All specimens studied here were collected by surface prospecting and screen-washing from 6 fossiliferous localities within three faunas: the DM16 fauna (including DM16, DM30, DM31), the DM01 fauna (including DM01, DM03), and the DM02 fauna (including DM02) in the Damiao area of Nei Mongol. The age and stratigraphic context of these localities can be referred to in Kaakinen et al. (2015). Observations and measurements were taken using a Leica M205 microscope with a precision of 0.01 mm. Measurements of the height of the horizontal ramus of the mandible followed those of Wang and Qiu (2018). All the measurements are given in millimeters. The images of specimens were generated by scanning with a Micro-X ray-CT: Xradia 520 Versa (Carl Zeiss X-ray Microscopy, Inc., Pleasanton, USA) at the Yunnan Key Laboratory for Palaeobiology, Institute of Palaeontology, Yunnan University, Kunming, China. Scanning parameters are as following: Beam strength: 50kV/4wW, no Filter, Resolution: 9.25µm. Images shown below were captured with the software Mimics (Version 17.0), processed with Adobe Photoshop CC 2018.
For convenience of comparison, all right-side teeth are figured as mirror images (reversed). Dental terminology in the description illustrated in Fig. 1 following Qiu and Li (2016). The proportion of specimens with a certain character was expressed as a ratio (*/*) of the variant to the total number of specimens. Upper teeth are shown with capital letters (P4, M1–M3) and lower teeth with lower case letters (m1–m3). In the Tables of measurements, length is abbreviated as L and width as W.
All studied specimens were deposited at the Institute of Vertebrate Paleontology and Paleoanthropology, Chinese Academy of Sciences (IVPP), Beijing, China.
Institutional Abbreviations: IVPP, Institute of VertebratePaleontology and Paleoanthropology, Beijing, China.
M1-2:
1.antercone; 2.anterostyle; 3.anterosinus; 4.paracone; 5.mesosinus; 6.mesostyle; 7.metacone; 8.posterosinus; 9. protocone; 10.protostyle; 11.enteroloph; 12.mesocone; 13.hypocone; 14. posterocone; 15. anteroloph; 16. protoloph I; 17. protoloph II; 18. mesoloph; 19. metaloph; 20. posteroloph; 21. sinus; 22. entoloph.
m1-2:
1.anteroconid; 2.metaconid; 3.psudomesolophid; 4.mesostylid; 5.entoconid; 6.posterosinusid; 7.metalophid II; 8. protoconid; 9.posterior crest of protoconid; 10.ectostylid; 11.mesoconid; 12.hypoconid; 13.posteroconid; 14. anterolophid; 15.metalophid I; 16.posterior arm of protoconid; 17.mesolophid;18.mesosinusid; 19.hypolophid; 20. posterolophid; 21.anterolophulid; 22.sinusid; 23.ectomesolophid; 24.ectolophid.
Systematic paleontology
Order Rodentia Bowdich, 1821
Family Dipodidae Fischer, 1817
Subfamily indet.
Genus Plesiosminthus Viret, 1926
Plesiosminthus barsboldi Daxner-Höck et Wu, 2003
Figs. 2, 3; Table 1
Referred Specimens:
DM16: IVPP V32963.1-2, left maxilla with P4-M1 (Fig.2a); right mandible with m1-3(Fig.2b).
DM31: IVPP V32959, right mandible with m1-3 (Fig.2c).
Locality and Age:
DM16 and DM31 Damiao Village, Siziwangqi, Nei Mongol, China; Early Miocene.
Measurements: see Table 1
Description
The specimen V 32963.1(Fig.2a) preserves P4-M1 and a small part of the zygomatic arch. The zygomatic arch originates just mesially from the anterior edge of the P4 crown. Based on what is preserved, the posterior border of the incisive foramen can extend to the posterior edge of P4.
P4(Fig.2a): Peg-like tooth with single root. The crown comprises a principal cusp in the mesiomedial position, two small cuspules on both the lingual and buccal sides, and a posterior cingulum connecting the two small cuspules. The buccal cuspule is slightly larger than the lingual one. The tooth is obliquely implanted, causing the crown to be tipped back toward M1.
M1(Fig.2a): It has an almost square occlusal outline. The anterior cingulum is weak and located at the base of the mesial wall of the tooth. The anterocone is barey visible. The anterior arm of the protocone merges with the anteroloph, which extends to the mesio-buccal edge. The connection between the protocone and the protoloph is weak. The mesoloph is long, extending to the edge of buccal side. The metaloph connectsthe hypocone. The posteroloph is broken. The sinus is directed forwardly. Three roots.
Mandible (Fig.2d, 2e): Both mandibles preserve a relatively complete horizontal ramus with m1-3 and part of the incisor, but not the asending ramus. The heights of horizontal ramus of mandibles are 2.33 mm (V32963. 2, Fig.2d) and 2.18 mm (V32959, Fig.2e), respectively. The ventral masseteric crest is pronounced and extends anterodorsally to below m1, whereas the dorsal masseteric crest is extremely weak. The mental foramen is located mesioventrally to m1 and is positioned below the anterior part of the diastema.
m1(Fig.2b, c): The anteroconid is pronounced and located at the medial axis of the tooth attaching to the base of the metaconid (V32963. 2, Fig.2b) or isolated from the base of the metaconid (V32959, Fig.2c). The protoconid is positioned anteriorly to the level of the metaconid. The metalophidⅡis U-shaped. The ectolophid is short and nearly longitudinal with a pronounced mesoconid. The mesolophid is obliquely directed mesio-lingually, reaching to the lingual border of the molar. The hypolophid is transverse and connected the anterior arm of the hypoconid. The posterolophid is strong, with an inflated posteroconid. The connection of hypoconid-posterolophid is constricted.
m2 (Fig.2b, c): It is rectangular in occlusal outline, wider than m1. The anteroconid is pronounced, with well-developed lingual anterolophid, while the buccal one is almost absent. The protoconid and hypoconid are respectively positioned distally to the metaconid and entoconid. The connection between the protoconid and anteroconid is either absent or weak. The posterior arm of protoconid is of medium length and connected the posterior wall of the metaconid inIVPP V32963. 2 (Fig.2b) or to the middle of the mesolophid in V32959 (Fig.2c). The mesolophid is long and obliquely mesiolingually. The ectolophid is more or less longitudinal and mesially connected to the posterior wall of the metaconid. The posterior part is similar to that of m1.
m3 (Fig.2b, c): It is not strongly reduced, with similar mesial part of m2 but a narrower distal part. The posterior arm of the protoconid is of medium length (V32963. 2, Fig.2b) or long (V32959, Fig.2c). InIVPP V32959, a short longitudinal crest connects the anteroconid and the middle of the posterior arm of the protoconid. The mesolophid is of medium length (V32963. 2) or extremly short (V32959).
Comparison and Discussion
The new specimens can be attributed to Plesiosminthus rather than those Late Oligocene to Early Miocene dipodids such as Parasminthus and Heterosminthus due to their lophodonty molar pattern, square M1 with three roots and the connection of metacone to hypocone rather than to the posteroloph. Additionally, the well-developed mesolophid on m1-2 sets it apart from Heterosminthus, Lidotonomys, and Sinodonomys.
Plesiosminthus has been documented across a wide geographic range, with species identified from Europe, Asia, and North America. At least ten European species have been identified, including P. schaubi (Viret 1926), P. myarion and P. promyarion (Schaub 1930, P. promyarion also found in Asia, Daxner-Höck and Wu 2003), P. bavaricus (Freudenberg 1941), P. winistoerferi (Engesser 1987), P. conjunctus (Ziegler 1994), P. meridionalis and P. admyarion (Comte 2000), P. moniqueae and P. margaritae (Freudenthal and Martín-Suárez 2017). Additionally, four Asian species have been identified: P. tereskentensis (Lopatin 1999), P. asiaticus, P. barsboldi (Daxner-Höck and Wu 2003), and P. vegrandis (Kimura 2010). There are also four North American species (Flynn 2008): P. grangeri, P. sabrae, P. cartomylos, and P. clivosus.
The new specimens are identified as P. barsboldi because they mostly similar to those of P. barsboldi by size (see Fig.3) and morphology. These shared characteristics include the lophodonty molar pattern, a weak anterior cingulum on M1, transversally compressed cusps, a short ectolophid with a distinct mesoconid on m1-3, a large anteroconid and a well-developed posterior arm of protoconid on m2-3, a high length ratio of m3/m1(DM16:0.85, DM31:0.87), and an unreduced m3.
There are some minor differences between materials of DM31 and DM16. Firstly, the posterior arm of protoconid on m2 in the DM31 specimen attaches to the middle of mesolophid whereas in the DM16 specimen, it connectsthe posterior wall of the metaconid. Secondly, there is a longer posterior arm of protoconid and a shorter mesolophid on m3 in DM 31. Thirdly, the metalophid II is present as a longitudinal ridge on m3 in DM31. The first two variations are also observed in previously described specimens of P. barsboldi (e.g., Daxner-Höck and Wu 2003; Kimura 2010) and the latter variation is documented in other species of Plesiosminthus (e.g., P. asiaticus, Daxner-Höck and Wu 2003). Therefore, these differences likely reflect intraspecific variation.
Compared to other species of Plesiosminthus from Asia, the new materials are larger than P. vegrandis (Kimura 2010) and P. tereskentensis (Lopatin 1999) and have a similar size to P. asiaticus and P. promyarion (Daxner-Höck and Wu 2003) (Fig.3). Morphologically, it differs from all of them by having a stronger posterior arm of protoconid on m2-3. Additionally, it differs from P. asiaticus by having a weaker anterior cingulum and anterocone of M1 and a larger anteroconid on m2.
Compared to the European species, P. barsboldi falls within the size range of P. schaubi (Viret 1926), P. myarion (Schaub 1930), P. winistoerferi (Engesser 1987), P. conjunctus (Ziegler 1994), P. meridionalis, and P. admyarion (Comte 2000), and is larger than other species based on the mean size of m1 (Fig.3). In comparison with the five larger European species, our materials differ from P. schaubi in the presence of the posterior arm of protoconid on m2-3; it differs from P. meridionalis by a long mesolophid on m2 and a posterior arm of protoconid on m2-3. Among all species of Plesiosminthus from Asia and Europe, the specimens described hereresemble P. winistoerferi from the Late Oligocene by the well-developed posterior arm of m2-3, more anteriorly inclined hypolophid on m2, larger m3, but differs by the shorter posterior arm of protoconid of m2, and having no secondary ridges between metalophid and mesolophid on m1.
Compared to these species from North America (Flynn 2008), P. barsboldi is different from the three large species, including P. grangeri (Wood 1935), P. sabrae (Black 1958), P. cartomylos (Korth 1987), by having more compressed cusps and an anterior ectolophid connecting to the protoconid instead of the metalophid on m1(Kimura 2010). P. clivosus is a smaller Plesiosminthus species from North America. According to descriptions and illustrations by Galbreath (1953), our materials have more compressed cusps on the lower molars and an unreduced m3.
So far, the generic allocation of these North American Plesiosminthus species has differing viewpoints. Some researchers support separating them into a distinct genus Schaubeumys due to morphological differences from the Eurasian Plesiosminthus (e.g., Black 1958; Qiu and Li 2016). Others retain them within the genus Plesiosminthus as synonyms based on overall similarity (e.g, Green 1977; Flynn 2008). A third perspective proposes splitting them into two genera - the three larger North American species into Schaubeumys and the smaller P. clivosus remaining in Plesiosminthus (e.g., Wilson 1960; Korth 1994). While a full revision of the North American Plesiosminthus species is outside the scope of the current study, further comparative work is needed to fully revise their systematics. For clarity, we follow Flynn (2008) in this study pending further revisions.
Data qouted: P. asiaticus (Daxner-Höck and Wu 2003), P. promyarion and P. barsboldi of the Valley of Lakes (Daxner-Höck et al. 2014), P. tereskentensis (Lopatin 1999), P. vegrandis and P. barsboldi of Gashunyinadege (Kimura, 2010), P. vegrandis and P. barsboldi of Aerban(L) (Qiu and Li 2016),P. barsboldi of DM16(this paper); the European species including P . promyarion of Pechdu Fraysse and Ruisseaudu Bey, P. meridionalis, P. winistoerferi, P. admyarion, P. schaubi, P. myarion, P. moniqueae, P. bavaricus, P. margaritae and P. conjunctus (Freudenthal and Martín-Suárez 2017).
Genus Sinodonomys Kimura, 2010
Emended Diagnosis:
Low-crowned bunodont; four-rooted M1-2; mesoloph(id) absent or vestigial on M1–2 and m1–2; mesial wall of M1 slightly expanded; paracone connecting to entoloph independently of protocone on M1; posterior arm of protocone(id) absent on M1–2 and m1–2; no anteroconid on m1; anterolophid of m2 isolated from anteriorly concave metalophid I; m2 subquadrate or rectangular; m3 reduced with two transverse lophids.
Sinodonomys cf. simplex Kimura, 2010
Fig. 4a, b; Table 1
Referred Specimens:
IVPP V32964, right mandible with m1-3. (Fig.4a, b)
Locality and Age:
DM16, Damiao Village, Siziwangqi, Nei Mongol, China; Early Miocene
Measurements: see Table 1
Description:
Mandible (Fig.4a): It preserves the horizontal ramus with m1-3. In the lateral view, the ventral rim of the mandible is convex under m1 and is concave under the last two molars. The single mental foramen is situated below the mesial root of m1 at the mid-depth of the jaw. The ventral masseteric crest extends transversely nearly paralleling to the molar alveolar margin, whereas the dorsal masseteric crest extends ventrally in an anterior direction. Both masseteric crests fade below the distal root of m1 and do not meet each other.
m1 (Fig.4b): The anteroconid is small and located at the base of the metaconid. The metaconid is positioned opposite to the protoconid. The metalophid II is straight, and there is no ectolophid present. The mesoconid is small and slightly higher than the base of the valley. A weak mesolophid extends mesiolingually from the mesoconid to a small but distinct mesostylid, which connectsthe distolingual base of metaconid by a short crest. A small ectostylid attaches to the base of protoconid. The entoconid is slightly mesial to the hypoconid, and the connection between the hypoconid and posterolophid is constricted.
m2 (Fig.4b): Both the lingual and buccal anterolophids are equally developed. An extremely weak anteroconid connectsthe protoconid through a very low and weak crest. The connection between the metaconid and the central part of protoconid is very low and weak. The short ectolophid approaches the medial axis of the tooth and connects anteriorly with the postero-ligual part of protoconid. The mesoconid is weak. The mesolophid is a weak vestigial transverse crest. The posterolophid is robuster than m1, and continues with the hypoconid.
m3 (Fig.4b): It is reduced and pear-shaped. Both arms of the anterolophid are present, and the anteroconid is distinct, fused with the anterolophulid. The protoconid and metaconid are linked via a transverse metalophulid I. The ectolophid extends obliquely from the distoligual side of the protoconid to the entoconid. The hypoconid fuses with the posterolophid to form a ridge, which extends along the distolingual edge and reaches the mateconid. The sinusid is wide, while the mesolophid and posterosinusid are lacking.
Comparison and discussion
The material described herein differs significantly from contemporary genera such as Parasminthus, Plesiosminthus, and Heterosminthus due to its extremely weak or absent mesolophid and the absence of the posterior arm of the protoconid on the lower molars (m1-3). Additionally, the weak ectolophid on m1 sets it apart from Litodonomys. Based on characteristics the extremely weak mesolophid and ectolophid, oppositional positioning of the metaconid and protoconid on m1, and absence of the mesolophid and anterolophulid on m2, we attribute this specimen to the genus Sinodonomys.
Sinodonomys, a monospecific genus identified by Kimura (2010) basing on specimens found in Gashunyinadege, Sonid Zuoqi, Nei Mongol. Our specimen shows some minor differences from the type species, S. simplex, including a shorter m2, and a more longitudinally oriented ectolophid on m2. Furthermore, our specimen differs from those referred to as S. cf. simplex by Qiu and Li (2016) in that there is no crest-like connection between the anteroconid and protoconid on m1 and there is only one mental foramen on mandible. Given that only the mandible is preserved, the identification as S. cf. simplex remains tentative.
Kimura (2010) delineated seven diagnostic characters of Sinodonomys to differentiate it from Litodonomys. However, three of these characters are also present in some specimens of Litodonomys, complicating the distinction. Three characteristics under consideration include: 1) the metaconid not shifted anteriorly on m1(observed in some specimens of L. huangheensis, Daxner-Höck et al. 2014), 2) the mesolophid absent or vestigial on m1–2 (observed in some specimens of L. huangheensis, Daxner-Höck et al., 2014), 3) the anterolophid of m2 isolated from anteriorly concave metalophid I (absence of anterolophuid) (seen in some specimens of L. lajeensis, Daxner-Höck et al. 2014; L. xishuiensis, Wang 2003 and L. zayssanensis, Lopatin and Zazhigin 2000). Despite these overlaps, our material is primarily classified as Sinodonomys instead of Litodonomys mainly because of the weak ectolophid on m1, a characteristic not described in any Litodonomys species. This suggests that the weak ectolophid on m1 could be a distinctive trait separating Sinodonomys from Litodonomys.
Genus Litodonomys Wang et Qiu, 2000
Litodonomys lajeensis Li et Qiu, 1980
Fig. 4c-e; Table 1
Referred Specimens:
IVPP V32960.1-2, right mandible with m1, right m2. (Fig. 4c-e)
Locality and Age:
DM31, Damiao Village, Siziwangqi, Nei Mongol, China; Early Miocene
Measurements: see Table 1
Description:
m1 (Fig.4d): The anteroconid attaches to the mesiobuccal base of metaconid. The mesoconid is distinct. The mesolophid extends to the lingual edge. The ectolophid is curved, extending continuously from the distomedial wall of the protoconid to the anterior arm of the hypolophid. The hypolophid connectsthe anterior arm of hypoconid. The posterolophid is long and slightly constricted behind the hypoconid.
m2 (Fig.4d): It is subquadrate in occlusal view. Both buccal and lingual branches of the anterolophid are well-developed, with the buccal one extending down forward from the top of the anteroconid to mesiobuccal edge, while the lingual one extends along the base of the anteroconid. The anteroconid is small and connects nearly symmetrically with both the protoconid and the metaconid via a longitudinal anterolophulid. The mesolophid is medium length and oblique mesiolingually. The hypoconid is continues with the robust posterolophid. The posterosinuid is transverse and open.
Comparison and discussion:
The new specimen is different from those of Plesiosminthus and Heterosminthus in having a simple occlusal pattern, reduced mesolophid on m2, and a reduced m3 with fused hypolophid and posterolophid. Furthermore, it differs from Sinodonomys in its well-deveoloped mesolophid and mesially-shift metaconid on m1, the well-developed ectolophid on m1-2 and the anterolophulid. These features are consistent with those of Litodonomys.
Within the genus Litodonomys, there are five recognized species: L. huangheensis (Wang and Qiu 2000), L. xishuiensis (Wang 2003), L. lajeensis (Li and Qiu 1980), L. minimus (Kimura 2010) and L. zaysanensis (Lopatin and Zazhigin 2000). Compared to these established species, our specimen presents a well-developed mesolophid on m1, distinguishing it from L. huangheensis; it is smaller in size, with a straight metalophid on m1 and a complete anterolophulid on m2, setting it apart from L. xishuiensis; and its smaller size also differentiates it from L. zaysanensis. The differences between our specimen and both L. lajeensis and L. minimus are less pronounced.
Daxner-Höck et al. (2014) noted the similarity of L. lajeensis with L. minimus and L. zaysanensis in dental pattern and size but did not provide further comments on the validity of the species. When considering all specimens from these three species, including materials from Lopatin and Zazhigin (2000), Kimura (2010), Daxner-Höck et al. (2014), and Li and Qiu (2016), the distributions of the measurements largely overlap. However, L. zaysanensis typically exhibits a larger size, covering the large end of the size range. Morphologically, Qiu and Li (2016) pointed out that L. minimus differs from L. lajeensis in having a weaker posteroloph and closed posterosinus on M2; differs from L. zaysanensis in the weaker mesolophid, the hypolophid connecting to the mesoconid rather than the distal portion of mesconid on m2. Limited by a lack of upper molars, a direct comparison with L. minimus and L. lajeensis regarding M2 is not possible. However, based on descriptions, L. lajeensis (Daxner-Höck et al. 2014)—which shares similarities with L. zaysanensis (Lopatin and Zazhigin 2000)—exhibits a more developed mesolophid and a hypolophid connecting to the distal portion of the mesconid.
Therefore, following Qiu and Li (2016), we support retaining the classification of these three species as distinct. The new material should be attributed to L. minimus, based on its small size, medium-length mesolophid, and the connection between the hypolophid and the distal portion of the mesioconid on m2.
Table 1 Measurements (in mm) of Plesiosminthus barsboldi, Sinodonomys cf. simplex and Litodonomys minimus from Damiao
|
|
|
M1
|
m1
|
m2
|
m3
|
Species
|
|
Catalog no.
|
L
|
W
|
L
|
W
|
L
|
W
|
L
|
W
|
Plesiosminthus barsboldi
|
DM16
|
V32963.1
|
1.30
|
1.28
|
|
|
|
|
|
|
|
|
V32963.2
|
|
|
1.18
|
0.84
|
1.2
|
0.92
|
1.0
|
0.84
|
|
DM31
|
V32959
|
|
|
1.23
|
0.91
|
1.23
|
1
|
1.07
|
0.91
|
Sinodonomys cf. simplex
|
DM16
|
V32964
|
|
|
1.17
|
0.89
|
1.13
|
1.02
|
0.71
|
0.81
|
Litodonomys minimus
|
DM31
|
V32960.1
|
|
|
1.19
|
0.84
|
|
|
|
|
|
|
V32960.2
|
|
|
|
|
1.12
|
0.92
|
|
|
Subfamily Zapodinae
Genus Sinozapus Qiu et Storch, 2000
Sinozapus damiaoensis sp. nov.
Fig. 5; Table 2
Holotype:IVPP V32977.1, right M1(Fig.5a)
Type Locality and Age:
DM02, Damiao Village, Siziwangqi, Nei Mongol, China; earliest Late Miocene
Referred Specimens:
DM01: IVPP V32968.1-10, 1 left M1, 1 left M2, 2 right m1, 1 left m1, 1 right m2, 3 left m2, 1 left m3;(Fig.5d, g, k, l, t)
DM03: IVPP V32974.1-6, 1 right M1, 1 left M1, 1 right M2, 3 right m1; (Fig.5c, e, h, m, n, q, s)
DM02: IVPP V32977.2-14, 2 right M1, 1 left M1, 1 left M2, 5 right m1, 2 left m1, 2 right m2. (Fig.5a, b, f, I, j, o, p)
Locality and Age:
DM01, DM03 and DM02, Damiao Village, Siziwangqi, Nei Mongol, China; latest Middle Miocene to earliest Late Miocene
Measurements: see Table 2
Etymology: Name after Damiao (large buddhist temple), Siziwangqi, where this new species was found.
Diagnosis:Small-sized Sinozapus; buno-lophodonty molars; longitudinal spurs rare on mesoloph, metaloph, and posteroloph, metaloph connected to hypocone on M1-2; presence of double protoloph on M2; ectolophid oriented parallel to long axis of tooth on m1-2; connection between the protoconid and anteroconid (anterior arm of protoconid) often absent, strong posterior arm of protoconid always attached to metaconid via metalophidⅡon m2; less reduced m3 with both mesolophid and hypolophid.
Differential diagnosis: Sinozapus damiaoensis sp. nov. differs from:
S. volkeri by smaller size; less developed longitudinal spurs on transversal lophs on M1-2; the metaloph connecting to the central of the hypocone on M1-2; absence of connection between the protconid and anteroconid on m2 in some case; the strong posterior arm of protoconid always attacthing to the metaconid instead of the mesolophid to form the metalophidⅡon m2; unreduce m3 with the well-developed posterior arm of protoconid.
S. parvus by the metaloph connecting to the central of the hypocone on M1-2; absence of connection between the protconid and anteroconid on m2 in some case; the strong posterior arm of protoconid always attacthing to the metaconid to form metalophidⅡon m2.
Table 2 Measurements (in mm) of Sinozapus damiaoensis sp. nov. from Damiao
|
|
|
Length
|
Width
|
|
Tooth
|
N
|
Range
|
Mean
|
Range
|
Mean
|
DM01
|
M1
|
1
|
|
1.21
|
|
1.15
|
M2
|
1
|
|
1.11
|
|
0.95
|
m1
|
3
|
1.07-1.29
|
1.15
|
0.78-0.9
|
0.83
|
m2
|
4
|
1.08-1.13
|
1.11
|
0.71-0.8
|
0.76
|
m3
|
1
|
|
0.89
|
|
0.66
|
DM03
|
M1
|
1
|
|
1.12
|
|
1.05
|
M2
|
1
|
|
1.02
|
|
0.77
|
m1
|
3
|
1.08-1.21
|
1.14
|
0.82-0.84
|
0.83
|
DM02
|
M1
|
4
|
1.06-0.99
|
1.04
|
0.78-1.03
|
1.00
|
M2
|
1
|
|
1.03
|
|
1.00
|
m1
|
7
|
1.09-1.23
|
1.16
|
0.76-0.89
|
0.81
|
m2
|
4
|
1.14-1.15
|
1.15
|
0.77
|
0.77
|
Description
M1 (Fig.5a-e): It has a square outline with a slightly wider mesial side. The protocone and hypocone are round and inflated. The paracone and metacone are usually sharp and higher than the two lingual main cusps. Five transverse lophs, including three slightly weak and short lophs (anteroloph, mesoloph and posteroloph), and two strong lophs (protoloph and meloph), extend to buccal edge of the molar. The protoloph connects the mesial part of the entoloph, and the metaloph connectsthe central part of the hypocone. A short spur between protoloph and anteroloph (protolophⅠ) is present in four of seven specimens: two specimens are complete (V32974.2, Fig.5e), and in two specimens, it is incomplete (V32974.1, IVPP V32968.1, Fig.2c, d). Longitudinal spurs between mesoloph-protoloph or mesoloph-metaloph have only been observed in two specimens from DM02. Three roots.
M2(Fig.5f-h): It is rectangular or square in outline. The overall morphology is similar to M1 but with stronger protolophⅠand Ⅱ connecting the protocone and paracone, and a shallower sinus. The lingual end of protolophⅡ is fused with the lingual end of mesoloph. Two out of three specimens have an extra spur between protolophⅠ and Ⅱ, either complete (IVPP V32968.2, Fig.5g) or incomplete(IVPP V32974.3, Fig.5h). Three roots.
m1(Fig.5i-n): The anteroconid is connected to the protoconid in 7 out of 14 specimens, while in one specimen, the anteroconid presents as three small cuspids sticking together in IVPP V32974.4: Fig. 5m). It is small in 2 out of 13 specimens and absent in 4 out of 13. The metaconid and protoconid are connected by a V-shaped loph, which is formed by metalophⅡ and the posterior arm of protoconid. Four out of 13 specimens develop the weak crest connecting metalophidⅡ and mesolophid (IVPP V32977.11, IVPP V32968.3: Fig.5j, k). The ectolophid extends along or parallel to the long axis with a mesial connection strong (6/13), weak or interrupted (7/13). The mesolophid extends obliquely to lingual edge. The posterolophid extends to the lingual edge but does not connect the entoconid in most specimens, and connectsthe entoconid in one specimen (IVPP V32968.3: Fig.5k). Two roots.
m2 (Fig.5o-s): The lingual branch of the anterolophid is well developed, whereas the buccal branch is absent. The metaconid connectsthe anteroconid by a strong metalophidⅠ. The protoconid-anteroconid connection (anterior arm of protoconid) is weak in 3 out of 6 specimens (Fig.5o, p) or interrupted in the remaining 3 out of 6(Fig.5q, r, s). The posterior arm of the protoconid is strong and reaches to the posterior wall of the metaconid in all specimens as in IVPP V32977.13 (Fig.5o), it connectsboth the metaconid and the middle of mesolophid (IVPP V32977.14: Fig.5p). The ectolophid extends parallel to the long axis. The posterolophid is relatively strong. Two roots.
m3 (Fig.5t): It resembles m2 with a narrower distal part. The metaconid is located at mesiolingual conner of the molar and connects directly to the large anteroconid without an anterolophid and metalophid. The metaconid, mesostylid, entoconid and lingual end of posterolophid are fused at the lingual margin to enclose all lingual valleys. Two roots.
Comparison and Discussion:
These new fossils can be attributed to Sinozapus rather than other contemporary dipodid taxa such as Lophocricetus and Sicista by their strong loph(id), square occlusal outline and three-rooted M1, and well-developed mesolophid on m1-2. They share similarities in molar structure with Plesiosminthus but can be distinguished from the latter by their strong loph(id).
In addition, the well-developed loph(id) and relatively simple cheek-teeth pattern are similar to these genera of subfamily Zapodinae, such as Eozapus, Pliozapus (Wilson 1936), Sminthozapus (Sulimski 1962), and Javazapus (Martin 1989). However, our specimens differ from Eozapus by presence of double strong protoloph on M2-3, longitudinally oriented ectolophid, and the presence of the posterior arm of protoconid on m2 (Qiu and Li 2016); differ from Sminthozapus by longitudinal spurs on meso-, meta-, and posteroloph on M1-2; differ from Pliozapus by a distinct anteroconid on m1 and the wide sinusid of m2; differ from Javazapus by the connection of metacone to hypocone rather than posteroloph on M1, and the presence of the posterior arm of protoconid on m2.
So far, two species within the genus Sinozapus have been identified: S. volkeri (Qiu and Storch 2000) and S. parvus (Qiu and Li 2016). The specimens from Damiao can be distinguished from the type species, S. volkeri, by their smaller size, rare longitudinal spurs on M1-2 , an extra spur connecting the metalophid and mesolophid on ml, a strong posterior arm of protoconid on m2, and mesolophid on m3. In comparison to S. parvus, they dispaly similarities in general dental morphology and size, but differ by the posterior arm of the protoconid contacting the metaconid, a weaker connection between the anteroconid and protoconid on m2, rather than to the middle of the mesolophid, and the metaloph connecting to the middle of the hypocone (isolated in some cases) on M1-2. Additionally, they differ from Sinozapus sp. (Qiu and Li 2016) by their smaller size, rare longitudinal spurs on transversal lophids, and the posterior arm of the protoconid contacting the metaconid on m2. Given these unique features, we propose these specimens represent a new species, Sinozapus damiaoensis sp. nov..
According to Qiu and Li (2016), the distinct posterior arm of the protoconid on m2-3, and weak longitudinal spurs on transversal loph(id) may represent a plesiomorphic evolutionary stage of Sinozapus. The same tendencies can be observed in the specimens from Damiao. In addition, the less reduced m3 of Damiao further indicates that these materials could represent a more plesimorphic species than the other two species in this genus.
Subfamily Sicistinae
Genus Sicista Gray, 1827
Sicista primus Kimura, 2010
Fig.6a; Table 3
Referred Specimens:
IVPP V32964, 1left M2. ( Fig.6a)
Locality and Age:
DM16, Damiao Village, Siziwangqi, Nei Mongol, China; Early Miocene
Measurements: see Table 3
Description
M2(Fig.6a): It has subquadrate outline, tapering distally. The anteroloph has a long buccal arm and an extremely weak lingual arm. The protoloph and metaloph are directed mesiolingually and connect theanterior arm of protocone and anterior arm of the hypocone, respectively. The mesocone is indistinct. The mesoloph extends transversely to the buccal edge but is low. The entoloph is more or less longitudinally from the protocone to the hypocone. Two short and weak spurs can be observed on the protoloph but do not form the enamel pit. A posterior concavity is presented behind the hypocone.
Comparison
This material can undoubtedly be referred into the genus Sicista based on its small-sized and low crowned buno-lophodont molar, subquadrate M2 with oppositely located main cusps. Compared with the species of Sicista, the single tooth closely resembles S. primus due to its less complicated occlusal pattern, absence of enamel pit, rare and weak secondary spurs, and a transverse and long mesoloph. However, the DM16 specimen differs from S. primus from the type locality in having a complete anterior entoloph connecting with the protocone (Kimura 2011). This minor difference likely reflects intraspecific variation based on other specimens of S. primus from Aoerban area of central Nei Mongol (Qiu and Li 2016).
Sicista ertemteensis Qiu et Li, 2016
Fig. 6b-n; Table 3
Referred Specimens:
DM01: IVPP V32969.1-2, 2 right m2;(Fig.6m, n)
DM03: IVPP V32975.1-5, 1 right M1, 2 right m1, 1 left m1, 1 right m2; (Fig.6b, e, l)
DM02: IVPP V32978.1-9, 1 right M2, 2 left M2, 1 broken mandible with m1, 2 right m1, 2 left m1, 1 right m2. (Fig.6c, d, f, g, h, i, j, k)
Locality and Age:
DM01, DM03 and DM02, Damiao Village, Siziwangqi, Nei Mongol, China; latest Middle Miocene to earliest Late Miocene
Measurements: see Table 3
Description
M1 (Fig.6n): The anterocone is distinct. The anteroloph is long and low terminating with a small anterostyle. The paracone points transversely to the center of protocone with a very low connection. An accessory spur extends into the middle of the mesosinus and connectthe weak and short mesoloph. The mesosinus is enclosed by a small but distinct mesostyle. The metaloph sthe center of hypocone. A longitudinal accessory spur extends from the metaloph to mesosinus. Two accessory spurs extend from the metaloph to the posterosinus. Two arms of posteroloph are both developed.
M2(Fig.6c, d): The anteroloph has a long and low buccal arm without anterostyle. The anterocone is indistinct. The protoloph I anterolingually connects anterior protocone. The mesoloph is short spur which points mesiobuccally to protoloph. A well-developed secondary crest extends mesiobuccally from the anterior arm of hypocone to buccal edge. Accessory spurs are observed on the protoloph extending to the anteroloph and/or the mesosinus. Accessory spurs also are observed on the metaloph which always extends to the mesosinus, and in one specimen (IVPP V32978.3: Fig.6d), it extends to the posteroloph. Both buccal and lingual arms of a posteroloph are present, but the latter one is relatively shorter.
m1(Fig.6e-j): The anteroconid is well-developed and located at the medial axis of the frontmost of the tooth connecting the lingual side of the protoconid by a longitudinal ridge. The protoconid and metaconid are compressed, and the latter one is directed more distobuccally. The mesolphid is long and terminated with a mesotylid, usually extending from mesial part of ectolophid insteading of mesoconid (7/8); in IVPP V32978.5(Fig.6f), it presents two mesolophids, which extend from anterior ectolophid and mesoconid, respectively, and converge into one at the lingual edge. The entoconid connectshypoconid. One or two accessory spurs from hypoloph and/or hypoconid into posterosinusid. A strong accessory spur is observed from the hypolophid into the mesosinusid and connecting to the mesolophid in most of specimens (7/8).
m2(Fig.6k-n): The anteroconid is distinct. The anteroloph is weak. The metaconid is shifted anteriorly connecting the anteroconid and the anterior arm of the protoconid. The hypolophid extends distobuccally attaching the hypoconid. The mesolophid is absent. The posterior arm of the protoconid is weak but extends to the lingual side, forming a pseudomesolophid and terminating with mesostylid. Two accessory spurs extend from hypoconid and/or hypolophid into the posterosinusid in all specimens.
Comparison:
These new materials possess typical characteristics of Sicista including its small size, bnuo-lophodont with accessory spurs, subqudrate M1-2 with main cusps opposite in arrangement, well-developed anteroconid connecting the protoconid on m1.
Among these contemporary species of Sicista in China, it is mostly comparable to Sicista ertemteensis (Qiu and Li 2016) owing to their complicated dental morphology characterized by well-developed accessory spurs, transverse protoloph on M1, less-developed mesoloph on M1-2, and the presence of posterior arm of protoconid on m2.
It can be easily distinguished from S. wangi (Qiu and Storch 2000) and S. primus (Kimura 2011) by its complicated occlusal structure with more accessory spurs. Furthermore, it differs from S. wangi (Qiu and Storch 2000; Qiu and Li 2016) by its smaller size, the transverse connection between the protoloph and protocone on M1, less-developed mesoloph on M1-2, and the presence of a posterior arm of protoconid of m2 in all specimens. These materials have a similar occlusal pattern with S. bilikeensis (Qiu and Li 2016), but they differ in the protoloph connecting the protocone rather than anterocone on M1 and shorter mesoloph on M1-2.
Table 3 Measurements (in mm) of Sicista from Damiao
|
|
|
|
Length
|
Width
|
Species
|
|
Tooth
|
N
|
Range
|
Mean
|
Range
|
Mean
|
S. primus
|
DM16
|
M2
|
1
|
|
0.98
|
|
0.94
|
S.ertemteensis
|
DM01
|
m2
|
2
|
0.96-1.07
|
1.01
|
0.73-0.79
|
0.76
|
DM03
|
M1
|
1
|
|
1.05
|
|
0.96
|
m1
|
3
|
1.02-1.07
|
1.05
|
0.80-0.87
|
0.83
|
m2
|
1
|
|
1.12
|
|
0.87
|
DM02
|
M2
|
3
|
0.99-1.04
|
1.02
|
0.91-0.98
|
0.94
|
m1
|
5
|
1.00-1.05
|
1.03
|
0.82-0.87
|
0.85
|
m2
|
1
|
|
1.14
|
|
0.89
|
Subfamily Lophocricetinae Savinov, 1970
Genus Heterosminthus Schaub,1930
Heterosminthus firmus Zazhigin et Lopatin, 2000
Fig. 7; Table 4
Referred Specimens:
DM16: IVPP V32966.1-5, 1 left maxilla with P4-M3, 1 right mandible with m1, 1 left mandible with m2-3, 1 left m1, 1 left m2; (Fig.7b, e, f, g)
DM30: IVPP V32958, 1 right mandible with m1; (Fig.7c, d)
DM31: IVPP V32961, 1 right maxilla with P4-M2. (Fig.7a)
Locality and Age:
DM16, DM30 and DM31, Damiao Village, Siziwangqi, Nei Mongol, China; Early Miocene
Measurements: see Table 4
Description
Two fragmentary maxillae (Fig.7a-b) are preserved, but all of them are very broken. The incisive foramen extends posteriorly to the level of the mesial root of the M1. The IVPP V32961(Fig.7a) preserves the root of the zygomatic arch which is anterior to P4 at a distance approximately equal to the length of the latter and is obivously different from Plesiosminthus (Fig.2a).
P4 has a single large main cusp surrounded distally by a distal ridge. This distal ridge has two small cuspules at the buccal end and the lingual end, respectively. An accessory cusp locates at the mesiolingual corner of the teeth.
M1(Fig.7a, b): It is rounded-rectangular in outline and buccolingually constricted slightly at the middle. The anterocone is distinct. The anterostyle is small but distinct. The posterior crest of protocone presents, but not form protostyle (a thickened rib). The protoloph II and metaloph direct backwardly. The protoloph II connects with the mesial part of entoloph. The posterior arm of hypocone and metaloph both connected with the posteroconid. The mesoloph is long and extends from the large mesocone to the weak mesostyle. The posteroloph has buccal and lingual arms. The entoloph continuously connects the posterior arm of protocone and anterior arm of hypocone. The mesocone is distinct. Four roots.
M2(Fig.7a, b): It is rectangular with a narrower distal end. The anterocone is large as in M1. A double anteroloph presents, but anteroloph II is very short. The posterior crest of protocone is weak as in M1. A weak protoloph I presents as a spur and points the anteroloph. The protoloph II connects with the entoloph. The metaloph connects with the anterior arm of hypocone. The mesoloph is long, extending from the large mesocone to the weak mesostyle. Four roots.
M3(Fig.7b): Three lingual cusps (anterocone, protocone and hypocone), four buccal cusps (anterostyle, paracone, mesostyle and metacone), a central cusp (mesocone) and five transverse lophs can be identifed. The anterocone connectsthe parastyle and the paracone by single anteroloph and protoloph, respectively. The mesostyle and the metacone connectsthe mesocone by the mesoloph and the metaloph respectively. The posteroloph extends from the hypocone to the metacone enclosing the posterosinus. The entoloph only remained a short, transverse distal part which connected the mesocone and the mesial part of the hypocone.
m1 (Fig.7c-e): The single anteroconid is isolated (2/4, as in IVPP V32958: Fig.7c) or connected both the protoconid and the metaconid by two weak crests (2/4, as in IVPP V32966.2: Fig.7e). The metalophid II is V-shaped. The entoconid is isolated (1/4, as in IVPP V32958), connectsmesial mesoconid (1/4, as in IVPP V32966.2) or to the mesoconid (2/4). The ectomesolophid extends to the buccal edge ending with the ectostylid. The posterior crest of protoconid presents in three specimens connecting the protoconid to the ectomesolophid (2/3, as in IVPP V32966.2) or to the mesoconid (1/3) and absent in one specimen (IVPP V32958). The posteroconid is distinctive. The posterolophid extends to the base of entoconid enclosing the posterosinusid.
m2 (Fig.7f, g): The lingual main cusps locate anteriorly to buccal ones. The anteroconid is distinctive and connected metaconid and protoconid. Both lingual and buccal branches of anterolophid are equally devoloped. The posterior arm of protoconid is long and attached the distolingual wall of the metaconid (2/3, as in IVPP V32966.5: Fig.7f) or lingual edge ending with the mesostylid (1/3, as in IVPP V32966.3: Fig.7g). An oblique crest connects the middle of the posterior arm of protoconid and the mesoconid. The short hypolophid connected with the mesoconid. The posteroconid is distinctive. The posterolophid extended to the base of entoconid enclosing the posterosinusid.
m3(Fig.7g): It is more triangular than m2 in outlin but resembles m2 in basical structure. It lacks the oblique connection between the middle of the posterior arm of protoconid and mesoconidthan m2. And a longitudinal short crest connects the middle of the posterior arm of protoconid and the metaconid.
Comparison and Discussion:
These specimens described here can be attributed to the genus Heterosminthus based on their well-developed mesoloph and connection between the protoloph and anterior entoloph on M1-2, distinct lingual branch of posteroloph on M1, an ectomesolophid and a connection where with the entoconid joins mesoconid on m1, and presence of the posterior arm of protoconid and absence of mesolophid on m2.
Heterosminthus has been found in Asia from the Late Oligocene to the Middle Miocene, including H. orientalis (Schaub 1930), H. gansus (Zheng 1982), H. lanzhouensis (Wang and Qiu 2000), H. firmus, H. honestus, H. jucundus, H. mongoliensis, H. nanus, H. mugodzharicus (Zazhigin and Lopatin 2000), H. saraicus (Zazhigin et al. 2002), H. erbajevae (Lopatin 2001) and H. intermedius (Wang 2003). Among these species, our specimens closely resemble H. firmus by sharing the following characteristics: the absence of the protostyle and weak posterior crest of protocone on M1-2, the presence of the posterior crest of protoconid presents on m1, an oblique connection between mesoconid and the posterior arm of protoconid on m2, and the long posterior arm of protoconid on m2–3. The specimens of H. firmus from the Valley of Lakes of Monglia (Daxner-Höck 2001) and Aoerban of Nei Mongol (Qiu and Li 2016) show a high frequency of the double protoloph on M2 (Daxner-Höck 2001: 44% (8/18) and Qiu and Li, 2016:100% (5/5)). In contrast, our specimens lack a complete double anteroloph and double protoloph on M2, but present complete anterolophⅠ+ rudimentary anterolophⅡand rudimentary protolophⅠ+complete protolophⅡ. Moreover, the hypolophid connects mesially to the mesoconid on m1(Fig.7e) is different from these previous specimens of H. firmus. This variation is previously described in H. intermedius (Wang 2003). However, our specimens are different from H. intermedius by the presence of a posterior crest of protoconid on m1 and the absence of ectomesolophid on m2. Despite these differences, based on the overall similarity in morphology and size, and considering that these variations are widespread in Heterosminthus, we include these new materialsinto the species H. firmus.
Compared to other species, it differs from H. orientalis from the type locallity by having a less distinct thickened protocone rib on M1–2, a more developed pseudomesolophid and posterior crest of protoconid on m1, an oblique connection between mesoconid and the posterior arm of protoconid on m2, and a less reduced m3 with an anterior arm of entoconid. It differs from H. nanus by the larger size, the absence of ectomesolophid, and a complete anterior ectolophid on m2. It differs from H. gansus by its long mesoloph on M1, well-developed psudomesolophid on m1, and a well-developed lingual branch of anterlophid and posterior arm of protoconid on m2. It differs from H. lanzhouensis by having a posterior crest of protoconid and a hypolohid connecting mesoconid on m1. It differs from H.mongoliensis by having less distinct thickened protocone rib on M1–2, posterior crest of protoconid on m1, and oblique connection between mesoconid and posterior arm of protoconid on m2. It differs from H. jucundus by the absence of ectomesolophid and the presence of oblique connection between mesoconid and posterior arm of protoconid on m2. It differs from H. honestus by more developed the posterior crest of protoconid on m1 and the absence of ectomesolophid on m2.
Heterosminthus nanus Zazhigin et Lopatin, 2000
Fig. 8; Table 4
Referred Specimens:
DM16: IVPP V32967.1-10, 1 right maxilla with M1-3, 1 left maxilla with M1-2, 1 right maxilla with M1-2, 1 right M1, 1 right mandible with m1-2, 1 left mandible with m1-2, 1 left mandible m1, 1 left m1, 1 left m2, 1 right m2; (Fig.8a-g)
DM31: IVPP V32962.1-2, 1 right mandible with m1-2, 1 left mandible with m2-3. (Fig.8h, i)
Locality and Age:
DM16 and DM31, Damiao Village, Siziwangqi, Nei Mongol, China; Early Miocene
Measurements: see Table 4
Description
M1(Fig.8a-c): It is rounded-rectangular in outline and buccolingually constricted slightly at the middle. The anterocone is absent in mostly specimens, or small in one specimens (IVPP V32967.2: Fig.8b). The anteroloph is single. The anterostyle is weak but distinctive. The posterior crest of protocone is absent. The protoloph II and the posterior arm of protocone directs backward and connects with the anterior part of entoloph. The mesocone is relatively large. The posterior arm of hypocone and metaloph both connected with the posteroconid. The mesoloph is long. The posteroloph with buccal and lingual arm. The entoloph continuously connects the posterior arm of protocone and the anterior arm of hypocone. Four roots.
M2(Fig.8a-c): The anterocone is large. The anteroloph is single. The posterior crest of protocone is absent. The protoloph connects with the entoloph. The metaloph connects transversely to the anterior arm of hypocone. The mesoloph is long, extending from the large mesocone to the weak mesostyle. Four roots.
M3(Fig.8a): It is rounded-triangular in outline. The anteroloph I is long and transverse. The protocone and the paracone are same size and connected with each other by a transverse crest which is formed by the posterior arm of protocone and the protoloph. The mesoloph and the ectoloph extend obliquely from the middle point of the transverse crest to the mesostyle and the hypocone, respectively. The protocone connects with the hypocone on lingual side to close the sinus. The metacone is isolated. The metaloph is lacking.
m1(Fig.8d-i): The single anteroconid is isolated (3/5, as in Fig.8d), or connected metaconid (2/5, as in Fig.8g). The metalophid II is V-shaped. Short hypolophid directs backward and connectsthe distinct mesoconid. The m1 has the pseudomesolophid (4/5), but its morphology showing variation (Fig.8d, e, g), or it lacks the pseudomesolophid but has short mesolophid in this case (1/5, Fig.8i). The ectomesolophid is low and extends to the buccal edge ending with ectostylid, but always interrupts at middle. The posteroconid is large. The posterolophid extends to lingual edge with posterosinusid open. The concavity behind the hypoconid is distinct.
m2(Fig.8e-i): Both lingual and buccal branches of anterolophid are equally devoloped. The anteroconid is distinct and connected the metaconid and the protoconid. The posterior arm of the protoconid is long and attaches the distolingual wall of the metaconid. An oblique short crest connects the middle of posterior arm of the protoconid and mesoconid in 4 out of 5 specimens. The mesocon is weak. The short hypolophid connected with mesoconid (3/5, as in Fig.8e, i) or anterior arm of hypoconid (2/5, as in Fig.8g). The posteroconid is distinct. The posterolophid extends to the base of entoconid closing posterosinusid. The ectomesolophid presents in two specimens (2/5, as in Fig.8g). The distal part is similar to m1.
Comparison and Discussion:
These materials resemble H. nanus rather than other species of Heterosminthus by their small size and dental characters. Specifically, it is smaller than all of other species of Heterosminthus. Morphologically, they can be distinguished from H. orientalis (Schaub 1930) H. jucundus H. mugodzharicus and H. mongoliensis (Zazhigin and Lopatin 2000) by the absence of a posterior crest of protocone and the protostyle on M1-2, the presence of oblique connection between mesoconid and the posterior arm of protoconid on m2. It differs from H. gansus (Zheng 1982) by long mesoloph on M1, well-developed posterior arm of protoconid on m2; differs from H. erbajevae (Lopatin 2001) and H. intermedius (Wang 2003) by the hypolophid connecting the mesoconid rather than the ecolophid.
The new materials show some variations that differ from other known specimens of H. nanus (Zazhigin and Lopatin 2000; Kimura 2010; Qiu and Li 2016): the metaloph connectsthe anterior arm of hypocone rather than the hypoconeon M2, and hypolophid connectsthe anterior arm of hypoconid rather than the mesoconid on m2 in two out of five specimens. These differences are more likely to represent intraspecific variation rather than interspecific variation. Firstly, the materials of H. nanus previously known are rare and may not display the full variability of dental characters. Furthermore, these variations are widely documented in other species of Heterosminthus, e.g., H. jucundus and H. honestus (Zazhigin and Lopatin 2000), H. lanzhouensis (Wang 2003), H. orientalis (Qiu and Li 2016). Thus, we assigned the above-described specimens into H. nanus.
Table 4 Measurements (in mm) of H. firmus and H. nanus from Damiao
Species
|
|
Catalog no.
|
M1
|
M2
|
M3
|
m1
|
m2
|
m3
|
|
|
|
L
|
W
|
L
|
W
|
L
|
W
|
L
|
W
|
L
|
W
|
L
|
W
|
H. firmus
|
DM16
|
V32966.1
|
1.43
|
1.12
|
1.26
|
1.11
|
0.81
|
0.9
|
|
|
|
|
|
|
|
V32966.2
|
|
|
|
|
|
|
1.48
|
1.06
|
|
|
|
|
|
V32966.3
|
|
|
|
|
|
|
|
|
1.52
|
1.18
|
1.15
|
0.91
|
|
V32966.4
|
|
|
|
|
|
|
1.45
|
1.02
|
|
|
|
|
|
V32966.5
|
|
|
|
|
|
|
|
|
1.5
|
1.11
|
|
|
DM31
|
V32961
|
1.5
|
1.16
|
1.32
|
1.02
|
|
|
|
|
|
|
|
|
DM30
|
V32958
|
|
|
|
|
|
|
1.61
|
1.0
|
|
|
|
|
H. nanus
|
DM16
|
V32967.1
|
1.13
|
0.84
|
1.03
|
0.82
|
0.63
|
0.72
|
|
|
|
|
|
|
|
V32967.2
|
1.31
|
1.02
|
1.24
|
0.93
|
|
|
|
|
|
|
|
|
|
V32967.3
|
1.21
|
0.98
|
1.08
|
0.95
|
|
|
|
|
|
|
|
|
|
V32967.4
|
|
|
|
|
|
|
1.21
|
0.93
|
1.24
|
0.99
|
|
|
|
V32967.5
|
|
|
|
|
|
|
1.23
|
0.85
|
1.22
|
0.89
|
|
|
|
V32967.6
|
|
|
|
|
|
|
1.11
|
0.84
|
|
|
|
|
DM31
|
V32962.1
|
|
|
|
|
|
|
1.27
|
0.83
|
1.27
|
0.93
|
|
|
Genus Lophocricetus Schlosser, 1924
Lophocricetus parvus sp. nov.
Fig. 9a-f
Holotype: IVPP V32979.4, left m1 (Fig.9d)
Type Locality and Age:
DM02, Damiao Village, Siziwangqi, Nei Mongol, China; earliest Late Miocene
Referred Specimens:
IVPP V32979.1-3, 5-8, 1 right M1, 1left M2, 4 left m1,1 left m2.
Etymology: From Latin parvus, small, in reference to its small size.
Measurement, mm. (length × width) of M1,1.38×0.82; M2, 1.26×0.85; m1:1.18×0.81, 1.39×0.87, 1.27×0.88; m2, 1.17×0.86.
Diagnosis:Small-sized Lophocricetus; lingual and buccal main cusps arranged alternatively; upper molars are relatively narrow; long mesoloph and well-developed posterior crest of protocone present on M1-2; double anteroloph on M2; mesoconid is weak on m1, posterior crest of protoconid connects protoconid to mesoconid, hypoconid connects hypolophid; m2 with posterior arm of protoconid.
Differential diagnosis: Lophocricetus parvus sp. nov. differs from:
L. grabaui by smaller size; the well-developed mesoloph on M1-2; the double anteroloph, well-developed mesocone on M2; presence of posterior crest of protoconid, weaker ectomesolophid on m1; presence of posterior arm of protoconid on m2.
L. xianensis by smaller size; presence of posterior crest of protoconid, weaker ectomesolophid and mesoconid, connection between entoconid and hypoconid on m1.
L. minuscilus by smaller size; well-developed mesoloph on M1; connection between protoconid and metalophⅡ, presence of posterior crest of protoconid, less development ectostylid on m1.
L. vinogradovi by smaller size; well-developed mesoloph on M1-2; connection between protoconid and metalophⅡ, presence of posterior crest of protoconid, less development ectostylid on m1. presence of posterior arm of protoconid, absence of buccal arm of the anterolophid on m2.
L. reliquus by smaller size; well-developed mesoloph on M1-2; double anteroloph, opened sinus on M2.
L. cimishliensis by smaller size; well-developed mesoloph on M1; distinct mesocone on M2; presence of posterior crest of protoconid, less development ectostylid, weaker ectomesolophid on m1; presence of posterior arm of protoconid on m2.
Description
M1(Fig.9a): It is elongated, the anterior part is slightly narrower than the posterior part. The lingual main cusps (protocone and hypocone) strongly shift forward. The protocone connects the anteroloph. No anterocone and anterostyle. A small pit is anterior to the anterolph. The posterior crest of protocone is strongly thickened, almost forming an independent protostyle. The mesoloph is long. The metaloph and the posterior arm of hypocone connect the posterocone, respectively. The posteroloph is low, its buccal and ligual branches developing equally. Four roots.
M2(Fig.9b): It is longitudinal extended and slightlynarrowerdistal part than mesial part. Three main cusps (protocone, hypocone and paracone) are round-shaped, and almost equal size, while the metacone is slightly smaller. The protocone conncets the anterostyle by double anteroloph without anteocone. The mesocone is distinct. The protoloph, mesoloph and metaloph extends transversely in parallel. The metaloph connectshypocone. Four roots.
m1(Fig.9c-e): The anteroconid is pronunced, isolated (2/4, as in Fig.9d) or connected the metaconid (2/4, as in Fig.9c). Two lingual mian cusps are mesial to two buccal mian cusps. The protoconid is isolated from metaloph Ⅱ (1/4, as in Fig.9d) or connected metaloph Ⅱ(3/4, as in Fig.9c). The posterior crest of protoconid presents and connects protoconid to mesoconid. The connection btween mesoconid and metaconid (anterior ectolophid) is contiuous. The mesoconid is weak. The ectomesolophid usually extends to the buccal edge but is very short in one case (Fig.9c). The psudomesolophid is weak or absent, but the mesostylid presents in all specimens. The hypolophid connects distal mesoconid. The anterior arm of hypoconid is connected hypolophid instead of mesoconid. The posterolophid connects the posterior arm of hypoconid and extends to the base of entoconid enclosing the posterosinusid. The concavity behind the hypoconid is distinct. Two roots.
m2(Fig.9f): The anteroconid connects metaconid to form a distobuccally oblique crest. The buccal arm of anterolophid is long, whereas the lingual arm is absent. the connection between the protocone and anterocone is very weak. There is no mesolophid and ectomesolophid. The posterior arm of protoconid is long, reaching to the lingual edge and conneting to the metaconid. The ectolophid extends from protoconid to hypolophid and is separated from the hypoconid. The posterolophid reaches to entoconid.
Comparison and Discussion:
These new materials can be easily distinguished from other dipodid taxa and classified as Lophocricetinae by its well-developed posterior crest of protocone on M1-2; absence of the mesolophid and presence of the ecomesolophid on m1; the entoconid connecting the mesoconid on m1-2. However, it is difficult to allocate these materials to a specific genus due to they exhibit b a suit of transitional morphological traits between Heterosminthus and Lophocricetus.
Many authors concurred that Heterosminthus and Lophocricetus were closely related, and the latter derived from the former (e.g., Zazhigin et al. 2002; Qiu et al. 2008, Delinschi 2014; Qiu and Li 2016). However, there is no consensus opinion on the generic allocation for those intermediate types with varied transitional dental characters between Heterosminthus and Lophocricetus. Zazhigin et al. (2002) listed several diagnostic characters to separate the two genera, mainly based on the different arrangement of the main cusps, including the connections between: 1) protocone-paracone (in Lophocricetus)/ -protoloph (in Heterosminthus) on M1-2; 2) hypocone-metacone/-posteroloph on M1; 3) hypoconid-entoconid/-mesoconid on m1; 4) protoconid-metaconid/-anteroconid on m2. Qiu et al. (2008) agreed with the opinion that arrangement of the main cusps is “important for serving as reference in definition of the two genera”, but stated these characters are “distinctly variable”. Meanwhile the developed level of protostyle on M1 and the presence or absence of posterior arm of protoconid on m2 are emphasized in separating the two genera (Qiu et al. 2008; Qiu and Li 2016). Based on different principles, those intermediate types, such as “Heterosminthus gansus”, “H. mugodzharicus”, “H. saraicus”, have been ascribed to Heterosminthus (Zazhigin et al. 2002) or Lophocricetus (Qiu et al. 2008, Delinschi 2014; Qiu and Li 2016).
These new materials possess typical features of Heterosminthus include small size, protocone-protoloph connection, long mesoloph on M1-2 and hypocone-posteroloph connection on M1, double anteroloph on M2, remnant of pseudomesolophid and posterior crest of protoconid on m1, the presence of posterior arm of protoconid on m2. Meanwhile, they also show some features of Lophocricetus including the developed protostyle on M1 and connection of hypoconid- entoconid on m1. Taking these characters together, the here described fossils can easily be distinguished from all of known species of Heterosminthus and Lophocricetus. Thus, these materials found from Damiao should represent a new species.
For its allocation of genus, we compared the characters discussed above among the known species of Heterosminthus and Lophocricetus found that most of them are frequently highly variable, excepting for the character: connection of hypoconid- entoconid on m1 which has been found mainly from some species of Lophocricetus. This character could represent an autapomorphy of Lophocricetus. However, a revision of the two genera is far beyond the scope of this study.
Therefore, based on the well-developed prostyle on M1 and connection of hypoconid- entoconid on m1, which are two important generic characters of Lophocricetus proposed by both Zazhigin et al. (2002) and Qiu et al. (2008), we allocate this material to Lophocricetus, as a new species, Lophocricetus parvus sp. nov..
Lophocricetinae gen. et sp. indet
Fig. 9g
Referred Specimens: IVPP V32980, right m1. (Fig.9g)
Locality and Age:DM02, Damiao Village, Siziwangqi, Nei Mongol, China; earliest Late Miocene
Measurement, mm. length × width of m1, 2.2 × 1.61.
Description
m1(Fig.9g): The size is large. The anteroconid is large and isolate. The isolated metaconid is larger than and the protoconid. A short ectolophid connecting the distolingual base of protoconid to the pronounced mesoconid. The ectomesolophid is reduced to an extrmely short process of the mesoconid. The ectostylid and the mesostylid are pronounced. There is no mesolophid. The entoconid joins the mesoconid nearly transeversely. The posterolophid is short but robust. There is a distinct concavity behind the hypoconid.
Comparison and Discussion:
A right m1 shows a suite of characters that correspond to Lophocricetinae including alternate arrangement of lingual and buccal main cuspids, distinct ectomesolophid, mesoconid and posteroconid, concaity behind hypoconid, and lack of mesolophid. However, it differs from all of Lophocricetine genera. It is smaller than most of species of Sibirosminthus, but larger than most of genera including Heterosminthus and Lophocricetus, Paralophocricetus, and falls into the large end of size range of Lophocricetus grabaui, Sibirosminthus seletyensis, Paralophocricetus ultimus and Lophosminthus (Zazhigin and Lopatin 2000; Zazhigin et al. 2002; Zazhigin and Lopatin 2002; Qiu and Li 2016). Compared to these similar size species, it can be easily separated from L. grabaui and S. seletyensis by its bunodont. In addition, it differs from other genera by its isolated metaconid, short ectolophid and reduced ectomesolophid. Besides that, it differs from Heterosminthus by its larger ectostylid; differs from Lophocricetus by the anteriorly connection of ectolophid to protoconid.
These differences suggest that this material could represent a new genus. We refrain to name a new taxon with the only tooth, and temporarily described as an indetermintate genus and species of Lophocricetinae.
The origin of the subfamily Dipodinae is debated. One hypothesis proposes that three-toed jerboas evolved from five-toed jerboas, based on the mid-Miocene dipodid fossil Mynsudipus usstjurtensis from Kazakhstan (Kordikova 2004; Qiu and Li 2016). However, some other previous studies have suggested that the subfamily Lophocricetinae is the immediate ancestor of Cardiocraniinae and, indirectly, of Dipodinae based on dental structure (Shenbrot et al. 1995; Zazhigin and Lopatin 2001, 2005). The new specimen exhibits a series of characters, including bunolophodont dentition, a large and anteriorly positioned isolated metaconid, and the absence of a mesolophid, which resemble features found in Dipus fraudator (Li and Qiu 2005) and some species of Dipodinae (Zazhigin and Lopatin 2001). Additionally, the shortened ectomesolophid and ectolophid in the new specimens appear to represent an intermediate stage of gradual reduction from Lophocricetinae to Dipodinae. Therefore, these observed characters further support the close relationship between Lophocricetinae and Dipodinae. Further research is necessary to validate and refine this hypothesis.
Subfamily Allactaginae Vinogradov, 1925
Genus Protalactaga Young, 1927
Protolactaga grabaui Young, 1927
Fig. 10; Table 5
Referred Specimens:
DM01: IVPP V32970.1-2, 1 left M1, 1 left m2; (Fig.10a, f)
DM02: IVPP V32981.1-4, 1 left M1, 1 right M2, 1 right m1, 1 left m2. (Fig.10b-e)
Locality and Age:
DM01 and DM02, Damiao Village, Siziwangqi, Nei Mongol, China; latest Middle Miocene to earliest Late Miocene
Measurements: see Table 5
Table 5 Measurements (in mm) of Protalactaga grabaui,Protalactaga lophodens and Paralactaga parvidens from Damiao
Species
|
|
Catalog no.
|
Tooth
|
Length
|
Width
|
Protalactaga grabaui
|
DM01
|
V32970.1
|
M1
|
1.65
|
1.43
|
|
V32970.2
|
m2
|
1.89
|
1.29
|
DM02
|
V32981.1
|
M1
|
1.66
|
1.41
|
|
V32981.2
|
M2
|
1.62
|
1.36
|
|
V32981.3
|
m1
|
2.00
|
1.42
|
|
V32981.4
|
m2
|
1.94
|
1.45
|
Protalactaga lophodens
|
DM01
|
V32971.1
|
M1
|
2.93
|
2.59
|
|
V32971.2
|
m2
|
3.04
|
2.27
|
DM02
|
V32982
|
M1
|
2.64
|
2.47
|
Paralactaga parvidens
|
DM01
|
V32972.1
|
m1
|
2.54
|
1.77
|
|
V32972.2
|
m2
|
2.55
|
1.71
|
DM02
|
V32983.1
|
M2
|
2.24
|
1.84
|
|
V32983.2
|
m2
|
2.52
|
1.80
|
Description
M1(Fig.10a, b): Small-sized tooth. The anterocone is small but distinct. The anteroloph ends with (IVPP V32970.1, Fig.10a) or without (IVPP V32981.1, Fig.10b) anterostyle. The protoloph is single and connectsthe posterior arm of protocone. The mesoloph is long. The metaloph adjoins backward with the posterocone. The posteroloph is short (IVPP V32981.1) or absent (IVPP V32970.1).
M2(Fig.10c): The anteroloph is long without distinctive anterocone and anterostyle. The protoloph I is slightly mesolingually directed to connect the anterior arm of protocone. The mesoloph is long and ends with a relatively inflated mesostyle. The entoloph is complete and more or less longitudinal. The metaloph is disolingually directed and joins to posteroloph. The posteroloph is well-developed.
m1(Fig.10d): Distinctly narrower in mesial part than distal part. No anteroconid. The metaconid is mesial to the protoconid and isolated. The protoconid connectsthe junction between mesoconid and mesolophid. The mesoconid is distinct. The mesolophid and ectomesolophid are well-developed. The hypoconid and the entoconid are large and both isolated from the mesoconid. The posterolophid is robust, and its connection with hypoconid is constricted leaving a concavity behind hypoconid.
m2(Fig.10e, f): The anteroconid is well developed, the buccal arm of anterolophid is weak. The metaconid and the protoconid both connect the anteroconid, but the latter connection (anterior arm of protoconid) is relatively low. The mesolophid is long, ending with a small but distinct mesostylid. The entoconid is separated (Fig.10f) or joined the mesolophid, and its junction fused with mesoconid (Fig.10e). The connection of ectolophid with hypoconid is weak or interrupted. theconcavity behind hypoconid is smaller than m1. The posterolophid reaches to entoconid.
Comparison and Discussion:
The distinct characteristics of these new specimens set them apart from the Miocene Allactagines, such as Paralactaga and Brachyscirtetes. They are distinguished by their smaller size, less developed loph(id) and lower crown, as well as the paracone connecting the posterior arm of the protocone on M1. Additionally, they feature a well-developed ectomesolophid on m1 and a mesolophid that separates from the entoconid on m1-2, which are traits unique to the genus Protalactaga.
Protalactaga includes seven identified species: P. grabaui (Young 1927), P. major (Qiu 1996), P. aenigmatica, P. shevyrevae (Zazhigin and Lopatin 2000), P. mynsuensis (Kordikova 2004), P. lantianensis (Li and Zheng 2005) and P. lophodens (Qiu and Li 2016). Our materials are similar to P. grabaui by its smaller size, buno-lophodont molars, isolated metaconid on m1. However, the protoloph slightly anterior to protocone on M2 may represent an intraspecies variation which hasn’t been descirbed in previous specimens (Qiu 1996; Qiu and Li 2016). It is significantly smaller and less lophodont than other three species, P. major, P. lantianensis and P. lophodens, which also found in China. Moreover, it differs from P. major in having relatively narrow anterior part, and connection between protoconid and mesoconid on m1; differs from P. lantianensis in hypolophid separating from mesolophid on m1-2; differs from P. lophodens in protoloph connecting to posterior arm of protocone on M1, isolated metaconid on m1.
These new materials also have similar molar morphology and size with P. aenigmatica and P. shevyrevae (Zazhigin and Lopatin 2000). Qiu and Li (2016) have pointed out the latter two may be the junior synonym of P. grabaui, but without providing any criteria for this statement. According to Zazhigin and Lopatin (2000): P. shevyrevae differs from P. grabaui by closed posterofossettids on m1-2, incomplete ectomesolophid with a ectostylid in some case, and occasional presence of the pseudomesolophid on m2; and P. aenigmatica differs from P. grabaui by a reduced posteroloph on M1-2 and by an isolated entoconid on m2. Together our materials and other known specimens of this species (Young 1927; Qiu 1996; Wu et al. 2009; Qiu and Li 2016), we can find that these characters discussed above are highly variable. Thus, we agree with Qiu and Li (2016)’s view that P. aenigmatica and P. shevyrevae should be the junior synonym of P. grabaui.
Protalactaga lophodens Qiu et Li, 2016
Fig. 11; Table 5
Referred Specimens:
DM01: IVPP V 32971.1-2, 1 left M1, 1 left m2; (Fig.11c-e)
DM02: IVPP V32982, 1 left M1. (Fig.11a, b)
Locality and Age:
DM01 and DM02, Damiao Village, Siziwangqi, Nei Mongol, China; latest Middle Miocene to earliest Late Miocene
Measurements: see Table 5
Description
M1(Fig.11a-d): The M1 is unilaterally high crowned, large size, and distinctly lophodont with a flat occlusal surface. Lingual main cusps (protocone and hypocone) are compressed as oblique crests extending distolingually, the latter one is continously merged to the entoloph. The anteroloph fuses with the paracone at the buccal margin. The protoloph and the mesoloph are transverse and lingually connect the protocone and mesocone respectively. The entoloph is complete connecting the posterior arm of protocone and the hypocone. The metaloph is fused with the really reduced posteroloph and connected to the posterior arm of hypocone; the sinus is deep and long, extending mesiobuccally to the medial axis of the tooth. In IVPP V32982 (Fig.10a), which is an almost unweared tooth, all buccal valleys but posterosinus are enclosed by a longitudinal ridge to form deep concavities.
m2(Fig.11e): The metaconid is fused with anteroconid, forming a strong transverse crest locating at mesiolingual edge of the tooth crown. The connection between the protoconid and the anteroconid (the anterior arm of protoconid) is narrow but complete. The protoconid-mesolophid(psudomesolophid), hypoconid-posterolophid connections are continous and fused into united crests, respectively. The mesconid is indistinct and merged in the ectolophid. The entoconid separates from mesolophid and connectsthe anterior arm ofhypoconid. The sinusid is deep and long, extending distolingually to the medial axis of the tooth. The posterosinusid is closed, whereas other lingual valleys are open.
Comparison:
These new materials are identical to Protalactaga lophodens by its size and morphology. It is distinctly larger than P. grabaui (Young 1927), P. lantianensis (Li and Zheng 2005), close to these large specimens of P. major (Qiu 1996; Zazhigin and Lopatin 2000; Qiu and Li 2016), and P. lophodens (Qiu and Li 2016). In morphology, they are similar with P. lophodens in having relatively higher crowned and lophodonty molars, connection between protoloph and protocone on M1, separation between mesolophid and hypolophid on m2. These materials show its size of some species of Paralactaga. However, the primitive characters, such as connection between protoloph and protocone, separation between protoloph and mesoloph, separation between mesolophid and hypolophid, make it easy to be distingushed from Paralactaga.
Genus Paralactaga Young 1927
Paralactaga parvidens Qiu et Li, 2016
Fig. 12; Table 5
Referred Specimens
DM01: IVPP V32972.1-2, 1 right m1, 1 right m2; (Fig.12b, c)
DM02: IVPP V32983.1-2, 1 left M2, 1 right m2. (Fig.12a, d)
Locality and Age:
DM01 and DM02, Damiao Village, Siziwangqi, Nei Mongol, China; latest Middle Miocene to earliest Late Miocene
Measurements: see Table 5
Description
M2(Fig.12a): The anteroloph is transverse and connected the small anterocone and anterostyle. The protoloph connectsthe junction of the mesoloph and the entoloph. The mesoloph is long and ends with the mesostyle. The entoloph is complete extending from the posterior arm of protocone to the hypocone, and more or less longitudinal. The metaloph is distolingually directed and joined to the junction of the hypocone and the posteroloph. The posteroloph is short.
m1(Fig.12b, c): Its mesial part is narrower than distal part. The anteroconid is absent. The mesolophid is fused with the metaconid into a lophid, which is subequal size and shape with the protoconid. the metaconid-mesolophid lopid, protoconid, entoconid and ecomesolophid all connect the mesoconid. The posterolophid is strong, continously extends from the hypoconid to the entoconid.
m2(Fig.12d): It is subrectangular in outline. The metaconid is compressed as a robust crest locating at the mesialligual edge of the molar and joined to the anteroconid. The protoconid is distal to the metaconid and connected mesially to the anteroconid. The mesolophid and entoconid are fused with mesooconid at the moderately wear stage. The mesolophid is shorter than entoconid. No ectomesolophid. The posterolophid is strong and inflated in middle part.
Comparison:
Although the dimensions of these materials are small, they exhibit typical characteristics of Paralactaga, such as the protoloph connecting the junction of the entoloph and mesoloph on M2 and the mesolophid joining the lingual base of hypolohid on m2.
In terms of size, this species is comparable to P. parvidens and P. varians (Savinov, 1970), and smaller than most species of Paralactaga, such as P. suni (Teilhard de Chardin and Young, 1931) and P. shalaensis (Qiu and Li, 2016). However, it is larger than P. minor (Zheng, 1982). Additionally, our materials are smaller than the type specimens of P. anderssoni (Young, 1927), but they fall within the size range of P. anderssoni from central Nei Mongol (Qiu and Li, 2016).
Morphologically, these new materials differ from P. suni and P. anderssoni by its protoloph and metaloph connecting to the base of the mesoloph and the posteroloph rather than the middle of mesoloph and the posteroloph on M2, respectively; differs from P. varians by the absence of anterolophid on m2; differs from P. minor by the well-developed ectomesolophid on m1, absence of anterolophid and shorter mesolophid than entoconid on m2. Compared to P. parvidens, these lower molars are slightly larger in size, and differs in morphology in having stronger posteroloph on M2. However, due to the current paucity of specimens and the fact that this morphological difference manifests itself as intraspecific variation in some taxa, e.g., Protalactaga grabaui. Therefore, we refer these materials to P. parvidens.