littor...@gmail.com wrote:
Growth and development of the third permanent molar in Paranthropus robustus from Swartkrans, South Africa
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https://www.nature.com/articles/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.
"The most easily observable and widely employed measure of dental
maturation in
modern humans, dental eruption, is in fact one of the least reliable1,2. Paradoxically,
however, molar eruption has been the measure of maturation most often used in comparative studies of humans with great apes and with fossil hominins."
"The initial mineralisation of the M3 tooth in both great apes and early
fossil hominins
occurs at a chronologically younger age than in modern humans. In Pan and Gorilla, the
age range reported, albeit for relatively few individuals (Supplementary
Text S1), spans
almost 2.5 years, from 2.9 to 5.3 years, but for a large number of modern humans the
range is much greater, at least 7 years, from 5 to 12 years."
"Clearly, there is insufficient evidence to determine a mean age for M3 initiation in early
hominins. However, the comparative information increasingly available
about the timing and sequence of these events in great apes and fossil specimens, notably for the stages
of dental development both before and after M3 initiation16,37, strongly suggests M3
initiation occurred in australopiths and early Homo sometime between 4 and
7 years of
age. Early hominin specimens aged ~ 4 years or less at death, such as the SK 63 P. robustus specimen from Swartkrans, or the A. africanus Taung
child, both from South Africa, show
no evidence of M3 initiation."
"The gradient of enamel formation through the cusps of this upper M3 are greater than
measured in a sample of modern human molars (Fig. 3), but similar to that
in some living
great apes and other early hominin molars."
"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. Given a likely 4–7-year age range for M3 initiation, it now
becomes possible to say that completion of the permanent dentition and
root apex
closure in SK 835 would have occurred sometime between 11 and 14 years of
age. This
estimate appears to compare well with what is known for dental maturation
in living
great apes."
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