of Sterkfontein and Swartkrans. For all three molars, classification and clustering scores demonstrate that DSM performs better at separating the A. africanus and P. robustus samples than does 3D GM. The EDJ provided the best results. Paranthropus robustus evinces greater morphological variability than A. africanus. The DSM assessment of the early Homo molar from Swartkrans reveals its distinctiveness from either australopith sample, and the "unknown" specimen from Sterkfontein (Stw 151) is notably more similar to Homo than to A. africanus.

Cochlear shape reveals that the human organ of hearing is sex-typed from birth

“…In a second step, in order to highlight local shape sex differences in torsion and curvature along the 3D cochlear curves that correspond to non-Euclidean spaces 36–38 , we computed the adult female and male Fréchet means for the observed cochlear curves sampled in this study (Fig. 2).…”

“…Our approach investigates for the first time 3D cochlear curves directly by their real (i.e., nonlinear and after a uniform scaling) shapes with two important advantages over previous methods 20–22 . First, our nonlinear elastic matching between shapes highlights local correspondences and differences by using full paths of deformations 35,38 . Second, our method allows the computation of optimal deformations in a 3D cochlear shape space.…”

Insect morphometry is reproducible under average investigation standards

“…To conclude, besides the above‐mentioned uncertainties that are common in regular practice in insect taxonomic research, morphometry has proven reproducible in our test setting. The best morphology, we believe, may be done through multimodal means, such as combining multiple microscopic and morphometric methods (e.g., Richter et al., 2019; Sarnat et al., 2019; Hita‐Garcia et al., 2019; Boudinot, 2019; Keklikoglou et al., 2019; Braga et al., 2019). Given the same size range of measured traits, the same range of observers' skill, and the same range of equipment, we expect the same reproducibility for other groups of arthropods, provided these have a similar exoskeleton stability and that specimens belong to a comparable developmental stage.…”

3D Data Analysis for Teeth Morphology Study

“…At a minimum it is possible to use general advantages of digital methods by application of traditional techniques to 3D reconstructions of tooth surfaces [12]. However more profound surface analysis methods have been used as well in odontological research [13,14,15,16].…”