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What do fossil samples actually represent? Dental facet and tooth representativity in performing repeatable dental microwear textural measurements



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5th International Conference on Surface Metrology
4th – 7th April 2016, Poznan University of Technology, Poland
What do fossil samples actually represent?
Dental facet and tooth representativity in performing
repeatable dental microwear textural measurements
IPHEP-CNRS, 6 rue Brunet, 86073 POITIERS, France
Keywords: dental microwear texture analysis, representativity
There are two different types of tooth wear, both resulting in the loss of small
fragments of enamel from the tooth itself: attrition, the wear resulting from tooth-
to-tooth contact, and abrasion, resulting from food-to-tooth contact (Kaiser et al.
2013). Dental microwear describes the latter, resulting in scars left by food on the
enamel surface. This microscopic wear varies according to the physical properties
of the food ingested over the last weeks of an animal’s life. The advent of 3D
acquisition techniques and automatic processing has allowed for surfaces to be
processed automatically (Scott et al. 2005; Schulz et al. 2010). Enamel surfaces are
scanned using a Leica DCM-8 confocal microscope at the iPHEP lab. The results are
processed through fractal analysis, allowing for 3D textures to be analysed in their
entirety (Dental Microwear Texture Analysis or DMTA; see Scott et al. 2005, 2006).
3D acquisition techniques and automatic processing have now become widespread
tools for reconstructing the past diet of primates and also other mammals and
even other vertebrates. However, although the advent of DMTA has allowed for
more precise and repeatable dietary reconstructions, few studies have ever
questioned the representativity of the signal analysed. Scan size, the choice of
dental facets or even the choice of tooth can all influence the microwear signal. Can
a scan, representing a fraction of the whole dental facet, truly reflect dietary
differences between groups? These matters remain to be characterized in a
controlled setting. In this study, we present the results of a DMTA on 30 ewes (Ovis
aries) in a controlled food experiment. Three groups were each given foods with
different physical properties (soft browse, tough grass, hard seeds). 3D-DMTA was
performed on occluding dental facets on both the upper and lower second molars
to test the representativity of scans of different sizes (50×50µm, 100×100µm and
200×200µm) and the most appropriate teeth to be analyzed (upper or lower
dentition). Two variables are considered in this study: anisotropy and complexity.
Complexity (Asfc or Area-scale fractal complexity) is a measure of the roughness at
5th International Conference on Surface Metrology
April 4–7, 2016 • Poznan University of Technology, Poland
5th International Conference on Surface Metrology
4th – 7th April 2016, Poznan University of Technology, Poland
a given scale. Anisotropy (epLsar or exact proportion of length-scale anisotropy of
relief) measures the orientation concentration of surface roughness. Data was
rank-transformed and then analysed using a repeated measures two-way ANOVA,
and corresponding posthoc analyses.
Results (Fig. 1) show that the three different groups display more significant
differences in both anisotropy and complexity when a scan of 200×200µm is
considered. DMTA highlights some significant differences between the upper and
lower teeth of the same dietary group, suggesting the studied facets do not carry
the same dietary signal. Furthermore, results on lower molars reveal more
differences between the three dietary groups than the analysis on the upper teeth.
Fig. 1. Asfc and epLsar according to dietary group and size of the scan analysed. Data gathered on
lower second molars.
[1] Kaiser, T. M., Müller, D. W., Fortelius, M., Schulz, E., Codron, D., & Clauss, M. (2013).
Hypsodonty and tooth facet development in relation to diet and habitat in
herbivorous ungulates: implications for understanding tooth wear. Mammal Review,
43(1), 34-46.
[2] Schulz, E., Calandra, I., & Kaiser, T. M. (2010). Applying tribology to teeth of hoofed
mammals. Scanning, 32(4), 162-182.
[3] Scott, R. S., Ungar, P. S., Bergstrom, T. S., Brown, C. A., Grine, F. E., Teaford, M. F., &
Walker, A. (2005). Dental microwear texture analysis shows within-species diet
variability in fossil hominins. Nature, 436(7051), 693-695.
[4] Scott, R. S., Ungar, P. S., Bergstrom, T. S., Brown, C. A., Childs, B. E., Teaford, M. F., &
Walker, A. (2006). Dental microwear texture analysis: technical considerations.
Journal of Human Evolution, 51(4), 339-349.
5th International Conference on Surface Metrology
April 4–7, 2016 • Poznan University of Technology, Poland
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Full-text available
Reconstructing the diets of extinct hominins is essential to understanding the paleobiology and evolutionary history of our lineage. Dental microwear, the study of microscopic tooth-wear resulting from use, provides direct evidence of what an individual ate in the past. Unfortunately, established methods of studying microwear are plagued with low repeatability and high observer error. Here we apply an objective, repeatable approach for studying three-dimensional microwear surface texture to extinct South African hominins. Scanning confocal microscopy together with scale-sensitive fractal analysis are used to characterize the complexity and anisotropy of microwear. Results for living primates show that this approach can distinguish among diets characterized by different fracture properties. When applied to hominins, microwear texture analysis indicates that Australopithecus africanus microwear is more anisotropic, but also more variable in anisotropy than Paranthropus robustus. This latter species has more complex microwear textures, but is also more variable in complexity than A. africanus. This suggests that A. africanus ate more tough foods and P. robustus consumed more hard and brittle items, but that both had variable and overlapping diets.
Mammals inhabit all types of environments and have evolved chewing systems capable of processing a huge variety of structurally diverse food components. Surface textures of cheek teeth should thus reflect the mechanisms of wear as well as the functional traits involved. We employed surface textures parameters from ISO/DIS 25178 and scale-sensitive fractal analysis (SSFA) to quantify dental wear in herbivorous mammals at the level of an individual wear enamel facet. We evaluated cheek dentitions of two grazing ungulates: the Blue Wildebeest (Connochaetes taurinus) and the Grevy's Zebra (Equus grevyi). Both inhabit the east African grassland savanna habitat, but they belong to fundamentally different taxonomic units. We tested the hypothesis that the foregut fermenting wildebeest and the hindgut fermenting zebra show functional traits in their dentitions that relate to their specific mode of food-composition processing and digestion. In general, surface texture parameters from SSFA as well as ISO/DIS 25178 indicated that individual enamel ridges acting as crushing blades and individual wear facets of upper cheek teeth are significantly different in surface textures in the zebra when compared with the wildebeest. We interpreted the complexity and anisotropy signals to be clearly related to the brittle, dry grass component in the diet of the zebra, unlike the wildebeest, which ingests a more heterogeneous diet including fresh grass and herbs. Thus, SSFA and ISO parameters allow distinctions within the subtle dietary strategies that evolved in herbivorous ungulates with fundamentally different systematic affinities but which exploit a similar dietary niche.
1. The evolution of high-crowned teeth or hypsodonty in herbivorous mammals is widely interpreted as a species-specific adaptation to increasingly wear-inducing diets and environments at evolutionary time scales, with internal abrasives (such as phytoliths in grasses) and/or external abrasives (such as dust or grit) as putative causative factors. The mesowear score (MS) instead describes tooth wear experienced by individual animals during their lifetime. 2. Under the assumption that the abrasiveness that causes the MS in individuals is the same abrasiveness to which species adapted by evolving hypsodonty, one would expect a close correlation between the MS and the hypsodonty index (HI). Alternatively, if these two measures reflect different aspects of wear, one would expect differences in the way that proxies of diet or environment/climate correlate with each parameter. 3. In order to test these hypotheses, we collated a dataset on the HI, MS, percentage of grass in the natural diet (%grass), habitat (open, intermediate, closed) and annual precipitation (PREC) in extant mammalian herbivores. The availability of a quantitative MS constrained the dataset to 75 species. Data were analysed with and without phylogenetic generalized least squares (PGLS). 4. Correlations with PREC were stronger for HI than for MS, whereas correlations with %grass were similar for HI and MS. Habitat had a significant influence on the relationship with %grass for HI but not for MS. Habitat also had a significant influence on the relationship between HI and MS. MS improved the predictive power of HI for %grass, but not for PREC. 5. These results suggest that while the MS indicates predominantly the wear effect of the diet (internal abrasives), HI represents an adaptation to a wear effect that comprises both diet and environment (external abrasives). The additional environmental wear effect must reduce tooth height without causing macroscopic changes in tooth facet development as described by the MS. 6. The most parsimonious explanation for the apparent discrepancy between HI and MS is that external abrasives of very fine particle size play a major role in naturally occurring tooth wear. The experimental testing of this hypothesis will enhance our understanding of the processes involved in tooth wear.
Dental microwear analysis is commonly used to infer aspects of diet in extinct primates. Conventional methods of microwear analysis have usually been limited to two-dimensional imaging studies using a scanning electron microscope and the identification of apparent individual features. These methods have proved time-consuming and prone to subjectivity and observer error. Here we describe a new methodological approach to microwear: dental microwear texture analysis, based on three-dimensional surface measurements taken using white-light confocal microscopy and scale-sensitive fractal analysis. Surface parameters for complexity, scale of maximum complexity, anisotropy, heterogeneity, and textural fill volume offer repeatable, quantitative characterizations of three-dimensional surfaces, free of observer measurement error. Some results are presented to illustrate how these parameters distinguish extant primates with different diets. In this case, microwear surfaces of Cebus apella and Lophocebus albigena, which consume some harder food items, have higher average values for complexity than do folivores or soft fruit eaters.