Comparative dental development and microstructure of ... - UCL

202 A. D. BEYNON ET AL.
from the estimated mean age of M1 emergence
based on data for brain size and M1
emergence in 23 species of primates (Kelley,
1997). In P. heseloni, M1 emergence was
estimated at 20·6 months with an approximate
range for the mean (based on the
confidence interval for brain size rather than
the error of the estimate) of between 19·6
and 21·6 months. Kelley (1997) concluded
that this age for M1 emergence falls at the
upper end of the range of means for all
extant nonhominoid catarrhines, many of
which are considerably larger on average
than P. heseloni. On this evidence Kelley
(1997) has cautiously suggested these
results indicate a more prolonged life history
for P. heseloni. But a considerable grade shift
may occur between Old World monkeys and
hominoids in many life history traits and in
dental development and predictions based
only on hominoid brain sizes, for example,
might result in estimates of M1 emergence
in P. heseloni in excess of those cited here.
Estimates for M 1 crown formation time in
P. heseloni in this study are around 14
months after birth (approximately 30 days of
M 1 formation occurred before birth). If root
extension occurred at the same rate as in M 2
and P 4 at an average 6·4 μm per day (and
poor data from one M 1 section suggests this
is likely) then M1 would have just less than
1·25 mm of root formed at 20·6 months but
close to 1·5 mm formed at 21·6 months.
These predictions are speculative but not
incompatible with the predictions of Kelley
(1997) based on 23 species of primates.
Future studies on dental development in P.
nyanzae and P. major might result in good
estimates for the age of emergence of M1 in
these larger bodied proconsulids and so provide
a better idea about the affects of body
size and tooth size on dental development
within Proconsul. This would make it easier
to judge whether there is evidence that
dental development and the overall maturational
profile was prolonged in Proconsul.
Histological studies on other Miocene primates
such as Victoriapithecus, Sivapithecus
and Lufengpithecus will also place the results
of this study into a better phylogenetic
perspective. It remains likely, however, that
postcranial, masticatory and life history
traits evolved in a mosaic fashion (Rae,
1997). Of these traits, those that probably
resulted from reduced adult mortality rates
(which include a prolonged developmental
period and a bigger brain) are likely to have
been the last to appear.
Conclusions
This is the first histological study of
Proconsul teeth from Rusinga Island, Kenya.
A chronology of the sequence of tooth
development in P. heseloni indicates M3 root
formation was complete between 6 and
7 years in this siamang-sized Miocene
primate. Crown formation times in an M 1
and M 2 attributed to the larger female
chimpanzee-sized P. nyanzae were between
30% and 40% less than the average values
for seven common chimpanzees studied in
the same way. The results reported here
suggest that both species of Proconsul from
Rusinga Island had thicker enamel than previously
described for P. africanus and P.
major from other older sites in western
Kenya and Uganda. Reports on the palaeoecology
of Miocene sites on Rusinga
together with future research on the evidence
for seasonality from accentuated
markings in teeth and from tooth microwear
studies may allow us to place studies of
enamel thickness in Proconsul into a more
secure dietary and functional context.
Certain microstructural features in enamel
and dentine appear, so far, to be unique to
Proconsul. P. nyanzae appears more derived
with respect to P. heseloni in the degree to
which these unique features are expressed.
Rates of enamel formation close to the EDJ
are higher than in other primates studied so
far. The ratio of dentine to enamel formed
close to the EDJ in the lateral aspects of the