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Pliocene hominid partial mandible from Tabarin Baringo Kenya.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 7221-37 (1987)
Pliocene Hominid Partial Mandible From Tabarin, Baringo, Kenya
STEVEN WARD AND ANDREW HILL
Department of Sociology and Anthropology, Kent State University,
Kent, Ohio 44242, and Human Anatomy Program,
Northeastern Ohio Universities College of Medicine, Rootstown, Ohio 44272
(S. W.); Department of Anthropology, Yale Uniuersity, Box 2114,
New Haven, Connecticut 06520 (A.H.)
KEY WORDS
Baringo, Chemeron, Hominid, Mandible, Pliocene
ABSTRACT
Sediments in the Tugen Hills, west of Lake Baringo, Kenya,
form one of the best fossiliferous successions known in Africa spanning the
period from 14 my to less than 4 my. Hominoid fossils have previously been
recovered from a number of localities in the region. We describe here a new
hominid mandible (KNM-TH 13150)from the site of Tabarin, in the Chemeron
Formation. Isotopic determinations on a tuff below the fossiliferous horizon
gives dates of 4.96 my and 5.25 my. The associated fauna is consistent with
these results and independently suggests a minimum age for the specimen of
4.15 my. Although fragmentary, the preserved morphology of the Tabarin
mandible is consistent with the diagnosis of the Pliocene hominid Australopithecus ufarensis. It can be distinguished from all other currently recognized
hominoid taxa.
Few hominoid fossils are known from Af- this paper, we describe the new specimen, its
rica between the relatively rich samples from chronostratigraphic placement, make comearly Middle Miocene sites, such as in west- parisons with other hominoid samples', and
ern Kenya up to 14 my in age, and those discuss problems of Chemeron stratigraphy.
from 3.7 my onwards, which include austra- This is the first of a number of papers in
lopithecines. This is unfortunate, especially which we will describe and review the anatsince new predictions of hominoid phyletic omy and context of hominoids from this area
branching events based on DNA-DNA hy- and time interval.
bridization studies (Sibley and Ahlquist,
GEOLOGY
1984) predict a hominid divergence at about
7 my ago.
Tabarin (Baringo Paleontological Research
A number of the hominoid specimens that Project site: BPRP K077: UTM 36N ZR
do come from this ten million year Miocene- 186844) is located about 2 km north of RonPliocene interval have been found in the area donin, northwest of Lake Baringo (Fig. 1). It
of the Tugen Hills, west of Lake Baringo in forms a n outcrop of Chemeron Formation
the northern Kenya Rift Valley. The Tugen sediments which are exposed in a half basin
Hills, and rocks in the eastern foothills, form developed against the Saimo Fault of the
one of the main recorded fossiliferous succes- main Tugen Hills scarp. They are part of a
sions spanning this important time period
(Bishop et al., 1971). Since 1981 the Baringo
Received April 25, 1985; revision accepted June 27,1986.
Paleontological Research Project (BPRP) has
'This sample includes all medium to large sized Proconsul
worked in this area, refining the time caliin the National Museums of Kenya; all the Swapithe
bration, reanalyzing the fauna, and collect- mandibles
cus mandibles from Pakistan in the care of the Geological Survey of Pakistan, and some of the Indian specimens at the Museum
ing additional fossils (Hill et al., 1985; Hill,
of the Geological Survey of India in Calcutta; most of the manCurtis, and Drake, in press). As a result of
dibles of Australopthecus afarensis from Hadar now in the Natthese efforts we have discovered a number of
ural History Museum, Addis Ababa; LH-4, the holotype of
Austrabpithecus afarensls from Laetoli, in the National Munew localities, including Tabarin, in the
seums of Kenya; and Austrabptthecus africanus specimens in
Chemeron Formation, from which has come the Department of Anatomy, University of the Witwatersrand,
a hominoid partial mandible (Hill, 1985). In and in the Transvaal Museum, South Africa.
0 1987 ALAN R. LISS, INC.
22
S. WARD AND A. HILL
discontinuous outcrop that strikes predominantly northsouth and is intermittently concealed by a piedmont regolith of lava boulders
and finer material.
A t the site, sediments lie on a flow of the
Kaparaina Basalt, which separates them
from the sediments of the Lukeino Formation beneath. The succession begins with approximately 20 m of fine to medium-grained
sediments, which are overlain and in places
baked by a n ignimbritic t d a c e o u s unit about
10 m thick. The upper part is a coarse pumice
tuff, and pumiceous patches also occur at the
- 0'30'N
base. Next follows about 17 m of mainly yellow and orange silts and sands, succeeded by
a dark brown, coarsely bedded ferruginous
conglomerate containing poorly sorted igneous clasts. This is the fossiliferous unit
Fig. 1. Map of the area to the west of Lake Baringo
from which, judging by location and matrix, showing
the Tugen Hills and the location of Tabarin.
come many of the surface fossil specimens,
including the hominid. There are other fossiliferous levels higher in the sequence of
sediments, which continue towards the
is important, as other Chemeron primates
Saimo fault of the main Tugen Hills range.
The history of description of the rocks come from the type area, and there is some
which now constitute the Chemeron Forma- doubt as to their precise dating. Two are the
tion has resulted in a relatively complicated type specimens of Paracolobus chemeroni, a
situation (Hill, Curtis, and Drake, in press). complete skeleton, and of Parapapio barinRocks lying on basalts, subsequently named gensis (Leakey, 1969; Birchette, 1982; Eck
the Kaparaina Basalts, in river valleys west and Jablonski, 1984); and another is a temof Lake Baringo were first named the Chem- poral of a hominid (Martyn, 1967; Tobias,
eron beds by McCall et al. (1967). More de- 1967)(see below; also Ward and Hill, in preptailed work by Martyn (1967, 19691, resulted aration, a).
In terms of stratigraphical position, interin his formalizing the unit and in allocating
additional outcrops (the Kipcherere outcrops) nal succession, and faunal content, a number
to the Formation, which he mapped north to of sites appear to correlate with Tabarin.
OO"45'. Sediments resting on Kaparaina These include sites about 20 km farther
Basalts were recorded north of this by Chap- south, such as Kibingor (BPRP KO01 = JM
man (1971). Subsequently Pickford (1975a) 511) and the Kipcherere sites (BPRP K004allocated these and other exposures to the KO10 = J M 4891, which are part of the ChemChemeron Formation. To a t least the Barge- eron Formation sensu Martyn (1967; 1969).
tyo River (approximately the latitude of Ya- Others nearer to Tabarin include Sagatia
tya in Fig. l), the sediments forming this (BPRP KO75 = 2/232), a site close to Ronnorthern extension lie on the Kaparaina donin. North of the Bargetyo River the relaBasalt, and this along with lithological simi- tionships of rocks forming the Chemeron
larities results in the correlation with the northern extension to those farther south are
Chemeron (Kipcherere) exposures to the less clear. Sites in this northern area include
south being little disputed. Further north, in BPRP KO37 (=2/211) (mistakenly recorded
areas where the Kaparaina Basalt is absent, as 2 / 2 0 in Pickford et al., 19831, from which
Pickford's reallocation of parts of the Kaper- has come a hominoid humerus (Pickford et
yon Formation (McClenaghan, 1971; Chap- al., 1983) (see below).
man, 1971; Pickford, 1975a) to the Chemeron
RADIOMETRIC DATING
unit has been questioned (Chapman and
G. Curtis and R. Drake have performed W
Brook, 1978; Chapman et al., 1978).
Problems also exist in the relation of the Ar isotopic determinations on samples of the
Kipcherere outcrops and their correlatives to tuffaceous unit underlying the main fossilithe Chemeron sequence in its type area. This ferous horizon. These give dates of 4.96 f
23
TABARIN MANDIBLE
0.03 my (on anorthoclase), and 5.25 f 0.04
my (on sanidine). The fossiliferous sediments
are apparently comformable on the surface
of this dated tuff, so the hominoid is unlikely
to be considerably younger than these isotopic dates.
PALEONTOLOGY AND DATING
An abundant and diverse fossil fauna
comes from Tabarin (Table 1).The same fossiliferous horizon crops out at Sagatia, a site
1.5 km south, and additional species from
there are also listed. The fauna includes
aquatic animals, such as molluscs, fish, turtles, crocodiles, and hippopotami, along with
others that might be found in the vicinity of
a lake or river, proboscideans, equids, rhinoceroses, suids, bovids, carnivores, and
primates.
The fauna is compatible with the age suggested by the radiometric dates and in addition independently provides a n upper limit
of 4.15 my. The most useful species for this
are the suid Nyanzachoerus jaegeri, and the
proboscideans, particularly A n a n c u s kenyensis.
T A B L E 1. Fossil fauna from Tabarin and Sagatia
Tabarin
Mollusca
Bivalvia
Pisces
Siluriformes
cf. Clarias
Reptilia
Chelonia
’ Testudinidae
Trionychidae
Pelomedusidae
Crocodilia
Mammalia
Primates
Cerocopithecidae
Papionini
Hominidae
A urtralopithecus
cf. afarensis
Carnivora
Hyaenidae
Felidae
cf. Panthera
Small species
Mustelidae
Enhydriodon cf campani
Another species; family
indet.
Proboscidea
Gomphotheriidae
Anancus kenyensis
Elephantidae
cf Primelephas
gomphotheroitlvs
Deinotheriidae
Deinotherium bozasi
Perissodactyla
Equidae
Hipparion
Rhinocerotidae
Artiodactyl a
Suidae
Nyanzachoerus jaegeri
Hippopotamidae
Bovidae
Several species
Rodentia
Hystricidae
Hystrix
Sagatia
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
24
S. WARD AND A. HILL
Nyanzachoerus jaegeri is so far only known
from sites older than 4.15 my. This age limit
is given by the Mursi Formation a t the Omo,
which is dated a t older than 4.15 my (corrected for new constants) (Brown and Lajoie,
1971; Harris and White, 1979). Among other
independently dated sites that the species
comes from is Aterir, which is likely to be
over 4 my (Bishop et al., 1971; Harris and
White, 1979). Ny. jaegeri is found in the Aramis Member and the Beearyada beds, which
underlie the Kalaloo beds of the Sagantole
Formation in the Middle Awash Complex
(Kalb et al., 1982a, 198213,1982~).
These units
are beneath the Cindery Tuff dated at 3.84.0 my (Clark et al., 1984; Hall et al., 1984).
There are sites just younger than 4 my where
it is not found. For example, it is not known
in the Hadar Formation (Harris and White,
1979; Johanson et al., 1982), nor in the Laetoli fauna at 3.7 my (Leakey et al., 1976;
Harris and White, 1979).
Despite assertions to the contrary in Coppens et al. (19781, Anancus kenyensis is not
known later than 4 my. It is not found in the
Hadar Formation (Johanson et al., 1982). In
the Omo sequence it is only known for the
Mursi Formation, dated a t older than 4.15
my (Brown and Lajoie, 1971; Beden, 1976).
Mebrate and Kalb (1985, Fig. 3) are mistaken in recording it as part of the Laetoli
fauna a t 3.7 my (Leakey et al., 19761, but it
is found in the older Lower Laetolil Beds (J.
Harris, personal communication). In the
Middle Awash, all but one specimen comes
from below the Cindery Tuff dated a t 3.8 and
4.0 my. Kalb reports a single specimen, which
was not collected, in the Kalaloo beds of the
Sagantole Formation, stratigraphically immediately above the Cindery Tuff horizon
(Kalb et al., 1982~).
Nothing else in the Tabarin fauna is so
explicit, but all of it is consistent with the
age suggested by the radiometric dating and
by the suids and gomphothere. There are
specimens of a n archaic elephant which are
insufficient to identify exactly but are probably Primelephas gomphotheroides, the species found a t other Chemeron Formation
sites. It is not known from Hadar or Laetoli;
Maglio (1973) considers it typical of about 5
my. The cercopithecid specimen is only a single molar tooth in a fragment of mandible,
but it can identified as papionine and there
is nothing to prevent its being early Pliocene
in age (MG Leakey personal communication). John Barry (personal communication)
identifies the Enhydriodon as resembling the
form found at Kanapoi; and the hyaenid is
also similar to the one from Kanapoi, which
also occurs in the Kaiso fauna.
CHEMERON HOMINOIDS
Two hominoids have previously been recorded from the Chemeron Formation.
The first to be discovered was a hominid
temporal bone from the Chemeron Formation in its type area (Martyn, 1967; Tobias,
1967). But this is not associated with the
fauna that typifies the Chemeron a t the majority of sites, including Tabarin. It is likely
to be considerably younger than this fauna
and than the other specimens discussed here.
Lavas above the sediments from which the
hominid was recovered are dated a t 1.57 and
2.07 my (Hill et al., 1985). This will be discussed in more detail in a following publication (Ward and Hill, in preparation a).
Pickford et al. (1983) described a proximal
humerus fragment from a site in Baringo
about 6.5 km north of Tabarin (BPRP KO37
= 2/211). They assigned it to the Hominoidea
and suggested that several features of the
specimen indicated hominid affinities. A
study by Senut (1983)stresses that a familial
allocation within the Hominoidea cannot be
justified. Although it has been referred to the
Chemeron Formation, the geological relationship of this site to the Tabarin sequence
and sites farther south is not yet clear. However, it may be a similar age, as Nyanzache
erus jaegeri is part of the associated fauna,
and a n isotopic date of 5.07 my has been
obtained on a tuff about 35 m beneath (Pickford et al., 1983). If there is only one species
of large hominoid living in this area at this
time period, and if the Tabarin specimen is
indeed a hominid, then it would support the
taxonomic suspicions of Pickford et al., (1983)
regarding the humerus.
THE NEW HOMINOID
Preservation
The Tabarin hominoid specimen (KNM-TH
13150;Fig. 2) was found by Kiptalam Cheboi.
It was a surface find a few meters downslope
from the outcrop of the ferruginous conglomeratic horizon. Matrix adhering to the specimen leaves no doubt that it derives from the
conglomerate and is associated with the
fauna from this unit.
The partial mandible is a fragment of the
right corpus, bearing the first and second
permanent molars, as well as portions of the
25
TABARIN MANDIBLE
A
C
B
Fig. 2. The hominid specimen from Tabarin (KNM-TH 13150):(A) buccal, (Bj lingual, and (Cj
occlusal views.
alveoli of Pm4 and M3. The mandible is that
of an adult hominoid, as can be determined
by the state of molar wear, and by the condition of the third molar socket. The entire
mesial wall of this alveolus is preserved, revealing a bifurcated root apex. From the state
of the apical impressions left by the root, and
from the presence of a distal interproximal
wear facet on M 2 , it is evident that the third
molar had completed its development.
Of the two molars preserved in the specimen, MI is moderately worn, and a small
portion of the protoconid has been lost me-
sially. The second molar has sustained extensive damage to its enamel cap, obscuring
details of its morphology. The crown is otherwise complete.
The corpus is broken along a horizontal
plane a t approximately the union of the alveolar process and the mandibular base. Below the distal root of the second molar, the
corpus is complete, permitting measurement
of its depth a t that point (Table 2). The lingual cortex is extensively damaged, but is in
good condition under the second molar, where
contours are well preserved. The alveolar
TABLE 2. Mandibular metrics
N
Tabarin
A. afarensis
A. africanus
S. siualensis
P. africanus
6
3
5
4
Corpus height M2
j r
Range
26.9
35.5
30.2
24.2
20.8
25.1-37.0
23.7-35.0
23.3-27.2
18.1-24.1
Alveolar
crest breadth M2
jr
Range
Corpus
X
breadth
M2
Range
11.2
14.0
14.9
12.5
08.4
19.8
23.7
23.1
18.7
14.5
18.8-28.5
22.0-25.1
16.3-19.1
11.1-19.2
-
12.1-17.5
14.9-15.0
11.1-14.1
08.1-11.0
-
26
S.W A R D A N D A.HILL
A
C
D
J
E
F
Fig. 3. Subocclusal morphology drawn from lateral radiographs: (A) Tabarin (KNM-TH
13150);(B) Proconsul africanus (KNM-RU 1855);(C) Proconsul major (KNM-SO 396);(D)S i u a p
ithecus indicus (AMNH 19413); (E) Siuupithecus sivalensis (GSP 4622); (F) A ustrnlopithecus
afarensis (AL 400). See text for details.
crest is intact on both the buccal and lingual below the lateral prominence, it is not possisides of the molars. There is some evidence ble to determine the relationships of the
of osteolytic alveolar resorption around the oblique line. A fairly discernible upwardly
concave line branches off the lateral promiroots of the first molar.
nence and terminates on the alveolar crest
Corpus morphology
adjacent to the distal socket of Pm4. This
The most striking feature of the Tabarin probably represents the line of buccinator
specimen is its shallow corpus depth yet very attachment.
In concert with a massive lateral promipronounced external contours. The lateral
prominence is strongly developed and ap- nence, the buccal surface of the Tabarin manpears to have attained its maximum expres- dible shows pronounced hollowing below the
sion adjacent to the contact between the first molar. This combination of relationships
second and third molars. It terminates below imparts a strong surface relief to the corpus
the distal root of MI. From these relation- and gives the impression of a strongly butships, and from the position of the fracture tressed corpus, despite its small size.
Although it is extensively damaged, the
a t the union of the ramus and the corpus, it
can be concluded that the third molar was lingual cortex is sufficiently well preserved
situated in a relatively posterior position, be- to reveal that there is only moderate relief
hind the anterior edge of the ramus. Since beneath the second molar. The alveolar crest
the external surface of the corpus is broken is robust above, and the lingual cortex be-
27
TABARIN MANDIBLE
comes concave a t midcorpus. From what is
preserved of the base, it appears that this
part of the corpus was transversely thick.
Occlusal morphology
Although the first molar is worn, and the
enamel on the occlusal surface of the second
molar is badly weathered, useful metric and
anatomical information is preserved (Table
3). From its wear configuration, as well as
from fractured surfaces on its protoconid, it
can be determined that the first molar has a
thick enamel cap. On both molars, the trigonid is slightly broader than talonid, and
the hypoconulid on MI is mesiodistally compressed. However, this compression is not the
result of interproximal wear, which affects
both molars at this contact. The metaconids
are the largest cusps on each molar, followed
by the protoconids. The buccal developmental grooves are well developed on each
molar, and despite wear and erosion, it appears that both the protoconids and hypoconids were prominently expressed. This finding
is reinforced by the strong bilobate appearance of the molar crowns when viewed from
their buccal surfaces.
Both molars show evidence of buccal flare,
while the lingual surfaces of the crowns are
vertical.
As has been noted, the first molar crown is
moderately worn, with wear being concentrated on the buccal cusps. A pair of hollowed
out dentin islands mark the positions of the
protoconid and hypoconid. These exposures
were in the process of coalescing a t the time
of death. They are bounded buccally by crescentic rims of enamel. Two other small dentin exposures are present: one a t the apex of
the metaconid, the other on the hypoconulid.
The metaconid is flattened by wear apically,
and its lingual rim is preserved as a sharp
edge. Despite its degree of wear, the metaconid is the tallest cusp on the crown. The
entoconid has also been flattened by wear,
but its dentin core has not been exposed.
Evidence of wear on the second molar has
been largely obliterated, but there is a small
surface of exposed dentin on the protoconid.
From the enamel tissue that remains, we
believe it is unlikely that the second molar
was as heavily worn as the first.
Of the three interproximal contacts that
are preserved, only the M1-Mz facet can provide any useful information concerning
interproximal wear. At this contact, a pro-
,--
E
28
S. WARD AND A. HILL
nounced interproximal facet has developed,
squaring off the distal surface of the MI hypoconulid and incising the mesial fovea of
the second molar. From what is preserved of
the mesial surface of the first molar, a
strongly developed interproximal facet is
seen to be present, the result of contact with
Pm4.
Although the third molar is missing, observation of the contiguous occlusal surfaces of
the first and second molars shows that a sinusoidal, or helicoidal, occlusal plane characterized the molars. Pitch reversal, or the
“pas helicoide” (Ackermann, 1953) occurs on
the talonid of Ma.
Subocclusal morphology
Lateral radiographs of KNM-TH 13150, in
addition to examination of its broken surfaces, provide important information on alveolar process relationships and subocclusal
anatomy (Fig. 3). Both molars have low, cynomorph pulp chambers. The pulp horns are
obtuse and do not extend up into the dentinal
mass of the crown. This probably indicates
that the dentin horns under the enamel were
likewise low and blunt, a factor that affects
the process of dentin exposure on the occlusal
surface (Martin, 1984).
The root furcation under the crown of both
molars is arch shaped, rather than narrow
and cleft-like. The roots of the first molar are
almost vertically inclined in lateral projection, and the distal root is thicker anteroposteriorly than the mesial. However, when
measured buccolingually, the mesial root is
broader than the distal. The same pattern
holds true for the second molar, except that
the roots are slightly more inclined distally.
In both molars, the roots are of equivalent
length when measured from the cervix.
Both partially preserved sockets, the distal
alveolus of the fourth premolar, and the mesial alveolus of the third molar, are sufficiently intact to determine basic subocclusal
relationships. From what is preserved of the
Pm4 socket, it is clear that this tooth had a
pair of roots, mesiodistally aligned. The apex
of the distal root is still present in its socket,
and it is bifurcated a t its tip. At the point
where the root is fractured, two root canals,
lingual and buccal, are visible. The total
length of this root is approximately the same
as the roots of both molars.
The mesial root socket of the third molar is
exposed in the posterior break and is notable
for its bifid apex. The socket is inclined distally with respect to the alveolar plane. In
coronal projection, its axial alignment is
strongly angled, with the apex inclined towards the buccal surface of the corpus and
the cervix angled towards the lingual cortical plate.
The molar roots occupy approximately 50%
of total corpus depth. The position of the inferior alveolar canal is not clear from either
the radiographs or direct inspection, so a precise measurement of alveolar process height
is not possible. However, it is possible that
the canal passes fairly close to, or even between, the bifurcated apices of the molar and
premolar roots. This would indicate that the
alveolar process-basal bone ratio is in the
neighborhood of 50-55%.
COMPARATIVE OBSERVATIONS
Comparative material
In order to identify the taxon to which the
Tabarin mandible belongs, it is necessary to
undertake phenetic comparisons with seven
known fossil samples: the medium to large
hominoids from the early Miocene of Kenya
and Uganda (Andrews, 1978; Martin, 1981;
Pilbeam, 1969); the South Asian Siuapithe
cus sample (Pilbeam et al., 1980; Ward and
Brown, 1985); Kenyapithecus from the middle to late Miocene of Kenya (Andrews and
Walker, 1976; Pickford, 1982; Ishida et al.,
1984); the Lothagam mandible (Patterson et
al., 1970); the Lukeino molar (Pickford,
1975b; Corruccini and McHenry, 1980); the
large A ustralopithecus afarensis sample from
Laetoli and Hadar (Leakey et al., 1976;
White, 1977; Johanson and White, 1980;
White and Johanson, 1982); and Austral@
pithecus africanus from Makapansgat and
from Sterkfontein, South Africa (Tobias,
1980). In addition there is a mandible from
Buluk, north Kenya (Leakey and Walker,
1985). There are taxonomic problems with
the Lukeino molar, and the Lothagam mandible is also difficult to classify taxonomically, for the same reasons the Tabarin
mandible is. However, the Lothagam specimen is usually attributed to Australopithe
cus (Patterson et al., 1970; Howell, 1978), and
we will support that practice here.
The early Miocene hominoid material from
Kenya and Uganda spans the time period
from about 23 my to about 14 my. The specimen from Buluk is about 17 my old. The
South Asian Sivapithecus is known from
11.8-7.2 my (Barry, in press). Kenyapithecus
is dated at around 15-14 my. Australopithecus afarensis ranges from 3.7-2.8 my.
29
TABARIN MANDIBLE
The isolated tooth from the Lukeino Formation, also in the Baringo area about 2.5
km north of Tabarin, is the oldest hominoid
specimen that might be a hominid. It was
tentatively referred to the Hominidae by Andrews (in Pickford, 1975b). This tooth is older
than the Tabarin specimen. Recent determinations have refined age estimates to about
5.6-6.2 my (Hill et al., 1985).
The fragment of hominid mandible with a
single tooth from Lothagam 1 is bracketed
radiometrically between 8.5 my and 3.8 my
(corrected for new constants) and paleontoloeical estimates focus on 5.5 mv (Patterson
etal., 1970; Maglio, 1973; Harris and White,
1979).
Corpus morphology
The strongly sculpted contours on the buccal surface of the Tabarin mandible are similar to a t least two Kenyapithecus mandibles
from Kaloma (KNM-MJ 5) (Pickford, 1982)
and Nachola (KNM-BG 91514) (Ishida et al.,
19841, a mandible fragment from Buluk
(KNM-WS 124) (Leakey and Walker, 1983,
and to all A. afarensis mandibles (White et
al., 1982). These specimens all have a prominently expressed lateral prominence, narrow
1
2
to moderately broad extramolar sulcus, and
pronounced hollowing of the corpus under
the premolars and first two molars.
The Tabarin specimen differs in these features from most equivalently sized Siuapithe
cus mandibles from India and Pakistan (Fig.
4). These latter specimens have a broader
extramolar sulcus and a transversely broader
corpus. The lateral prominences of the Siuapithecus mandibles, while pronounced, are
not as strongly sculpted as in the Tabarin
mandible but tend to have greater definition
of the buccinator line.
Despite extensive size diversity, few Pre
consul mandibles bear any resemblance to
the Tabarin mandible. Of those that do, most
are large. For example, KNM-LG 452, a female F! major mandible (Martin, 1981), exhibits a strongly sculpted lateral corpus but
is considerably larger than KNM-TH 13150.
A smaller mandible from Rusinga (KNM-RU
1678) is metrically similar to the Tabarin
mandible with respect to corpus depth under
the second molar but shows considerably less
surface relief, including a weakly inflated
lateral prominence and oblique line. Finally,
the mandibular corpora of all Proconsul species are transversely narrow, while those of
3
4
a 0
J
0
Fig. 4. Transverse and coronal sections of the mandibular corpus: ( A l , B1) the Tabarin
mandible (KNM-TH 13150);(A2, B2) Proconsul africanus (KNM-RU 7290);(A3, B3) Siuapithecus
siualensis (GSP 4622);(A4, B4) Australopithecus afarensis (AL 198). Buccal surface is to the left;
lingual to the right.
30
S. WARD AND A. HILL
hominids and the late Miocene hominoids
from South Asia are transversely broad
(Fig. 4).
The Lothagam mandible (KNM-LT 329)
also differs from the Tabarin fragment in
terms of corpus relief. In the Lothagam specimen, the lateral prominence is weakly developed, the extramolar sulcus is very
narrow, and there is little evidence of hollowing under the first and second molars. Finally, the oblique line is only faintly visible,
and no useful details concerning its topography can be discerned.
Lateral corpus contours preserved on KNMTH 13150 are quite similar to the general
pattern described for the Laetoli and Hadar
mandibles (White et al., 1981). This pattern
involves hollowing of the corpus anterior to
the lateral prominence and a high origin of
the ramus, with consequent narrowing of the
extra molar sulcus.
Both the Tabarin mandible, as well as the
mandibles of A. afarensis, are distinguished
from the South African Australopithecus africanus mandibles in the extent of corpus
inflation and in the size of the extra molar
sulcus. A. africanus mandibles are characterized by diminished surface relief (MLD 18,
MLD 40, Sts 7, Sts 36, Sts 52b), a lower origin
of the ramus at its junction with the corpus,
and a broader extramolar sulcus (MLD 40,
Sts 7, Sts 36, Sts 52b) (see White, 1977).
The lingual surface of the hominoid mandibular corpus has few useful diagnostic features. As noted previously, the Tabarin
mandible shows bulging of the alveolar process into the oral cavity, with a longitudinal
concave furrow below the level of the alveolar canal. This furrow is the posterior continuation of the intertoral sulcus that extends
posteriorly from the genial fossa in the symphyseal region. Early Miocene hominoids
lack a n intertoral sulcus, and they also lack
a n inferior symphyseal torus. Except for
large individuals, such as male l? major
(KNM-SO 396) there is very little relief on
the lingual side of the corpus. In addition, no
Proconsul mandibles evince the internal
twisting of the alveolar process that produces
the characteristic bulging of its internal surface. The Sivapithecus mandibles from India
and Pakistan all have a n intertoral sulcus,
and on some larger specimens, the mylohyoid line is discernible. The Lothagam mandible has a pronounced intertoral sulcus, a
very well-defined mylohyoid line, and a
deeply excavated submandibular fossa. The
extent of internal surface relief in the Lothagam specimen in fact approaches the pattern that characterizes Homo. However, a
long, matrix-filled crack in the lingual cortical plate of this specimen may indicate a
certain amount of buckling that could increase the surface relief.
The Hadar and Laetoli mandibles show
variations in lingual corpus relief, but all
conform to one basic pattern. Within this
pattern, shallow mandibles (AL 145, AL 288)
tend to have a large transverse dimension
which is associated with less internal relief.
Larger specimens (AL 277, AL 333w-60)have
more clearly defined intertoral sulci and mylohyoid attachments. However, all the A.
afarensis mandibles show pronounced buccal-lingual twisting of their alveolar processes, and where preserved, a n undercut
area under the third molars, the subalveolar
fossa of Weidenreich (1936). White (1977)has
described the medial surface of the mandibular corpus of A. africanus is some detail and
noted that it presents, on one hand, a pattern
of contours intermediate between that of
many early Homo specimens from East Africa and, on the other, “robust” australopithecines (MLD 18, MLD 40, Sts 52b).
Occlusal morphology
Despite wear and damage, occlusal anatomy of the two molars present in the Tabarin
mandible are clearly distinct from the P r e
consul lower molar pattern of buccal cingula,
thin enamel, and separated, conical cusps.
Thus the only hominoid dental samples that
provide useful comparisons are the Kenyapithecus specimens, Siuapithecus, the Lothagam first molar, the Lukeino molar, and the
Laetoli-Hadar sample.
Direct comparison between the few Kenyapithecus mandibular molars and the Tabarin
teeth is difficult, due to wear and damage to
much of the published material. From the
observations that are possible, the Tabarin
molars share with Kenyapithecus thickened
enamel, reduction of cingula, low, blunt
cusps, and less certainly, accumulation of differential molar wear. In these features, both
Tabarin and the Kenyapithecus teeth from
West Kenya and Nachola are similar to Siuapithecus molars from South Asia. However, there are also differences. A particularly useful comparison involves the Tabarin
mandible and GSP 4622, a S. siualensis mandible from the Potwar Plateau in Pakistan.
TABARIN MANDIBLE
Both specimens are almost identical in depth
dimensions beneath the second molar, and
making minimal allowances for interproxima1 wear, are very similar in first and second
molar mesiodistal length. Other similarities
include thickened enamel and differential
tooth wear. However, they differ in the manner in which occlusal wear accumulates. In
the Tabarin mandible, which shows slightly
less wear on MI than does GSP 4622, a dent i n exposure has appeared on the metaconid,
while the dentin horn of the metaconid of the
small Siuapithecus mandible is not yet exposed. Also, the Tabarin M1 metaconid has
been flattened apically, while metaconid
wear on the M1 of GSP 4622 has accumulated on the buccal flanks of the cusp. These
differences in the topography of metaconid
wear may indicate concomitant differences
in mandibular trajectories during the terminal phases of the occlusal contact sequence.
It may also suggest differences in the extent
of maxillary-mandibular canine interlock in
the two taxa, although Kay (1981) asserts
that canine interlock does not limit mandibular movement in Siuapithecus. Our interpretation of molar macrowear in all currently
recognized species of Siuapithecus and Australopithecus leads us to conclude that protoconid and metaconid attrition accumulates
differently in the two genera. Siuapithecus
metaconids sustain wear in a manner quite
similar to Kenyapithecus, with this cusp generally configured as a tall, angular peak as
the protoconid is progressively flattened. It
is tempting to impute the pattern of metaconid flattening characteristic of hominids at
least partially to canine reduction, but there
is at present insufficient evidence to support
this functional interpretation.
Other differences between the Tabarin
teeth and small Siuapithecus molars include
greater buccal flare in the African specimen,
which mediates broader crown dimensions
in molars of equivalent length, evidence of
greater cusp lobation in the Tabarin teeth, a
distinct hypoconulid in the Tabarin MI, and
apparent expansion of the M2 trigonid.
The Lukeino molar, which is probably a n
MI, is excellently preserved but is slightly
smaller than the Tabarin first molar. While
the worn condition of the Tabarin MI. precludes a detailed morphologic comparison,
the Lukeino molar shares with Tabarin buccal flare and pronounced cusp lobation. It is
doubtful that repeatable crown component
measurements can be made on the Tabarin
31
teeth, but it does appear that both first molars share a common pattern of cusp size
ratios, with the metaconid being the largest
cusp, followed by the protoconid hypoconid,
entoconid, and hypoconulid. This pattern
seems to be in accord with the data on hominid molar cusp areas reported by Wood et al.
(1983). These workers showed that this pattern of cusp size relationships was characteristic of South and East African Australopzthecus, while the areas of the protoconid
and metaconid are reversed in East African
early Homo specimens. The data reported by
Wood and his colleagues (Wood and Abbot,
1983; Wood et al., 1983) also shows a general
reduction in the size of the trigonid cusps and
enlargement of talonid elements. This trend
does not appear evident in the Tabarin first
molar, which is likely to possess the primitive condition for hominids.
A comparison of first molar morphology
in the Tabarin mandible, the Lothagam
mandible (KNM-LT 3251, and the combined
Sterkfonteinhfakapansgat sample from
South Africa generally confirms the conclusions of White et al. (1981), Kimbel et al.
(1985) and White (1985) that a basic and fundamental pattern of differences separate the
South and East African “gracile” australopithecines at the species level. In the Tabarin-Lothagam comparison, both first
molars share a swollen hypoconid and a pronounced bilobate buccal cusp configuration.
The hypoconulid is mesially appressed in
both teeth, but is more strongly expressed as
a distinct element in the Tabarin mandible.
The degree of occlusal wear as determined
by dentin exposure is similar in both molars,
with the Lothagam molar showing slightly
more exposed dentin. In both teeth the dentin islands are deeply incised, with lingually
concave crescentic rims of enamel delimiting
the exposed dentin. Metaconid and entoconid
wear contours are also congruent in the two
molars, especially apical flattening of the
former.
Features of first lower molar anatomy and
wear described for the Tabarin and Lothagam mandibles are concordant with the A.
afarensis first molar sample (White et al.,
1981, White, 1985, Johanson, 1985). Buccal
molar flare associated with bulbous buccal
cusps is a consistent feature in A. afarensis,
as is squaring off of the distal margin of MI.
This trait is due largely to mesial appression
of the hypoconulid in this species. Apical flattening of the metaconid is characteristic of
32
S. WARD AND A. HILL
A. afarensis lower first and second molars. In
later stages of wear, the inflated protoconid
and hypoconid mediate the development of a
bicrescentic rim that circumscribes a deeply
incised dentin exposure (LH 4, AL 198-1, AL
400-lb).
The South African lower molar sample
from Sterkfontein and Makapansgat is distinguishable from the Pliocene East African
specimens in several respects. Cusp relief is
generally lower, imparting an inflated, massive appearance to the entire crown. As noted
by White et al. (1981) the bilobate buccal
configuration that is diagnostic of A. afarensis is either attenuated or lacking entirely.
There also seems to be a considerable amount
of anatomical heterogeneity in A. africanus
lower molars, and some discussion has been
devoted to the very robust molars of MLD 2,
leading to speculation that it may have affnities with A. robustus (see Tobias 1980 for
details). MLD 2 has massive cusps, very
prominent metaconids, and strongly expressed protostylids. Protostylids are also
prominent on some other Transvaal hominids, especially Sts 18. In addition, A. africanus molars are generally broader than the
Pliocene hominid lower molars from East
Africa.
It is likely that these anatomic features are
implicated in the pattern of molar wear accumulation that differentiates South and
East African early hominids. Molar attrition
in A. africanus begins in the typical hominoid fashion, with faceting appearing early
on the buccal cusps, followed by exposure of
small dentin islands on the protoconid and
hypoconid. At a slightly later stage, and in
the absence of pathological occlusion, wear is
spread more evenly over the entire crown,
while in A. afarensis, Tabarin and Lothagam, wear is concentrated for a longer period
on the buccal half of the occlusal surface. The
pattern of early molar wear that characterizes A. africanus is rather similar to that of
A. robustus (SK 6, SK 843, SK 1587, SK 23,
and others), which shows a generalized flattening of the crown in early stages of wear,
with dentin esposures occurring after much
of the original crown relief has been lost
(Wallace, 1978). Radiographically determined enamel thickness data recently reported by Sperber (1985) indicates that the
enamel caps of A. robustus are on average
thicker than those of A. africanus. This, coupled with differences in cusp disposition and
fissure patterns, may account for differences
in the time required to expose substantial
areas of dentin in the two species.
Dental metrics
Length and breadth dimensions for the Tabarin molars and a comparative sample of
Miocene hominoid and Plio-Pleistocene
hominid molars are presented in Table 3.
The Tabarin M1 falls within the A. afarensis
sample range for length, but below the
Hadar-Laetoli range for breadth. It is quite
similar in both dimensions to the Lothagam
MI, and larger than the Lukeino molar.
When compared to the A. africanus molar
sample, the Tabarin mandible proves to be
considerably smaller, falling well below the
length and breadth dimensions reported for
the Sterkfontein and Makapansgat australopithecines. When compared to r( africanus
and S. siualensis, the Tabarin MI is larger in
both dimensions but is approximately the
same size as the S. sivalensis MI.
The Tabarin second molar falls within the
low end of the A. afarensis length racge, but
falls below the Hadar-Laetoli range for
breadth. When compared to A. africanus, the
Tabarin M2 falls well below the South African sample ranges for both length and
breadth. It is slightly longer than the second
molar of S. sivalensis, but falls within the
breadth range for this species. When compared to the second molars of I.! africanus,
the Tabarin M2 is larger in both dimensions.
Conversion of the dental metric data in
Table 3 to crown shape indices is shown
graphically in Figure 5. The Tabarin M1 falls
into a “hominid” cluster with A. afarensis,
A. africanus, and the Lothagam mandible.
The smaller Lukeino molar falls below this
group at a shape index of 99, while the two
Miocene hominoid species fall well above it.
Figure 5b shows crown shape indices for
the same specimens and samples as presented in Figure 5a, with the exception of the
Lukeino molar. The crown shape index for
the Lothagam M2 was calculated from measurements taken from the roots of this tooth,
which are broken quite close to the cervical
line of the missing crown. However, the value
shown here should be regarded only as a n
estimate.
The Tabarin M2 yields a crown-shape index
of 118, which most closely approximates the
crown shape index of P africanus. A. afarenis and A. africanus fall below a value of
110, as does the estimated Lothagam index.
The S. sivalensis value is slightly over 110.
33
TABARIN MANDIBLE
-
M, SHAPE INDEX
P africanus
*
S indicus
Lukeino
Lothagam
A
Tabarin
A afarensis
0
--
I
I
A africanus
-
M, SHAPE INDEX
P africanus
S sivalensis
Lothagam
w
0
Tabarin
A afarensis
-
1
A africanus
1
a
1
-
4
I
I
I
I
I
I
I
I
I
90
95
100
105
110
115
120
125
130
Fig, 5. Molar crown shape indices. See text for discussion.
These data show that the Tabarin first molar falls within currently known Pliocene
hominid ranges for length, breadth, and
shape, while the second molar does not. The
Tabarin M2 is transversely narrow, and this
accounts for its high shape index. Postmortem loss of enamel on both its buccal and
lingual surfaces may partially account for its
small buccolingual diameter, since the contours of the crown show evidence of the buccal expansion that characterizes the mandibular molars of A. afarensis. In addition,
White (1985) has noted that there are no
small individuals represented in the 3.7-my
Laetoli sample, and that this bias may skew
the distribution of dental metrics and crown
shape indices in Pliocene hominids. It is
therefore uncertain what effect a small A.
afarensis M2 would have on combined sample metrics. Finally, it is possible that buccal
expansion of Pliocene hominid lower molars
began as a trend prior to 3.7 my.
Subocclusal morphology
Since the subocclusal morphology of KNMTH 13150 is undistorted, it proves to be a
useful source of phenetic information (Fig. 3).
Lateral periapical radiographs were made on
mandibles of medium- to large-sized Proconsul (I? africanus, €? nyanzae, ?? major), on
samples of Sivapithecus, Australopithecus
afarensis, and A. africanus.
This radiographic survey demonstrates
that the Tabarin mandible differs markedly
from the Proconsul specimens in its enamel
thickness, pulp chamber morphology and
volume, root furcation pattern, root canal
volume, and the topography of the alveolar
process. The Proconsul molars vary in
enamel thickness (Martin 1985), but they
share a common pattern of capacious pulp
chambers with tall and acute pulp horns.
The pulp chamber floors are quite thin, and
the roots diverge away from the furcation a t
a pronounced angle. As noted above, the Tabarin molars have low, vertically compressed
pulp chambers. The pulp chamber floor rises
to a high position, almost touching the ceiling and markedly constricting the chamber
a t this point. Where the root canals in Pre
consul mandibular molars tend to be wide,
even in adult specimens, they are very narrow in the Tabarin teeth. Proconsul molar
roots are relatively longer than those in the
Tabarin mandible, and as a consequence, the
alveolar canal takes a lower course through
34
S. WARD AND A. HILL
the corpus than it does in KNM-TH 13150.
The Tabarin and Proconsul molars share a
pattern of symmetrical root lengths from the
distal root of Pm4 through the mesial root
of M3.
When the subocclusal anatomy of the Tabarin mandible is compared to the South
Asian Sivapithecus sample, considerably
more similarities emerge than was the case
in the Tabarin-Proconsul comparison. Both
Tabarin and the two best sampled Sivapithecus species, S. indicus and S. sivalensis, share
thick molar enamel, low pulp chambers with
a n intrusive and thick floor, low, blunt pulp
horns, and narrow root canals. Also similar
is an anteroposteriorly thickened distal molar root, and roots diverging from the furcation a t a small angle. Finally, both Tabarin
and Asian Sivapithecus share a pattern of
bifid root apices.
There are also some notable differences.
The most consistent subocclusal feature that
segregates the Tabarin specimen from all
known Sivapithecus mandibles is a pronounced change in root lengths between the
first and second molars. S. indicus and S.
sivalensis always show this shift in molar
root lengths; in fact, it is one of their most
diagnostic features. The Tabarin mandible
has a distal Pm4 root, as well as first molar
roots that are equivalent in length to the
second molar roots, and to the mesial root
of M3.
There are also differences in the transverse
dimensions of the molar roots. Sivapithecus
mesial molar roots are equal in buccolingual
breadth to their distal counterparts, or are
slightly narrower. Conversely, in the Tabarin mandible the mesial molar roots are
always broader than the distal.
Although there are differences in surface
contours that distinguish the Tabarin and
Lothagam mandibles, they share a common
subocclusal pattern. The Lothagam mandible preserves the roots of the first and second
molars and the mesial roots of the third molar. Although the Lothagam MI is larger
than that of Tabarin, the pulp chamber in
both is low and constricted. The floor of the
pulp chamber in the Lothagam MI contacts
its ceiling. This severe vertical compression
is in part the result of secondary dentin deposition in response to heavy occlusal wear.
There are in fact no notable differences in
the subocclusal morphology of these two
mandibles. While it appears that the roots of
the Lothagam MI are shorter than those of
the second and third molars, this difference
is the result of MI having been partially extruded from its socket during fossilization.
Other similarities include bifid root apices
and small root canals and a pattern of root
angulation not found in any earlier hominoid. Both the Lothagam and Tabarin mandibles share a complex arrangement of molar
root orientation in which the mesial root,
which is always broader than its distal counterpart, is more vertically implanted than
the distal. The latter root is axially aligned
in frontal projection, with its apex slanting
towards the buccal cortical plate of the alveolar process. This combination of broad and
vertically oriented mesial roots, along with
narrower, angled distal roots, produces a serrate root system when the molars are viewed
lingually. The only other hominoid taxon
with this subocclusal molar pattern is Australopithecus afarensis.
Although they span a large size range, the
mandibles from Laetoli and Hadar share a
common pattern of subocclusal anatomy, a
pattern that they also share with the Tabarin mandible. Despite the extensive size
variation, the Hadar mandibles have low,
vertically constricted pulp chambers, narrow
root canals, roots of equal length throughout
the premolar-molar series, bifid molar root
apices, molar roots broader mesially than
distally, vertically implanted mesial roots,
and bucally inclined distal roots, producing a
pronounced serrate molar root pattern (Ward
et al., 1982). All of these characters are particularly well shown in LH-4, the type specimen of A. afarensis. In these features it is
not possible to distinguish the Tabarin and
Lothagam mandibles from A. afarensis
mandibles.
The subocclusal pattern of Australopithecus africanus mandibles from Makapansgat
and Sterkfontein is somewhat more variable
than that presented by the Pliocene specimens from East Africa. Several of the South
African specimens, but particularly MLD 18,
show some similarities to Australopithecus
robustus. These similarities are manifested
in transversely broad and massive roots, tall
and projecting pulp horns, and absence or
weak development of the serrate molar root
configuration. A. africanus molars also show
evidence towards a n increase in root length,
which also characterizes A. robustus, and A.
boisei.
TABARIN MANDIBLE
35
erecting a new taxon to accommodate it.
When new material is available from TaAs the recent history of the Ramapithecus barin, or if A. afarensis is revised, then the
issue has amply demonstrated, accurate tax- attribution may change.
onomic assignment of fragmentary hominoid
mandibles can be difficult without a suffiACKNOWLEDGMENTS
cient sample of synchronic specimens that
This work forms part of the Baringo Pacollectively preserve all major parts of the
leontological
Research Project, based a t Harfacial skeleton. Our assessment of the morvard
and
Yale
Universities, and carried out
phologic characters preserved in KNM-TH
jointly
with
the
National Museums of Kenya.
13150, coupled with a trait-by-trait comparaIt
was
financed
by grants from the National
tive analysis of all known major Miocene
hominoid and Plio-Pleistocene hominid col- Science Foundation to David Pilbeam, John
lections, directs us to the conclusion that it Barry and Kay Behrensmeyer (# BNS 81is a hominid mandible. The specimen is 408181, and to David Pilbeam, John Barry,
clearly not attributable to any currently rec- and Andrew Hill (# BNS 84-075751, and by a
ognized Miocene hominoid taxon, but does grant from the Louise Brown Foundation to
satisfy, in the parts preserved, the diagnosis Andrew Hill and David Pilbeam. Steven
of A. afarensis developed by Johanson et al. Ward acknowledges support from the Na(1979). The hypodigm of A. afarensis, as con- tional Science Foundation (# BNS 84-08126)
structed by its describers, in fact embraces a and from the Foundation for Research into
capacious envelope of mandibular size and the Origins of Man. For their help with the
shape. It is sufficiently accommodating to field work a t Tabarin we thank Kiptalam
contain both the Tabarin and Lothagam Cheboi, John Kimengich, and Sally Mcmandibles. Although the validity of the Jo- Brearty. Richard Leakey and the National
hanson et al. diagnosis for A. afarensis is Museums of Kenya provided much logistical
currently a matter of contention (Tobias, support, and we are grateful to the Govern1980; Olson, 19851 we believe that it accu- ment of the Republic of Kenya for research
rately defines the spectrum of Pliocene hom- permission. Mary Leakey kindly allowed us
to examine the Laetoli hominid material;
inoid mandibular variation.
To this point in our discussion, we have Richard Leakey gave permission to examine
excluded the possibility that the Tabarin the Lothagam mandible; Donald Johanson
mandible might represent a n unknown gave access to the Hadar material a t the
taxon. However, different criteria apply in Cleveland Museum of Natural History;
simply identifying a particular specimen as Philip Tobias permitted the examination of
a known taxon than in describing a new specimens a t the University of the Witwataxon on the basis of a particular specimen: tersrand, as did Elisabeth Vrba a t the Transa hypodigm of a new species should show vaal Museum. We are grateful to Garniss
diagnostic evidence of belonging to a partic- Curtis and Robert Drake for the results of
ular family if it is to be referred to it; a n isotopic age determinations which they perindividual specimen need not. The Tabarin formed. We thank John Barry, Bobbie Brown,
mandible accords with descriptive criteria for Jay Kelley, and David Pilbeam for their
the Hominidae, even if it cannot be shown to helpful comments on the manuscript. Sally
fulfill all definitive diagnostic criteria. This McBrearty drew Figure 1; Figures 3 and 4
are by Bobbie Brown.
was the view taken in Hill (1985).
Although the Tabarin mandible is incomLITERATURE CITED
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