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The Face of SiamopithecusNew Geometric-Morphometric Evidence for Its Anthropoid Status.

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THE ANATOMICAL RECORD 292:1734–1744 (2009)
The Face of Siamopithecus: New
Geometric-Morphometric Evidence for
Its Anthropoid Status
CHRISTOPH P.E. ZOLLIKOFER,1* MARCIA S. PONCE DE LEÓN,1
YAOWALAK CHAIMANEE,2 RENAUD LEBRUN,1 PAUL TAFFOREAU,3
SASIDHORN KHANSUBHAAND,2 AND JEAN-JACQUES JAEGER4
1
Anthropological Institute, University of Zürich, Zürich, Switzerland
2
Department of Mineral Resources, Paleontological Section, Bureau of Paleontology and
Museum, Bangkok, Thailand
3
European Synchrotron Radiation Facility, Grenoble, France
4
Institut International Paléoprimatologie et Paléontologie Humaine, Evolution
et Paléoenvironments, Université de Poitiers, Poitiers, France
ABSTRACT
Amphipithecids assume a key position in early primate evolution in
Asia. Here we report on new maxillofacial and associated mandibular
remains of Siamopithecus eocaenus, an amphipithecid primate from the
Late Eocene of Krabi (Thailand) that currently represents the most
complete specimen belonging to this group. We used synchrotron microtomography and techniques of virtual reconstruction to recover the three-dimensional morphology of the specimen. Geometric-morphometric analysis
of the reconstructed specimen within a comparative sample of recent and
fossil primates clearly associates Siamopithecus with the anthropoids. Like
modern anthropoids, Siamopithecus displays a relatively short face and
highly convergent and frontated orbits, the lower rim of which lies well
above the alveolar plane. The cooccurrence of spatially correlated anthropoid features and classical anthropoid dental characters in one individual
represents a strong argument to support the anthropoid status of Siamopithecus. It is, thus, highly unlikely that amphipithecids are specialized adapiforms exhibiting complete convergence with anthropoids. Anat Rec,
C 2009 Wiley-Liss, Inc.
292:1734–1744, 2009. V
Key words: primate
evolution;
virtual
reconstruction;
geometric morphometrics; synchrotron tomography;
Amphipithecidae
Siamopithecus eocaenus from the late Eocene lignite
deposits of Krabi (Southern Thailand) was described as
a new genus and species (Chaimanee et al., 1997) and,
subsequently, attributed to Amphipithecidae (Jaeger
et al., 1998). Based on a suite of mostly dental diagnostic
characters, S. eocaenus was identified as an anthropoid
of relatively large body size (Chaimanee et al., 1997;
Chaimanee et al., 2000a; Egi et al., 2004). Various
derived dental features suggest, on a functional level,
dietary specialization (Chaimanee, 2004; Kay et al.,
2004a). Siamopithecus shares its anthropoid characters
with Pondaungia, Amphipithecus, and Myanmarpithecus, three late middle Eocene primate genera, from the
Pondaung Formation in Myanmar (Chaimanee et al.,
1997; Takai et al., 2001; Chaimanee, 2004). These SouthC 2009 WILEY-LISS, INC.
V
Additional Supporting Information may be found in the
online version of this article.
Grant sponsors: Swiss National Science Foundation, The Fyssen
Foundation, The Department of Mineral Resources (Bangkok), The
C.N.R.S.-T.R.F. Biodiversity project, The C.N.R.S. ‘‘Eclipse-1 & 2’’
Research Program, and The ESRF; Grant sponsor: Swiss NFS;
Grant numbers: N 205321-102024/1and 205320-109303/1.
*Correspondence to: Christoph P.E. Zollikofer, Anthropological
Institute, University of Zürich Winterthurerstrasse 190, CH8057 Zürich, Switzerland. Fax: þ41 44 635 6886. E-mail: zolli@
aim.uzh.ch
Received 12 May 2009; Accepted 15 June 2009
DOI 10.1002/ar.20998
Published online 28 August 2009 in Wiley InterScience (www.
interscience.wiley.com).
SIAMOPITHECUS RECONSTRUCTION
1735
Fig. 1. S. eocaenus specimen TF 7624-7625 from the Late Eocene locality of Krabi, Thailand. A: Associated maxillary and mandibular remains before physical preparation (site photograph). B: Occlusal view
of the maxillae following preparation. Scale bar is 1 cm.
east Asian primates exhibit an array of peculiar synapomorphies, which warrant their inclusion in Amphipithecidae (Jaeger et al., 1998; Chaimanee et al., 2000a;
Beard, 2002; Chaimanee, 2004; Jaeger et al., 2004; Takai
and Shigehara, 2004).
Consensus has now been reached concerning the
monophyly of that group (Kay et al., 2004b; Jaeger and
Marivaux, 2005; Marivaux et al., 2005; Seiffert et al.,
2005). Jaeger et al. (2004) argue that the two largest
taxa, Amphipithecus and Pondaungia, display no significant morphological difference other than size (but see
Takai and Shigehara, 2004, for an opposing view), such
that they might represent males and females of a single,
sexually dimorphic species. Pronounced sexual dimorphism, which among primates is only known in anthropoids (Kelley and Qinghua, 1991; Simons and Plavcan,
1999; Simons et al., 2007), would thus represent an
additional anthropoid character of amphipithecids.
In an alternative interpretation, amphipithecids are
associated with the adapiforms (Kay et al., 2004a), specifically with the notharctids (Ciochon and Holroyd,
1994; Ciochon and Gunnell, 2004), and are seen as an
early evolutionary convergence with anthropoids (Gunnell and Miller, 2001; Kirk and Simons, 2001). Following
this ‘‘complete convergence’’ hypothesis, the hypothesized specialization to hard diets (Chaimanee et al.,
1997; Jaeger et al., 1998; Chaimanee, 2004) would represent a major evolutionary constraint shaping the entire
amphipithecid cranial morphology (Kay et al., 2004a).
However, an analysis of dental enamel microstructure
by means of synchrotron microtomography (SR-lCT)
suggested that, at least in Siamopithecus, a diet based
on hard food is improbable (Tafforeau, 2004): Siamopithecus has thin, radial enamel, which is not in accordance with high-pressure resistance implied by hard food
items.
The ‘‘complete convergence’’ hypothesis has other difficulties. For example, amphipithecids share no single
derived dentognathic character with notharctids (Kay
et al., 2004a). Another disputed point concerns a postcranial partial skeleton (humerus, calcaneus, and ulna fragment; NMMP 20), which has tentatively been associated
with the largest Pondaung amphipithecids and which is
described as exhibiting notharctid affinities (Ciochon
et al., 2001). However, a talus (NMMP 39) from the Pondaung Formation displays all the diagnostic derived
characters of anthropoids (Marivaux et al., 2003).
Because the talus is considered to represent a critical
postcranial element for primate phylogeny (Gebo et al.,
2000), the evidence from the Pondaung talus is in stark
contrast with that from the other postcranial elements.
The recent discovery of a diversified sivaladapid community in the Pondaung Formation (Beard et al., 2007;
Marivaux et al., 2008) renders more probable the hypothesis that the NMMP 20 postcranial remains belong
to a large and dentally still undocumented sivaladapid.
Moreover, a frontal bone fragment (NMMP 19), which
was ascribed to a small-sized Pondaung amphipithecid
(Gunnell et al., 2002; Shigehara et al., 2002; Takai et al.,
2003) and was considered to provide evidence for
absence of postorbital closure, is most likely of nonmammalian origin, as evinced by a detailed comparative
reanalysis of its anatomy (Beard et al., 2005).
Here, we examine the ‘‘anthropoid’’ versus ‘‘complete
convergence’’ hypotheses of amphipithecid origins in the
light of previously undescribed fossil evidence bearing
on Siamopithecus. The 1996 excavations in the Bang
Mark pit of the Krabi lignite mine yielded a new specimen of S. eocaenus, consisting of a right mandible (TF
7625; Chaimanee et al., 2000a) preserved in anatomical
occlusion with midfacial remains (TF 7624). The latter
comprise both maxillae including the hard palate and,
on the left side, parts of the orbital rim formed by the
zygomatic bone (Fig. 1). The upper dentition is represented on both sides by P3-M3 and the alveoli of P2 and
C. The premaxillary bone and incisors are lacking. The
specimen directly confirms the association of upper and
lower molars proposed earlier on the basis of isolated
gnathic elements (Chaimanee et al., 1997; Ducrocq,
1999) and permits a first view of amphipithecid facial
morphology.
This study has two aims: the first is to present the
results of a virtual reconstruction of the face of Siamopithecus and to identify key features of amphipithecid
facial morphology. The second is to perform a comparative geometric-morphometric analysis of its restored
morphology. Geometric-morphometric methods are independent of classical methods of craniodental character
1736
ZOLLIKOFER ET AL.
analysis, such that the phenetic analyses presented here
yield valuable complementary data to test taxonomic
and phylogenetic hypotheses.
MATERIALS AND METHODS
State of Preservation of the Specimen
Like many other fossils recovered from coal deposits,
the TF 7624/7625 specimen underwent distortion
through fragmentation and taphonomic compression.
Because physical preparation would involve unnecessary
risk and correction of plastic deformation is impractical,
we performed a virtual reconstruction. Given the small
size and high degree of mineralization of the TF 7624/
7625 remains, we used SR-lCT (Tafforeau et al., 2006)
with a monochromatic beam at 65 keV to acquire digital
volume data at an isotropic voxel size of 45.71 lm. When
compared with conventional lCT, the high-energy, highflux, monochromatic beam of SR-lCT has the advantage
of yielding cross-sectional images with high spatial and
contrast resolution and free of beam hardening artifacts
(Tafforeau et al., 2006; Fig. 2).
Semiautomated image segmentation procedures were
applied to separate virtual fossil parts along major
cracks and to diagnose the specimen’s external and internal state of preservation. The dental arcades of the
right and left maxillae remained undisturbed postmortem, with the exception of the left M3 (as evinced by
mirror-image matching of the two sides). The preserved
right mandibular corpus underwent substantial lateral
compression, which resulted in fracturing and mediolateral flattening. During this process, fragments were
crushed and driven apart in a vertical direction, such
that the original height of the mandibular corpus (except
for the symphyseal region) cannot be reconstructed with
confidence.
Virtual Reconstruction
Fig. 2. Synchrotron microtomographic parasagittal cross-section of
the right maxilla of S. eocaenus TF 7625. Scale bar is 1cm.
The isolated parts served as a basis for four virtual
reconstructions, which were carried out independently
by four team members (CZ, MPL, RL, PT), following the
general principles outlined in Zollikofer and Ponce de
Fig. 3. Stages of virtual reconstruction of Siamopithecus TF 7624/25
(reconstruction protocol 1; see text). A: establishment of dental occlusion (right
lateral view). B: Correction of deformation of the mandibular corpus (basal
view; transparent: distorted original morphology). C, D: correction of plastic
deformation in the right palatal area (transparent: distorted original morphology) and reposition of the left zygomaxillary fragment. Scale bar is 5 cm.
SIAMOPITHECUS RECONSTRUCTION
Fig. 4. Virtual reconstruction of S. eocaenus TF 7624-7625 following protocol 1 (see text). A: right lateral view; B: frontal view; C, D: superior views; E, F: inferior views. Colors indicate electronically isolated
parts; mirror-image completions are transparent; scale bar is 5 cm.
1737
1738
ZOLLIKOFER ET AL.
TABLE 1. Comparative sample
Family
Strepsirrhines
Cheirogaleidae
Indriidae
Lemuridae
Lepilemuridae
Galagidae
Lorisidae
Adapidaey
Notharctidaey
Archaeolemuridaey
Haplorrhines
Tarsiidae
Aotidae
Atelidae
Cebidae
Pitheciidae
Cercopithecidae
Hylobatidae
Pliopithecidaey
Omomyidaey
y
TABLE 2. Landmarks
N
18
8
9
9
16
24
7
1
6
4
6
11
17
10
41
4
1
1
Extinct.
León (2005). Reconstructions 1–3 started with reestablishment of dental occlusion between the well-preserved
right maxilla and the isolated mandibular teeth (Fig. 3).
Subsequently, the isolated fragments of the mandibular
corpus and ramus were adapted using dental positions
as a guide. The left and right maxillary halves exhibit
anatomical contact along the palate. The noticeable plastic deformation of the right side of the palate was corrected with reference to the better-preserved left side,
such that the maxillae could be oriented relative to the
midsagittal plane of the skull. Positioning the maxillae
with the reconstructed right mandible and its mirror
image in dental occlusion showed that the mandibular
symphysis was crushed mediolaterally during fossilization (Fig. 4). To recover the midfacial architecture of the
specimen, maxillary fragments on the right side were
repositioned through comparison with mirror-imaged
matching regions on the left side. The position and orientation of the orbital rim fragment was evaluated by
adapting a mirror image of the isolated left zygomatic
bone to the well-preserved right zygomatic process of the
maxilla and subsequent adjustment of the corresponding
fragmentary region on the left side. During reorientation
and relocation of the zygomatic bone, the mandibular
coronoid process served as an additional positional clue.
This structure must fit into the temporal fossa, the anterolateral extent of which is constrained by the zygomatic bone. In comparison with its displaced in situ
position, the reconstructed zygomatic fragment is elevated relative to the alveolar plane and assumes a more
anteroposterior orientation.
The fourth virtual reconstruction (Tafforeau 2004) also
started with the reconstruction of the mandible, but it
used undistorted mandibles of Pondaungia (NMMP 24)
and of Amphipithecus as templates to reconstruct the dental arch of Siamopithecus. This reconstruction is based on
the hypothesis that the specific ‘‘parabolic’’ shape of the
mandibular dental arch is a synapomorphy of amphipithecids. The vertical curvature (i.e., the shape of the occlusal
plane) was reconstructed according to the morphology of
the relatively undistorted Siamopithecus mandibular frag-
Number
Maxillofacial
1
2, 3
4, 5
6, 7
8, 9
10, 11
12, 13
14, 15
Mandibular
16
17
18, 19
20, 21
22, 23
24, 25
26, 27
Landmark definition
Prosthion
Frontomalare orbitale
Orbitale
Point between fmo and o
Buccalmost point on canine
Buccalmost point on P2 or P3
Buccalmost point on M1
Buccalmost point on M3
Infradentale
Gnathion
Buccalmost point on
Buccalmost point on
Buccalmost point on
Buccalmost point on
Foramen mentale
canine
P2 or P3
M1
M3
ment TF3634. The reconstructed right mandible was then
complemented with its mirror image, and the mediolateral inclination of the hemimandibles was adjusted
according to the NMMP24 mandible of Pondaungia. The
resulting mandibular reconstruction exhibits a narrower
symphyseal region compared with reconstructions 1–3.
The upper jaws were reconstructed with the better-preserved right maxilla and its mirror-image; these were
placed in anatomical position relative to each other and
put in occlusion with the mandibular dentition. Finally,
the left zygomatic bone was adapted to the maxillary
reconstruction and mirror imaged to the right side.
The consensus of reconstructions 1–3 is shown in Fig.
4 (individual reconstructions are shown in Supporting
Information Figs. S1 and S2). The spatial consistency of
dental occlusion in the reconstructive variants was
checked with 3D-hardcopies produced by means of 3Dprinting technology. Reconstructions 1–3 converged with
reconstruction 4 in an important feature: the shape of
the mandibular dental arcade (which resulted from dental occlusion with the undistorted right maxilla) is similar to that of the undistorted Pondaungia mandible,
which served as a starting point for reconstruction 4.
Reconstruction 4 resulted in some degree of anisognathy
(i.e., different widths of upper and lower dental arcs):
masticatory movements inferred from this reconstruction—as well as the radial enamel structure revealed by
SR-lCT—suggest a folivorous diet in Siamopithecus
(Tafforeau, 2004).
Morphometric Analysis
We used geometric-morphometric methods to assess
the phenetic position of the reconstructed Siamopithecus
morphology (as represented by the four reconstructive
variants) within a comparative sample of N ¼ 194 extant
and fossil prosimian and anthropoid primate skulls
(Table 1; details in Supporting Information Table S1).
Craniomandibular form was quantified with 15 maxillofacial and 12 mandibular anatomical landmarks, whose
location could be determined reliably on the virtual
reconstructions of Siamopithecus (Table 2).
Patterns of shape variation in the sample were analyzed using principal components (PC) analysis of shape
SIAMOPITHECUS RECONSTRUCTION
1739
Fig. 5. Principal components analysis of craniomandibular and cranial shape. A: craniomandibular shape variability along the first three
PCs. B: corresponding virtual morphologies at extreme values of PC1
(0.25; 0.21), PC2 (0.21; 0.25), and PC3 (0.15; 0.09). C: cranial
shape variability along the first three PCs. D: corresponding virtual
morphologies at extreme values of PC1 (0.22; 0.21), PC2 (0.22;
0.28), and PC3 (0.11; 0.22). Arrows in A and C show axes of sizerelated shape variation in strepsirrhines and anthropoids. In B and D,
crania of Lepilemur ruficaudatus AIMZ11054 (negative pole of PC1)
and Saimiri sciureus AIMZ9159 (all other cases) are used to represent
shape transformations. Grey/black symbols: strepsirrhines/haplorrhines. Horizontal rectangles: lemuriforms; vertical rectangles: lorisiforms; diamonds: Archaeolemur; open triangles: Adapinae; filled
triangle: Notharctus; stars: catarrhines; circles: platyrrhines; z: Tarsius;
þ: Microchoerus; square: Pliopithecus; X: four reconstructive variants
of Siamopithecus.
in Linearized Procrustes space (Dryden and Mardia,
1998), after shape variation was constrained to bilateral
symmetry (Zollikofer and Ponce de León, 2002). Analysis
and visualization of patterns of shape variation were
performed with the interactive geometric morphometrics
software package MorphoTools (Specht, 2007; Specht
1740
ZOLLIKOFER ET AL.
of reference. The orbital plane (O) is determined by
three landmarks on the zygomatic portion of the orbital
rim (frontomalare orbitale, orbitale, and a point between
these landmarks). The cranial midplane M is evaluated
by averaging the positions of bilateral landmark pairs
and calculating the plane through these points. The
alveolar plane A is defined as containing the landmarks
on maxillary canines and third molars (see Table 2 for
landmark definitions). Using a standard definition of
vector geometry, the orientation of planes O, M, and A is
given by the corresponding normal vectors nO, nM, nA
(Fig. 7B). These vectors permit measurement of orbital
convergence (angle between nO and nM) and frontation
(angle between nO and nA).
RESULTS
Fig. 6. Phenetic trees depicting morphological affinities of Siamopithecus with extant and extinct primate families. A: UPGMA phenetic
tree computed for the skull. B: UPGMA phenetic tree computed for
the cranium. Siamopithecus branches within anthropoids. Grey: prosimians (Strepsirrhini þ Tarsiiformes); black: anthropoids.
et al., 2007; Lebrun, 2008). Size-related shape variation
(size allometry) was quantified by evaluating allometric
shape vectors, which were obtained by multivariate
regression of PC scores on log centroid size. Because
strepsirrhine and anthropoid primates differ widely in
patterns of craniomandibular shape variation, groupspecific allometric shape vectors were evaluated. To
quantify morphological affinities between Siamopithecus
and the comparative primate sample, Procrustes shape
distances were computed between Siamopithecus and
extant and fossil primate taxa (for extant taxa, familyspecific mean shapes were used). By using PHYLIP (Felsenstein, 1989), the resulting distance matrix was represented as a phenetic tree.
To evaluate the Siamopithecus reconstruction within
the comparative framework of earlier studies using conventional distance- and angle-based morphometrics, we
derived various linear and angular measurements from
the 3D landmark data. The definitions of orbital frontation and convergence proposed by Ross (1995) cannot be
applied here directly, because the neurobasicranial and
medial orbital morphology of Siamopithecus is missing.
To calculate analogous variables, we used three planes
Graphing the first three PCs, which account for 74% of
the total shape variability in the sample, shows clear separation between prosimian (Tarsius þ strepsirrhines) and
anthropoid morphologies (Fig. 5A,B). This demonstrates
that a small but reliable set of maxillofacial and mandibular landmarks provides sufficient analytical sensitivity to
resolve taxonomically relevant morphological differences
in the sample. Similar analyses were performed on maxillofacial morphology alone (Fig. 5C,D), permitting inclusion of various fossil crania without associated mandibles,
such as those of early primates (Adapiformes and Omomyidae) and of subfossil lemurs (Archaeolemur). Inclusion
of Archaeolemur in the comparative sample is of interest,
because overall craniofacial form in this genus exhibits a
high degree of convergence with anthropoid primates
(Forsyth-Major, 1896).
In all analyses, Siamopithecus groups with the anthropoids (Figs. 4–6). As an additional check of consistency
of the virtual reconstruction, we tested whether Siamopithecus could be reconstructed to fit the morphology of
known fossil and extant nonanthropoid primates. To this
end, the Siamopithecus landmark configuration was
transformed into the closest possible hypothetical prosimian configuration, and the virtual fossil parts were
accommodated accordingly. All reconstructions toward
strepsirrhine morphologies led to anatomically impossible configurations, involving overlap and/or disruption of
anatomical continuity between neighboring fragments.
Among the fossil specimens, adapiform primates (Magnadapis, Leptadapis, Adapis, and Notharctus), whose
nonanthropoid status is undisputed, are more closely
associated with prosimians, whereas Pliopithecus groups
with the anthropoids. As expected, the archaeolemurs
present a morphology that is closer to that of anthropoids than other strepsirrhines, but all of them are
clearly distinct from anthropoids in shape space (Fig.
5C). Furthermore, Siamopithecus exhibits smaller shape
distances to most anthropoid families than to extant or
extinct prosimian families (Fig. 6).
Major morphological contrasts within the sample can
be visualized by shape transformation of an average primate face towards extreme shape values of the data
scatter along each PC (Fig. 5B,D). PC1, which accounts
for most of the differences between anthropoids and prosimians, reveals contrasts between the short-snouted,
comparatively narrow anthropoid face with a high mandibular corpus and orbital cavities that are elevated
above the alveolar plane and exhibit a high degree of
SIAMOPITHECUS RECONSTRUCTION
Fig. 7. Relative orbital dimensions and orbital orientation. A: relative
biorbital width (fmo: absolute biorbital width; M3C: distance between
buccal alveolar borders of M3 and C; M1: distance between buccal
alveolar borders of M1). B: orbital frontation versus orbital convergence (in degrees). Frontation (f) is the angle between nA and nO;
1741
convergence (c) is the angle between nO and nM (see text). Open/
closed circles: diurnal/nocturnal anthropoids; open/closed squares:
diurnal/nocturnal strepsirrhines. Diamonds: Archaeolemur. X: reconstructive variants of Siamopithecus; Y: Pliopithecus; Z: Tarsius; open
triangles: Adapinae; filled triangle: Notharctus; þ: Microchoerus.
1742
ZOLLIKOFER ET AL.
TABLE 3. Morphological characters of Siamopithecus and amphipithecidsa
Anthropoid characters of Siamopithecus eocaenus shared with large Amphipithecidae from Pondaung
Cingulum-derived hypocone
Almost continuous crista obliqua and short trigone’s basin
Well-developed hypoparacrista
Upper premolars unwaisted in occlusal view
Reduction or absence of labial cingula on molars
Deep horizontal branch of lower jaw
Strong bunodonty with molar talonid displaying nearly the same elevation as trigonid
Reduced, single-rooted P/2
P/3-P/4 exodaenodont and obliquely oriented
P/4 with well-developed metaconid cusp
Straight cristid oblique
Presence of X-facet on lower molars
Vertical symphysis
Synapomorphies of Amphipithecidae
Crest linking protocone to hypocone on upper molars
Very high horizontal branch of the mandible
Short dental rows indicating short muzzles
Parabolic tooth rows in occlusal view
Convex upper occlusal surface and corresponding concave lower occlusal surface
Wrinkled enamel
Absence of paraconid
Waisted lower molar outline in occlusal view
Reduced to absent hypoconulid
Slanted buccal and lingual upper molar walls
M/3 surface smaller than M/2
Entoconid reduced and distally displaced
Cingulids reduced to absent
Plesiomorphic characters of Amphipithecidae
Presence of P2/2
Unfused symphysis
a
Jaeger and Marivaux, 2005, Science 310:244–245; Seiffert et al., 2005, Science 310:300–304.
TABLE 4. Linear and angular dimensions of the virtual reconstruction of Siamopithecus
Method of measurement
Dimensiona
Bi-frontomolare orbitale (fmo)
Between alveolar midpoints
Between alveolar midpoints
Between lingual alveolar borders of M1
Between buccal alveolar borders of M1
Between buccal alveolar borders of M1
Infradentale-gnathion
See text
See text
(fmo-fmo)/(M1-M1)
(fmo-fmo)/(M3-C)
52.9 mm
29.7 mm
33.9 mm
20.6 mm
39.1 mm
35.6 mm
25.7 mm
81.6 degrees
72.1 degrees
1.35 mm
1.78 mm
Variable
Biorbital width
C-M3 (maxilla)
C-M3 (mandible)
Palate width (at M1)
Maxillary width at M1
M1-M1 buccal width (mandible)
Symphyseal height
Frontation
Convergence
Biorbital width rel. to bimaxillary width
Biorbital width rel. to maxillary length
a
Mean values of the four reconstructive variants.
convergence, as opposed to the long, low prosimian face
with a low mandibular ramus, and with orbits at the
level of the alveolar plane and exhibiting a low degree of
convergence. PC2 mostly accounts for allometric shape
variation in the sample, which influences the degree of
prognathism and of orbital frontation, while PC3
expresses a pattern of variation similar to that of PC2,
but independent of size.
Although the orbital morphology of Siamopithecus is
only partially preserved, it is possible to derive various
functionally relevant measurements. Siamopithecus had
orbits of moderate size compared with gnathic dimen-
sions (Fig. 7A), and it clearly falls within the anthropoid
range of variation of frontation and convergence.
DISCUSSION
In the context of the current discussion of an anthropoid versus strepsirrhine affiliation of the amphipithecids, the reconstructed face of Siamopithecus provides
new data on previously unknown aspects of amphipithecid morphology, which are relevant for taxonomic and
phyletic inferences. First and foremost, the shape of the
face of Siamopithecus, as quantified by a configuration
1743
SIAMOPITHECUS RECONSTRUCTION
of 3D anatomical landmarks, clearly falls within the variation displayed by extant and fossil anthropoids and
outside the variation displayed by extant and fossil prosimians. A suite of correlated characters can be identified in these analyses, which all group Siamopithecus
with anthropoids as follows:
1. The position of Siamopithecus in shape space (Figs. 5,
6) suggests that its overall facial morphology is close
to that of a generalized anthropoid.
2. The lower margin of the orbit is well above the alveolar
plane, as in most anthropoids, and unlike in prosimians.
3. The high degree of orbital frontation and convergence
inferred from the reconstruction (Fig. 4B and Table 3)
place Siamopithecus within the variation displayed by
anthropoid primates and outside the range of variation of prosimians.
4. Biorbital distance (Fig. 7A and Table 4) suggests that
the orbits were of moderate relative size and in the
range of diurnal anthropoids (Kay and Kirk, 2000).
5. The mandibular morphology of Siamopithecus is reminiscent of that of the undeformed Burmese amphipithecid specimens (Chaimanee et al., 2000b; Jaeger
et al., 2004). The mandibular dental arch displays a
parabolic shape between P3 and M3, the ramus is
high, and the canines were large (judging from the
preserved parts of the right lower canine and from
the reconstructed border of the alveolar socket of the
right upper canine).
The face of Siamopithecus is, thus, best described as
that of a generalized anthropoid. Moreover, the combination of facial features seen in Siamopithecus (short maxilla, high degree of orbital convergence and frontation,
and relatively small orbits well above the alveolar plane)
is also characteristic for moderate-sized extant anthropoid primates, such as cebids, atelids, langurs, and small
cercopithecines (e.g., Miopithecus). However, Siamopithecus differs from langurs and atelids by exhibiting
larger canines and a more robust zygomatic process.
The virtual reconstruction presented here also permits
a first comparative assessment of midfacial variability
within amphipithecids. The only other known amphipithecid midface—that of Pondaungia cotteri (NMMP 18;
Shigehara et al., 2002)—is less complete than that of S.
eocaenus, but several features can be compared directly.
In both specimens, the facial surface of the maxilla bulges
laterally and, in inferior view, overarches the posterior
dental arcade. The height of the maxillary body, as estimated from the distance between the alveolar margin and
the orbital rim, suggests a large maxillary sinus in both
specimens. A robust zygomatic process is also a feature
found in both amphipithecid specimens, but the root of
the zygomatic arch is located more mesially in Siamopithecus (above M1) than in Pondaungia (above M2), indicating a shorter face in the former species.
Combining the new morphometric data with classical
morphological evidence provides additional support for
the hypothesis that Siamopithecus, and its related Burmese amphipithecids are anthropoid primates. A list of
dentognathic characters of Siamopithecus and the large
amphipithecids compiled from previous publications
(Jaeger and Marivaux, 2005; Seiffert et al., 2005) is provided in Table 3. This character complex is indicative of
an anthropoid affiliation of the amphipithecids. Although
some of these ‘‘anthropoid’’ characters also occur in several prosimians (Ciochon and Gunnell, 2004), the large
array of concurrent anthropoid dentognathic features
shared by all amphipithecids is so far documented only for
undisputed anthropoid primates. Amphipithecids also exhibit an array of plesiomorphic features characteristic of
primitive primates (Table 3). Interestingly, however, some
of these features are also present in several Fayum late
Eocene anthropoids (Beard, 2002; Rasmussen, 2002) and
may be related to the greater geological age of these fossils compared with lower Oligocene Fayum crown anthropoids displaying more derived character states.
Our analyses also confirm that craniofacial convergence
of Archaeolemur with anthropoids is only superficial. The
archaeolemurid specimens examined here all have anteroposteriorly short faces; however, geometric-morphometric analysis clearly groups them with strepsirrhines. A
recent virtual reconstruction of the subfossil archaeolemurid Hadropithecus also displays a cranial morphology
reminiscent of anthropoids (Ryan et al., 2008), and it
remains to be examined whether this peculiar morphology
matches the pattern of morphological convergence
of archaeolemurids toward anthropoid morphologies
revealed by our geometric-morphometric analyses.
Overall, the ‘‘complete convergence’’ hypothesis of
amphipithecids toward anthropoids is less parsimonious
than the ‘‘anthropoid origins’’ hypothesis, and the postulated association of amphipithecids with prosimian primates seems to be an effect of incomplete fossil evidence
and of misattribution of some fragmentary cranial and
postcranial specimens. Additional comparative fossil evidence is, thus, required to better understand the phyletic relationships and the dietary and locomotor
specializations of this early Asian anthropoid group.
ACKNOWLEDGMENTS
We greatly acknowledge Dr. med. K. Geissmann’s support with medical CT. We thank the staff of beamlines
ID19 and ID17 (European Synchrotron Radiation Facility, ESRF), and Peter Wyss (EMPA) for help with microtomography. Special thanks to Matthias Specht for
collaborative implementation of MorphoTools. We thank
Suzanne Jiquel and Monique Vianey-Liaud (I.S.E.M.),
Edmée Ladier (Musée d’Histoire Naturelle de Montauban), Jacques Cuisin, Marc Godinot and Pascal Tassy
(Museum National d’Histoire Naturelle de Paris) for
access to primate specimens.
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