UR 501, the Plio-Pleistocene hominid from Malawi

0 Acadbmie
des sciences / Elsevier, Paris
Paleontology
/ Pal6ontologie
(Human
paleontology
/ Palkonfologie humaine)
UR 501, the Plio-Pleistocene
Malawi.
Analysis of the microanatomy
hominid
of the enamel
UR 507, /‘hominid6
du Plio-PEistocSne
Analyse de la microanatomie
de I’kmail
Fernando
1
V. RAMIREZ ROZZI,’
Chnire depak?oanthropologie
ou Laboratoire
’ Department
’ Department
d‘anthropologie.
Tim BROMAGE~
et pr@histoire.
Call&e
musBe de I’Homme.
of Anthropoloo% Hunter
of Geoloe)? Paleontology
and Friedemann
de France,
from
du Malawi.
SCIIRENK~
station Marcelin-Berthelot,
30, au. Marcelin-Berthelot,
92360 Meudon-la-For@<
17, place du Trocadho, 75016 Paris. France;
College, CUM
and Mineralogy,
695 Park Ave, New York. NY 10021,
Hessisches Landesmuseum
LISA:
Darmstadt,
Friedensplatz
1, D-6106
Darmstadt,
German??
ABSTRACT
The study of enamel microstructure characteristics was carried out in a mandible of a Plio-Pleistocene
hominid (UR 501), found in Chiwondo Beds at Uraha (northern Malawi), dated around 2.5-2.3 Myrs,
and attributed to Homo rudolfensis. It indicates that UR 501 dental development shares many patterns
with other Plio-Pleistocene hominids, i.e. similar crown formation time in premolars and molars.
Nevertheless, differences were found, especially in the lateral enamel thickness. In premolars, lateral
enamel is as thin as in early Homo, and in molars it is as thick as in robust australopithecines
from
East Turkana. The difference between enamel lateral thickness in premolars and molars in UR 501,
which is not found in another specimen attributed to H. rudolfensis (KNM-ER 1802), may indicate
inter-populational
variation in H. rudolfensis.
Keywords:
Striae
of Reizius,
Crown
formation
time,
Enamel
thickness,
H. rudolfensis
L&de
des caracthistiques
de I’&nail a &t& effect&e dam une mandibule d’un hominid6 pliopl&sto&ne (UR 501) trouu@ dans les couches de Chiwondo d Uraha (nerd du Malawi), dat6 d ‘environ
232,3 Ma et attribuk d Homo rudolfensis. Elle indique que plusieurs aspects du dheloppement
dentaire de LIR 501 sont semblables d ceu.x d’autres hominidt%plio-plCstoc&es,
par exemple la durie
semblable deformation de la couronne despr@molaires et molaires. Cependant, des d@rences ont
&tt?obse&es dans I’kpaisseur laterale de 1’6mail. Dans les prtimolaires, lZmai1 est aussi mince que
chez lespremiers Homo, tandis que, dans les molaires,
il est aussi 6pais que chez les australopithGques
robustes de 1’Est Turkana. La di@rence dans 1’6paisseur
Iat&-ale de /‘@mail entre pr6molaires et
molaires d’lJR 501, qui n’a pas Bt@ observ&e chez d’autres specimens attribuh ti H. rudolfensis
(XNWER 1802), peut indiquer une variation inter-populationnelle
au sein de cette espe‘ce.
Mots
cl&
: Stries de Retrius,
Durke
de formation
de la couronne,
ipaisseur
latkale,
H. rudolfensis
Note pr&sent& par YvesCoppens
Note remix le 3 f&rier 1997,accept& aprCls r&vision le 7 awil 1997
C. R. Acad. Sci. Paris, Sciences
1997, 325,23 l-234
de la terre
et des plan&es
/ Earth & Planetary
Sciences
231
F.V. Ramirez Rozzi et al.
Introduction
The importance of Malawi was pointed out by Clark et al.
(1966) and Tobias (1971) for its geographic
placement
between the two regions arboring Plio-Pleistocene
fossil
hominids,
East and South Africa. During the 1991 and
1992 field seasons, carried out by the Hominid
Corridor
Research Project, a mandible of a Plio-Pleistocene
hominid was found in Chiwondo
Beds at Uraha (Bromage et al.,
1995). The mandible
came from Unit 3A (Betzler and
Ring, 1995) and was dated around 2.5-2.3 Myrs by means
of fauna1 correlation
to radiometrically
dated biostratigraphic horizons in eastern Africa (Bromageet
al., 1995). The
description
of the mandible,
the paleobiogeographic
context as well as the paleoecology
of this region and the
biogeographic
implication
of this finding for hominid evolution and its relation with climate change were discussed
elsewhere (Bromage et al., 1995). Total or partial left and
right P3-M2 crowns are preserved in the mandible.
The
presence of some naturally
broken crowns in UR 501
enables the study of deeper layers of enamel.
Dental enamel reveals two kinds of incremental
lines,
i.e. cross-striations
and Striae of Retzius, with a circadian
and an approximately
circaseptan
periodicity,
respectively. Hunter Schreger bands are also revealed as an optical
phenomenon
related to prism decussation (Boyde, 1976;
Dean, 1989). The characteristics
of the incremental
lines
and bands are the results of processes implicated
in tooth
formation. Therefore, enamel microanatomy
reflects patterns of dental development
(Ramirez Rozzi, 1992, 1993a,
b).
Mandible
and tooth morphology
of UR 501 is a mosaic
of characteristics
shared by several Plio-Pleistocene
hominid species. However, UR 501 morphology
is close to that
of KNM-ER 1802, which was attributed to Homo rudolfensis (Wood, 1991). Bromage et al. (1995) has thus assigned UR 501 to H. rudolfensis.
There are differences in dental development
between
Plio-Pleistocene
hominid
species (Beynon and Wood,
1987; Grine and Martin,
1988; Ramirez Rozzi, 1992,
1994). The taxonomic value of these differences remains,
nevertheless, uncertain because previous taxonomic attributions need to be reconsidered
in light of new discoveries and reinterpretation
of fossils from East Africa. Although enamel microstructural
characteristics
could be
used to define hominid
taxa, the lack of comparative
studies between great apes and modern humans prevents
us from knowing which enamel microstructural
characteristics have high taxonomic valency.
The aims of this study are to estimate dental developmental patterns in UR 501 and to compare them to those
found in other Plio-Pleistocene
hominids given our current understanding.
Materials
and methodology
Right and left P3, Ml and M2 present postmortem broken
lateral tooth faces. The strong wear in Ml and the condi-
232
tion of left M2 prevent an accurate measurement
of the
enamel thickness. Deeper layers of the enamel have been
analysed in mesial and distal faces of left P3, and in lingual
and distal faces of right M2. The cervix is preserved only in
the mesial face of right M2, thus the fracture plane in this
face was used to count the imbricational
striae.
Several enamel
microstructural
characteristics
were
analysed. The Striae of Retzius were counted from the
apparent dentine horn to the cervix. The arrangement
of
striae in the enamel enables us to divide the crown into an
appositional
stage, where striae do not reach the enamel
surface, and an imbricational
stage, where s(triae reach the
enamel surface and form perikymata (Beynon and Wood,
1987). The first stria reaching the enamel surface was
identified and the number of striae of each crown stage
was obtained as well as the proportion
of the imbricational
striae in the enamel.
An estimation
of seven crossstriations between adjacent striae was assumed in calculations of crown formation time (Ramirez Rozzi, 1992,
1995). The slope of striae to the enamel-dentine
junction
(EDJ) (Beynon and Wood, 1986) was obtained
in the
occlusal,
middle and cervical one third of the lateral
enamel. The characteristic
of the course of the striae was
described (e.g. Ramirez Rozzi, 1992). The lateral enamel
thickness (Beynon and Wood, 1986), the heights of the
apparent dentine cap as well as the heights of each crown
stage measured (e.g. Ramirez Rozzi, 1992:, 1993~). The
curvature of Hunter Schreger bands was calculated (e.g.
Beynon and Wood, 1986). These characteristics
enable us
to estimate the following
aspects of dental development:
the active life of the first ameloblasts,
the transition to a
higher activity of the enamel organ, the activity of ameloblasts, the extension rate, the number of ameloblasts
active at a given time, the shape of the matrix forming front of
enamel (Ramirez
Rozzi, 1992, 1993c), and the prism
decussation (Beynon and Wood, 1986).
Results
Results are presented in table I. In M2, the fracture plane of
the distal face is also passing through the hypoconulid
dentine horn. In this cusp, eight striae of Retzius were
formed before the enamel was secreted on the distal
border. The corrected number of appositional
striae corresponds to the striae counted in the lateral fracture plane
plus the eight striae in the hypoconulid.
We have assumed
that a similar number of striae were formed in the premolar
cusps before enamel secretion in borders. The slopes of
striae to the ED] as well as the Hunter Schreger band
curvature are presented in table II. The fracture planes are
rarely perpendicular
to the EDJ; for this reason, attention
was given not to absolute angles but to the variation of
angles along the EDJ (Ramirez Rozzi, 1992). The angles of
the striae to the EDJ become more obtuse toward the
cervix. Nevertheless, the last striae in the distal face of left
P3 show a slope more acute (32”) than that of the other
C. R. Acod. Sci. Paris, Sciences de la terre et des plan&tes / E&h & Plonefary Sciences
1997.325.231-234
UR 501,
Table
.:’
I. Enamel
microstructural
Caract&istiques
characteristics
de /a microstructure
LP3
RM2
de /‘6mai/
ISr
No Sr
45
53
66
119
60
LJ
Tfm
2.28
Angles
striae-/ED
and
et courbure
Hunter
124
Schreger
des bandes
band
de Hunter
2.38
Sr-EDJ
df
If
13
31
34
30
40
39
44
df
16
30
45
2-3
Sr-EDJ:
angle
stria-enamel-dentine
band curvature.
%-ED]
: angles
stries-jonction
courbure
des bandes
de Hunter
junction;
de /‘@mail
Schreger.
2-3
curv:
avec
Hunter
la
1424
5083
2313
46
1419
4873
2364
49
df
2938
2135
4414
4779*
2218
50
62
one
third.
The
an occluso-cervical
Schreger
dentine;
CUN :
striae
present
orientation.
a
These
results
enable
us to suggest
some
patterns
of
dental
development.
The
number
of Striae
of Retzius
indicates
a time of premolar
crown
formation
very close to
C. R. Acad.
1997.325,231-234
Sci.
Paris,
of prism
decussation
Sciences
de
la terre
is low.
et des
plan&es
;
:
/ Earth
Ethiopia.
striae
contained
The number
of imbricational
and
and
the total
numbser
of striae
in
within
the variability
observed
in
the appositional
stage
is lower
in the UR 501 premolar
than in the Omo
premolarsfrom
Member
13 to Submember
G8, whereas
this proportion
in the UR 501 molar
corresponds
to the lowest
limit
of the variation
in the Omo
sample.
It indicates
that the number
of ameloblasts
active
at any given
time
was very
low. The
premolar
lateral
enamel
is thinner
and the molar
lateral
enamel
is thicker
in
UR 501 than the average
lateral
enamel
for the same faces
in teeth from Member
B to Submember
G8 from Omo.
The
height
of the appositional
stage indicates
that the number
of ameloblasts
active
at any given
time was not high. The
matrix
forming
front
of the enamel
presented
a concave
degree
: nombre
de stries appositionnelles
corrigk
la couronne
; LJ : 6paissewde
/‘@mail
; Hdc
de la hauteur
de la partie
appositionnelle
molar
crown
formation
is,
respectivlely,
2.53
and
2.24-2.65
yrs, which
is close to the results obtained
from
actual
counts.
Some characteristics
of UR 501 are different
from other
Plio-Pleistocene
hominids.
The proportion
of the height
of
before
half of the crown
was formed
in premolars,
whereas in molars
it was registered
approximately
when
half of
the crown
was completed.
The extension
rate decreases
toward
the cervix
in both premolars
and molars,
but it is
possible
that it increases
in the latest
period
of crown
formation.
Approximatively
50% of the proportion
of the
The
striae
corrected;
ISr: number
of
of the apparent
dentine
cap; Ha:
Mesures
of ILT, Hdc and Ha in
the Omo sample
(teeth from Member
B to Submember
G8)
(Ramirez
Rozzi,
1993d,
1995).
Ramirez
Rozzi
(1993d,
1995)
suggested
an algorithm
to calculate
the total number of striae from the number
of imbricational
striae.
If this
algorithm
is applied
to UR 501, the time of premolar
and
that for molars.
The active
life of the first ameloblasts
is
longer
in molars
(1.23 yrs) than
in premolars
(1.02
yrs),
and it could
be the reason
for thicker
enamel
in molars.
The strongest
activity
of the enamel
organ
took
place
shape.
appositional
Hdc: height
*: estimated.
2’374
molars
(Kronfeld,
1935).
The interval
between
the beginning of enamel
secretion
in different
cusps is not known
in
great
apes.
Therefore,
it is not possible
to estimate
the
variability
in Plio-Pleistocene
hominids.
If we assume
intervals
similar
to those
in modern
humans,
the time of
molar
crown
formation
could
reach 2.61 yrs.
Almost
all results for UR 501 are close to those for teeth
from
Omo,
appositional
UR 501 are
striae
of the cervical
concave
course
with
% ha
the striae
in the hypoconulid
do not represent
the first
periods
of crown
formation.
However,
the largest
interval
between
the first enamel
secretion
in the protoconid,
i.e.
the first cusp formed,
and the hypoconulid
is never greater
than
3 months
(12 striae)
in modern
hurnan
permanent
curv
23
mf
14
iHa
The time of crown
formation
was obtained
from the number of appositional
striae of the hypoconulid.
This cusp is
almost
always
the last part to be formed
(Kraus,
1965),
thus
M2
CUN
Hdc
Discussion
curvature.
Schreger.
P3
Sr-EDJ
Malawi
df
mf: mesial
face; df: distal
face; If: lingual
face; A%: number
of appositional
striae;
ASrc: number
of
imbricational
striae;
NoSr: total number
of striae; Tfm: time of crown
formation;
LT: lateral
thickness;
height
of the appositional
stage; %ha: proportion
of the height
of the appositional
stage in crown;
microns.
mf : face m&ale
; df : face distale
; If : face lingua/e
; ASr : nombre
de stries appositionnelles
; ASrc
I.% : nombre
de stries imbricationnelles
; N” Sr : nombre
total de stries ; Jfm : d&e
de formation
de
hauteur
de la pointe
apparente
de la dentine
; Ha : hauteur
de la partie
appositionnelle
; %ha : proportion
dans /a couronne
; * : estimke.
stria-EDJ
from
mf
If
Table
II. Angles
*, !
hominid
de UR 501.
ASrc
64
Plio-Pleistocene
of UR 501.
ASr
56
the
& flonefafy
Sciences
233
F.V. Ramirez
Roui et al
premolar
lateral enamel thickness of UR 501 is close to
the average lateral enamel thickness of early Homo premolars from East Turkana, but the molar lateral enamel
thickness of UR 501 is close (probably
higher) to the
average of robust Australopithecine
molars from East Turkana.
Bromage et al. (1995) have attributed
UR 501 to H.
rudolfensis
because UR 501 morphology
is close to that of
KNM-ER 1802. In this specimen, however, lateral enamel
thickness in premolars (1.5 mm) is very close to that in
molars (1.7 mm) (Beynon and Wood, 1986), while UR
501 is characterized
by a discrepancy
of lateral enamel
thickness values.
Conclusion
UR 501 resembles other Plio-Pleistocene
hominids
in
having similar premolar and molar crown formation times.
UR 501 is unique, however, in having thicker molar lateral
enamel than that of premolars, probably as a result of a
longer active life of the first ameloblasts. This difference is
not found in other Plio-Pleistocene
hominids, not even in
those that are close morphologically
(e.g. KNM-ER 1802).
If we assume that UR 501 belongs to H. ruddfensis
hypodygm, then the differences observed between UR 501 and
KNM-ER 1802 could reflect inter-populational
variation in
this species.
Acknowledgements:
Fieldwork
was conducted
by permission
of the Malawi
Government
under the auspices
of the Department
of Antiquities.
Major
funding
was provided
by the Deutsche
Forschungsgemeinschaft,
the L.S.B. Leakey
Foundation,
and the
National
Geographic
Society
Laboratory
work was partially
supported
by a National
Science
Foundation
(DBR-9512373)
grant to
the Analytical
Microscopy
and Imaging
Center
in Anthropology
(AMICA)
of Hunter College,
CUNY.
Kraus B.S. 1965. Morphogenesis
of deciduous
man. Dental Anthropology,
Brothwell
D.R. (ed).
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