Comparative dental development and microstructure of ... - UCL

DENTAL DEVELOPMENT IN PROCONSUL
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angles among living hominoids are found
in Pongo but while there is considerable
variation among hominoids both between
different aspects of the same tooth and
between different tooth types, none
approach those reported here for P. nyanzae.
Some striae in Proconsul are strongly
‘‘S-shaped’’ and resemble those in Pongo
and H. (Symphalangus) syndactylus. Dean &
Shellis (1998) have considered the geometry
of ‘‘S-shaped’’ striae and have proposed a
developmental model to account for their
formation. ‘‘S-shaped’’ striae are formed
when three things occur together. (i) When
daily rates of enamel formation increase
steadily from the EDJ and reach maximal
rates close to the enamel surface. (ii) When
enamel prisms in the same plane as striae
either remain the same width or decrease in
width as they run towards the surface and
(iii) when enamel prisms turn cervically in
their course towards the enamel surface. At
present all that can be said is that the
underlying developmental processes are the
same in the few specimens of Proconsul, H.
(Symphalangus) syndactylus and Pongo that
have been studied. In the future it will be
interesting to report on ‘‘S-shaped’’ striae in
other extant primates and fossils such as
Lufengpithecus and Sivapithecus.
While the pattern of increase in cuspal
enamel formation rates appears to resemble
that documented here for M 2 sinPan and
Homo, the fast rates of enamel formation in
outer lateral enamel resembles in Pongo.
Cross striation repeat intervals between
regular striae of Retzius in humans and great
apes and in Papio and Theropithecus (and in
one siamang available to us) are usually in
the range of 7–9 days with outliers recorded
at six and 11 days or more (Swindler &
Beynon, 1992; Dean, 1995b; FitzGerald,
1995; Reid & Dirks, 1997). Intervals in
macaques are reported to be four or five
days (Bowman, 1991) and they are four
days based on a limited sample of gibbons
(Dirks et al., 1995). It appears that the
repeat intervals in these specimens of P.
heseloni are closer to those known for small
species of gibbons and small monkeys.
Those in P. nyanzae are hard to interpret in
the absence of more data. Larger samples of
teeth from different individuals may extend
the range of these counts in both species and
data for larger samples of great apes in
particular are needed.
Of the primates studied so far, the greatest
rate of dentine formation occurs in the tallest
cusps of teeth (Dean, 1995b; Dean &
Scandrett, 1995, 1996; Liversidge et al.,
1993). Low squat tooth crowns are likely to
have slower rates of dentine formation than
tall crowns with high cusps. Their spacing
reveals slow daily rates of cuspal dentine
formation in premolar and molar teeth.
They are below the range known for humans
and modern great apes. They are also below
the range of daily cuspal rates of dentine
formation determined experimentally in
macaques (Bowman, 1991; Dean, 1993;
Molnar et al., 1981). In all these primates
cuspal rates of dentine formation are closer
to 4 or 6μm per day. However, the
measurements in both species of Proconsul
match measurements made in the same way
and in the same tooth types of H. moloch. In
the gibbon M 2 , for example, the mean value
for the spacing of daily lines was 2·6 μm
(S.D.=0·23, range=2·3–3·2).
The combined effect of large fast rates of
enamel formation at the EDJ and slow rates
of dentine formation at the EDJ in both
P. heseloni and P. nyanzae give these teeth
a unique histological appearance at the
cervical EDJ. Interestingly, Andrews &
Martin (1991) illustrate sections of other
Miocene hominoids and some (e.g., Heliopithecus)
appear, at least superficially, similar
to Proconsul in this respect. A constant
extension rate in both enamel and dentine at
the EDJ is maintained by combining a very
low angle of inclination of the mineralizing
front in the dentine with a very high angle
of inclination of the mineralizing front in