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第56卷 第3期
2018年7月
古 脊 椎 动 物 学 报
VERTEBRATA PALASIATICA
DOI: 10.19615/j.cnki.1000-3118.180226
Reappraisal of Endotherium niinomii Shikama, 1947,
a eutherian mammal from the Lower Cretaceous Fuxin
Formation, Fuxin-Jinzhou Basin, Liaoning, China
WANG Yuan-Qing1,2,3 KUSUHASHI Nao4 JIN Xun1 LI Chuan-Kui1
SETOGUCHI Takeshi5 GAO Chun-Ling6 LIU Jin-Yuan6
(1 Key Laboratory of Vertebrate Evolution and Human Origins of Chinese Academy of Sciences, Institute of Vertebrate
Paleontology and Paleoanthropology, Chinese Academy of Sciences Beijing 100044, China)
(2 CAS Center for Excellence in Life and Paleoenvironment Beijing 100044, China wangyuanqing@ivpp.ac.cn)
(3 College of Earth Sciences, University of Chinese Academy of Sciences Beijing 100049, China)
(4 Department of Earth’s Evolution and Environment, Graduate School of Science and Engineering, Ehime University
Matsuyama, Ehime 790-8577, Japan nkusu@sci.ehime-u.ac.jp)
(5 Kyoto Municipal Science Center for Youth Kyoto 612-0031, Japan)
(6 Dalian Museum of Natural History Dalian, Liaoning 116023, China)
Abstract Reinvestigation of the Early Cretaceous eutherian mammal Endotherium niinomii
Shikama, 1947, based on the impressions of the type specimen, casts made from the impressions,
and the original description, indicates that E. niinomii is characterized by the following characters:
decrease in size from the m1 to the m3; a moderate height difference between the trigonid and
talonid of lower molars; blunt lower molar cusps; the protoconid being the largest among the
trigonid cusps; the paraconid being as tall as the metaconid; the p3 not being obviously reduced;
and possible possession of five lower premolars. These characteristics clearly distinguish E.
niinomii from other known Cretaceous eutherians, and support its taxonomic identity as a valid
taxon.
Key words Fuxin, Liaoning; Early Cretaceous; Fuxin Formation; Mammalia, Eutheria,
Endotherium niinomii
Citation
Wang Y Q, Kusuhashi N, Jin X et al., 2018. Reappraisal of Endotherium niinomii
Shikama, 1947, a eutherian mammal from the Lower Cretaceous Fuxin Formation,
Fuxin-Jinzhou Basin, Liaoning, China. Vertebrata PalAsiatica, 56(3): 180–192
1 Introduction
Endotherium niinomii Shikama, 1947, is a eutherian mammal discovered from the Fuxin
Formation at the Xinqiu (Hsinchiu) coal mine in Fuxin (Husin), Fuxin-Jinzhou Basin, Liaoning
Province, northeastern China. The report was subsequently reviewed by Shikama (1948),
Chow (1953), and Patterson (1956). However, as was pointed out by Clemens et al. (1979) and
Kielan-Jaworowska and Cifelli (2001), the original descriptions of Shikama (1947) were not
adequate, and gures were relatively uninformative. It is unfortunate that important parts of the
中国科学院战略性先导科技专项(B类)(编号：XDB18000000)、国家自然科学基金(批准号：41688103,
41541015)、大连市科技计划项目(编号：20044J9JH238)和日本学术振兴会科学研究费补助金若手研究(B) (编号：
24740349, 16K17830)资助。
收稿日期：2017-11-07
pp. 180–192
gs. 1–3
181
Wang et al. - Reappraisal of Endotherium from the Lower Cretaceous of China
type specimen, including the molars, were lost after Shikama’s report. Therefore, except for
brief listing as a eutherian taxon in review work (e.g., Zhang, 1984; Wang et al., 1995; Meng,
2014), Endotherium has rarely been mentioned in recent papers on Mesozoic eutherians,
and is sometimes treated as a nomen dubium (Kielan-Jaworowska and Cifelli, 2001; Kielan-
Jaworowska et al., 2004). Recently, Meng et al. (2015) suggested that Endotherium niinomii
is a valid taxon and its diagnosis should be revised. The species was originally thought to
be of Late Jurassic in age. Although the Fuxin Formation is now generally considered as
late Early Cretaceous in age (Aptian–Albian; e.g., Li et al., 2005; Kusuhashi et al., 2009a,
b), Endotherium is still an early member of eutherians and is important to the study of the
evolution and diversication of eutherians in the Early Cretaceous.
Since the early 1990s, researchers of the Institute of Vertebrate Paleontology and
Paleoanthropology, Chinese Academy of Sciences (IVPP) have conducted eld investigations
at Fuxin and neighboring areas, which later resulted in a major China-Japan joint research
project. So far, more than one hundred fossil mammal specimens have been collected from the
Shahai and Fuxin formations. These specimens represent eutriconodontans, multituberculates,
spalacotheriid symmetrodonts, stem zatherians, and eutherians (e.g., Hu et al., 2005a, b; Li et
al., 2005; Kusuhashi et al., 2009a, b, 2010, 2016; see also Wang et al., 1995). See Kusuhashi et
al. (2009a, b) for the geologic setting of these formations. Among these specimens, about one-
third belong to eutherian mammals. Since many of the eutherian specimens have been collected
from the same rock unit as the type specimen of E. niinomii, it is necessary to reinvestigate E.
niinomii before the study on these recently collected eutherian specimens is carried out. Early
in 1953 when he reviewed the discovery of Endotherium and Manchurodon, Chow (1953)
discussed their geological age and suggested geologists and paleontologists to pay special
attention to the small Mesozoic mammal remains during the eld work. We dedicate this paper
to Prof. Dr. Zhou Mingzhen (Minchen Chow) in commemoration of the centenary of his birth.
The type specimen of Endotherium was reported to be preserved on “a slab of very
fragile coal”, together with the type of a lacertilian Teilhardosaurus (Shikama, 1947). The
sample coal bearing the specimens (catalogue number D0247) is currently housed in the
Dalian Museum of Natural History, Dalian, Liaoning Province, China. Although most parts of
the type specimen of Endotherium niinomii had been lost, its impressions, particularly those
of the teeth, are fortunately preserved on the sample coal, or exactly speaking on a very thin
intercalated layer of carbonaceous mudstone within the coal layers (Fig. 1). Reexamination
of the specimen together with review of Shikama’s (1947) original description reveals some
additional characteristics of E. niinomii, which help to clarify its taxonomic status.
During the restudy of the specimen, casts were made for easier examination and for SEM
photographing. At first, a silicone mold was made from the real specimen. Then a counter
silicone mold was made from the rst mold. Finally, both resin casts and counter casts were
produced from these molds. In the descriptions below, we follow the dental terminology
for tribosphenic molars from Bown and Kraus (1979) and Kielan-Jaworowska et al. (2004).
182 Vertebrata PalAsiatica, Vol. 56, No. 3
Fig. 1 The sample coal bearing the type specimens of Endotherium niinomii
and Teilhardosaurus carbonarius (D0247)
A. the whole sample block; B. close-up view corresponding to the white box in A
Lower premolars and molars are abbreviated as lower case p and m, respectively, followed
by numbers indicating tooth locus counting from the mesial to the distal end of each. For
premolars, numbers only denote tooth position and do not necessarily indicate homology of
teeth.
2 Description
2.1 The rock specimen
We compared the rock specimen (D0247; Fig. 1) with gures of Shikama (1947:gs. 1–3)
and confirmed that it is what the type specimen of Endotherium niinomii was preserved in.
On the exposed surface of D0247, the type specimen of a lizard Teilhardosaurus carbonarius,
contemporaneously described by Shikama (1947) with E. niinomii, is still preserved (Fig. 1B),
but its posterior part, including several teeth, seems to have been missing. Close to it and at
the very position gured by Shikama (1947:g. 3), there is an impression of a right dentary
with teeth of E. niinomii (Fig. 1B). Adjacent to it, another impression of a lower jaw is present
with only the broken anterior part preserved. The impression is from the left lower jaw of E.
niinomii, which was only mentioned by Shikama (1947) (Fig. 1B). In addition to these jaw
remains, there are some bone fragments scattered around them. They were partly illustrated
by Shikama (1947:g. 3), but some of the illustrated bones seem to have been damaged and/or
lost. Shikama (1947) included fragments of scapula and humeri into the holotype of E. niinomii
without any description or specic illustration. However, they could not be recognized from
the fragmentary bones. In the following descriptions, we focus only on the right and left lower
jaws and teeth of E. niinomii, which are mainly known from impressions.
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Wang et al. - Reappraisal of Endotherium from the Lower Cretaceous of China
2.2 Lower jaws of Endotherium niinomii
The major part of the right lower jaw of the type specimen has been lost, and only its
impression can now be observed (Fig. 2). The impression of the mandibular corpus of the right
dentary is about 15.0 mm long. On its posterior part, there are impressions of three tribosphenic
molars; the mesial two are clearly observable, whereas the distalmost one is not very distinct.
They clearly correspond to the m1–m3 described by Shikama (1947), and hereafter we follow
Fig. 2 The right and left lower jaws of Endotherium niinomii (D0247, holotype)
A. SEM image of a resin cast of the specimen in current condition
(mainly impressions of some teeth and both dentaries, except fragmentary anterior part of left dentary);
B. SEM image of a resin counter cast of the specimen; C. an interpretive sketch on B
Hatched areas are teeth (enclosed by solid lines) and possible teeth (enclosed by dotted lines)
184 Vertebrata PalAsiatica, Vol. 56, No. 3
his identication for these molars. There are impressions of at least two antemolar teeth dorsal
to the impression of the right dentary, which were not mentioned by Shikama (1947).
The left mandibular corpus is preserved partially as an impression (middle to posterior
portion, about 9.1 mm long) and as a fragment (anterior portion, about 4.2 mm long). No tooth
impression remains. According to Shikama’s (1947) descriptions and gures, the major part
of the left lower jaw was already missing when he observed the specimen, and the present
condition probably remains the same as what he described.
Shikama’s (1947) descriptions and figures suggest that the posterior part of the
mandibular corpus and the anteriormost part of the coronoid process of the right dentary
were preserved when he observed the specimen. These structures are now missing and only
the anterior part of the broken coronoid process is still partially preserved, but it is seriously
damaged. The angle between the mandibular corpus and the coronoid process is unknown.
In the impression of the left dentary, the posteriormost part of the dorsal margin slightly
curves posterodorsally; this part is probably the anterior base of the coronoid process. The
dentary is slender. Measured from the impressions, the right and left dentaries are about 2.4
and 2.6 mm deep, respectively, at the deepest part of each jaw. Shikama (1947) described
that the mandibular corpus of the right dentary is about 3 mm deep. As mentioned by him,
the ventral margin of the preserved portion of the dentary is relatively straight, and slightly
convex ventrally. There is a small depression on the right dentary, which seems to be a mental
foramen (Fig. 2). It is positioned about 1.3 mm above the ventral margin of the impression of
the dentary, and below the point mesial to the impression of the m1 (probably below the point
between the mesial root of the p5 and the distal root of the p4). Other features of the dentary,
such as additional mental foramina, the ascending ramus, and the Meckelian groove, cannot be
recognized from the impression, and Shikama (1947) did not describe them either.
The specimen preserved no incisors, canines, and premolars when Shikama (1947)
examined it, but, as mentioned above, there are impressions of at least two antemolar teeth
(Figs. 2, 3A). The mesial most impression is of the distal base of a slightly procumbent
tooth. Because the impression of the tooth is very limited, it is difficult to confirm whether
the tooth is a canine or a premolar. Distal to it and separated by a short diastema, there is an
impression showing nearly the complete shape of the protoconid of a tooth, but the basal
part of the tooth impression is not clear. It is undoubtedly an impression of a mesial premolar
with fully premolariform crown (Figs. 2, 3A). The tooth is erected, and the mesial margin of
the protoconid gently recurves toward the tip, which is comparatively steeper than the distal.
The presence of other cusps and a distobasal heel on the tooth is unknown. Judging from its
crown morphology and size, this tooth was likely to be single-rooted. Between this impression
and the impression of the m1, there is another impression (Figs. 2, 3A). Because this is not
well impressed, it is difficult to determine whether or not this is a tooth impression. If it is
of a tooth, it seems to be an impression of the protoconid of a premolariform premolar; this
tooth probably was taller than and mesiodistally longer than the mesial premolar (the second
185
Wang et al. - Reappraisal of Endotherium from the Lower Cretaceous of China
impression described above); there is no sign of a paraconid at the mesial base of the tooth, and
the paraconid is, therefore, thought to be absent or shifted (at least slightly) lingually. There are
two shallow and procumbent hollows above the anteriormost part of the dentary impression;
they are, however, too vague to determine whether or not they are related to procumbent
incisors.
No tooth impressions are observed for the left dentary, but six broken alveoli with partial
root fragments are seen on the anterior preserved part of the dentary (Fig. 2, 3C), as described
by Shikama (1947). The mesialmost alveolus is larger and extends more ventrally than the
others, and leans mesially. This alveolus is likely to be of a canine, although the possibility that
this alveolus is of a single-rooted mesial premolar is not eliminated. Distal to it, there are ve
sub-equal in size and nearly vertical alveoli, which are interpreted to be of one single-rooted
(mesial one) and two double-rooted (distal four) premolars. Judging from these alveoli, at least
the distal two double-rooted premolars were subequal in size.
Shikama (1947) described the right three molars. Some parts of his descriptions are
difcult to understand and here we compare them with our observations of the characteristics
that we could observe on the impressions, and select or interpret understandable and
informative descriptions for the characteristics that we could not observe. Based on Shikama’s
(1947) descriptions and measurements, the three molars were undoubtedly tribosphenic,
although we can now only know the morphology of the protoconid and hypoconid (or a part
of talonid only for m2) of them from the tooth impressions. At least some of the measurement
values of molars presented by Shikama (1947:80) are quite certainly inaccurate; for example,
mesiodistal length of the m1, m2, and m3 were reported as 4, 3.5, and 3.7 mm, respectively.
Based on our measurement, the impressions of the m1 and m2 are 1.6 and 1.7 mm long,
respectively, and thus the true molar lengths (estimated about 2 mm) should be slightly more
than these values, but not up to the numbers given by Shikama (1947).
The impressions of the m1 and m2 are better preserved and much more clear than those of
the antemolars and the m3. Judging from the impression, the m2 is displaced posterolingually,
as shown in the gure 4 of Shikama (1947), and the m1 is slightly displaced mesially. The m3
is apparently present more labially than the m1 and m2 (Fig. 2); this is probably because of the
damage to the dentary. The impression of the m1 is only of its labial side including protoconid
and hypoconid. The situation is similar for the impression of the m2, but because the tooth
was slightly more labially displaced, the hypoconulid, the labial side of the entoconid, and the
talonid basin are also partly impressed (Fig. 3B). For the m3, we can only observe parts of its
protoconid, hypoconid, and hypoconulid (Fig. 3B). The impressions suggest that the m1 and
m2 are larger than the m3, although the impression of the m3 is not measurable. Shikama (1947)
suggested that the molar sizes diminish distally, as shown in his measurements, although the
measurement values are inaccurate as mentioned above. Based on our observation, the m1 is
possibly as large as or slightly larger than the m2. As mentioned above, the impression of the
m2 is slightly longer mesiodistally than that of the m1, but these values may not represent their
186 Vertebrata PalAsiatica, Vol. 56, No. 3
Fig. 3 Images (left) and interpretive sketches (right) of a resin counter cast of the holotype
of Endotherium niinomii (D0247, holotype) (as in Fig. 2B)
A. the right ?p1 and ?p4; B. the right m2 and m3 (SEM image); C. the anteriormost part of the left dentary
with alveoli for ?c–?p3. Abbreviations: cob. cristid obliqua; hyd. hypoconid; hyld. hypoconulid;
prd. protoconid; pregd. precingulid; tadb. talonid basin
true length; the size difference between these two molars, however, is difficult to ascertain
from the impressions, because they show different degrees of lingual displacement.
Although Shikama (1947) only clearly described the m1, at minimum both the m1 and
m2 had blunt cusps. Shikama (1947:79) described that the “protoconid in buccal view, with
very at wall and sharp crest” for the m2, but the impression of the protoconid of the m2 shows
that the cusp was obviously blunt, not sharp. Additionally, in all lower molars, the trigonid is
higher than the talonid as mentioned by Shikama (1947); the height difference between the
talonid and trigonid is, however, not as great as seen in other eutherians reported from the
Late Jurassic to Early Cretaceous strata. In our measurement of the impressions, the heights
of protoconid and hypoconid of the m1 are 1.3 and 0.8 mm, respectively, and those of the
187
Wang et al. - Reappraisal of Endotherium from the Lower Cretaceous of China
m2 are 1.4 and 0.8 mm, respectively. According to the diagnosis of Endotherium by Shikama
(1947), the hypoconid is the largest of the talonid cusps, and thus the height of the hypoconid
can represent the height of the talonid. The protoconid seems to be worn especially in the m1,
as Shikama (1947) described, but it is probably not much greater than the measurement value.
He described that the m3 trigonid is low relative to the talonid, but the impression of the tooth
shows that the trigonid is still notably higher than the talonid.
As described by Shikama (1947), the m1 and m2 were probably similar in morphology.
His measurements show that the m3 has a distally elongated talonid, which suggests that this
tooth is the last molar; this can be conrmed by the hypoconulid impression situated distally
and slightly medially to the hypoconid. According to Shikama’s (1947) diagnosis of the genus
Endotherium, the protoconid is the largest among three trigonid cusps, and the paraconid
and the metaconid are subequal in size. The three trigonid cusps of the m2 were illustrated
as being subequal in size in the figure 4 of Shikama (1947), but the impression shows that
these drawings of molars are most likely inaccurate. Shikama (1947:79) described that the
“paraconid and metaconid running paralled (likely to be a typographical error of ‘parallel’)
to each other” for the m1. This probably means that the paraconid is not shifted buccally and
both the paraconid and metaconid are situated lingually as commonly seen in Early Cretaceous
eutherians, but this cannot be confirmed based on the impressions. He did not mention this
characteristic for the m2 and m3 (the paraconid and metaconid of the m3 were missing when
he observed the specimen). He described “peripheral cusps” or a “parastyle” at the mesiolabial
part of the trigonid of the m2 (he actually wrote the “outer side” of the “anterior border” of the
trigonid, which is here interpreted as the mesiolabial part). There are no impressions of such
cusps, but there is an impression of a precingulid at the mesiolabial base of the crown (Figs.
2, 3B), and we think that this is what he described. The impression of the m1 shows that a
precingulid was present on the tooth, but Shikama (1947) did not mention it. According to him,
the precingulid was absent on the m3; this part of the tooth is not impressed on the matrix, and
we could not conrm this. The impressions of the m1 and m2 show that a labial cingulid is
absent on these teeth. Judging from the impressions, the distal trigonid wall is nearly vertical.
Other trigonid features such as the cusp e and the angle of the trigonid are not known from
either the impressions or Shikama’s (1947) description.
The impressions show that the hypoexid is deeply formed on the molars, and that the
m2 talonid was well developed and basined (Fig. 3B). According to the measurements of
Shikama (1947), the talonid was as wide as the trigonid in the m2. The talonid of each molar
probably had three talonid cusps, although Shikama (1947) did not mention the entoconid of
the m1. Terms of the talonid cusps are confused in the descriptions of Shikama (1947); we
here interpret that the “endoconid” and “endoconulid” in his descriptions are the entoconid and
hypoconulid, respectively. According to the diagnosis of the genus as well as descriptions by
Shikama (1947), the hypoconid is larger than the other two talonid cusps, and the hypoconulid
is the smallest at least on the m2; the impression of the m2 shows that the hypoconulid of the
188 Vertebrata PalAsiatica, Vol. 56, No. 3
tooth is probably smaller than the hypoconid (Fig. 3B). He also described that the hypoconid
and hypoconulid were situated at the distolabial corner of the m1 talonid. This is, again,
unlikely. The impression of the m1 clearly shows that there was only one cusp, the hypoconid,
at that part, and thus the hypoconulid was surely placed more medially. Given that the m1 is
morphologically similar to the m2 as described in Shikama (1947), its hypoconulid is probably
not very closely positioned with the entoconid, similar to that in the m2 where the hypoconulid
is approximately placed at the labiolingual midline of the crown (Fig. 3B). The postcingulid is
absent on at least the m1 and m2.
3 Discussion and concluding remarks
Although not certain, tooth designations on the basis of impressions (the right lower jaw)
and alveoli (the left lower jaw) of antemolars are highly probable. The distance between the
mesialmost impression of a tooth (except for anterior unobvious two) and the impression of
the m1 on the right dentary is about 5.7 mm. Taking into account the molar length, about 2
mm, and premolar size estimated based on impressions to be much shorter mesiodistally than
molars, there were probably ve teeth. Five lower premolars are commonly present in early
eutherians (Kielan-Jaworowska et al., 2004). The mesialmost impression is, therefore, likely
to have been left by the canine. The second one is probably of the p1, and the third one might
be of the p4, if it is an impression of a tooth. If our identication of the right p1 is correct,
the mesialmost alveolus for a procumbent root on the left dentary is more likely to be of the
canine than a premolar, because the right p1 is erected. This view is supported by the fact that
this alveolus is slightly larger than the distal ones. The distance between this alveolus and the
posterior end of the impressed mandibular corpus of the left dentary is about 12.0 mm, which
does not exceed, but rather is compatible with the distance between the mesialmost tooth
impression and the posterior end of the mandibular corpus of the right dentary (12.5 mm).
Therefore, the ve more distal alveoli are probably for the single-rooted p1 and double-rooted
p2 and p3, respectively. This suggests that the p3 of Endotherium niinomii was probably
subequal to the p2 in size and not being obviously reduced. This is still the case even if the
mesial alveolus is of a single-rooted p1.
We agree with the Shikama’s (1947) attribution of Endotherium to Eutheria because of
the following characteristics: the slender dentary, which is more common among eutherians
than among metatherians in the Cretaceous; the presence of five teeth (five premolars)
implied by a relatively long distance between the canine and the rst preserved molar rather
than four (three premolars and a molar) as seen in most metatherians except for Sinodelphys;
and especially, the absence of the twinning of the entoconid and hypoconulid, in contrast to
twinned entoconid and hypoconulid common in most Cretaceous metatherians except for
primitive forms. The fact that there is no denite metatherian material in the fossil assemblage
with over one hundred mammalian specimens collected from the Shahai and Fuxin formations
further supports this view.
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Wang et al. - Reappraisal of Endotherium from the Lower Cretaceous of China
Informative characters of Shikama’s (1947) original diagnosis for the genus Endotherium
include: 1) lower jaw is not stout; 2) lower molars decrease their size distally from the m1 to
m3; 3) the trigonid and the talonid are transversely wider than long; 4) the protoconid is the
tallest among the trigonid cusps, and the paraconid is as tall as the metaconid; 5) the hypoconid
is larger than the other two talonid cusps (the “hl’s” in the diagnosis of Shikama, 1947, is
here interpreted as the hypoconulid); and 6) the precingulid is present (the “parastyle” in his
diaginosis is here reinterpreted as the precingulid as mentioned above) on the lower molars.
Among these characters, we concur 1), 2), and 6), although 6) is not diagnostic because
the precingulid is present in the molars of most Cretaceous eutherians. In addition to these
characters, our observation of the impressions reveals that Endotherium has some additional
features: the trigonid taller than the talonid in the lower molars, but their height difference is
not remarkable; and blunt lower molar cusps. Moreover, it is highly probable that Endotherium
has ve lower premolars, the p3 not being smaller than the p2.
The above mentioned character combination of Endotherium is sufciently diagnostic,
so that we consider Endotherium niinomii as a valid taxon. Endotherium is obviously different
from other Jurassic to Early Cretaceous eutherians (Acristatherium, Eomaia, Holoclemensia,
Juramaia, Montanalestes, Murtoilestes, Prokennalestes, and Sasayamamylos) in having:
blunt molar cusps; the molar diminishing in size from the m1 to m3; and the trigonid not
greatly taller than the talonid (Slaughter, 1968; Kielan-Jaworowska and Dashzeveg, 1989;
Sigogneau-Russell et al., 1992; Cifelli, 1999; Averianov and Skutschas, 2001; Ji et al., 2002;
Hu et al., 2009; Davis and Cifelli, 2011; Luo et al., 2011; Kusuhashi et al., 2013). The former
two characteristics, as well as the protoconid being the tallest among trigonid cusps and that
the paraconid is almost as tall as the metaconid, distinguish Endotherium from most Late
Cretaceous taxa. Blunt lower molar cusps characterizing Endotherium are also found in some
Late Cretaceous species, such as Paranycotoides and zhelestids (Nessov et al., 1998; Archibald
and Averianov, 2001, 2012; Averianov and Archibald, 2005, 2013; Gheerbrant and Astibia,
2012; Montellano-Ballesteros et al., 2013). In the lower molars of these eutherians, however,
the metaconid is much larger than the paraconid, and the protoconid is only slightly larger
than the metaconid. Endotherium is, therefore, clearly distinguishable from these species.
Shikama (1947) compared Endotherium with Zalambdalestes, one of few eutherians known
from Asia at the time, and suggested their comparatively close relationship. Endotherium is,
however, different from Zalambdalestes in many aspects as partly noted by Shikama (1947),
for example the possible possession of ve lower premolars, the talonid as wide as the trigonid
in the molars, and the protoconid being taller than the metaconid. Zalambdalestids are known
to have an enlarged incisor, the root of which is extended distally to below the level of the m1
(Fostowicz-Frelik and Kielan-Jaworowska, 2002), but the anterior preserved portion of the
left dentary of Endotherium does not show any sign of such an enlarged incisor. Therefore,
it is unlikely that the two genera are closely related to each other in today’s view. Due to the
condition of the type specimen of Endotherium, substantial comparison is difficult at the
moment, but better preserved specimens will provide more information about the genus.
190 Vertebrata PalAsiatica, Vol. 56, No. 3
Acknowledgments We are grateful to Wang Zhiyan and Shen Caizhi (Dalian Museum of
Natural History) for help in accessing and observing the type specimen of Endotherium.
We thank Wang Zhao (IVPP) for making the cast from the impression of the type specimen.
Thanks also go to Terufumi Ohno, Satoshi Maruyama (Kyoto University, Japan), Masahiro
Ichida (Toyohashi Museum of Natural History, Japan), and Mao Fangyuan (IVPP) for their
help in SEM photographing. Specimen observation in Russia was kindly supported by
Alexander O. Averianov (Zoological Institute, Russian Academy of Sciences). We are also
grateful to the reviewers for their critical reading and informative suggestions, and to Yinmai
O’Connor for improving the English text. This research was supported by Grants-in-Aid for
Young Scientists (B) (no. 24740349, 16K17830) from Japan Society for the Promotion of
Science awarded to N.K., the Strategic Priority Research Program (B) of Chinese Academy
of Sciences (no. XDB18000000), the National Natural Science Foundation of China (no.
41688103, 41541015), and the Dalian Municipal Science and Technology Plan Projects (no.
20044J9JH238).
辽宁下白垩统阜新组真兽类哺乳动物新带氏远藤兽
(Endotherium niinomii Shikama, 1947)再研究
王元青1,2,3 楠桥直4 金 迅1 李传夔1 濑户口烈司5 高春玲6 刘金远6
(1 中国科学院古脊椎动物与古人类研究所，中国科学院脊椎动物演化与人类起源重点实验室 北京 100044)
(2 中国科学院生物演化与环境卓越创新中心 北京 100044)
(3 中国科学院大学地球科学学院 北京 100049)
(4 日本爱媛大学大学院理工学研究科 爱媛 790-8577)
(5 日本京都市青少年科学中心 京都 612-0031)
(6 大连自然博物馆 辽宁大连 116023)
摘要：根据正型标本的印痕和翻制的模型，重新研究了产于辽宁阜新新邱阜新组的早白垩
世真兽类哺乳动物新带氏远藤兽(Endotherium niinomii Shikama, 1947)。对照Shikama (1947)
的原始描述，将E. niinomii的主要特征归纳为：下臼齿从m1到m3逐渐变小；下三角座和下
跟座的高差不是很大；下臼齿的齿尖钝；下原尖比下前尖和下后尖大, 并且下前尖和下后
尖高度相近；可能有5个前臼齿, 并且p3不明显变小。这些特征组合足以将该属与其他的白
垩纪真兽类相区别，因此认为Endotherium niinomii是一个有效种。
关键词：辽宁阜新, 早白垩世, 阜新组, 哺乳动物, 真兽类, 远藤兽
中图法分类号：Q915.873 文献标识码：A 文章标号：1000–3118(2018)03–0180–13
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