Whilst a study of lower first molars of a selected sample of African Plio-Pleistocene teeth may not be expected to resolve any of the challenges facing palaeoanthropologists concerned with the issue of species and species concepts as discussed in 1.3 above, it is possible to draw some conclusions in respect of taxonomy from these teeth. If the currently cited diagnostic features that distinguish first molars of different fossil hominin species from the African Plio-Pleistocene are accepted, by comparing two lower first molars together, it should be possible to distinguish the molar of one currently-accepted species from the molar of another, if those diagnostic features are present. For example, it is possible to distinguish a molar from, for example, SK 6, an individual attributed to Paranthropus robustus, from that of KNM-ER 992, an individual attributed to Homo ergaster (Homo erectus) (see photographs of these specimens above in Figures 2.8 and 2.9): the size ranges for both species do not overlap, and the cusp arrangements are significantly distinct from each other. It is also possible to distinguish a difference between the lower first molar of OH 7 (the holotype of Homo habilis) and LH 4 (the holotype of Australopithecus afarensis): in this case the great width
(“squareness”) of LH 4 contrasts with the significant narrowness of OH 7, as well as the extreme difference in the angle of the buccolingual groove (almost perpendicular to the mesiodistal axis in the case of LH 4 and at a very steep angle in the case of OH 7). Although at times it is difficult to be specific as to what a specimen “is”, and whether
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fossil species designations themselves are valid or not, it is often possible when
comparing between individual specimens to say what a specimen “is not”. Researchers would, however, be in no doubt as to the fact that these are specimens from different species, even if cranial and/or post-cranial remains for specimens attributed to these species were not available for study. If this kind of distinction can be quantified so that these significant phenotypic differences can be visualised graphically and so that the specimens can be statistically and visually separated using classification techniques, not only will the diagnostic traits of these teeth be useful for adding further quantitative information to the datasets that already exist, particularly those of cranial elements, but also there might be instances where individual specimens produce anomalous results in terms of their currently accepted species allocation, and these individuals can be
highlighted for further study.
Since many of the palaeoanthropological discoveries to date have consisted of
fragments of mandibles or maxillae or isolated teeth (including lower first molars), it is not illogical to address the question of phenotype diagnostics, based on occlusal crown morphometric data aimed at determining which particular features and metrics of lower first molars contribute towards the taxonomical analysis of these remains, based on our current understanding of fossil species. The diagnostic traits of the teeth being studied ultimately provide the rationale for the next phase of the study: the
landmarking of these traits to capture all the major features that add necessary information to the morphometric study.
However, even if a landmark placement model can be found that leads to the ability to differentiate between fossil species as they are designated at present, there is no
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guarantee that these fossil species delineations are in any way comparable to “biological species” (see, for example, De Queiroz 2007; Wright 1940; Mayr 1942; (Dobzhansky 1950), the only examples of which are extant species. By also subjecting these metrical data a priori to testing on extant species, patterns of variation within known genetic species might be seen to be repeated in the fossil record.
If shape analyses and “shape-and-size” analyses based on the landmark experiments result in a “landmark model” that adequately produces visual and statistical
representations of boundaries and/or overlaps between “known” (extant) species, by applying the same protocols to the fossil specimens, if any anomalous specimens are identified that fail to group with their presumed species (or in a group close to the holotype for their presumed species), these can be scrutinised in more detail. Further statistical analytical methodologies can also be applied to the data to confirm or falsify the anomalous results, with the expectation that if specimens by and large consistently group together generally as they would be predicted to do (into their presumed species groups) and that if certain specimens consistently fail to group together with their predicted groups, no matter which of the chosen analyses is employed, even if it
remains difficult to draw firm conclusions about species boundaries and the reasons for the failure of certain specimens to group with their currently accepted species
groupings, a certain degree of confidence can at least be placed on the “inputs” – the landmark model (the placement of the landmarks).
What should also become possible from the testing of the landmark model on the extant species alone is whether patterns can be perceived regarding shape differences within and between individual species, size differences within and between individual species
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and shape-and-size differences together within an individual species. Sexual dimorphism is often cited as an explanation for very wide variability between
specimens within individual species (see 1.3 above). If it can be established whether this sexual dimorphism is manifested in extant species as a size difference or as a size- and-shape difference in highly dimorphic species (such as gorillas), it might inform the way species groupings are perceived for the fossil species (for instance, the way that teeth group for gorillas on a principal components plot might provide some information as to how to assess the principal components output for groupings for the fossil
species). If two teeth are distinct both in size and in shape, does this necessarily imply (as in the case of SK 6 and KNM-ER 992 as discussed above) that they are from a different species, or could one single species encompass such vastly different traits?
It is therefore vital that the “inputs” to all the analyses (the choice of placement of the landmarks used in the analyses) provide enough information to be able to assess the “outputs” (the charts, tables and statistical results from the analyses) in light of the above. Chapters Three and Four of this study therefore place a great deal of emphasis on both the inputs and the outputs.
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CHAPTER THREE – MATERIALS AND METHODS