Person:

Worthington, Steven

Analyses of hominin dental remains conventionally include measurements of tooth crown sizes and descriptions of occlusal morphology such as minor accessory cusps, fissure patterns, and ridges (e.g., Wood, 1981, Aiello and Dean, 1990 and Bailey, 2006). Following Dahlberg, 1951 and Turner et al., 1991 developed a formal system for dividing these ‘non-metric’ aspects of dental morphology into discrete categories. This system, termed the Arizona State University Dental Anthropology System (ASUDAS), is an effective tool for discriminating among modern human populations and for assessing inter-population relationships (Scott and Turner, 1997). Other researchers later used ASUDAS to examine the evolutionary relationships of various Pleistocene hominins (Irish and Guatelli-Steinberg, 2003 and Martinón-Torres et al., 2007; but see Bailey et al., 2009). Most recently, Irish et al. (2013) used ASUDAS to assess the phylogenetic position of Australopithecus sediba, finding support for both an Au. sediba + Au. africanus clade and a clade uniting South African australopiths with Homo. However, as Kimbel (2013) has argued, there are theoretical issues with applying ASUDAS to assess phylogenetic relationships from small samples of fossil hominin dental remains. Here we explore the suitability of applying a method developed for partitioning among modern human populations to assess interspecies relationships among fossil hominins. We then discuss the ramifications of different choices made during phylogenetic estimation, including those pertaining to character weighting, clade support, and outgroup composition. We find that slight alteration of phylogenetic assumptions leads to numerous equally possible evolutionary reconstructions for Au. sediba.

Background: Developmental processes that underpin morphological variation have become a focus of interest when attempting to interpret macroevolutionary patterns. Recently, the Dental Inhibitory Cascade (dic) model has been suggested to explain much of the variation in mammalian molar size proportions. We tested the macroevolutionary implications of this model using anthropoid primate species (n=100), focusing on overall morphological patterns, as well as predictions made about molar size variability, direct developmental control, and diet. Results: Of the species sampled, 56 % had centroids that fell within regions of molar proportion morphospace consistent with the dic model. We also found that the third molar had greater variation in size than either the first or second molars, as expected by the model. Some dic model predictions were not supported, however, such as the expected proportion of M2/M1 when the third molar is absent. Furthermore, we found that some variability in third molar size could not be explained by the influence of the inhibitory cascade. Overall, we found considerable clade-specific differences in relative molar sizes among anthropoid primates, with hominoids and cercopithecins strongly divergent from dic model predictions, and platyrrhines, colobines, and papionins more consistent with the inhibitory cascade. Finally, we investigated reasons why some clades deviated from dic model expectations. Adaptations for frugivory (e.g., bunodont cusp relief) appeared to be one driver of relatively larger second molars and have evolved independently in multiple lineages of anthropoids. Conclusions: The dic model explains some of the variation in anthropoid primate molar proportions. However, there are interesting deviations away from this broad mammalian pattern, particularly in hominoids and cercopithecins, which suggest the model is only one of multiple mechanisms determining morphological variability in mammalian teeth. Electronic supplementary material The online version of this article (doi:10.1186/s12862-016-0673-5) contains supplementary material, which is available to authorized users.