Growth and development of the third permanent molar in Paranthropus robustus from Swartkrans, South Africa

Christopher Dean,Mirriam Tawane,Adeline Le Cabec,Roberto Macchiarelli,Jan Garrevoet,Clément Zanolli,Arnaud Mazurier

doi:10.1038/s41598-020-76032-2

Abstract

Third permanent molars (M3s) are the last tooth to form but have not been used to estimate age at dental maturation in early fossil hominins because direct histological evidence for the timing of their growth has been lacking. We investigated an isolated maxillary M3 (SK 835) from the 1.5 to 1.8-million-year-old (Mya) site of Swartkrans, South Africa, attributed to Paranthropus robustus. Tissue proportions of this specimen were assessed using 3D X-ray micro-tomography. Thin ground sections were used to image daily growth increments in enamel and dentine. Transmitted light microscopy and synchrotron X-ray fluorescence imaging revealed fluctuations in Ca concentration that coincide with daily growth increments. We used regional daily secretion rates and Sr marker-lines to reconstruct tooth growth along the enamel/dentine and then cementum/dentine boundaries. Cumulative growth curves for increasing enamel thickness and tooth height and age-of-attainment estimates for fractional stages of tooth formation differed from those in modern humans. These now provide additional means for assessing late maturation in early hominins. M3 formation took ≥ 7 years in SK 835 and completion of the roots would have occurred between 11 and 14 years of age. Estimated age at dental maturation in this fossil hominin compares well with what is known for living great apes.

Highlights

Third permanent molars (M3s) are the last tooth to form but have not been used to estimate age at dental maturation in early fossil hominins because direct histological evidence for the timing of their growth has been lacking
The regular daily changes in Ca concentration do not, occur abruptly at cross striations in the synchrotron X-ray fluorescence (SXRF) scans but more closely resemble the circadian fluctuations reported[68] for plasma ionized Ca2+. In this SXRF study, unlike previous studies[65], there was no discernible variation in phosphorus concentration where the circadian rhythm has been reported to peak twice in a day[68]
The estimate of ≥ 7 years for the time taken to grow an M3 in a representative of P. robustus, based on the data collected from the upper third molar specimen SK 835 from the 1.5–1.8 Mya site of Swartkrans, South Africa, represents the first direct evidence of this kind for any early hominin

Summary

Introduction

Third permanent molars (M3s) are the last tooth to form but have not been used to estimate age at dental maturation in early fossil hominins because direct histological evidence for the timing of their growth has been lacking. Mineralised tooth tissues (enamel and dentine), on the other hand, contain daily increments of growth that are often well-preserved, even in fossil teeth[9,10] These have been used to calibrate the eruption times of first permanent molars (M1s) in fossil hominins and this has proved to be a reasonably successful method for assessing the time and patterning of dento-skeletal maturation in a broad comparative c ontext[6,11,12,13,14,15,16], if perhaps a less successful proxy for other life history v ariables[17,18,19,20]. The age at death of another fossil specimen with an empty M3 crypt, the Early Pleistocene early Homo KNM-ER 820 from Koobi Fora, Kenya[44,45], is yet to be determined

Methods

Results

Conclusion