New Eocene nimravid carnivoran from Thailand

Abstract

Dental remains assigned to nimravid carnivores have been discovered in southern Thailand. These specimens come from the upper Eocene Krabi Basin that has already yielded numerous vertebrate taxa important for the knowledge of mammal evolution. The fossils described here are among the oldest remains belonging to the Nimravidae, and they are attributed to Nimravus cf. intermedius and Hoplophoneus sp. The occurrence of nimravid carnivores in Southeast Asia implies exchanges between Asia and North America during the Late Eocene, and it supports a larger geographical distribution and an origin of the family older than previously known.

Full text

157
Journal of Vertebrate Paleontology 20(1):157–163, March 2000
q
2000 by the Society of Vertebrate Paleontology
EOCENE NIMRAVID CARNIVORANS FROM THAILAND
STE
´PHANE PEIGNE
´
1
, YAOWALAK CHAIMANEE
2
, JEAN-JACQUES JAEGER
3
, VARAVUDH SUTEETHORN
2
, and
STE
´PHANE DUCROCQ
3
1
Laboratoire de Ge´obiologie, Biochronologie et Pale´ontologie Humaine, EP 1596 CNRS,
Faculte´ des Sciences Fondamentales et Applique´es, Universite´ de Poitiers, 40 avenue du Recteur Pineau,
86022 Poitiers cedex, France;
2
Department of Mineral Resources, Geological Survey Division, Rama VI Road, Bangkok 10400, Thailand;
3
Institut des Sciences de l’Evolution, UMR 5554 CNRS, Case 064, Universite´ de Montpellier II,
34095 Montpellier cedex 5, France
ABSTRACT—Dental remains assigned to nimravid carnivores have been discovered in southern Thailand. These
specimens come from the upper Eocene Krabi Basin that has already yielded numerous vertebrate taxa important for
the knowledge of mammal evolution. The fossils described here are among the oldest remains belonging to the Nim-
ravidae, and they are attributed to Nimravus cf. intermedius and Hoplophoneus sp. The occurrence of nimravid car-
nivores in Southeast Asia implies exchanges between Asia and North America during the Late Eocene, and it supports
a larger geographical distribution and an origin of the family older than previously known.
INTRODUCTION
The Nimravidae are sabre-toothed carnivores that lived in the
northern hemisphere from late Eocene to late Miocene (Bryant,
1991). Most of early authors (Cope, 1880; Matthew, 1910; Piv-
eteau, 1931; Hough, 1952; Ginsburg, 1979) placed them close
to the Felidae due to the cat-like morphology of their teeth,
their claws and their skull. The study of the basicrania brought
new insights on the phylogenetic relationships of the families
included within the Order Carnivora (Hunt, 1974, 1987, 1989,
1991; Tedford, 1976; Neff, 1983). For example, the peculiar
anatomy of the auditory region of the Nimravidae resulted in a
reappraisal of the relationships of the family among Carnivora
(Neff, 1983; Hunt, 1987; Flynn et al., 1988; Bryant, 1991).
Nonetheless, the taxonomic position of Nimravidae within Car-
nivora is still discussed (see Flynn et al., 1988; Bryant, 1991
for such a discussion).
North American localities have yielded most of the known
Paleogene nimravid specimens; hundreds of them have been
collected from late Eocene to late Oligocene sites. The Paleo-
gene European record is more sparse; most specimens come
from Quercy (Piveteau, 1931; Ginsburg, 1979; Bonis and Cirot,
1995; Peigne´ and Bonis, 1999) and the Aquitaine Basin in
France (Brunet, 1970, 1972; Ringeade and Michel, 1994a, b).
Compared to the American and European taxa, the Paleogene
Asian nimravids are poorly known, and include only some teeth
fragments (Chow, 1958; Ding et al., 1977; Gabounia, 1966;
Tang and Qiu, 1979) and a few mandibles from Mongolia as-
signed to Nimravus (Gromova, 1959; Toohey, 1959; Dashzev-
eg, 1996). Therefore, the discovery in the late Eocene of Thai-
land of new nimravid material is very important because it im-
proves our knowledge of the Nimravidae and of the paleobi-
ogeographical history of this family.
The material described in this note comes from the upper
Eocene Krabi Basin in southern Thailand (Fig. 1). This basin
has already yielded numerous new taxa that contributed to our
understanding of the relationships and the paleobiogeographical
history of anthracotheriid and suoid artiodactyls (Ducrocq,
1994a, b, 1997; Ducrocq et al., 1996, 1997, 1998), primates
(Ducrocq et al., 1995a; Chaimanee et al., 1997), megachirop-
terans (Ducrocq et al., 1993), dermopterans (Ducrocq et al.,
1992a) and carnivores (Ducrocq et al., 1992b). The new dental
remains described here have been collected in the Wai Lek pit
(Krabi Basin) in the main lignite seam that yielded almost all
of the mammal remains. The carnivore remains include speci-
mens that are among the oldest known nimravids, and they
provide the best evidence for the presence of nimravids in
Southeast Asia. Two genera are distinguished in the Thai ma-
terial. We confirm the presence of Nimravus in the late Eocene
of Asia (already noticed by Ducrocq et al., 1995b). The genus
Hoplophoneus is also recorded by dental remains of a young
individual with dental morphology close to those of the most
primitive Hoplophoneus species from North America.
The following abbreviations are used in the text: AMNH,
American Museum of Natural History, New York; MNHN QU,
Collections of Phosphorites of Quercy, Muse´um national
d’Histoire naturelle, Paris; TF, Thai Fossil at the Department
of Mineral Resources, Bangkok.
SYSTEMATIC PALEONTOLOGY
Order C
ARNIVORA
Bowdich, 1821
Family N
IMRAVIDAE
Cope, 1880
Genus N
IMRAVUS
Cope, 1879
Type Species—Nimravus brachyops (Cope, 1878).
N
IMRAVUS
cf.
INTERMEDIUS
(Filhol, 1872)
(Figs. 2–4)
Referred Material—A fragmentary left mandible with p4–
m1 (TF 2682, Figs. 2, 3), and a right one with damaged p4–
m1 (TF 2683), a fragmentary maxilla with right P3–P4 (TF
2684, Fig. 4). All specimens are housed in the collections of
the Department of Mineral Resources, Bangkok.
Locality—Wai Lek lignite pit, Krabi Basin, southern Thai-
land (latitude: between 7
8
54
9
49
0
N and 8
8
12
9
16
0
N; longitude: be-
tween 98
8
11
9
35
0
E and 99
8
8
9
35
0
E).
Horizon—Upper level of the main lignite seam of Wai Lek
pit (Formation B2, see Bristow, 1991). The mammalian fauna
associated with the carnivore remains indicates a late Eocene
age (see Ducrocq et al., 1995b).
Description—Unfortunately, all the specimens are badly pre-
served. They are referred here to Nimravus cf. intermedius and

158 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 20, NO. 1, 2000
FIGURE 1. Map of Thailand showing the location of the Krabi Basin.
The letter F represents the mine that yielded the nimravid remains. FIGURE 2. Nimravus cf. intermedius, left mandible, TF 2682. Pho-
tograph of the dentition in labial view. Dotted line
5
reconstruction of
the shape of the cuspids. Scale equals 1 cm.
TABLE 1. Dental dimensions of the Thai Nimravus compared to those of N. intermedius and N. brachyops (in mm). L
5
length; W
5
width;
N
5
sample size; x¯
5
mean; np
5
not preserved.
Thai specimens
TF 2682 TF 2684 TF 2683
Nimravus brachyops
Nx¯ Range
Nimravus intermedius
Nx¯ Range
Lp4
Wp4
L/Wp4
Lm1
Wm1
L/Wm1
15.8
7.5
2.11
19.7
7.7
2.56
—
—
—
—
7.8
—
—
—
—
—
—
—
14
13
13
16
18
15
18.4
7.8
2.36
25.6
8.9
2.83
16.1–21
6.6–9.7
2.09–2.6
22.2–28.3
8.0–11.1
2.45–3.09
42
47
42
58
58
55
16.1
6.7
2.46
21.25
7.6
2.82
10.9–19.4
4.5–9.2
2.11–3.13
16–26.7
5.2–9.7
2.49–3.24
LP3
WP3
L/WP3
LP4
WP4
L/Wp4
—
—
—
—
—
—
—
—
—
—
—
—
17.2
8.4
2.05
20.4
np
—
25
21
18
21
18
15
19.4
8.3
2.37
24.5
13.8
1.77
16.8–21
7.0–9.7
2.0–2.71
22.4–27.1
11.8–16.6
1.59–2.05
7
10
7
21
17
17
16.6
7.2
2.26
20.5
11.75
1.74
12.2–18.6
5.4–8.0
2.0–2.44
16.2–22.9
8.8–14.0
1.48–1.87
probably belong to the same individual because they were re-
covered together and have the same state of preservation.
p4 is not elongated compared to those of others Nimravus
(see Table 1); it has two large accessory cuspids; the posterior
one is slightly larger and wider. On the right tooth row, the
main cusp of p4 is well preserved; it is large and lanceolate.
The protoconid of m1 is slightly higher than the paraconid,
there is no metaconid, and a talonid partly broken. Posterior to
m1, an alveolus is present, presumably for a single-rooted m2.
On the maxillary fragment (Fig. 4), the top of P3 is broken
but the tooth was probably high. There is likely no anterior
accessory cusp but a strong and trenchant posterior one is pre-
sent. P4 has no parastyle; the protocone, which projects anter-
olingually, is not well preserved but it is similar in size to the
smallest European Nimravus. The alveoli of M1 are not pre-
served. Measurements are in Table 1.
Comparisons—The absence of the metaconid on m1 and the
large size of p4 and P3 support the generic assignation to Nim-
ravus. However, the specific attribution remains to be confirmed
by additional material. Three species of Nimravus are currently
recognized by most workers. N. brachyops (Cope, 1878) from
North America and N. intermedius (Filhol, 1872) from Europe,
both come from Oligocene deposits and are represented by nu-

159PEIGNE
´ET AL.—EOCENE NIMRAVIDS FROM THAILAND
FIGURE 3. Nimravus cf. intermedius, left mandible, TF 2682. Ster-
eophotographs of the dentition in occlusal view. Scale equals 1 cm.
FIGURE 4. Nimravus cf. intermedius, right maxilla, TF 2684. Ster-
eophotographs of the dentition in occlusal view. Scale equals 1 cm.
merous remains in paleontological collections; ‘‘N. mongolien-
sis’’ (Gromova, 1959) from late Eocene to early Oligocene de-
posits of Asia (from ‘‘Ergilian’’ to ‘‘Hsandagolian’’ in Mon-
golia, Meng and McKenna, 1998) is represented by only few
fragmentary remains (Gromova, 1959; Toohey, 1959; Gaboun-
ia, 1966; Mellett, 1968; Dashzeveg, 1996). The small size of
the Thai specimen (Table 1) prevents assignment to N. brach-
yops, which is a large species. The differences between N. in-
termedius and ‘‘N. mongoliensis’’ are less obvious. The type of
’’N. mongoliensis’’ is only known from a drawing in Gromova
(1959:fig. 1) and later in Dashzeveg (1996:fig. 7). After those
authors, the Mongolian species differs from N. intermedius by
its slightly smaller size, the diastema between c and p1, the
presence of p1, and a well-developed m2 (see Dashzeveg, 1996:
7). A comparison with European material does not confirm such
differences. Thus, some of the specimens assigned to N. inter-
medius are smaller than ‘‘N. mongoliensis’’ (compare Table 1
in this paper with table. 2 in Dashzeveg, 1996). Moreover, at
least two specimens from Quercy (MNHN QU 9499 and
AMNH 105390) display a p1, separate from the canine by a
diastema; m2 is present in most specimens assigned to N.
brachyops and N. intermedius and the published illustrations
and the study of material (AMNH 21638) indicate no structural
differences compared to the lower carnassial of the other spe-
cies assigned to Nimravus. Therefore, pending systematic re-
view of the genus Nimravus, it seems better to assigned the
Thai specimen to Nimravus cf. intermedius that is a better
known and well-established species.
Genus H
OPLOPHONEUS
Cope, 1874
Type Species—Hoplophoneus oreodontis (Cope, 1874).
H
OPLOPHONEUS
sp.
(Figs. 5–7)
Referred Material—An isolated left m1 (TF 2692, Fig. 5,
Table 2), and an upper right deciduous canine (TF 2700, Fig.
6), collections of the Department of Mineral Resources, Bang-
kok.
Locality—Wai Lek lignite mine, Krabi Basin, southern Thai-
land (latitude: between 7
8
54
9
49
0
N and 8
8
12
9
16
0
N; longitude: be-
tween 98
8
11
9
35
0
E and 99
8
8
9
35
0
E).
Horizon—Upper level of the main lignite seam of Wai Lek

160 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 20, NO. 1, 2000
FIGURE 5. Hoplophoneus sp., left m1, TF 2692. A, labial view. B,
lingual view. C, stereophotographs of the occlusal view. Scale equals 1
cm. FIGURE 6. Stereophotographs of Hoplophoneus sp., upper right de-
ciduous canine, TF 2700. A, labial view. B, lingual view. Scale equals
1 cm.
TABLE 2. Dental dimensions of the Thai Hoplophoneus sp. compared to other species of Hoplophoneus (in mm). L
5
length; W
5
width; *
5
decidual tooth; np
5
not preserved; sic.
5
sicarius; dak.
5
dakotensis;N
5
sample size; x¯
5
mean; R
5
Range.
Thai specimens
TF 2692 TF 2700
H. sic.
Type
H. dak.
Type
H. occidentalis
Nx¯R
H. primaevus/mentalis
Nx¯R
Lm1
Wm1
L/Wm1
20.5
8.2
2.5
—
—
—
18.9
9.4
2.01
23.2
10.1
2.3
5
5
5
22.06
10.16
2.17
21.1–24.5
9.9–10.5
2.06–2.4
34
36
34
18.2
7.6
2.38
16.5–19.9
6.7–9.4
2.01–2.6
LC
WC
L/WC
—
—
—
*21
*7.6
*2.76
np
np
np
np
np
np
1
1
1
26
14
1.86
—
—
—
23
15
15
13.86
7.07
1.95
11.2–18.1
5.9–8.8
1.77–2.08
pit (Formation B2, see Bristow, 1991). The mammalian fauna
associated with the carnivore remains indicates a late Eocene
age (see Ducrocq et al., 1995b).
Description—The teeth belong to a young individual but not
necessarily to the same one. m1 is especially well preserved. It
is a massive wide tooth with a high, pointed protoconid. The
posterior ridge of the protoconid is serrated, and the paraconid
is lower, with a shorter blade, and the anterior ridge of the tooth
is slightly convex and serrated. The carnassial notch is shallow;
the metaconid is reduced and trenchant, and located on the lin-
gual side of the posterior ridge of the protoconid; the talonid is
trenchant, short, low and oriented toward the labial side.
The measurements of the upper deciduous canine are sum-
marized in Figure 7 and Table 3. It is a large, flattened tooth
with a length/width ratio of 2.76 that is less than in Eusmilus
but more than in Hoplophoneus mentalis and H. primaevus.
However, this ratio might be slightly overvalued because of
preservation and lateral compression of the tooth. The posterior
and anterior ridges are serrated as are the deciduous upper ca-
nines of other species of Hoplophoneus and Eusmilus; the pos-
terior border is nearly rectilinear as in Hoplophoneus; a wide
but shallow groove is present on the lingual side, presumably
for the eruption of the permanent canine as noted by Brunet
(1972).
Comparisons—The material from Krabi can be compared
with the various species assigned to Hoplophoneus and Eus-
milus. Both genera are characterised by strongly developed sa-
bretooth features, especially on the skull (Bryant, 1996), by a
shearing dentition with an upper canine elongated and flattened,
a reduction in the number of premolars, and by a massive but
high lower carnassial with a metaconid and a talonid reduced.
Eusmilus is more derived than the species assigned to Hoplo-
phoneus: on the lower carnassial, the metaconid and the talonid
form a single short blade, and the upper canine is proportionally
more elongated, more curved and more flattened than in Ho-
plophoneus (Table 3). In addition, the NorthAmerican Eusmilus
is much smaller than any species assigned to Hoplophoneus.
The structure of m1 in the most derived Hoplophoneus (H. si-
carius and H. dakotensis) is close to those in Eusmilus (Bryant,

161PEIGNE
´ET AL.—EOCENE NIMRAVIDS FROM THAILAND
TABLE 3. Upper canines dimensions of the Thai Hoplophoneus (in
mm) compared with Eusmilus from Villebramar, France. The sample
size is indicated in parentheses.
Length Width Length/
width Height Height/
length
TF 2700 21 7.60 2.76 64 3.04
Eusmilus sp.
permanent canines 20.80 (4) 7.15 (4) 2.92 (4) 71.70 (4) 3.50 (4)
Eusmilus sp.
deciduous canines 19.95 (2) 6.50 (2) 3.08 (2) 66.50 (1) 3.24 (1)
FIGURE 8. Geographical distribution of Nimravinae in Asia. Black
dot
5
Nimravus: 1,Nimravus? sp., Gabunia, 1966; 2, cf. Nimravus sp.,
Mellett, 1968; 3,Nimravus mongoliensis, Gromova, 1959; Dashzeveg,
1996; 7,Nimravus cf. intermedius, this contribution. Black square
5
Eusmilus: 4, cf. Eusmilus sp., Chow, 1958; 6,Eusmilus? sp., Ding et
al., 1977. Open square
5
Hoplophoneus: 5,Hoplophoneus? sp., Tang
and Qiu, 1979; 7,Hoplophoneus sp., this contribution.
FIGURE 7. Outline of an upper canine illustrating the measurements
taken in this study.
1996). On the Thai specimen, m1 is more primitive with a
distinct metaconid and talonid, more like those of Hoplopho-
neus mentalis and Hoplophoneus primaevus, which are the most
primitive species of Hoplophoneus (Bryant, 1996). The decid-
uous canine is usually more flattened than the permanent one
(Table 3) and it is serrated; a groove is present on the internal
face of the tooth for the eruption of the permanent canine. Bru-
net (1972) noticed this peculiar groove and figured a well-pre-
served maxilla with deciduous and permanent canine of Eus-
milus from Villebramar (France). The generic assignation of TF
2692 and TF 2700 is supported by the distinct talonid and meta-
conid on m1, the size and the shape of the upper deciduous
canine. Compared with the species from North America, the
Thai Hoplophoneus is slightly larger than all the specimens that
can be assigned to Hoplophoneus mentalis and primaevus (Ta-
ble 2). The proportion and the structure of m1 are also similar
to those species. However, the size and shape of the upper de-
ciduous canine suggests that the Thai Hoplophoneus had a
much more flattened and larger upper permanent canine than
those of primitive Hoplophoneus (see dimensions in Table 2).
Although they are fragmentary, these specimens provide the
best evidence of the presence of Hoplophoneus in Asia.
DISCUSSION AND CONCLUSIONS
The Krabi fauna is one of the richest of South Asia. Many
taxa show that Southeast Asia played an important role in the
history of mammalian groups such as artiodactyls and anthro-
poid primates (see Ducrocq et al., 1995b for a summary). In
spite of our discovery, the fossil record for Paleogene Nimrav-
idae remains relatively poor in Asia (see the geographic distri-
bution on Fig. 8). The oldest known nimravids are represented
by some late Eocene specimens assigned to the genera Dinictis
and Hoplophoneus in North America (Bryant, 1996) and Asia
(Ding et al., 1977; this article); to the genus Nimravus in Mon-
golia (Dashzeveg, 1996; Meng and McKenna, 1998; this arti-
cle); and to the genus Eusmilus in China (Chow, 1958). The
presence of Hoplophoneus sp. in Thailand shows that the dis-
persal of Nimravidae between Asia and North America via Ber-
ingia during the late Eocene was possible, as previously sup-
posed for the Krabi fauna (Ducrocq, 1994a). Nonetheless, the
direction of the dispersal of nimravid carnivores is difficult to
establish because of the scarcity of the material.
Hoplophoneus has never been found in Europe but a review
of the European material is necessary in order to confirm this
assumption. In the late Eocene in Asia and North America, and
the early Oligocene in Europe, sabre-toothed carnivores be-
longing to the family Nimravidae suddenly appeared, including
very derived forms like Eusmilus bidentatus. That suggests a
much earlier origin of the family. The most recent systematic
review (Bryant, 1991) stated that a relationship sister-group be-

162 JOURNAL OF VERTEBRATE PALEONTOLOGY, VOL. 20, NO. 1, 2000
tween Aeluroidea (
5
Feliformia) and the Nimravidae is the most
plausible interpretation, although this is supported by only few
characters (position of the palatine canal, morphology of the
retractile claws, and morphology of P4 and M1). This hypoth-
esis is consistent with molecular evidences. Thus, Wayne et al.
(1989) suggest that Feloidea (extant families of Feliformia) and
Canoidea (
5
Canidae and their close relatives) may have di-
verged about 56–57 Ma ago, the different families of Feloidea
at about 40 Ma. More recently, an Artiodactyl/Cetacea age of
divergence at 55–60 Ma has been considered (Arnason and
Gullberg, 1996; Ledje and Arnason, 1996) to suppose that Can-
iformia and Feliformia diverged at about 50 Ma. It is therefore
possible to consider an origin of Nimravidae between 50 and
40 Ma, i.e., during Eocene. Discoveries of additional remains
is therefore needed, and investigations in Southeast Asia should
also be conducted in deposits older than late Eocene in order
to better understand the evolutionary history of carnivores as
well as other groups of mammals.
ACKNOWLEDGMENTS
We wish to thank people who gave us access to the collec-
tions under their care: J. Alexander, M. C. McKenna, R. H.
Tedford, X. Wang (New York), L. D. Martin, D. Miao
(Lawrence), R. M. Hunt, R.G. Corner (Lincoln), J. Rensberger
(Seattle), M. A. Turner (New Haven), T. Daeschler (Philadel-
phia), L. de Bonis (Poitiers), P. Tassy (Paris). Many thanks to
M. Brunet (Poitiers) for the loan of unpublished material from
Villebramar. We want to express our gratitude to the anonymous
reviewers for their constructive comments and critical review
that greatly improve the manuscript. This study was financially
supported by the American Museum of Natural History (Col-
lections Study Grant Program) and the Laboratoire de Ge´o-
biologie, Biochronologie et Pale´ontologie Humaine (Poitiers,
S.P.), and the Mission Pale´ontologique Franc¸aise en Thaı¨lande
(Ministe`re des Affaires Etrange`res). Drawings have been per-
formed by Sabine Riffaud (Poitiers). This is contribution ISEM
n
8
2000-01, CNRS.
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