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operating at 200 kV. A Link ISIS energy-dispersive x-ray
analysis system, equipped with a Link Pentafet ultrathin
window energy-dispersive spectrometer, was used to
obtain in situ mineral analyses with the Cliff-Lorimer
thin film approximation for data reduction. Experimen-
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transform of digital HRTEM images.
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Institute of Meteoritics, University of New Mexico.
9 June 1999; accepted 21 July 1999
Equatorius: A New Hominoid
Genus from the Middle Miocene
of Kenya
Steve Ward,
1
Barbara Brown,
2
Andrew Hill,
3
Jay Kelley,
4
Will Downs
5
A partial hominoid skeleton just older than 15 million years from sediments in
the Tugen Hills of north central Kenya mandates a revision of the hominoid
genus Kenyapithecus, a possible early member of the great ape–human clade.
The Tugen Hills specimen represents a new genus, which also incorporates all
material previously referable to Kenyapithecus africanus. The new taxon is
derived with respect to earlier Miocene hominoids but is primitive with respect
to the younger species Kenyapithecus wickeri and therefore is a late member
of the stem hominoid radiation in the East African Miocene.
An important issue in hominoid systematics
concerns the origin of the great ape and human
clade. Estimated divergence times among the
lineages of extant great apes and humans based
on comparative genetics suggest that the last
common ancestor of this clade may have lived
during the Middle Miocene (about 16 to 11
million years ago) (1). The African Middle
Miocene hominoid Kenyapithecus has been
considered to be either an early member of the
clade or its sister taxon (2–4). Most recent
analyses, however, consider Kenyapithecus to
be too primitive to be closely related to extant
great apes and humans (1,5–7).
Two species of Kenyapithecus are cur-
rently recognized: K. wickeri, from the type
locality at Fort Ternan in western Kenya, and
K. africanus, from several localities in west-
ern Kenya, the Tugen Hills, and Nachola in
the Samburu region (Fig. 1). All sites produc-
ing fossils referable to the genus range in age
between 15.5 and ⬃14 million years ago. The
fossils from Fort Ternan, at ⬃14 million
years, are younger in age than all known K.
africanus specimens. The genus Kenyapithe-
cus has been controversial since its initial
diagnosis (8), in part because of the small
sample of K. wickeri specimens but also be-
cause of a paucity until recently of similarly
aged large hominoid fossils from Africa and
elsewhere. Consequently, the congeneric sta-
tus of K. wickeri and K. africanus, as well as
hypotheses that place either of these taxa in
the ancestry of modern apes and humans have
been questioned (1,5–7,9–14).
Here we describe a partial hominoid skel-
eton from locality BPRP 122 at Kipsaramon,
a Middle Miocene site complex in the Mu-
ruyur Formation that is exposed along the
northern crest of the Tugen Hills, west of
Lake Baringo in central Kenya. The skeleton,
KNM-TH 28860, provides new evidence re-
garding the taxonomic diversity and phyloge-
netic relationships of Middle Miocene homi-
noids in Africa.
KNM-TH 28860 is the first Middle Mio-
cene hominoid with associated teeth and
postcranial remains (Figs. 2 and 3; Table 1).
The specimen includes most of a mandible
preserving all teeth except the right central
incisor, right canine, and right second molar.
Also included are the left maxillary central
incisor and both lateral incisors. Postcranial
elements include portions of the scapula and
sternum, a clavicle, numerous rib fragments,
most of the right humerus and the head of the
left humerus, a complete right radius, half of
the right ulna and parts of the left ulna and
radius, five carpal bones, and portions of
several fingers. Also preserved are one com-
plete lower thoracic vertebra and other frag-
mentary thoracic vertebrae. Regressions of
dental and long bone dimensions on body
mass in a variety of extant primates (15)
suggest a body mass of approximately 27 kg.
The maxillary central incisor crown is
relatively broad mesiodistally in proportion
to its height (Fig. 2). There is a low but
distinct basal lingual tubercle and a distinct,
continuous lingual cingulum on the mesial,
distal, and basal margins. The I
2
crown is
highly asymmetrical, with a lingual cingulum
that “spirals” apically from the mesial to the
distal margins of the crown (Fig. 2). The
mandibular canine is low-crowned relative to
basal crown dimensions, and its size and
morphology indicate that KNM-TH 28860
1
Department of Anatomy, Northeastern Ohio Univer-
sities College of Medicine, Post Office Box 95, Roots-
town, OH 44272, USA, and Division of Biomedical
Sciences, Kent State University, Kent, OH 44242,
USA.
2
Department of Orthopaedic Surgery, North-
eastern Ohio Universities College of Medicine, Roots-
town, OH 44272, USA.
3
Department of Anthropology,
Yale University, New Haven, CT 06520, USA.
4
Depart-
ment of Oral Biology, College of Dentistry, University
of Illinois at Chicago, Chicago, IL 60612, USA.
5
Bilby
Research Center, Northern Arizona University, Flag-
staff, AZ 86011, USA.
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was a male (16). Molar buccal cingulae are
absent except for tiny remnants at the base of
the buccal cleft and on the mesiobuccal sur-
face of the protoconid of M
1
. The mandible
preserves a well-developed inferior trans-
verse torus and a long sublingual planum
(Fig. 3).
The sternebrae are broad and flat. The
preserved portions of the scapula are suffi-
cient to determine that the acromion project-
ed well beyond the glenoid and that the axil-
lary margin was longer than the vertebral.
The clavicle is not markedly twisted along its
long axis. The humerus has a posteriorly
oriented and flattened head. There is a pro-
nounced deltopectoral crest, the shaft is ret-
roflected, and the medial epicondyle is pos-
teriorly deflected and relatively small. The
ulnar olecranon is proximally elongate, the
radial notch faces laterally, and the styloid
process is long and contacts the proximal
carpal row. The hamate is distinctive in the
depth of the pit for the piso-hamate ligament
and the depth and degree of twist of the
triquetral groove. The metacarpal heads are
palmarly broad, and pits for the collateral
metacarpo-phalangeal ligaments are dorsal,
almost meeting in the midline. The shafts of
all digits are relatively gracile and minimally
curved. The body of the thoracic vertebra is
heart-shaped in proximal and distal views,
and there is a discernible ventral keel anteri-
orly. Overall, the morphology of the body
and position of the zygapophyses suggest a
long, flexible vertebral column.
The new skeleton from Kipsaramon, com-
bined with previously described specimens
from other sites, demonstrates that K. africa-
nus and K. wickeri represent two distinct
genera. The description of the new genus is as
follows: Order Primates Linnaeus 1758; Sub-
order Anthropoidea Mivart 1864; Superfam-
ily Hominoidea Gray 1825; Genus Equato-
rius gen. nov. Diagnosis: Mandible with
well-developed inferior transverse torus,
proinclination of the sublingual planum, and
robust corpora; mandibular canine low-
crowned in relation to basal dimensions;
mandibular molar length sequence M
1
⬍
M
2
⬍M
3
;I
1
relatively broad mesiodistally in
relation to height, with a broad but low lin-
gual tubercle and a distinct, continuous, but
low-relief lingual cingulum and marginal
ridges; I
2
with a highly asymmetric mesial-
to-distal “spiraled” lingual cingulum; mini-
mal expression of cingulae on both maxillary
and mandibular premolars and molars; max-
illary premolars buccolingually and mesio-
distally expanded relative to M
1
; reduction of
maxillary premolar cusp heteromorphy; low
broad origin of the zygomatic root off the
alveolar process and expanded pneumatiza-
tion of the maxillary alveolar process to in-
clude the premolar segment. The general
body plan is similar to that of large Early
Miocene hominoids, with a long flexible ver-
tebral column; a humerus with a small, flat-
tened, and posteriorly oriented head and pos-
teriorly retroflected shaft; ulnar styloid con-
tact with carpus; and free os centrale. It dif-
fers from Proconsul and Afropithecus in
forelimb features relating to elbow and fore-
arm mobility, including a proximal radio-
ulnar joint facing more laterally. The femur
has a high neck/shaft angle and relatively
small femoral head; there is a robust, ad-
ducted hallux.
Etymology: The name reflects the prox-
imity to the equator of all localities from
which the genus has been recovered. Type
species:Equatorius africanus (Leakey,
1962). Diagnosis: As for genus. Holotype:
BMNH M 16649, a partial left maxilla with
P
3
-M
1
and roots of M
2
.Hypodigm: Large
hominoids previously referred to K. africanus
from the Maboko Formation on Maboko Is-
land, Ombo, Majiwa, Nyakach, and Kaloma.
Also assigned to Equatorius africanus are
large hominoids from the Aiteputh and Na-
chola Formations at Nachola and the Mu-
ruyur Formation in the Tugen Hills (17).
Junior synonyms: Proconsul africanus Ma-
cInnes 1943; Sivapithecus africanus LeGros
Clark and Leakey 1951; Dryopithecus (Siva-
pithecus) sivalensis Simons and Pilbeam
1965; K. africanus Leakey 1967; Gripho-
pithecus africanus Begun 1987. Discussion:
Equatorius shares a number of features with
Proconsul and Afropithecus that are primitive
for large hominoids. These include the mor-
phology of the sternum, proximal humerus,
distal ulna, scaphoid/centrale complex, and
vertebral column. All three genera possess
the primitive molar size sequence M
1
⬍
M
2
⬍M
3
and mandibular canines that are
relatively low-crowned with respect to basal
crown dimensions, with roots that converge
toward the midline. Features shared with
Afropithecus alone include the lingual mor-
phology of the maxillary central incisor, a
distinct, inferior symphyseal torus on the
mandible, and a robust mandibular corpus.
Hindlimb bones attributable to Equatorius
from Maboko and Nachola are similar in
most respects to those of Afropithecus and
Proconsul, with the exception of the proxi-
mal femur, lateral femoral condyle, distal
fibula, and hallux (5,18–20).
Equatorius also manifests an array of de-
rived features, which collectively distinguish it
from the Early Miocene genera. Premolar and
molar cingulae are much reduced (6). This
latter feature also distinguishes Equatorius
from Griphopithecus, known from the Middle
Miocene of southeastern Europe (21). The
maxillary premolars of Equatorius, although
enlarged relative to the first molars as in Afro-
pithecus, are metrically and morphologically
distinct from the latter. Equatorius premolars
from Maboko and Kipsaramon are longer rela-
tive to breadth than are those of Afropithecus,
and cusp height is more nearly equal. The de-
ciduous third premolar preserved in a juvenile
Equatorius mandible from Maboko (KNM-MB
20573) possesses a distinct metaconid and lacks
significant cingulum formation (22). Derived
features in the postcranial skeleton of Equato-
rius relative to Early Miocene hominoids in-
clude (in addition to those enumerated in the
diagnosis) a robust and relatively straight clav-
icle, reduced and posteriorly deflected humeral
medial epicondyle, reduced radial fossa relative
to the coronoid fossa on the distal humerus,
ulna with heavily buttressed coronoid process
and strongly developed supinator crest (23),
Fig. 1. Digital elevation models
showing the geography of Middle
Miocene hominoid localities in
present-day Kenya, with northern
Kenya and the adjacent Uganda
highlands indicated in the inset
area below and shown expanded
to the right. The Maboko-Ma-
jiwa-Ombo-Kaloma site cluster
has been slightly expanded for
clarification.
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dorsal position of metacarpo-phalangeal liga-
ment pits, minimal to no sesamoid “fluting” on
the metacarpal heads, and a femoral head that
projects proximal to the greater trochanter.
Equatorius retains a number of primitive
characters for which K. wickeri expresses a
derived condition. The type maxilla of E.
africanus differs from that of Kenyapithecus,
KNM-FT 46, in having a lower origin of the
zygomatic root off the alveolar process of the
maxilla (4,9,11,13,14), a morphology
found also in maxillae from Nachola (24). In
addition, the maxillary sinus of BMNH M
16649 excavates the alveolar process to a
greater degree and, unlike the Fort Ternan
maxilla, extends into the premolar region. In
these features, Equatorius shares with all
Proconsul species and Afropithecus a similar
pattern of midfacial anatomy. These differ-
ences in maxillary alveolar and zygomatic
topography are important characters in defin-
ing the derived morphology of the younger
Fort Ternan material with respect to earlier
Equatorius. The new material from the Tu-
gen Hills reinforces evidence from previous
collections that these differences extend to
the dentition as well.
Although there is not a uniform pattern of
character polarity in the dentition as a whole,
Kenyapithecus exhibits derived maxillary in-
cisor and mandibular canine morphologies.
The Kenyapithecus upper central incisor from
Fort Ternan, KNM-FT 49, has a singular
morphology for African Middle Miocene
hominoids (Fig. 4). The lingual marginal
ridges are massively inflated, to the point that
they begin to envelop the more basal part of
the lingual crown surface and obliterate the
foveae that typically flank the lingual tuber-
cle. Apically, the marginal ridges turn out
from the lingual surface of the crown at
abrupt, nearly 90° angles. This morphology
contrasts with the more primitive Equatorius
configuration, in which the lingual topogra-
phy is more muted and a lingual tubercle is
set off from the gradually emergent, low-
relief marginal ridges by distinct foveae (25).
There is also an upper lateral incisor from
Fort Ternan, KNM-FT 3637, that lacks the
spiral lingual cingulum characteristic of
Equatorius from Maboko and Kipsaramon
(Fig. 4). Instead, it has a nearly symmetrical
arrangement of the lingual cingulum and a
narrow, median lingual pillar. Determining
character polarity in lateral incisor morphol-
ogy is difficult, but a morphology similar to
that of Equatorius is found in Early Miocene
Fig. 2. KNM-TH 28860. (A) Left mandibular corpus. (B) Left maxillary central incisor. (C) Right
maxillary lateral incisor. (D) Right mandibular corpus fragment. (E) Right scapula. (F) Right clavicle.
(G) Left proximal humerus. (H) Right humerus with first rib attached. (I) Right hand (hamate;
trapezium; trapezoid; scaphoid; pisiform; metacarpals II, III, and V; and phalanges). (J) Sternum. (K)
Right radius. (L) Right proximal ulna. (M) Right distal ulna. (N) Lowest thoracic vertebra.
Fig. 3. Mandibular dentition of KNM-TH 28860.
(A) Left mandibular corpus with C-M
3
.(B) Sym-
physeal fragment with left I
1
and I
2
and right I
2
.
(C) Right corpus fragment with P
4
and M
1
.
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Proconsul major and perhaps P. africanus.
A male lower canine from Fort Ternan,
KNM-FT 28, is high-crowned in relation to
its mesiodistal length. This is an unusual
morphology among Early and Middle Mio-
cene hominoids and sets Kenyapithecus apart
from nearly all contemporaneous and earlier
genera (26). This morphology contrasts with
the primitive, relatively blunt male canine
known for Equatorius from Kipsaramon and
Nachola (10) and for Afropithecus and Procon-
sul (Fig. 5), but it is similar to the canines of
Late Miocene hominoids such as Dryopithecus
and Lufengpithecus (27). There are no male
lower canines available from Maboko. Single
female upper canines are known from both Fort
Ternan and Maboko, but the Maboko canine is
heavily worn distally, making comparison un-
reliable. There is a relatively unworn male up-
per canine of Kenyapithecus (KNM-FT 39), but
the few male upper canines available for Equa-
torius are all heavily worn or broken, again
precluding meaningful comparison.
Lower third premolars of Kenyapithecus
from Fort Ternan retain a distinct, continuous or
nearly continuous lingual cingulum, whereas
those of Equatorius are more derived, having
only small vestiges of cingulae. An upper
fourth premolar in the type maxilla of K. wick-
eri has an unusual morphology (28). The buccal
and lingual cusps are united by a prominent
crest, which divides the occlusal surface into
mesial and distal foveae. The crest is divided in
the middle, creating a small central fovea. Up-
per fourth premolars of Equatorius have a more
or less continuous mesiodistally oriented fissure
separating the two cusps. One or more crests
are variably developed, but these crests never
unite to obliterate completely the fissure and
partition the occlusal surface into mesial and
distal foveae.
A distal humerus from Fort Ternan
(KNM-FT 2751) is the only postcranial spec-
imen considered likely to be attributable to K.
wickeri. It is derived toward the great ape
condition in the depth of the olecranon fossa,
well-developed lateral supracondylar crest,
and in details of the capitular and trochlear
anatomy, including a well-developed zona
conoidea (5,28).
Most investigators have viewed the mor-
phology preserved in the type maxilla of E.
africanus as primitive for large hominoids
and that present in the Fort Ternan maxilla as
derived toward the extant great ape condition
(4,9,11,13,14). An alternative is that the
differences in the E. africanus and K. wickeri
maxillae fall within an acceptable range of
variation for a single genus and that the K.
wickeri morphology is not more derived (6).
This argument would extend to the apparent-
ly derived features of the K. wickeri dentition
as well. Although the number of presumed
derived features of K. wickeri is fairly large,
the very small sample and consequent limited
variation at Fort Ternan have made it difficult
to refute the alternative view. However, the
overall morphological pattern of K. wickeri is
now also known to be present in one of the
two species represented at the Middle Mio-
cene site of Pas¸alar in Turkey.
The more common of the two Pas¸alar
species has been assigned to G. alpani,
whereas the other has not yet been named
(29–31). This second species possesses the
derived, high-crowned lower canine (32) and
a variant of the highly derived upper central
incisor morphology expressed by K. wickeri
(29,33). It lacks the spiraled upper lateral
incisor morphology found in Equatorius (33)
and instead has an I
2
morphology resembling
that of K. wickeri. It also shares with K.
wickeri features of the premolar dentition
(33) and the maxilla (31). The presence of the
presumed autapomorphic features of K. wick-
eri in a second Middle Miocene hominoid
species supports the interpretation that Keny-
apithecus sensu stricto is derived with respect
to Equatorius and that morphological differ-
ences between the Fort Ternan samples and
the other African Middle Miocene samples
are not artifacts of small sample size. Thus,
Kenyapithecus as it has been constituted is
paraphyletic. Separating K. wickeri and E.
africanus at the generic level is required to
maintain Kenyapithecus as a monophyletic
taxon.
Recognition of the generic distinctiveness of
Equatorius and Kenyapithecus supports the
view that catarrhine diversity in the Middle
Miocene was considerable (13). The higher tax-
onomic affiliations of Equatorius are still un-
certain, but our work supports the suggestion
that it is a derived member of the tribe Afro-
pithecini (12), generally regarded to be stem
hominoids. Kenyapithecus is more derived to-
ward later Miocene hominoids and extant great
Fig. 4. Middle Miocene hominoid maxillary in-
cisor morphology. All teeth are shown in lingual
view. The upper row shows maxillary central
incisors, and the lower row shows maxillary
lateral incisors. (A) KNM-TH 28860. (B) KNM-
MB 104. (C) KNM-FT 49. (D) KNM-TH 28860.
(E) KNM-MB 9729. (F) KNM-FT 3637. (A) and
(D) are from Kipsaramon; (B) and (E) are from
Maboko; (C) and (F) are from Fort Ternan. Note
the marked expansion of the marginal ridges on
(C) and the “spiral” cingulum (arrows) on (D)
and (E).
Fig. 5. Mandibular canine crowns in lingual
view. (A) KNM-WK 17010 from Kalodirr (Afro-
pithecus). (B) KNM-TH 28860 from Kipsar-
amon. (C) KNM-FT 28 from Fort Ternan. All
teeth are unworn and are aligned on a horizon-
tal plane that passes through their mesial and
distal cervixes. (B) has been reversed to facili-
tate comparison with the other teeth. Note the
blunter, more robust crowns and greater lingual
relief in (A) and (B) when compared with (C).
Table 1. Dental and selected postcranial measure-
ments of KNM-TH 28860. MD, mesiodistal; BL,
bucco-lingual; L, length; AP, anteroposterior; ML,
mediolateral. All measurements are in millimeters.
AP and ML dimensions were taken at mid-shaft.
Maxillary teeth
MD BL
I
1R
10.2 7.5
I
2L
6.5 7.1
Mandibular teeth
I
1L
5.0 6.1
I
2L
5.5 8.3
I
2R
4.9 8.2
C
L
12.4 9.5
P
3L
8.8 12.9
P
3R
8.8 12.7
P
4L
8.0 8.2
P
4R
7.5 8.5
M
1L
⫺8.9
M
1R
9.9 8.5
M
2L
11.3 10.5
M
3L
13.9 11.3
M
3R
14.4 11.2
Postcranials
LAPML
Humerus* 225 18.3 13.8
Ulna†218 10.9 13.8
Radius 193 8.5 10.7
*Humerus length was estimated by aligning the left proxi-
mal humerus with the shaft of the right humerus and
correlating the position of deltopectoral crests. †Ulna
length was determined by creating a composite ulna using
the right and left partial ulnae and then calibrating the
composite using the proximal and distal radio-ulnar articu-
lations as landmarks.
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apes and provides evidence of a link between
African and Eurasian hominoids in the Middle
Miocene (34).
References and Notes
1. D. Pilbeam, Mol. Phylogenet. Evol.5, 155 (1996).
2. S. C. Ward and D. R. Pilbeam, in New Interpretations
of Ape and Human Ancestry, R. L. Ciochon and R. S.
Corrucini, Eds. (Plenum, New York, 1983), pp. 325–
351.
3. L. Martin, in Major Topics in Primate and Human
Evolution, B. Wood, L. Martin, P. Andrews, Eds. (Cam-
bridge Univ. Press, Cambridge, 1986), pp. 151–187.
4. P. Andrews and L. B. Martin, J. Hum. Evol.16, 101
(1987).
5. M. L. McCrossin and B. R. Benefit, in Integrated Paths
to the Past: Paleoanthropological Advances in Honor
of F. C. Howell, R. S. Corrucini and R. L. Ciochon, Eds.
(Prentice-Hall, New York, 1994), pp. 95–122.
6. M. L. McCrossin and B. R. Benefit, in Function, Phy-
logeny, and Fossils, D. R. Begun, C. V. Ward, M. D.
Rose, Eds. (Plenum, New York, 1997), pp. 241–267.
7. D. R. Begun, C. V. Ward, M. D. Rose, in Function,
Phylogeny, and Fossils, D. R. Begun, C. V. Ward, M. D.
Rose, Eds. (Plenum, New York, 1997), pp. 389–415.
8. L. S. B. Leakey, Ann. Mag. Nat. Hist.13, 689 (1962).
9. M. Pickford, J. Hum. Evol.14, 113 (1985).
10. 㛬㛬㛬㛬 ,Z. Morph. Anthropol.76, 117 (1986).
11. 㛬㛬㛬㛬 ,inPrimate Evolution, vol. 1, J. G. Else and
P. C. Lee, Eds. (Cambridge Univ. Press, Cambridge,
1986), pp. 163–171.
12. P. Andrews, Nature 360, 641 (1992).
13. T. Harrison, Primates 33, 501 (1992).
14. D. R. Begun, Yrbk. Phys. Anthropol.37, 11 (1994).
15. C. B. Ruff, J. Hum. Evol.17, 687 (1988); K. L. Rafferty,
A. Walker, C. B. Ruff, M. D. Rose, P. Andrews, Am. J.
Phys. Anthropol.97, 391 (1995); G. Conroy, Int. J.
Primatol.8, 115 (1987).
16. J. Kelley, Am. J. Phys. Anthropol.96, 365 (1995).
17. Preliminary descriptions of large hominoid specimens
from Nachola (20,23,24) and differences in the
morphology of hominoid humeri from Maboko (35)
suggest that material herein assigned to Equatorius
may comprise more than one species.
18. M. D. Rose, in Function, Phylogeny, and Fossils,D.R.
Begun, C. V. Ward, M. D. Rose, Eds. (Plenum, New
York, 1997), pp. 79–100.
19. C. V. Ward, in Function, Phylogeny, and Fossils,D.R.
Begun, C. V. Ward, M. D. Rose, Eds. (Plenum, New
York, 1997), pp. 101–130.
20. M. Nakatsukasa, A. Yamanaka, Y. Kunimatsu, D.
Shimizu, H. Ishida, J. Hum. Evol.34, 657 (1998).
21. The entire type series of G. darwini from Devı´nska
Nova´ Ves, Slovakia, consists of four isolated molar
teeth, which limits comparisons with other taxa.
However, the two lower molars in the series (includ-
ing the type specimen) have prominent buccal cin-
gulae, a primitive hominoid character that is rare in E.
africanus (6).
22. M. L. McCrossin and B. R. Benefit, Proc. Natl. Acad.
Sci. U.S.A. 90, 1962 (1993).
23. M. D. Rose, Afr. Std. Monogr. Suppl.24, 3 (1996).
24. H. Ishida, M. Pickford, H. Nakaya, Y. Nakano, ibid. 2,
73 (1984).
25. McCrossin and Benefit (6) propose that new upper
central incisors from Maboko extend the range of
variation from the site to encompass the morphology
of the Fort Ternan incisor. However, details of these
specimens have not yet been described, and we
emphasize that the morphology of the several avail-
able incisors from Maboko, Majiwa, and Kipsaramon
is remarkably uniform and discretely different from
that of K. wickeri at Fort Ternan.
26. J. Kelley, Am. J. Phys. Anthropol.96, 390 (1995).
27. KNM-FT 28 has an index of relative canine height
(crown height divided by maximum mesiodistal
length) of 1.62. The value for the canine belonging to
KNM-TH 28860 is 1.35. Ranges of values and sample
sizes for Proconsul and Afropithecus are, respectively,
1.40 to 1.50 (9) and 1.14 to 1.18 (2); for Dryopithecus
and Lufengpithecus values are 1.54 to 1.61 (3) and
1.56 to 1.85 (7).
28. P. Andrews and A. Walker, in Human Origins: Louis
Leakey and the East African Evidence, G. Ll. Isaac and
E. R. McCown, Eds. ( W. A. Benjamin, Menlo Park, CA,
1976), pp. 279–304.
29. L. Martin and P. Andrews, in Species, Species Con-
cepts, and Primate Evolution, W. H. Kimbel and L.
Martin, Eds. (Plenum, New York, 1993), pp. 393–427.
30. P. Andrews, T. Harrison, E. Delson, R. L. Bernor, L.
Martin, in The Evolution of Western Eurasian Neogene
Mammal Faunas, R. L. Bernor, V. Fahlbusch, H.-W.
Mittman, Eds. (Columbia Univ. Press, New York,
1996), pp. 168–206.
31. J. Kelley, unpublished data. Features by which the
two species can be differentiated have been identi-
fied in all of the anterior teeth, all the premolar teeth
except P
4
, and the maxilla.
32. Two distinct lower canine morphologies are now
recognized in the Pas¸alar sample. Canines belonging
to the unnamed species are uniformly higher
crowned in relation to length than those assigned to
G. alpani.
33. B. Alpagut, P. Andrews, L. Martin, J. Hum. Evol.19,
397 (1990).
34. Andrews (12) proposed phyletic links between Keny-
apithecus and all Turkish Middle Miocene taxa. We
would restrict this relationship to the second, un-
named species at Pas¸alar.
35. M. L. McCrossin, Am. J. Phys. Anthropol. Suppl.24,
164 (1997).
36. This research forms part of the work of the Baringo
Paleontological Research Project (BPRP), in collabo-
ration with the National Museums of Kenya. We
thank the Office of the President of the Republic of
Kenya for permission to carry out research in Kenya.
BPRP has been supported by grants to A.H. from NSF
(SBR-9208903), the Louise H. and David S. Ingalls
Foundation, the Louise I. Brown Foundation, Clayton
Stephenson, and Yale University. J.K. acknowledges
support from NSF grant SBR-9408664 and thanks B.
Alpagut who kindly gave permission to study the
Pas¸alar hominoids. We thank E. Mbua and the staff of
the National Museums of Kenya for their support; P.
Andrews, T. Harrison, and D. Pilbeam for valuable
insights regarding the relationships of Kenyapithecus;
B. Kimeu, who discovered KNM-TH 28860; K. Cheboi
for assistance in the field; and S. McBrearty, who
suggested the generic name. L. Anderson, J. Kingston,
and M. Tomasco prepared the figures.
29 June 1999; accepted 30 July 1999
Fossil Plants and Global
Warming at the
Triassic-Jurassic Boundary
J. C. McElwain,* D. J. Beerling, F. I. Woodward
The Triassic-Jurassic boundary marks a major faunal mass extinction, but
records of accompanying environmental changes are limited. Paleobotanical
evidence indicates a fourfold increase in atmospheric carbon dioxide concen-
tration and suggests an associated 3° to 4°C “greenhouse” warming across the
boundary. These environmental conditions are calculated to have raised leaf
temperatures above a highly conserved lethal limit, perhaps contributing to the
⬎95 percent species-level turnover of Triassic-Jurassic megaflora.
The end-Triassic mass extinction event
[205.7 ⫾4 million years ago (Ma)] was the
third largest in the Phanerozoic, resulting in the
loss of over 30% of marine genera (1) and 50%
of tetrapod species (2), ⬎95% turnover of
megafloral species (3,4), and marked micro-
floral turnover in Europe (5) and North Amer-
ica (6). Many causal mechanisms have been
suggested (7), but because of the paucity of
Triassic-Jurassic (T-J ) oceanic sediments suit-
able for geochemical analyses (7–9), the asso-
ciated environmental conditions remain poorly
characterized and the causal mechanism or
mechanisms equivocal. Here we provide a tem-
porally detailed investigation of the atmospher-
ic CO
2
and climatic conditions associated with
the T-J mass extinctions, from analyses of the
ecophysiological characteristics of fossil mega-
floras preserved in composite terrestrial T-J sec-
tions in Jameson Land, East Greenland (10),
and Scania, southern Sweden (11).
Evidence from sedimentary facies (12),
paleosols (13), sea-level change (14), and
flood basalt volcanism (15) all indirectly sug-
gest perturbation of the T-J global C cycle.
Therefore, likely atmospheric CO
2
variations
across the T-J boundary were determined
from the stomatal density (SD, number of
pores per unit area) and stomatal index (SI,
proportion of pores expressed as a percentage
of epidermal cells) of fossil leaf cuticles from
7 genera from 16 beds in Jameson Land and
11 genera from 13 beds in Scania. SD and SI
are inversely related to the ambient CO
2
con-
centration during growth (16,17) and can be
used to reconstruct geological time series of
atmospheric CO
2
(18,19). The standardized
SD and SI records for both localities were
obtained and corrected for changes in species
composition between individual beds to re-
move taxonomic bias (20) (Fig. 1).
High-resolution records of SD and SI
from both sites showed reductions during the
middle Rhaetian persisting into the Hettang-
ian and then returning to preexcursion values
(Fig. 1). The reductions are consistent with an
increase in atmospheric CO
2
concentration
across the T-J boundary, in agreement with
inferences from geochemical analyses of fos-
Department of Animal and Plant Sciences, University
of Sheffield, Sheffield, S10 2TN, UK.
*To whom correspondence should be addressed. E-
mail: J.McElwain@sheffield.ac.uk
REPORTS
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