A new method to quantify mandibular corpus shape in extant great apes and its potential application to the hominoid fossil record

Abstract

Objectives: Mandibular corpus robusticity (corpus breadth/corpus height) is the most commonly utilized descriptor of the mandibular corpus in the great ape and hominin fossil records. As a consequence of its contoured shape, linear metrics used to characterize mandibular robusticity are inadequate to quantify the shape of the mandibular corpus. Here, we present an alternative to the traditional assessment of mandibular shape by analyzing the outline of the mandibular corpus in cross-section using landmarks and semilandmarks. Materials and methods:Outlines of the mandibular corpus in cross-section between M 1 and M 2 were quantified in a sample of hominoids and analyzed using generalized Procrustes analysis, Procrustes ANOVA, CVA, and cluster analysis. Corpus breadth and width were also collected from the same sample and analyzed using regression, ANOVA, and cluster analysis.Results: Analysis of corpus outline shape revealed significant differences in mandibular corpus shape that are independent of size and sex at the genus level across hominoids. Cluster analysis based on the analysis of corpus outline shape results in almost all specimens grouping based on taxonomic affinity (99.28% correct classification). Comparison of these results to results using traditional measures of mandibular robusticity shows that analysis of the outline of the corpus in cross-section discriminate extant great apes more reliably. Conclusion:The strong taxonomic signal revealed by this analysis indicates that quantification of the outline of the mandibular corpus more fully captures mandibular corpus shape and offers the potential for greater power in discriminating among taxa in the hominoid fossil record.

Mandibular corpus shape is a taxonomic indicator in extant hominids

“…The results of our study contrast with those of Pitirri and Begun (), who achieved correct classification rates of over 99% at the species level among extant hominids, compared to 82% in our study, using a measure of corpus outline at M 1 /M 2 . In our study, between 72–92% of specimens were classified correctly at a species level when GPAs were performed for each genus separately, though no similar analyses were provided by Pitirri and Begun ().…”

“…The results of our study contrast with those of Pitirri and Begun (), who achieved correct classification rates of over 99% at the species level among extant hominids, compared to 82% in our study, using a measure of corpus outline at M 1 /M 2 . In our study, between 72–92% of specimens were classified correctly at a species level when GPAs were performed for each genus separately, though no similar analyses were provided by Pitirri and Begun (). Our study measured mandibular corpus outline at a slightly different region of the mandible (i.e., at the midpoint of M 1 , in contrast to between M 1 and M 2 as was adopted by Pitirri & Begun, ), but it is unlikely that this small difference in the region quantified would account for the different results.…”

“…In our study, between 72–92% of specimens were classified correctly at a species level when GPAs were performed for each genus separately, though no similar analyses were provided by Pitirri and Begun (). Our study measured mandibular corpus outline at a slightly different region of the mandible (i.e., at the midpoint of M 1 , in contrast to between M 1 and M 2 as was adopted by Pitirri & Begun, ), but it is unlikely that this small difference in the region quantified would account for the different results. Details about whether cross‐validated analyses were performed as part of the cluster analysis procedure that was used to discriminate among taxa were not provided by Pitirri and Begun (), and it is therefore possible that differences in the level of test conservatism between the two studies may explain the discrepancy between the two sets of findings.…”

“…Our study measured mandibular corpus outline at a slightly different region of the mandible (i.e., at the midpoint of M 1 , in contrast to between M 1 and M 2 as was adopted by Pitirri & Begun, ), but it is unlikely that this small difference in the region quantified would account for the different results. Details about whether cross‐validated analyses were performed as part of the cluster analysis procedure that was used to discriminate among taxa were not provided by Pitirri and Begun (), and it is therefore possible that differences in the level of test conservatism between the two studies may explain the discrepancy between the two sets of findings. The results of the present study show that a relatively high number of Pongo specimens were misclassified as Pan , which is consistent with Pitirri and Begun's () finding that mandibular corpus shape was not effective in distinguishing either of the Pongo species from Pan trogodytes , which intuitively is at odds with their high classification rates.…”

“…Attempts to use similar methods to classify specimens at the species level among Gorilla and Pongo using mandibular corpus morphology have yielded less encouraging results, with only 76% percent of Gorilla , and 80% of Pongo specimens being correctly classified (Lague et al, ). A recent study attempting to classify extant hominid species using mandibular corpus shape between M 1 and M 2 has yielded classification rates as high as 99% when incorporating six extant hominid species into a canonical variation analysis of outline shape coordinates (Pitirri & Begun, ). These authors further found that Pan paniscus corpus shape is significantly different from all other taxa, and although Pongo pygmaeus and Pongo abelii could be distinguished from Gorilla gorilla based on mandibular corpus shape, this morphological region was not effective for distinguishing either of the Pongo species from Pan troglodytes .…”