A new Early Cretaceous eutherian mammal from the Sasayama

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A new Early Cretaceous eutherian mammal from the Sasayama
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A new Early Cretaceous eutherian
mammal from the Sasayama Group,
Hyogo, Japan
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Research
Cite this article: Kusuhashi N, Tsutsumi Y,
Saegusa H, Horie K, Ikeda T, Yokoyama K,
Shiraishi K. 2013 A new Early Cretaceous
eutherian mammal from the Sasayama Group,
Hyogo, Japan. Proc R Soc B 280: 20130142.
http://dx.doi.org/10.1098/rspb.2013.0142
Received: 20 January 2013
Accepted: 7 March 2013
Subject Areas:
palaeontology, evolution
Keywords:
Early Cretaceous, Eutheria, Sasayamamylos
kawaii gen. et sp. nov., Sasayama Group,
Japan, early eutherian diversification
Authors for correspondence:
Nao Kusuhashi
e-mail: [email protected]
Yukiyasu Tsutsumi
e-mail: [email protected]
†
Author for correspondence for radiometric
dating part.
Electronic supplementary material is available
at http://dx.doi.org/10.1098/rspb.2013.0142 or
via http://rspb.royalsocietypublishing.org.
Nao Kusuhashi1, Yukiyasu Tsutsumi2,†, Haruo Saegusa3,4, Kenji Horie5,
Tadahiro Ikeda4, Kazumi Yokoyama2 and Kazuyuki Shiraishi5
1
Department of Earth’s Evolution and Environment, Graduate School of Science and Engineering,
Ehime University, Ehime 790-8577, Japan
2
Department of Geology and Paleontology, National Museum of Nature and Science, Ibaraki 305-0005, Japan
3
Institute of Natural and Environmental Sciences, University of Hyogo, Hyogo 669-1546, Japan
4
Museum of Nature and Human Activities, Hyogo 669-1546, Japan
5
National Institute of Polar Research, Tokyo 190-8518, Japan
We here describe a new Early Cretaceous (early Albian) eutherian mammal,
Sasayamamylos kawaii gen. et sp. nov., from the ‘Lower Formation’ of the
Sasayama Group, Hyogo Prefecture, Japan. Sasayamamylos kawaii is characterized by a robust dentary, a distinct angle on the ventral margin of the
dentary at the posterior end of the mandibular symphysis, a lower dental
formula of 3 –4 : 1 : 4 : 3, a robust lower canine, a non-molariform lower
ultimate premolar, and a secondarily reduced entoconid on the molars. To
date, S. kawaii is the earliest known eutherian mammal possessing only
four premolars, which demonstrates that the reduction in the premolar
count in eutherians started in the late Early Cretaceous. The occurrence of
S. kawaii implies that the relatively rapid diversification of eutherians in
the mid-Cretaceous had already started by the early Albian.
1. Introduction
Eutherians represent ca 94 per cent of extant mammalian species [1,2], and are thus
important for understanding the evolutionary and ecological history of mammalian faunas. However, because of deficiencies in the fossil record, the early
evolution of eutherians remains obscure, and their mode of early diversification
has yet to be clearly understood. The recent discovery of a Jurassic eutherian
from China, Juramaia sinensis, indicates that the appearance of eutherians dates
to at least the Middle Jurassic [3]. This raises the interesting possibility that eutherians have been a minor element of mammalian faunas throughout the Jurassic and
early Early Cretaceous, at least in Asia, in contrast to their relative abundance in
Late Cretaceous and Cainozoic faunas in the Northern Hemisphere. The 13
genera and 16 species of mammals reported from the Yixian Formation (Barremian–Aptian) of northeastern China include only two genera and two species
of eutherians, Eomaia scansoria and Acristatherium yanensis [4,5]. It is probable
that this rich mammalian fossil assemblage somewhat reflects the true composition of its component clades; if so, the mammalian records of the Yixian
Formation suggest that eutherians were poorly diversified in the mid-Early Cretaceous. The absence of a eutherian record in other Late Jurassic and early Cretaceous
fossil faunas at least in the Northern Hemisphere seems to support this view. In
light of the diversity attained by the Late Cretaceous, this observation suggests a
comparatively rapid eutherian diversification in the mid-Cretaceous. Some molecular-based studies indicate that the early diversification of placental mammals
(a clade including the common ancestor of all extant eutherians) occurred in the
mid-Cretaceous [6–8]; if so, the placental diversification can be considered to
have taken place nearly contemporaneously with the eutherian diversification,
although studies based on fossil records generally contest this view and propose
later diversification of placentals [9,10].
& 2013 The Author(s) Published by the Royal Society. All rights reserved.
35° N
Kobe
Kyoto
Osaka
‘Upper Fm.’
106.4 ± 0.4 Ma
mammalian fossils,
112.1 ± 0.4 Ma
accretionary
complex
Figure 1. (a) Location of Sasayama City. (b) Schematic stratigraphic table of the Sasayama Group (adapted from Yoshikawa [24]) and U–Pb ages obtained in this study.
This hypothetical diversification of eutherians is difficult to
examine on the basis of the fossil record. There are far more
Late Cretaceous mammalian fossil localities than those of the
Early Cretaceous [11]. It is thus difficult to determine whether
the apparent increase in eutherian diversity in the fossil record
subsequent to the Barremian–Aptian Jehol biota represents an
actual diversification or simply an increase in the number of
fossil localities.
Dental formula, especially the premolar count, provides
one possible clue for ascertaining patterns of eutherian diversification; in Cretaceous eutherians, the premolar count is
better known than the incisor count, which is rarely known.
The primitive eutherian premolar count is generally considered to be five [12,13], as shown by early eutherian taxa,
such as Acristatherium, Eomaia and Juramaia [3– 5]. Other
Early Cretaceous eutherians with known premolar counts
(Prokennalestes and unnamed eutherians from the Lower Cretaceous Shahai and Fuxin formations of northeastern China)
also have five premolars [14–16]. In contrast to this consistent
retention of the five premolars observed in Jurassic and Early
Cretaceous eutherians, the premolar count varies from three
to five in Late Cretaceous eutherians [10,11,17,18]. This
increase in variation of dental formula is apparently coincident with the diversification of the eutherians (the number
of described Late Cretaceous eutherian species are more
than quintuple that of the Jurassic to Early Cretaceous) and
is possibly related to it.
Here, we report on the first known Early Cretaceous eutherian mammal with only four lower premolars, which we have
named Sasayamamylos kawaii, and we briefly discuss the implications of this finding for the timing of early eutherian
diversification. We would like to emphasize that our new
material is well dated; thus, the information will contribute
to future studies on the early evolution of eutherians. In this
study, we follow the premolar labelling of Cifelli [19], the
dental terminology for tribosphenic molars of Bown & Kraus
[20] and Kielan-Jaworowska et al. [11], and the wear facet designation of Davis [21] modified from Crompton [22]. Lower
incisors, canines, premolars and molars are abbreviated by
lower case i, c, p and m, respectively, followed by numbers
indicating the order of each tooth in each tooth class. For
the phylogenetic relationships of eutherians, we basically
follow those of Luo et al. [3] modified from Wible et al. [10,18].
Holoclemensia is treated as a eutherian following Davis & Cifelli
[23]. Ambondro, ausktribosphenids and henosferids are also
taken into account in comparison (but not in discussion because
their relationship with eutherians is contentious).
2. Geologic setting
The mammalian specimens reported on here were collected
from the ‘Lower Formation’ of the Cretaceous Sasayama
Group, distributed in the eastern part of Hyogo Prefecture,
Japan (figure 1). The Sasayama Group unconformably overlies
a Permo–Triassic accretionary complex, and is unconformably
overlain by the Upper Cretaceous Arima Group [24]. The
group, which consists of the ‘Lower’ and ‘Upper’ formations,
is mainly composed of terrestrial clastic rocks ([24]; figure 1);
the ‘Lower Formation’ consists mainly of conglomerates,
sandstones and mudstones, and intercalated acidic tuff beds
in the lower part, whereas the ‘Upper Formation’ consists
mainly of andesites, andesitic pyroclastic rocks, and tuffaceous
sandstones and mudstones. The present materials were recovered from a red siltstone bed in the ‘Lower Formation’, at the
Miyada microvertebrate site in Sasayama City, which contains
fragmentary remains of dinosaurs, at least four different
species of Scincomorpha lizards [25], and one anuran species.
The depositional age of the Sasayama Group is still unclear,
although some zircon fission-track ages and biostratigraphical correlations have been reported from the group [26,27].
We obtained high-precision U–Pb ages of zircons using a sensitive high resolution ion microprobe (SHRIMP II) installed at
the National Institute of Polar Research (NIPR), Japan.
The samples for dating were collected from a tuff bed in the
lower part of the ‘Lower Formation’, and an andesite in
the lower part of the ‘Upper Formation’ (figure 1). The U–Pb
analyses gave deposition/eruption ages of the tuff and the
andesite of 112.1 + 0.4 Ma (95% confidence interval, CI) and
106.4 + 0.4 Ma (95% CI), respectively (figure 2). The acidic
tuff sample was collected from a tuff bed a few metres below
the fossil-bearing bed, at the same outcrop. The age of the fossils is, therefore, considered to be nearly the same as that of the
tuff bed (ca 112 Ma), which is approximately correlated with
the Aptian–Albian boundary [28]. Strictly speaking, the age
of the fossils is bounded by the dates obtained for the tuff
and the andesite, which are 112 and 106 Ma, respectively
(early Albian). See the electronic supplementary material file
Proc R Soc B 280: 20130142
34° N
‘Lower Fm.’
Sasayama
2
Arima Group
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36° N
(b)
Sasayama Group
136° E
Lower Cretaceous
135° E
pre-Cret.
(a)
Upper
Cret.
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(a) 118
3
111 mean = 106.4 ± 0.4 Ma
n = 22, MSWD = 1.13
109
114
112
110
(Ma)
(Ma)
107
105
103
101
106
99
Figure 2. U – Pb age distribution plots and mean ages of zircons from (a) an acidic tuff bed in the lower part of the ‘Lower Formation’ and (b) an andesite in the
lower part of the ‘Upper Formation’. Error bars show 2s ranges. The uncertainties of the mean ages are the 95% confidence limits of the t-distribution. MSWD,
mean standard weighted deviation.
for a description of the methodology and a more detailed
discussion of the age results.
3. Systematic palaeontology
Infraclass: Eutheria Gill [29]
Order: Asioryctitheria Novacek, Rougier, Wible, McKenna,
Dashzeveg & Horovitz [30]
Family: Incertae sedis
Sasayamamylos kawaii gen. et sp. nov. Kusuhashi and Saegusa
(a) Etymology
Sasayama, after the Sasayama Group, from which the present
materials were collected, and also after Sasayama City, where
the fossil locality is situated; mylos (Greek), meaning ‘mill’,
referring to the tribosphenic molars with fully developed
talonids; kawaii, in honour of Dr Masao Kawai, a primatologist born in Sasayama City who explored the origins and
evolution of humans through ecological studies of primates.
(b) Holotype
A right dentary, with alveoli for i1 and i2, and well preserved
crowns of i3, c, p1–p4 and m1–m3 (Museum of Nature and
Human Activities, Hyogo (MNHAH) D1-030444; figures 3a–c
and 4; electronic supplementary material, S3, S6, S8). The dentary is almost completely preserved; a small piece of the dorsal
part of the coronoid process is missing.
(c) Other referable specimens
We also refer to this new species a fragment of horizontal ramus
of left dentary, with i1–i3, c, p1–p4 and m1 (MNHAH
D1-030445; figure 3d–f; electronic supplementary material,
S4), a horizontal ramus of left dentary with i1–i4, c, p1–p4
and m1–m3 (MNHAH D1-030446; figures 3g–i and 5, electronic supplementary material, S5, S7, S8); and a fragment of
horizontal ramus of right dentary, with p3 and p4 (MNHAH
D1-030447; figure 3j–l; electronic supplementary material, S4).
(d) Locality and horizon
All specimens were recovered from the lower part of the
‘Lower Formation’ of the Sasayama Group, Miyada, Sasayama
City, Hyogo Prefecture, Japan (358050 3400 N, 1358100 4000 E).
This horizon is considered as early Albian (Early Cretaceous)
in age.
(e) Diagnosis
A eutherian mammal distinguished from any other Mesozoic
eutherians by the following combination of characteristics:
dentary robust; ventral margin of dentary having a distinct
angle at posterior end of mandibular symphysis; Meckelian
groove and labial mandibular foramen absent; lower dental
formula 3– 4 : 1 : 4 : 3; canine robust; ultimate lower premolar
non-molariform; in lower molars, protoconid much taller
than paraconid and metaconid; paraconid as tall as, or
slightly shorter than, metaconid; paraconid displaced labially
relative to metaconid in occlusal view; cusp e absent; talonid
much lower than trigonid; entoconid variably present. Differing from Late Cretaceous eutherians including those from
Asia such as Maelestes, zalambdalestids, zhelestids and
asioryctitherians in having lower molars with much taller
protoconid than the other two trigonid cusps, and large trigonid–talonid height difference. Differing from Acristatherium,
Bishops, Bobolestes, Eomaia, Eozhelestes, Henosferus, Juramaia
and Prokennalestes in having four lower premolars rather
than five or six; from Ambondro, Bobolestes, Holoclemensia,
Montanalestes, ausktribosphenids and many Late Cretaceous
eutherians in having a non-molariform ultimate lower premolar; from Sheikhdzheilia in absence of a lingual cingulid on the
ultimate lower premolar; from Acristatherium and Prokennalestes
in absence of a cusp e on the lower molars; from ausktribosphenids in having small lingual–labial height difference of the
molar crowns and a labially displaced paraconid relative to
the metaconid; and from Ambondro, ausktribosphenids, henosferids, in absence of an anterolingual cingulid on molars.
Differing from Juramaia, ausktribosphenids, henosferids and
Late Cretaceous eutherians in having a medially inflected angular process of the dentary; from Asioryctes, Barunlestes, Bishops,
Bobolestes, Cimolestes, Daulestes, Eoungulatum, Gypsonictops,
Henosferus, Juramaia, Kulbeckia, Maelestes, Paranyctoides, Parazhelestes, Zalambdalestes and Zhelestes in having a distinct angle
of dentary at posterior end of mandibular symphysis; from
Bobolestes, Eomaia, Henosferus, Prokennalestes and ausktribosphenids in the absence of a Meckelian groove on dentary; and from
henosferids in absence of a postdentary trough on dentary.
Proc R Soc B 280: 20130142
108
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116
(b)
mean = 112.1 ± 0.4 Ma
n = 24, MSWD = 0.65
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(c)
(a)
4
(d)
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(e)
2 mm
(b)
(f)
(g)
2 mm
( j)
(l)
(h)
(k)
2 mm
2 mm
Figure 3. Sasayamamylos kawaii gen. et sp. nov. (a–c) Holotype (MNHAH D1-030444); a right dentary with i3, c, p1–p4, and m1–m3. (d–f ) MNHAH D1-030445; a
fragment of the left dentary with i1–i3, c, p1–p4 and m1. (g–i) MNHAH D1-030446; a fragment of the left dentary with i1–i4, c, p1–p4, and m1–m3. ( j–l ) MNHAH
D1-030447; a fragment of the right dentary with p3 and p4. (a,d,g,j) Labial views, (b,e,h,k) lingual views and (c,f,i,l ) occlusal views (stereopairs).
(c)
(a)
2 mm
(b)
Figure 4. Posterior premolars (p3, p4) and molars of Sasayamamylos kawaii gen. et sp. nov. Holotype (MNHAH D1-030444). Surface rendering images. (a) Labial view, (b)
lingual view and (c) occlusal views (stereopair). Computed tomography (CT) scanning was performed using TOSCANER-32300mhd installed at the Technology Research
Institute of Osaka Prefecture, Japan, and CT images were rendered using AMIRA v. 5.3.3 software at the MNHAH.
4. Description
We provide here only a brief description of Sasayamamylos
kawaii gen. et sp. nov.; see the electronic supplementary
materials for additional description.
(a) Dentary
The dentary of S. kawaii is robust, and the Meckelian groove
and labial mandibular foramen are absent. The angular process
is rod-like and projects posteroventrally and is slightly inflected
medially. The mandibular symphysis extends posteriorly to a
point below the posterior root of p1. The longitudinal axis of
the symphysis is at an approximately 308 oblique angle to the
ventral margin of the horizontal ramus; thus, the ventral
margin of the dentary bends upward with a distinct angle at
the posterior end of the symphysis.
(b) Lower incisors
Sasayamamylos kawaii shows a variable numbers of incisors;
D1-030444 and D1-030445 have only three incisors, whereas
Proc R Soc B 280: 20130142
(i)
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(c)
(a)
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(b)
broken
Figure 5. Posterior premolars ( p3, p4) and molars of Sasayamamylos kawaii gen. et sp. nov. MNHAH D1-030446. Surface rendering images. (a) Labial view, (b)
lingual view and (c) occlusal views (stereopair).
D1-030446 has four. The anterior three incisors (i1 –i3) are
subequal in size and morphologically similar to one another,
with spatulate crowns and distally projecting tiny cuspules.
The i4 is present only in D1-030446, and is much smaller
than in the other three incisors (figure 3g– i; electronic
supplementary material, S5).
(c) Lower canine
The canine is a robust tooth, and is the tallest of the entire
lower tooth series; it slightly recurves posteriorly; its anterior
margin bends posterodorsally at about half the height of the
crown. It is double-rooted in D1-030444 and D1-030445, but is
single-rooted in D1-030446 (see the electronic supplementary
material, figure S9).
long as, slightly narrower than, and nearly half as tall
as the trigonid. The entoconid is absent on the molars of
D1-030444 and D1-030445. In D1-030446, the entoconid is
present, but is smaller than the other two talonid cusps; it
is more distinct on the m1 than on the m2 and m3.
Wear facets are best observed on the m1 of D1-030445.
The wear facet 1 is developed on the posterior surface of the
protoconid and the hypoflexid. The anterior surface of the protoconid is also worn (facet 2). Facet 3 is observed on the cristid
obliqua from the tip of the hypoconid and on the hypoflexid.
Facet 4 is not clearly observed, but facet 5 is clearly present
on the posterior surface of the metaconid. There is a small
wear facet on the cristid at the position where the entoconid
is thought to usually be present in the other eutherians, and
this is probably facet 6.
(d) Lower premolars
There are four lower premolars and all of them are doublerooted. The lower premolars increase in size posteriorly
from p1 to p3; p4 is as large as p3 (see the electronic supplementary material, table S1). The ultimate premolar ( p4)
is not submolariform. The heel of p4 is placed higher than
that of p3, and has a more dorsally projecting heel cusp;
the heel of p4 is also transversely wider than that of p3, but
the distinct basin is not developed.
(e) Lower molars
The three lower molars are subequal in size; the m2 is slightly
larger than the m1 and m3 (see the electronic supplementary
material, table S1). The trigonid cusps are trenchant. The protoconid is much taller than the other two trigonid cusps. On
the m1, the metaconid is taller than the paraconid, whereas
on the m2 and m3, the paraconid is nearly as tall as the metaconid. In occlusal view, the paraconid is displaced labially
relative to the metaconid; it is more labially situated on the
m1 than on the m2 and m3. The paraconid is also slightly
procumbent anteriorly. The cusp e, which is present in the
molars of Prokennalestes and Acristatherium [5,14], is absent.
The talonid is well developed and basined; it is nearly as
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2 mm
5
5. Comparison and discussion
Specimens of S. kawaii show variability in some characters. D1-030446 can be distinguished from D1-030444 and
D1-030445 in having a single-rooted canine rather than
double-rooted, four lower incisors rather than three, and
small but distinct entoconids on the molars. The size difference
between p1 and p2 is slightly less in D1-030446 than in D1030444 and D1-030445 (see the electronic supplementary
material, table S1). The determination of whether these differences are intraspecific or interspecific, based on data from only
four specimens, is difficult. However, in general, the specimens
are morphologically similar; for example, they possess a robust
dentary and a distinct angle of the ventral margin of the dentary
at the posterior end of the oblique mandibular symphysis, which
are features uncharacteristic of other Mesozoic eutherians, as
mentioned below. In addition, the specimens were collected
from the same bed. Based on these observations, we tentatively
assign all of the specimens to one species (S. kawaii).
The possession of three to four lower incisors, four lower
premolars and three tribosphenic lower molars with fully developed talonids indicates that S. kawaii belongs to the Eutheria
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6
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mammals, such as in Bobolestes, Eomaia and Prokennalestes
[4,14,39,40,42,44,45]; it shares the absence of this groove
with Juramaia, Montanalestes and many Late Cretaceous
eutherians [3,33]. The lower incisor count of S. kawaii is
also probably apomorphic. Two specimens of S. kawaii
(D1-030444 and D1-030445) possess only three lower incisors,
whereas D1-030446 has four lower incisors, the last of which
(i4) is considerably smaller than the other three. We consider
that the i4 of D1-030446 is unlikely to be a deciduous tooth,
because m3 of this specimen is fully erupted and is even
slightly worn. This condition suggests that S. kawaii probably
shows a transitional developmental pattern. The primitive
lower incisor count for eutherians is possibly four [11,47].
Several early eutherians, such as Bobolestes, Eomaia, Eozhelestes
and Juramaia have four lower incisors [3,4,44]; there are
also some later forms, such as Asioryctes, Kulbeckia and
Ukhaatherium, which have four lower incisors [30,48,49],
although one of the earlier eutherian mammals, Acristatherium,
has only three incisors [5]. Sasayamamylos kawaii shows a
polymorphic intermediate state between primitive forms
with four lower incisors and those with three; this indicates that the reduction in the incisor count in the lineage to
which Sasayamalestes belongs started from i4. This manner
of reduction is different from that presumed for the Zalambdalestidae, in which the i2 was lost first [49]. The presence of
S. kawaii and Acristatherium in the Early Cretaceous indicates
that the reduction of lower incisor counts from four to three
occurred independently at least several times, and probably in several different ways, during the early evolution of
the eutherians.
The lower premolar count of S. kawaii is four rather than
five; this is another derived feature of S. kawaii, as the primitive
number of lower premolars for eutherians is probably five
[3,12,13,17], as observed in all previously reported Jurassic
to Early Cretaceous definite eutherians with known premolar
counts (Acristatherium, Eomaia, Juramaia, Prokennalestes and
unnamed eutherians from the Lower Cretaceous Fuxin
Formation of northeastern China [3–5,14–16]). The earliest
Late Cretaceous eutherians, such as Bobolestes and Eozhelestes,
also retain five lower premolars [44]; however, in two dentary
specimens (described as ‘Zhelestidae’ indet., unnamed large
sp. A [44, and references cited therein]) from the lower
Cenomanian of Uzbekistan, the px has been lost and a long
diastema is present between p2 and p3. Sasayamamylos kawaii
lacks such a diastema, and is considered to show more derived
condition than the unnamed Uzbekistan specimens. Five lower
premolars are also known for some later Late Cretaceous
eutherians, such as Gypsonictops, Maelestes, Paranyctoides and
at least some zhelestids [10,50–54]. Premolar reduction is
thought to be a common trend in eutherian lineages [17] and
fossil records, including those for Sasayamamylos, suggest
that the loss of a premolar occurred independently in at least
several eutherian lineages. Sasayamamylos kawaii is currently
the earliest known eutherian mammal in which a premolar
has been completely lost, demonstrating that the reduction in
premolar count had already started among eutherians in the
late Early Cretaceous.
The robust dentary and the ventral margin with a distinct
angle at the posterior end of the symphysis, relating to the
large canine roots, are unique characteristics of Sasayamamylos
among Mesozoic eutherians, whereas they are comparatively
not uncommon in early metatherians as seen in Alphadon,
Deltatheridium and Eodelphys [34,55,56]. Dentaries of many
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[11]. Our tentative phylogenetic analyses using the data matrix
of Luo et al. [3] modified from Wible et al. [10,18] with S. kawaii
also support this view (see the electronic supplementary
material). Although our analyses also suggest the close affinity
of S. kawaii to the Asioryctidae (sensu Archibald & Averianov
[31]), we here conservatively classify S. kawaii into the Asioryctitheria. The angular process of the dentary of S. kawaii is slightly
inflected medially, which is a character seen in most metatherians including Cretaceous members [11,32]. However, an
inflected angular process is also present in some early eutherians such as Eomaia, Montanalestes and Prokennalestes [4,14,33],
and the angular process in these taxa is also rod-like, being morphologically similar to that of S. kawaii. Compared with the
angular process of the basal metatherian Deltatheridium, the
rod-like angular process of S. kawaii is clearly different from
the shelf-like process of Deltatheridium [34]. The angular process
of the oldest known definitive eutherian, Juramaia, is apparently
not inflected [3], and even the oldest known metatherian, Sinodelphys, also lacks the inflected angular process [35]. The rod-like
inflected angular process is possibly an apomorphic character
for eutherians, seen only in early members.
Sasayamamylos kawaii differs from all known Mesozoic
eutherians in absence of entoconids on the lower molars,
although one specimen (D1-030446) has distinct entoconids.
The entoconid is not present on the lower molars of some
stem boreosphenidans, such as Kielantherium and Hypomylos,
although the other two talonid cusps are present in these
genera [36,37]. This indicates that the absence of the entoconid in S. kawaii can be interpreted as a primitive condition.
However, the lower molars of Jurassic–Cretaceous eutherians
known thus far including those of taxa older than S. kawaii,
such as in Acristatherium, Eomaia, Juramaia and Murtoilestes,
possess distinct entoconids [3– 5,38]. Ausktribosphenids and
henosferids also have entoconids [39–42], whereas Ambondro
does not have a distinct entoconid [43]. Here, taking into
account some advanced characteristics mentioned below,
we consider that the entoconid of S. kawaii is a secondarily
reduced condition.
Sasayamamylos kawaii is clearly distinguished from most
Late Cretaceous eutherians by lower molars with much taller protoconids than paraconids and metaconids, and the great height
difference between the trigonid and the talonid. These are features characteristic of early eutherians, such as in Acristatherium,
Eomaia, Holoclemensia, Juramaia, Montanalestes, Murtoilestes and
Prokennalestes [3–5,14,23,33,38]; thus, they can be regarded as
primitive for eutherians. Within these early eutherians, the general configuration of lower molars of Sasayamamylos is similar
to those of Holoclemensia, but the former genus is distinguished
from the latter by the more open trigonid angle and the wider
talonid of m2 [23]. Sasayamamylos also differs from Holoclemensia
in having a non-molariform ultimate lower premolar [23], which
also distinguishes Sasayamamylos from ausktribosphenids and
some mid-Cretaceous eutherians, such as Bobolestes (including
Otlestes, which is suggested as a junior synonym of Bobolestes by
Averianov & Archibald [44]) and Montanalestes [33,39,40,44–46].
On the other hand, Sasayamamylos shares a non-molariform
ultimate lower premolar with Acristatherium, Eomaia, Juramaia,
Prokennalestes and Sheikhdzheilia [3–5,14, 44]; this characteristic is
also thought to be plesiomorphic for eutherians.
In contrast to its primitive features, S. kawaii possesses
some advanced features for early eutherians. For example,
S. kawaii does not have a Meckelian groove, which is present
in Henosferus, ausktribosphenids, and some early eutherian
Downloaded from http://rspb.royalsocietypublishing.org/ on August 13, 2017
be supported by Holoclemensia and Montanalestes from the
Aptian–Albian of North America, which have molariform
ultimate lower premolars [23,33], although their premolar
counts are unknown. The molariform ultimate premolar is common for Late Cretaceous eutherians, but is not known among
other Jurassic to Early Cretaceous definite eutherian taxa. This
is another example of the increased variation of the dental
morphology among eutherians in the late Early Cretaceous.
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We wish to express our sincere gratitude to K. Adachi for his discovery of the Miyada microvertebrate site and to Y. Takahashi for her
preparation of fossil specimens. Thanks are also owing to staff and
volunteers of MNHAH, and to staff of Sasayama City, for their support and assistance. We thank M. Shigeoka for her help in sample
preparation and SEM analyses for zircon U– Pb dating. U– Pb
dating was contributed by the NIPR SHRIMP Laboratory, Japan.
Specimen observation in the Museum Victoria was supported by
Thomas H. Rich, David A. Pickering and Erich M. G. Fitzgerald.
This paper was greatly improved by various significant suggestions
of Brian M. Davis, another anonymous reviewer, and an associate
editor, Zhe-Xi Luo. The programme TNT 1.1 is being made available
with the sponsorship of theWilli Hennig Society (the result of the
analysis is mentioned in the electronic supplementary material).
This work was supported in part by a Grant-in-Aid for Scientific
Research (B) (no. 20340145) from the Japan Society for the Promotion
of Science (JSPS) awarded to H.S. and N.K., a Grant-in-Aid for Young
Scientists (B) (no. 21740366 and 24740349) from JSPS to Y.T. and N.K.
(respectively), a Grant-in-Aid for Research Promotion from Ehime
University, Japan, to N.K., a Grant-in-Aid for Research Promotion
from University of Hyogo, Japan, to H.S., and Projet International
de Coopération Scientifique (DINASIA) from Centre National de la
Recherche Scientifique, France, to N.K.
7
rspb.royalsocietypublishing.org
Mesozoic eutherians are rather slender, and most Mesozoic
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