Three-dimensional molar enamel distribution and thickness in Australopithecus and Paranthropus

Abstract

Thick molar enamel is among the few diagnostic characters of hominins which are measurable in fossil specimens. Despite a long history of study and characterization of Paranthropus molars as relatively 'hyper-thick', only a few tooth fragments and controlled planes of section (designed to be proxies of whole-crown thickness) have been measured. Here, we measure molar enamel thickness in Australopithecus africanus and Paranthropus robustus using accurate microtomographic methods, recording the whole-crown distribution of enamel. Both taxa have relatively thick enamel, but are thinner than previously characterized based on twodimensional measurements. Three-dimensional measurements show that P. robustus enamel is not hyper-thick, and A. africanus enamel is relatively thinner than that of recent humans. Interspecific differences in the whole-crown distribution of enamel thickness influence crosssectional measurements such that enamel thickness is exaggerated in two-dimensional sections of A. africanus and P. robustus molars. As such, two-dimensional enamel thickness measurements in australopiths are not reliable proxies for the three-dimensional data they are meant to represent. The three-dimensional distribution of enamel thickness shows different patterns among species, and is more useful for the interpretation of functional adaptations than single summary measures of enamel thickness.

Two-dimensional patterns of human enamel thickness on deciduous (dm1, dm2) and permanent first (M1) mandibular molars

“…Three-dimensional values for RET from Australopith taxa differed to previous published 2D values. 13 This was due in part to differences in enamel distribution over the molars, which was much thicker at the tips in the Australopiths compared to humans. Because of this, measurements through the tips were erroneously exaggerated in the 2D sections.…”

“…1 Measures of the final thickness of that coating have provided important insights into primate taxonomic status [2][3][4][5][6][7] and dietary adaptations. [8][9][10][11][12][13] Others have related variations in enamel thickness upon a single tooth and along the molar row to functional and morphological interpretative models.…”

Is the deciduous/permanent molar enamel thickness ratio a taxon-specific indicator in extant and extinct hominids?

“…The existence of interspecific differences in molar enamel thickness has been ascertained within the australopith clade (e.g., Grine and Daegling, 2017;Grine and Martin, 1988;Olejniczak et al, 2008b;Pan et al, 2016;Skinner et al, 2015), but their consideration here is far beyond the specific purposes of our present work. Details about the composition and origin of the mandibular dm2 and M1 specimens/samples are provided in Table 1.…”

“…In this sense, our study attempts to do this for the first time. While occlusal enamel topography is more directly informative in terms of functional activity and adaptive responses (e.g., Guy et al, 2013;Kono, 2004;Kono and Suwa, 2008;Olejniczak et al, 2008b), lateral enamel thickness is also involved in dissipating occlusally-related stresses (Benazzi et al, 2013a(Benazzi et al, , 2013b. Lateral enamel also resists wear, tooth height loss and maintains interproximal tooth-tooth contacts during the late stages of tooth wear after dentine exposure over the occlusal surface.…”

“…Martin for measurement procedure and standardization (Martin, 1985), the bi-three-dimensional assessment of tooth enamel thickness has become routine in taxonomic and adaptive/evolutionary studies of fossil and extant primates (e.g., Alba et al, 2013;Kono, 2004;Kono et al, 2014;Macchiarelli et al, 2004Macchiarelli et al, , 2009Macchiarelli et al, , 2013Olejniczak et al, 2008aOlejniczak et al, , 2008bOlejniczak et al, , 2008cOlejniczak et al, , 2008dPan et al, 2016;Skinner et al, 2015;Smith et al, 2003Smith et al, , 2005Smith et al, , 2011Smith et al, , 2012Suwa et al, 2009;Zanolli et al, 2015Zanolli et al, , 2016a. Commonly used to infer durophagy and considered as a proxy of the dietary niches exploited by extinct species (e.g., Constantino et al, 2011Constantino et al, , 2012Lucas et al, 2008;Martin et al, 2003;Schwartz, 2000a;Teaford, 2007;Teaford and Ungar, 2015;Vogel et al, 2008), occlusal enamel thickness is seen as intimately related to dietary abrasiveness and selectively responsive to lifetime dental wear resistance (Pampush et al, 2013;Rabenold and Pearson, 2011).…”