While several studies have been conducted on fossil teeth based on occlusal
morphology (Wood & Abbott 1983; Wood et al. 1983; Martinón-Torres et al., 2006; Gómez-Robles et al., 2007, 2008, 2012, 2013), the materials and/or the methods used in these previous studies differ from those being used in the present study. This study, in particular, includes the following:
- Unique landmark placement model optimising information from worn
molars and including size, perimeter shape and full cusp description
including direction of cusps: an experimental landmark model was developed,
using five “anatomical” landmarks, the remainder being primarily “pseudo” or “constructed” landmarks (Dryden & Mardia, 1998, pages 3-6; see 3.5 below) which are geometrically calculated landmarks that can be independent of anatomical structures on the tooth itself. Included in the sample are some teeth that are effectively “blank canvasses” except for the perimeter edge which preserves the intersections of the five cusps. Using just the overall length, relative width and spatial “weighting” of the tooth (larger or smaller talonid relative to the geometric centre of the tooth) as well as the direction, length and width of each cusp, geometry can be employed to optimise what might seem at first glance to be sparse information from differentially worn and damaged occlusal crown enamel surfaces. Landmarks are divided into three polygon groups (external (dimensions); peripheral (shape); inner (cusp arrangement)) to
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provide a calculated balance in the analysis to areas of taxonomic diagnostic assessment normally considered by morphologists when differentiating between molars from fossil specimens (see Aiello & Dean, 1990; Hillson, 1996 and Wood & Abbott 1983; Wood et al, 1983 for general descriptions). Provision has been made for the landmarking of a) mesiodistal-buccolingual diameters (relative size together with relative width of the molar); b) general perimeter shape; c) the presence of a sixth cusp or “tuberculum sextum”; and d) the internal cusp arrangement including the width of the individual cusps, relative length of the individual cusps and direction of the individual cusps from the mesiodistal- buccolingual diameter intersection at the centre of the tooth. This differs from the work done by Wood & Abbott (1983) and Wood et al. (1983), in that their work concentrated on overall morphology of the crown and a morphometric analysis of cusp areas. The value of landmarking the width, length and direction of each cusp is that a) cusps that have wide arcs along the perimeter edge of the molar, but that are fairly “shallow” in terms of their general projection
(perimeter distance) from the centre of the tooth might have the same surface area as cusps that are very narrow along the perimeter edge but which project well out from the centre of the tooth, so this is a way to differentiate between “wide, shallow cusps” and “narrow, long cusps” that might have the same area but are spatially distinct; and b) each cusp can be evaluated in terms of its general direction from the centre of the crown: for example, is the hypoconid very buccally-oriented, or is it angled more distally? Cusp direction can be as diagnostic between species as cusp size (see below, Chapter Two). The
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these features to be included in the analyses even if the features themselves are absent due to wear or damage;
- Focus on Plio-Pleistocene molars, and use of original data rather than
casts: researchers such as Gómez-Robles et al. (2007, 2008, 2012, 2013) and
Martinón-Torres et al. (2006) have published articles on Geometric
Morphometric analyses of the post-canine dentition of hominins, but this study differs in that a) the focus of this study is on African Plio-Pleistocene hominins, rather than primarily on Lower Pleistocene hominins (designated primarily as Homo heidelbergensis and Homo neandertalensis) and b) original photographs of the bulk of the Plio-Pleistocene specimens in the study have been taken from personal visits to collections in Nairobi, Dar es Salaam, Pretoria and
Johannesburg, rather than using casts for these specimens as the cited researchers have done. Additionally, although Aida Gómez-Robles has
mentioned (in personal correspondence) that a study of lower first molars is in progress, this has not yet been published. My own study focuses on lower first molars.
- Unique test for the angle of tilt in occlusal photography: this study utilises a similar photographic and image-processing methodology to that used by Gómez- Robles et al. and Martinón-Torres et al. in their various studies of post-canine teeth (ibid.). However, in this present study, a novel technique has been
employed to calculate the error of levelling of the teeth to ensure that the teeth are directly orthogonal to the lens above them (and that the images produced are therefore true occlusal images of the teeth). By splicing three individual shots of a single specimen (taken on different days) into a 3D image of the same tooth (kindly supplied by Professor José Braga in Toulouse), differences in levelling
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along 3 axes in space can be calculated and compared, to test for significant observer error during the photograph capturing process. This test is explained in more detail in 3.3.2.3. below.
- Inclusion of extant species to test for variability in known species’ lower
first molar occlusal crown morphology: the study seeks to test some aspects
of species variability and to assess whether or not these can be quantified statistically and probabilistically. The study differs from the occlusal crown image-based dental studies mentioned above, in that the question of species variability is tested in a preliminary way using specimens selected from a sample of occlusal images of lower first molars of extant species. Original images of specimens of gorilla, chimpanzees, bonobos and modern Homo sapiens were captured in Tervuren, Belgium and in Johannesburg, South Africa for inclusion in the study for comparative purposes where possible. This is not the first study to include both extant ape species and fossil hominins for shape analyses.
However, most studies that include both extant and fossil comparisons
concentrate on cranial comparisons (e.g., Wood et al. 1991; Thackeray & Prat, 2009; Lordkipadnidze et al. 2013, Gordon & Wood, 2013). Those that compare dental features often concentrate on non-metric traits (e.g., Bailey & Wood 2007; Irish & Guatelli-Steinberg 2003; Guatelli-Steinberg & Irish 2005) or enamel thickness (e.g., Rabenold & Pearson 2011) or some other feature such as microwear (e.g., Grine et al. 2006) or diet and dental topography (e.g. Ungar 2004). Skinner et al. ( 2008a; 2008b) and Braga et al. (2010) have studied mandibular molars to examine taxonomic parameters that allow for
discrimination between known species (extant species), comparing the findings to fossil hominins. The main dental elements used in those studies are molar
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teeth as in this study, but in those cases, the focus is on the enamel-dentine junction (EDJ). This present study focuses on crown outer enamel surfaces and seeks to maximise the amount of information that can be gleaned from the crown itself, using a minimum number of anatomical features (those common to both worn and unworn teeth), in an attempt to be able to discriminate adequately between species even when teeth are worn. Although one of the stated
advantages of using the EJD rather than the outer enamel surfaces (OES) is that the EDJ in moderately worn teeth remains pristine, the problem remains that many gorilla teeth, bonobo teeth, and more importantly, fossil teeth (including some holotypes or mandibular proxies for holotypes) are more than moderately worn. Once the enamel has worn through to the EDJ surface, the EDJ then disappears and that specimen becomes unusable for an EDJ study or it reduces the information available for such. In the case of this present study, the only disqualifying factor for a badly worn tooth to be included is in instances where the points at which the individual cusp arcs meet at the perimeter of the tooth are absent. Even severely worn teeth can be used provided that those junctions are visible at the perimeter. This study is therefore able to include some teeth that are badly worn. Thus, not only has the study included some specimens of extant species that would sometimes be excluded from studies, but some important specimens such as Peninj 1, which is a proxy for the holotype of Paranthropus boisei, have also been able to be included.
- Use of numerous analytical methodologies to test robustness of initial
results: the study juxtaposes various analytical methodologies that investigate
shape (and shape-size) comparisons from quite different approaches. A similar approach has been taken by Valeria Bernal (Bernal 2007), wherein geometric
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morphometric techniques are compared to more traditional analyses. This study, based on lower first molars, uses some different methodologies. One of these is the “Log sem” regression-based probability approach, pioneered by
Professor F. Thackeray (Thackeray et al. 1997; Thackeray 1997; Braun et al. 2004; Thackeray 2005; 2007a; Thackeray & Odes 2013), to determine the probability of pairs of individuals being conspecific. By first subjecting the data to geometric morphometric analyses and a principal components analysis (based on a Procrustes “form-space” analysis (Mitteroecker et al. 2004) – see 4.2.3), followed by a discriminant function analysis, a framework exists to determine, a priori, whether species groupings can be discriminated visually and statistically, and whether the likelihood of misclassifications exists for any of the individual specimens studied. Thereafter, the application of the Log sem method would
provide a different, independently-based approach to confirm or falsify these results. In doing so, the Log sem methodology itself might also be able to be
confirmed by methodologies more widely used at present by researchers (geometric morphometric methods and statistical analyses such as principal components analyses), and if the results largely concur, this would confirm the methodology’s usefulness in contributing a probabilistic approach to the
examination of the “mathematical species concept” (bearing in mind the fact that the species concept itself is a subject of great discussion (Hublin 2014); see also Kimbel & Martin 1993; De Queiroz 2007).
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