FRFossil RecordFRFoss. Rec.2193-0074Copernicus PublicationsGöttingen, Germany10.5194/fr-21-291-2018First occurrence of Duboisia (Bovidae, Artiodactyla, Mammalia) from ThailandFirst occurrence of Duboisia from ThailandNishiokaYuichironishioka@aoni.waseda.jpVidthayanonChavalitWaseda Institute for Advanced Study, Waseda University, Shinjuku,
169-8050, Japanindependent researcher: Bangkok, 10900, ThailandYuichiro Nishioka (nishioka@aoni.waseda.jp)24October201821229129910May201812August20184October2018This work is licensed under the Creative Commons Attribution 4.0 International License. To view a copy of this licence, visit https://creativecommons.org/licenses/by/4.0/This article is available from https://fr.copernicus.org/articles/21/291/2018/fr-21-291-2018.htmlThe full text article is available as a PDF file from https://fr.copernicus.org/articles/21/291/2018/fr-21-291-2018.pdf
The first fossil record of Duboisia (Boselaphini, Bovidae) from
Thailand confirms that this genus is no longer endemic to Java, Indonesia.
The new fossil material is a calvarium with horn cores (older than the Middle
Pleistocene) collected from a sandpit at Tha Chang, Nakhon Ratchasima
Province, north-eastern Thailand. The present specimen is provisionally
allocated to a species of Duboisia aff. D. santeng, which
has weaker precornual ridges and anterior keels on the horn cores than
D. santeng from Early and Middle Pleistocene deposits of Java, but
these species share basic characteristics of horn cores as follows: the lower
half inclined backwards; the upper half curved upwards; cross section rounded
triangular, antero-posteriorly compressed, and with medial and lateral keels.
Morphological similarities between D. aff. santeng and D. santeng support a strong faunal interchange between continental South East Asia
and Java before the Middle Pleistocene, and suggest that the genus
Duboisia diverged from the other genera of Boselaphini in the
“Siva-Malayan” region.
Introduction
South East Asia is geographically divided from the Indian subcontinent by the
Himalayan–Tibetan Plateau, the Indo-Burma Range, and the Ganges and Brahmaputra
rivers. These barriers form a zoogeographical boundary for terrestrial
mammals between the Indo-Chinese and Indian subregions (Corbet and Hill, 1992).
The Neogene fauna of continental South East Asia is similar to that of the
Siwaliks, in the Indo-Pakistan area (e.g. Stamp, 1922; Colbert, 1938), but recent
studies based on mammalian fossils from the Irrawaddy beds (Middle Miocene to
Early Pleistocene) of Myanmar have demonstrated a faunal difference between
the Siwaliks and Irrawaddy beds, which had increased since the Late Miocene
or Early Pliocene due to forming zoogeographical barriers (Nishioka et al.,
2015, 2018b; Takai et al., 2016).
Mammalian endemism in South East Asia had increased during the
Plio-Pleistocene (e.g. Van den Bergh et al., 2001; Van der Geer et al.,
2010). The latest Pliocene and Early Pleistocene Satir fauna from Java,
Indonesia, is impoverished and unbalanced and includes mammals of
“Siva-Malayan” origin, such as Hexaprotodon sivajavanicus
(Hippopotamidae, Artiodactyla) and Sinomastodon bumiajuensis
(Proboscidea) (Sondaar, 1984). These mammals have been traditionally believed
as endemic species to Java (Van der Maarel, 1932; Hooijer, 1950), but to what
degree island species differ from the original continental species from the
Siwaliks and the Irrawaddy beds is insufficiently studied (Van der Geer et
al., 2010).
The “Stegodon–Homo erectus fauna” from the Early–Middle
Pleistocene of Java (e.g. Trinil HK and Sangiran dome) is characterized by
a relatively low degree of endemism and is composed of Siwalik species such
as Pachycrocuta brevirostris (Carnivora) and Javanese species such
as Stegodon trigonocephalus (Proboscidea) derived from mammals of
the “Siva-Malayan” origin (Van der Geer et al., 2010). Duboisiasanteng is an endemic species of Boselaphini (Bovidae, Artiodactyla)
in the Early–Middle Pleistocene fauna of Java, which represents a somewhat
isolated environment (Hooijer, 1958; Aimi and Aziz, 1985; Van den Bergh et
al., 2001; Rozzi et al., 2013). D. santeng is phylogenetically
related to living Indian boselaphins, Boselaphus tragocamelus and
Tetracerus quadricornis (Stremme, 1911; Pilgrim, 1939; Hooijer,
1958). However, when a putative ancestor of D. santeng colonized
Java is still debatable (Rozzi et al., 2013), owing to the absence of certain
fossil evidence on continental South East Asia to show a relationship between
Javanese D. santeng and Indian Boselaphini.
Geological map of the Khorat basin, Nakhon Ratchasima, north-eastern
Thailand, and the locality of Tha Chang sandpit no. 8 (modified from
Department of Mineral Resources, 1999).
Out of Java, Moigne et al. (2016) reported three isolated teeth belonging to
the genus Duboisia from the Upper Pliocene Masol Formation of the
Siwaliks, India, although they did not mention detail morphological
comparisons on the basis of apomorphic characteristics of each taxon. The
presence of Duboisia in continental South East Asia was mentioned by
Hooijer (1962), based on some dental and postcranial remains from Quaternary
cave deposits at Tambun, Peninsular Malaysia. However, we are not sure if
this identification is valid because of the fragmentary nature of the
remains. Furthermore, all specimens of D. santeng in the Hooijer
collection are missing (Lim, 2013). A partial cranium of Duboisia
was recently found from central Myanmar (but without geological context),
which gives preliminarily evidence suggesting the existence of the genus
on continental South East Asia (Nishioka et al., 2018b). In the present study, we
describe the first specimen of Duboisia from Thailand. The specimen
is a skull with complete horn cores that are useful for taxonomic comparisons
of Duboisia remains between continental and island specimens from
South East Asia.
Geological settings
The Tha Chang sandpits (15∘05′ N, 102∘20′ E) are
located 22 km east of Nakhon Ratchasima (or Khorat) City, north-eastern
Thailand (Fig. 1). More than 10 sandpits have been mined since 1985 and have
exposed Neogene and Quaternary fluvial deposits along the Mun River, a
tributary of the Mekong River. One of the fossil localities, Tha Chang
sandpit no. 8 (or Somsak sandpit), is well-known for its yield of Miocene
orangutans, Khoratpithecuspiriyai (Chaimanee et al.,
2004). The sediments at sandpit no. 8 are an exposed outcrop of about 30 m
depth from the ground to the bottom when the sandpit was first mined
(Shinji Nagaoka, personal
communication, 2002; see also Thasod et al., 2011).
Sediments at sandpit no. 8 have been divided into two units: a lower unit of
organic greyish sand and gravel, and an upper unit of yellowish sand
(Chaimanee et al., 2006, 2007). The basal part of the lower unit overlies on
the Mahasarakham Formation including the evaporitic layer around 40 or 50 m
depth (Chaimanee et al., 2006). The lower unit is formed by grey
conglomeratic sandstone with some thin silt and clay layers. The sandstone shows
clear cross-bedding, suggesting channel deposits, and contains abundant
fossils of tree trunks, leaves and vertebrate animals. The upper part of the
lower unit is characterized by the inclusion of peat (ca. 0.5 m thick) and
gravel layers (ca. 0.5–1 m thick) around 10 m depth from the ground. Plant
fossils have been found in all layers.
The upper oxidized unit starts from about 8 m depth from the ground, based
on the stratigraphic column by Chaimanee et al. (2006). Some tektites have
been collected from the lowermost part of the upper unit (ca. 8 m depth),
which originate from the widespread tektite debris field at 0.8 Ma in
north-eastern Thailand (Howard et al., 2003). Haines et al. (2004) estimated
that the sediments of the Tha Chang sandpits are dated to Quaternary, because
those tektites were also collected from the lower unit.
Mammalian fossils have been collected primarily from the lower unit. An
isolated molar of the proboscideans Stegolophodon cf.
stegodontoides and a skull of the anthracothere
Merycopotamusthachangensis were collected in situ from the
silt (ca. 13.5 m depth) and sandstone (ca. 18 m depth) layers of sandpit
no. 8, respectively (Hanta et al., 2008; Thasod et al., 2011). Geological
information for other mammalian fossils from the Tha Chang sandpits is
unavailable, but most of them were probably from the lower unit because the
fossils are covered by greyish sandstone with pyrite (iron sulfide).
To date, the mammalian fossil assemblage from sandpit no. 8 includes 4
orders, 11 families, and 21 genera: Primates (Khoratpithecus);
Artiodactyla (Hippopotamodon, Propotamochoerus,
Hexaprotodon, Merycopotamus, Microbunodon,
Bramatherium and several species of bovids); Perissodactyla
(Hipparion, Chilotherium, Brachypotherium,
Alicornops and Aceratherium); and Proboscidea
(Prodeinotherium, Deinotherium, Gomphotherium,
Tetralophodon, Sinomastodon, cf. Protanancus,
Stegodon, Stegolophodon and Elephas) (Chaimanee
et al., 2004, 2007; Saegusa et al., 2005; Hanta et al., 2008; Thasod et al.,
2011; Deng et al., 2013; Nishioka et al., 2014). Another sandpit (no. 10)
near Takut Khon village also recovered a similar mammalian fossil assemblage
including Zygolophodon (Duangkrayom et al., 2017). The previous
studies, as listed above, suggested that these genera are biostratigraphically
correlated with those in the Dhok Pathan fauna (10.1 to ca. 3.5 Ma; Barry et
al., 2002) from the Siwaliks, Indo-Pakistan. Chaimanee et al. (2006) strictly
estimated the age of the lower unit being between 9 and 6 Ma if hippos
(Hexaprotodon) are absent in the fossil assemblage, and between
7.4 and 5.9 Ma if hippos are present. However, proboscideans include both
Neogene taxa (e.g. Prodeinotherium) and Quaternary taxa (e.g.
Elephas) (Thasod et al., 2011). The fossil assemblage from sandpit
no. 8 is most likely dated to the Late Miocene, but it sometimes includes
several fauna from different ages owing to collection bias.
Material and methodsMaterial
The examined fossil specimen (PPN 01-000109) that is housed at the Sukhothai
Elephant World Museum, Thailand, was originally found in a private collection
of Kamol Chaivanich in Bangkok. According to him, the specimen was
collected from a sandpit in Tha Chang before 2007 when sandpit no. 8 was
being mined. The fossil surface was originally covered by greyish sandstone
with many pebble- or granule-sized gravel stones under 10 mm in diameter
(Fig. 2). Gravel layers at Tha Chang sandpit no. 8 lie between 7 and 14 m
depth from the ground, which includes both upper and lower units (Chaimanee
et al., 2006). A combination of accompanying both greyish sandstone with
pyrite and many pebbles on the fossil surface indicates that this fossil
might have been retrieved from the lower unit, similar to mammalian fossils
from the Miocene rather than the Pleistocene.
Posterior view (right side) of PPN 01-000109 before preparation.
PPN 01-000109 was compared with two skulls of Duboisia santeng from
the Pleistocene of Java. One specimen (MGB.SA 290779) is a calvarium with
right and left horn cores recovered from the Early–Middle Pleistocene
deposits at Pucung, Sangiran Dome area (Aimi and Aziz, 1985). The other
specimen (MGB.Dbs) on display at the same institute is a neurocranium with
horn cores probably from Sangiran. Furthermore, we also referred to pictures
of D. santeng skulls from Trinil, including the lectotype, which are
housed at Naturalis Biodiversity Centre in Leiden, the Netherlands (Stremme,
1911; Hooijer, 1958; Tshen Tze Lim,
personal communication, 2016). The other species belonging to Boselaphini
used in this study were as follows: Boselaphus tragocamelus (BNHS
18183; 18185; 18186); Boselaphus namadicus (BMNH
36851 = holotype by Rutimeyer, 1878; M486) and Tetracerus quadricornis (BNHS 17986; 18125).
Calvarium with right and left horn cores of Duboisia aff.
santeng (PPN 01-000109). (a) Anterior view;
(b) cross section of the left horn core (an., anterior; me.,
medial.); (c) left lateral view; (d) posterior view;
(e) schematic drawing of the posterior surface; (f) dorsal
view; (g) schematic drawing of the dorsal surface.
Institutional abbreviations
BMNH, British Museum of Natural History (currently, Natural History Museum,
London), UK; BNHS, Bombay Natural History Society, Mumbai, India; MGB,
Museum Geologi Bandung, Java, Indonesia; PPN, Prasert Prasartthong-Osoth
Natural, Sukhothai Elephant World Museum, Sukhothai, Thailand.
Systematic palaeontology
Order Artiodactyla Owen, 1848
Family Bovidae Gray, 1821
Subfamily Bovinae Gray, 1821
Genus Duboisia Stremme, 1911
Duboisia aff. santeng (Dubois, 1891)
(Fig. 3)
Examined material: PPN 01-000109, a calvarium with
right and left horn
cores.Locality: A
sandpit at Tha Chang, Nakhon Ratchasima City, north-eastern Thailand.
Age: Younger than the Middle Miocene
and older than the Middle Pleistocene.
Measurements:
Antero-posterior diameter of the left horn core: 18.5 mm; medio-lateral
diameter of the left horn core: 33.6 mm; length of brain case (between
bregma and superior nuchal line): 65.1 mm; maximum width of brain case:
68.7 mm; height of the occipital without the foramen magnum: 38.1 mm (see
also Table 1).
Description: The
present specimen is a calvarium with complete right and left horn cores,
including the frontal, parietal, temporal and occipital bones. The
basioccipital part is broken. The dorsal surface on the fronto-parietal
region is flat and faintly depressed. The frontal surface is rugose, with
weak precornual ridges extending from the anterior ridge of the horn cores
(Fig. 3g). Supraorbital foramina are not found on the preserved parts: that
means they are situated further towards the front than the horn pedicels. The
postero-dorsal rims of the orbits weakly project outwards. Temporal crests
are moderately developed, slightly overhanging along the superior nuchal line
on the occipital. The external occipital protuberance is projected
posteriorly. The median nuchal line forms a sharp ridge and gently inclines
below the centre. The brain case is squared in dorsal view
(length/width = 0.95). The height of the occipital without the foramen
magnum is low relative to the brain-case width (height/width = 0.55). The
mastoid processes project laterally, with shallow squamosal shelves.
Comparison of skulls between Duboisia aff. santeng
(A–B: PPN 01-000109) and D. santeng (C–D: MGB.SA 290709) from
Pucung (Java). Panels (a) and (c), left lateral view;
(b) and (d), anterior view and cross section of the left
horn core. Abbreviations: an., anterior; me., medial.
The horn cores insert caudolaterally from the orbits, strongly inclined
backwards being parallel to the frontal surface at the base, and bend upwards
at the middle part. In the antero-dorsal view (Fig. 3a), the horn cores
diverge at an angle of 95∘ at the base, and curve inwards after the
middle part. The horn core is weakly twisted clockwise (right side). Right
and left horn cores have a wide interval at the frontal surface (56.0 mm
between the medial keels). The cross section of the horn cores has a rounded
triangular outline antero-posteriorly compressed (Fig. 4b). There are medial
and lateral keels from the base to the top. The anterior surface is convex
but does not have a clear
keel.Remarks: Most
antelope-sized species of Bovinae were traditionally included in the tribe
Boselaphini (McKenna and Bell, 1997). However, it is currently accepted that
the tribe encompasses only the following crown genera: Boselaphus,
Tetracerus, and Duboisia (Bibi, 2009; Bibi et al., 2009).
There are some bovines whose taxonomic positions are formally pending.
Schlosser (1903) proposed Paraboselaphus ameghinoi, referring to
some isolated cheek teeth from China, but his materials and description are
insufficient to define an independent taxon based on apomorphies of the
genus. Matsumoto (1915) described Proboselaphus from the Pleistocene
of Sichuan, China, as a primitive form of living Boselaphus, but the
holotype skull of P. watasei has neither cranial nor tooth
characteristics of Bovidae, but rather those of Cervidae (Nishioka et al.,
2018a). According to Pilgrim (1939), Sivaportax from the Neogene
Irrawaddy beds of Myanmar is phylogenetically related to
Boselaphus, but our recent studies indicated that the former shares
horn core morphology with Miocene Tragoportacini or Stem Bovini rather than
Boselaphini (Nishioka et al., 2018b). In this study, we accept that
Duboisia is the only extinct genus included in Boselaphini.
Comparison of brain-case dimensions (in mm) among boselaphins.
Measuring points (L, length; W, width; H, height) are shown in Fig. 3.
Regarding cranial morphology, PPN 01-000109 has the precornual ridges
extending from the anterior keels (or ridges) of the horn cores which are a
synapomorphy of Boselaphini. Rugosity on the fronto-parietal surface is a
symplesiomorphy of Tragoportacini (e.g. Protragocerus,
Helicoportax, Tragoportax, and Miotragocerus) and
Boselaphini, but that of PPN 01-000109 is finer than that of the
tragoportacin genera. Moreover, PPN 01-000109 has relatively weaker temporal
crests on the parietals and an antero-posteriorly shorter brain case than the
tragoportacin genera listed above. According to previous studies (Stremme,
1911; Hooijer, 1958) and our observation, a combination of the following
characteristics of the horn cores is diagnostic to distinguish the genus
Duboisia from all of the other genera of Bovinae: the lower half
inclined backwards; the upper half curved upwards; and the cross section
rounded triangular, compressed antero-posteriorly, and with medial and
lateral keels. Boselaphus namadicus is known as an extinct form of
boselaphins from the older alluvium (Middle Pleistocene) of the Narmada
basin, India (Rütimeyer, 1878; Pilgrim, 1939; Chauhan, 2008), and from
the Plio-Pleistocene deposits of Sardhok, Pakistan (Siddiq et al., 2017). The
horn core of the holotype (BMNH 36851) of B. namadicus has
equilateral triangular cross section with a sharp anterior keel and is less
compressed antero-posteriorly than that of PPN 01-000109 or the genus
Duboisia. In measurements (Table 1), the brain case of PPN 01-000109
is antero-posteriorly shorter than that of Boselaphus tragocamelus
and approximately as long as that of Boselaphus namadicus or
Tetracerus quadricornis based on dividing length by width. In the
occipital height divided by width, PPN 01-000109 is intermediate between
Boselaphus and Tetracerus. Based on the qualitative and
quantitative comparisons in horn core and brain-case morphology, as mentioned
above, PPN 01-000109 is distinguished from Boselaphus and
Tetracerus.
The genus Duboisia formally contains two species i.e. D. santeng (Dubois, 1891) and D.? sartonoi Geraads, 1979
(or D. saatensis Von Koenigswald, 1934), although the latter was established based
on some isolated teeth. Van den Bergh (1988), in his unpublished master
dissertation, suggested that Cervus problematicus Von Koenigswald,
1933, from Bumiayu, Java, should be combined with the genus Duboisia
based on a large skull fragment. Both ?D. sartonoi and D. problematicus are likely larger than D. santeng, but these species
are still debatable taxonomically (Rozzi et al., 2013). PPN 01-000109 is
almost as large as the specimen MGB.Dbs. of D. santeng from Sangiran
(Table 1). The horn cores of PPN 01-000109 are primarily similar to those of
D. santeng, in having keels on the medial and lateral sides and
cross section compressed antero-posteriorly, but there are some minor
differences between PPN 01-000109 and D. santeng (MGB.SA 290709)
from Pucung. Compared to the latter, the former has slender horn cores,
curved prominently, and incomplete anterior and medial keels (Fig. 4).
Moreover, precornual ridges of PPN 01-000109 are weaker than those of MGB.SA
290709. These differences of horn cores are possibly due to intraspecific
variation. Although a species identification of PPN 01-000109 should be
discussed in detail based on additional specimens from Thailand and comparing
with a sufficient number of specimens of D. santeng from Java, PPN
01-000109 has a strong affinity with D. santeng.
Discussion
What Duboisia species were distributed on the continental region of
southern Asia has been discussed in previous studies (Hooijer, 1962; Moigne
et al., 2016), but these fossil records based only on isolated teeth are
still debatable in taxonomy because cranial (and possibly dental) morphology
should be diagnostic of the genus Duboisia (Stremme, 1911; Hooijer,
1958). Recently, we described an incomplete calvarium of Duboisia
from an indeterminate horizon at Bagan, central Myanmar (Nishioka et al.,
2018b). The present finding of D. aff. santeng from Thailand
in conjunction with the fossil record from Myanmar confirmed the existence of
Duboisia on continental South East Asia, and provides a certain fossil
evidence indicating a morphological affinity of Duboisia species
between continental and island South East Asia. Rozzi et al. (2013) discussed
if D. santeng had been downsized and acquired unique characteristics
in postcranial bones as a result of insular processes (see also Rozzi, 2018)
and/or an adaptive shift to dense forest. Although we have not examined
strict size and shape differences between Duboisia species from Thailand
and Java, our results suggest that basic cranial characteristics of the genus
Duboisia (e.g. a squared brain case, weak temporal crests and
antero-posteriorly compressed horn cores with medial and lateral keels) had
been already acquired before or at the beginning of the Early Pleistocene on
continental South East Asia.
The occurrence of the genus Duboisia from Thailand supports either
of the two hypotheses. If PPN 01-000109 was collected from the upper unit
that includes Early Pleistocene tektites (0.8 Ma by Howard et al., 2003),
this age corresponds approximately with the D. santeng horizon on
Java, or the Early–Middle Pleistocene ages of Sangiran and Trinil H.K. (ca.
0.9–0.5 Ma by Joordens et al., 2015; Falguères et al., 2016). In fact,
the Pleistocene fauna share many mammalian species between continental and
island regions in South East Asia (Tougard, 2001; Louys et al., 2007;
Zin-Maung-Maung-Thein et al., 2010; Suraprasit et al., 2016). Morphological
affinities between the Thai and Javanese forms of Duboisia are
consistent with what existed contemporaneously in South East Asia. On the
other hand, greyish sandstone and pyrite coated the surface of PPN 01-000109
(Fig. 2) are the same as those in the lower unit dated to the Miocene. If PPN
01-000109 was collected from the lower unit, as most mammalian fossils from
Tha Chang sandpits were (Chaimanee et al., 2004, 2007; Hanta et al., 2008;
Thasod et al., 2011; Deng et al., 2013; Nishioka et al., 2014; Duangkrayom et
al., 2017), undeniable morphological differences between D. aff.
santeng and D. santeng should be found, which coherently
explains geographical and chronological gaps of these species. At the present
state, however, we have no more evidence from which horizon PPN 01-000109 was
recovered.
The present study clarifies that the genus Duboisia is undoubtedly a
member dispersed via the “Siva-Malayan” route and supports a low degree of
endemism of the Early–Middle Pleistocene fauna of Java (e.g. Van der Geer et
al., 2010). Bovines on the southern Asian continent were dominated by
Tragoportacini (e.g. Protragocerus, Helicoportax, and
Tragoportax) and Stem Bovini (Selenoportax and
Pachyportax) during 9 and 6 Ma (Gentry et al., 2014), and after the
latest Miocene the Irrawaddy fauna of central Myanmar began being dominated
by Irrawaddy-endemic taxa by the formation of geographical barriers, such as
the Indo-Burma Range, between South Asia and South East Asia (Nishioka et al.,
2018b). There is an exceptionally incomprehensible fossil record of
Duboisia from the Pliocene of India (Moigne et al., 2016), but based
on certain cranial fossil records of Boselaphus namadicus (Pilgrim,
1939) and Duboisia species (Nishioka et al., 2018b; this study),
geographical isolation at the Indo-Burma boundary could be a possible factor
explaining the divergence of Boselaphins between the South Asian lineage
(Boselaphus+Tetracerus) and the South East Asian
lineage (Duboisia) (Fig. 5).
Distribution of Boselaphini and zoogeographical barriers between
South Asia and South East Asia. Fossil localities: 1, Siwaliks; 2, Bagan; 3,
Tha Chang; 4, Java.
Conclusions
A species of Duboisia aff. D. santeng newly discovered from
a sandpit at Tha Chang, north-eastern Thailand, supports a faunal interchange
between continental South East Asia and Java before the Middle Pleistocene. The
finding of Duboisia on the South East Asian continent suggests that
the genus was diverged from the other Boselaphini (Boselaphus and
Tetracerus) owing to a geographical isolation on the continent
rather than insular processes of South East Asian islands.
All material examined in this study is accessible in the listed institutions and all data are described in the text and table.
YN and CV designed this study and prepared the examined fossil material.
YN prepared the manuscript with contributions from all co-authors.
The authors declare that they have no conflict of
interest.
Acknowledgements
We thank Pratueng Jintasakul (Nakhon Ratchasima Rajabhat University,
Thailand) for his helpful fossil preparation at his institute.
Kamol Chaivanich (Bangkok, Thailand), Prasert Prasarttong-Osoth (CEO of
Bangkok Airways and the owner of the Sukhothai Elephant World Museum, Thailand),
Porpan Vachajitpan (Bangkok, Thailand) and Sakchai Juan-Ngam (Sukhothai
Elephant World Museum, Thailand) kindly gave us a chance to study the present
fossil material. Comparative material was obtained from the National History
Museum, London (UK), the Museum Geologi Bandung (Indonesia) and from
pictures by Tshen Tze Lim (University of Cambridge, UK). We are thankful to
the helpful comments of the reviewers Roberto Rozzi and
Dimitris S. Kostopoulos. The present study was financially supported in part
by a Waseda University Grant for Special Research Project [2018K-462] and by
the Japan Society for the Promotion of Science KAKENHI, Grant-in-Aid for
Scientific Research [16K17828], [18K06444], and [18H01327]. Edited by: Johannes Müller Reviewed by:
Roberto Rozzi and Dimitris S. Kostopoulos
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