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453

ISSN 0031-0301, Paleontological Journal, 2009, Vol. 43, No. 4, pp. 453–462. © Pleiades Publishing, Ltd., 2009.Original Russian Text © A.V. Lopatin, A.O. Averianov, E.N. Maschenko, S.V. Leshchinskiy, 2009, published in Paleontologicheskii Zhurnal, 2009, No. 4, pp. 92–100.

INTRODUCTION

The docodont

Sibirotherium rossicum

is a memberof the Early Cretaceous Shestakovo Vertebrate Assem-blage of Western Siberia, which has been studiedintensely during the past years (Maschenko and Lopa-tin, 1998; Maschenko et al., 2002, 2003; Lopatin et al.,2005; Lopatin and Averianov, 2007; see also referencesin these papers). This species was described based onthree mandibular fragments, one of which (holotypePM TGU, no. 16/5-22) contains two deciduous premo-lars and

å

1

and others have more posterior molars, pre-sumably identified as M

2

and M

3

in specimen PM TGU,no. 16/5-14 and as M

3

–M

6

in specimen PM TGU,no. 16/5-2 (Maschenko et al., 2003). The holotypeadditionally has four alveoli of two relatively smalldouble-rooted teeth; therefore, the lower cheek teethformula of


was initially deter-mined as P

4(?)

M

6(?)

. In 2004, E.N. Maschenko (PIN) andA.S. Rezvyi (SPbGU) found a new dentary fragment of

Sibirotherium

(specimen PM TGU, no. 120/9-34) in theShestakovo 1 locality. This specimen has M

1

–M

2

, threeposterior premolars, the posterior alveolus of thecanine, and six small alveoli (with partially preservedroots) of three anterior premolars. Close to the dentaryfragment in the matrix, there were three isolated pre-molars and a fragment of the symphyseal region of thedentary, with a strongly damaged large double-rootedtooth, which was identified as the canine, and an alveo-lus of a single-rooted tooth anterior to it (which pre-sumably contained the ultimate incisor). As the speci-men was prepared, the breaks of roots of the isolatedpremolars fit into the fragments preserved in the alve-oli; thus, these teeth were glued to the dentary. The den-

tary fragment with the canine, alveolus of the incisor,and partial symphysis also belongs to this hemimandi-ble; they come in contact just anterior to the alveoli ofthe anterior premolar. This allowed the establishment ofthe structure and number of the lower premolars of

Sibirotherium

and variation in the structure of the lowermolars and correctness of identification of M

2

. Anotherimportant find is a maxillary fragment with the penulti-mate and ultimate molars (specimen PM TGU,no. 120/5-Sh1-5), which were found by P.P. Skuchas(SPbGU), A.S. Rezvyi, and other researchers in 2005 inthe Shestakovo 1 locality. In addition, in 2007, Shesta-kovo 1 yielded another isolated upper molar (specimenPM TGU, no. 120/7-Sh1-3). These specimens allow thestructure of the upper molars of

Sibirotherium

to bedescribed for the first time. The present study revisesthe morphology of

Sibirotherium

based on all knownspecimens, including new finds listed above and others.

In the description, we use the dental nomenclatureand terminology of Docodonta developed by Butler(1997) and modified by us (Averianov et al., 2009; seealso Fig. 1) based on subsequent works (Hu et al., 2007;Luo and Martin, 2007). The letter designations of den-tal cusps correspond to those accepted by some otherresearchers (Kermack et al., 1987; Sigogneau-Russell,2001, 2003; Kielan-Jaworowska et al., 2004): in thelower molars, cusp

a

is the main cusp, cusp

b

is themesiolabial cusp, cusp

c

is the distolingual cusp, cusp

g

is the mesiolingual cusp, cusp

bb

is the anterior cusp,cusp

e

is an additional anterolingual cingular cusp, cusp

d

is the distolabial cusp or labial talonid cusp, cusp

dd

is the lingual talonid cusp; in the upper molars, cusp

A

is the mesiolabial cusp, cusp

B

is the labial cingularcusp, cusp

C

is the distolabial cusp, cusp

D

is the distal

Early Cretaceous Mammals of Western Siberia: 2. Tegotheriidae

A. V. Lopatin

a

, A. O. Averianov

b

, E. N. Maschenko

a

, and S. V. Leshchinskiy

c

a

Borissiak Paleontological Institute, Russian Academy of Sciences, Profsoyuznaya ul. 123, Moscow, 117997 Russiae-mail: [email protected]

b

Zoological Institute, Russian Academy of Sciences, Universitetskaya nab. 1, St. Petersburg, 199034 Russia

c

Tomsk State University, pr. Lenina 36, Tomsk, 634050 Russia

Received July 21, 2008

Abstract

—New specimens of the tegotheriid docodont


Maschenko et al., 2003, includ-ing a maxillary fragment with two posterior teeth, an isolated upper molar, and mandibular fragments with teeth fromthe Early Cretaceous Shestakovo locality are described. The dental formula of

Sibirotherium

is I

1 + ?

C

1

P

6

M

6?

. Theupper molars of

Sibirotherium

, with two main labial and three lingual cusps, are convergently similar to themolars of tribosphenic mammals. In the dentary, the symphysis is short and Meckel’s groove is reduced.

Sibirotherium

is similar in the structure of lower teeth to

Tegotherium

from the Upper Jurassic of Mongolia; itis the latest known representative of Docodonta.

DOI:

10.1134/S0031030109040157

Key words

: Docodonta, Tegotheriidae, Lower Cretaceous, Western Siberia.

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No. 4

2009

LOPATIN et al.

stylar cusp, cusp

E

is the mesial stylar cusp, cusp

X

isthe mesiolingual cusp, and cusp

Y

is the distolingualcusp (homologues of the cusp

Z

have not been recog-nized). The crests between particular cusps are desig-nated as follows:

a–b, a–g, a–c, a–d, b–g, b–bb, bb–g,c–d, d–dd, dd–c, A–B, A–C, C–D, B–E, X–Z

, and

X–Y

.We use the following abbreviations: (L) maximum

length of the tooth crown; (W) maximum width of thetooth crown, (H) height of the tooth crown along thelabial side of the cusp

a

; and (h) depth of the horizontalramus of the lower jaw on the labial side.

The material described and discussed in the presentstudy is stored in the Paleontological Museum ofTomsk State University (PM TGU), Tomsk, Russia.

SYSTEMATIC PALEONTOLOGY

Order Docodonta Kretzoi, 1946

Family Tegotheriidae Tatarinov, 1994

Tegotheriidae: Tatarinov, 1994, p. 104; Maschenko et al., 2003,p. 76.

Ty p e g e n u s.

Tegotherium

Tatarinov, 1994,Upper Jurassic of Mongolia.

D i a g n o s i s. Upper molars with cusp

Z

, withoutcrest

A–X

; lower molars with cusp

bb

and largepseudotalonid basin bordered by crests

a–b

,

b–bb, bb–g

,and

a–g

, without crest

b–g

.G e n e r i c c o m p o s i t i o n. In addition to the

type genus,

Krusatodon

Sigogneau-Russell, 2003 fromthe Middle Jurassic of England and

Sibirotherium

Maschenko, Lopatin et Voronkevich, 2003 from theLower Cretaceous of Western Siberia.

C o m p a r i s o n. The family Tegotheriidae differsfrom other docodonts in the presence of the cusp

Z

andthe absence of the crest

A–X

on the upper molars, thepresence of the cusp

bb

and the formation of walls ofthe pseudotalonid basin by the crests

a–b

,

b–bb, bb–g

,and

a–g

combined with the absence of a crest

b–g

onthe lower molars.

R e m a r k s. The family Tegotheriidae was consid-ered to include

Tashkumyrodon

Martin et Averianov,2004 from the Middle Jurassic of Kyrgyzstan and

Itat-odon

Lopatin et Averianov, 2005 from the MiddleJurassic of Western Siberia (Martin and Averianov,2004; Lopatin and Averianov, 2005; Averianov andLopatin, 2006). At present, based on new phylogeneticanalysis, we regard these genera as Docodonta incertaesedis, assuming a basal position of

Itatodon

amongdocodonts (Averianov et al., 2009).

Genus

Sibirotherium

Maschenko, Lopatin et Voronkevich, 2003

Sibirotherium

: Maschenko et al., 2003, p. 76.

Ty p e s p e c i e s.


Maschenko, Lopatin et Voronkevich, 2003, Lower Cre-taceous of Western Siberia.

D i a g n o s i s. Upper molars with strongly trans-versely expanded lingual projection. In lower molars,cusp

c

larger than cusp

b

, crest

b–bb

short, crest

bb–g

incomplete, reduced, crest

b–e

or distinct cusp

e

absent,crest

d–dd

well developed, crest

dd–c

present or absent,supplementary crests of posterior basin absent, lingualcingulid well developed.

S p e c i e s c o m p o s i t i o n. Type species.C o m p a r i s o n.

Sibirotherium

differs from

Tegotherium

in the shorter crest

b–bb

, reduced crest

bb–g

, and well-developed crest

d–dd.

It differs from

Krusatodon

in the more strongly expanded lingual pro-jection of the upper molars, the reversed size ratio ofcusps

c

and

b

, the absence of crest

b–e

and distinct cusp

e

, possible presence of crest

dd–c

, the absence of sup-plementary crests of the posterior basin, and in thewell-developed lingual cingulid of the lower molars.


Maschenko, Lopatin et Voronkevich, 2003

Plate 12, figs. 1–6

Sibirotherium rossicus

[sic, incorrect ending of the species epi-thet]: Maschenko et al., 2003, p. 77, text-figs. 1–4.


: Lopatin and Averianov, 2007, p. 263.

H o l o t y p e. PM TGU, no. 16/5-22, left dentaryfragment with DP

5

, DP

6

, M

1, and alveoli of (D)P3 and(D)P4; Russia, Kemerovo Region, Chebulinskii Dis-trict, Shestakovo 1 locality (55°54'12'' N, 87°57'28'' E,1.5 km downstream from the village of Shestakovo onthe right bank of the Kiya River); Lower Cretaceous,Ilek Formation.

D e s c r i p t i o n (Figs. 2–6). The upper teeth areknown from a maxillary fragment with two posteriormolars (specimen PM TGU, no. 120/5-Sh1-5) and anisolated upper molar (specimen PM TGU, no. 120/7-Sh1-3). Although tentative number of lower molars ofSibirotherium has been determined (presumably six,see Maschenko et al., 2003), we refrain from thehomologization of the posterior upper molars, since, indocodonts, the number of upper and lower molars maydiffer by one or even two (Kielan-Jaworowska et al.,

B A C

D

E

X

ZY

bb

g c

dd

b a d(‡) (b)

Fig. 1. Designations of dental cusps of Docodonta, usingSibirotherium rossicum Maschenko, Lopatin et Voronkevich,2003 as an example: (a) left upper molar and (b) left lowermolar.

PALEONTOLOGICAL JOURNAL Vol. 43 No. 4 2009

EARLY CRETACEOUS MAMMALS OF WESTERN SIBERIA: 2. TEGOTHERIIDAE 455

2004). The isolated molar under study occupied a moreanterior position in the jaw than the two posterior.

The upper molars are transversely expanded, theectoflexus is weak (Pl. 12, figs. 1, 2; Fig. 2). The labialpart of the crown is formed of the central cusps A and Cand the anterior cusp B in specimen PM TGU,no. 120/7-Sh1-3. In this specimen, the cusp B is locatedon a well-developed anterolabial projection (Pl. 12,fig. 1; Fig. 2a), which is an analogue of the parastylarlobe of the tribosphenic teeth. Lingual to the cusp B, theanterior cingulum has a well-developed cingular cuspE. The space between the walls of the cusps A, B, and Eis occupied by a deep and extensive depression, whichforms a fissure in the anterior wall of the cusp A just lin-gual to the crest A–B. The anterior wall of the cusp Ahas subvertical enamel crenulations (Fig. 2b). In bothposterior molars of specimen PM TGU, no. 120/5-Sh1-5,the anterolabial projection of the crown is completelyabsent, the cusp B is reduced to a small cingular prom-inence, which is positioned anterolabial to the cusp A(Pl. 12, fig. 2; Fig. 2f). In the penultimate molar, thecusp B is positioned close to the stylar cusp E, the api-ces of these cusps are connected in a single crestlikestructure; however, a superficial groove separating thebases of these cusps is observed on the anterior side(Fig. 2g). In the ultimate molar, the apices of the cusps

B and E are differentiated and connected by a shortcrest.

On the labial side of specimen PM TGU, no. 120/7-Sh1-3, the cusps A and C are separated by a well-pro-nounced triangular depression (Fig. 2d). In specimenPM TGU, no. 120/5-Sh1-5, this depression is weaker;however, it has a distinctive narrow depression at thebase of the anterior side of the cusp A, which is partic-ularly well developed in the penultimate tooth (Fig. 2h).

The cusp C is approximately half as high as the cuspA in specimen PM TGU, no. 120/7-Sh1-3 and in thepenultimate tooth of specimen PM TGU, no. 120/5-Sh1-5 (in the ultimate tooth, it is damaged). The cingu-lar cusp D is very small, hardly discernible. In speci-men PM TGU, no. 120/7-Sh1-3, the crest C–D is verystrong, with a wear facet along the entire length. Lin-gual to the bases of the labial cusps, the crown is nar-rowed considerably longitudinally (at the point of nar-rowing, the length is 48% of the labial length of thecrown in specimen PM TGU, no. 120/7-Sh1-3 and 57%in the penultimate tooth of specimen PM TGU,no. 120/5-Sh1-5; at the same time, the length of the lin-gual projection is accordingly 48 and 63% of the labiallength of the crown, respectively). The center of the lin-gual projection of the crown is occupied by a large cuspX, which is approximately equal in height to the cusp C

(‡) (b) (c)

(d)

(e)

(f) (g)

(h)0 1 mm

Fig. 2. Sibirotherium rossicum Maschenko, Lopatin et Voronkevich, 2003: (a–e) specimen PM TGU, no. 120/7-Sh1-3, isolated leftupper molar: (a) occlusal, (b) anterior, (c) posterior, (d) labial, and (e) lingual views; (f–h) specimen PM TGU, no. 120/5-Sh1-5,left upper penultimate and ultimate molars: (f) occlusal view; (g) penultimate molar, anterior view; (h) molars, labial view; Shesta-kovo 1 locality; Lower Cretaceous, Ilek Formation.

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LOPATIN et al.

(Fig. 2e). Two smaller cusps are located at the base ofthe cusp X: the anterolabial cusp Z and posterolabialcusp Y. The cusp Z is somewhat larger than the cusp Y.Labially, the cusps Z and Y are connected by transversemarginal crests to the anterior and posterior portions ofa strong cingulum, respectively; in specimen PM TGU,no. 120/5-Sh1-5, this cingulum borders the entire labiallobe of the crown (anteriorly, posteriorly, and labially).In specimen PM TGU, no. 120/7-Sh1-3, the labial cin-gulum is restricted to the posterior half of the crown; itsanterior end has a distinct cingular cuspule opposite themiddle of the cusp A. Between the labial and lingualcusps and transverse cingula, the central part of thecrown is occupied by an extensive basin. In specimenPM TGU, no. 120/5-Sh1-3, the cusps Z and Y have nar-row crests inside the basin, which are analogues of themedial wings of conules of tribosphenic mammals(Pl. 12, fig. 1; Fig. 2a). In specimen PM TGU, no.120/5-Sh1-5, either tooth has only a weak anterolabialprojection of the cusp Y (Pl. 12, fig. 2; Fig. 2f).

The crests A–B and C–D and the anterior portion ofthe crest A–C are well developed. A long transversecrest, which extends from the apex of the cusp A andborders anteriorly a superficial basin of the lingual pro-jection (anterolingual crest: Sigogneau-Russell, 2003;it is better developed in specimen PM TGU, no. 120/5-Sh1-5), is also distinct. Other crests (X–Z, X–Y, and theposterior portion of A–C of three available molars, thelabial crest of the cusp A, and the crest B–E of the ulti-mate molar of specimen PM TGU, no. 120/5-Sh1-5)are shorter and weaker. In specimen PM TGU, no.

120/5-Sh1-5, the ultimate molar is similar in structureto the penultimate, differing from it in the considerablysmaller size, the distinctly reduced cusp C, and theabove-described relationship between the cusps B andE (Pl. 12, fig. 2; Fig. 2f).

The teeth are slightly worn. In specimen PM TGU,no. 120/7-Sh1-3, only the above-mentioned facet on thecrest C–D is distinct. In the penultimate molar of spec-imen PM TGU, no. 120/5-Sh1-5, the apical facets ofcusps A, C, and Y, the labial facets at the apices of cuspsX and Z, the facets of the crest A–B, and an extendedwear facet along the anterior cingulum, which occupiesthe united apex of cusps B + E, are distinct. In the ulti-mate molar, the apical facets of the cusps A and Z andthe labial facet at the apex of the cusp X are distinctlypronounced.

The upper molars have three widely spaced roots,with the lingual root smaller than the others.

The horizontal ramus of the dentary is low, gradu-ally increasing in depth posteriorly (Pl. 12, fig. 6;Fig. 3). A small anterior mental foramen is located inline with the anterior root of C1; the posterior mentalforamen is larger, located under the anterior root of P2.The symphysis is weak and relatively short, extendsposteriorly to the space between P2 and P3. Meckel’sgroove is poorly outlined, its anterior extension widelyvaries in length (in specimen PM TGU, no. 120/9-34, itterminates under the posterior region of DP6, while, inspecimen PM TGU, no. 16/5-2, under M4).

M2M1 DP6

P5P4

C1

Ix

(‡)

(b)

0 1 mm

P3P2

P1

Fig. 3. Sibirotherium rossicum Maschenko, Lopatin et Voronkevich, 2003, specimen PM TGU, no. 120/9-34, right dentary fragmentwith C1, P1–P5, DP6, M1, M2, and root of the ultimate incisor: (a) labial and (b) lingual views; Shestakovo 1 locality; LowerCretaceous, Ilek Formation.



In specimen PM TGU, no. 120/9-34, the tooth rowretains the alveolus of the ultimate incisor (IX, which isprobably I4), double-rooted C1 with a somewhat dam-aged crown, and eight double-rooted cheek teeth: P1–P5, DP6, M1, and M2. The lengths change as follows:ê1 � P2 � P3 � P4 = P5 ≈ M2 ≈ DP6 < M1 (Fig. 4b).

The lower canine is relatively large, positioned ver-tically in the jaw. The roots are closely spaced, the ante-rior root is slightly longer (anteroposteriorly) than theposterior root. At the crown base, there are distinct lin-gual and labial cingulids and a relatively small addi-tional posterior cuspule (the anterior part of the crownis damaged). The tooth has a strong posterior rib and anextensive facet labial to it, which is formed by theocclusion with the upper canine.

The isolated tooth (specimen PM TGU, no. 120/5-Sh1-6; Pl. 12, fig. 3; Figs. 5a–5c) is identified as adeciduous lower canine (DC1). It corresponds in sizeand structure to C1 described above. DC1 is double-rooted, with widely spaced roots. The doubled structureextends from the root to the crown base (better pro-nounced on the labial side). The posterior root is some-what longer (anteroposteriorly) and wider than theanterior root. In the terminal part, the roots curve con-siderably posteriorly, which is particularly well-pro-nounced in the posterior root. The crown is conical,with a lingually curved central apex and very smallanterior and posterior additional cuspules. The anterioradditional cuspule is located much higher than the pos-terior one. A distinct posterior rib of the crown extendsfrom the apex of the canine to the posterior cuspule.The cingulid is continuous, better developed on the lin-gual side. The posterolabial side of the crown has anextended subvertical wear facet. The assignment of thistooth to the deciduous generation is supported by itswidely spaced roots and their considerable posteriorcurvature in the terminal region; this may result fromthe position of the tooth in a low dentary of a youngindividual.

P1–P5 are similar in structure and differ considerablyin size, gradually increasing from P1 to P4 and slightlydecreasing in P5. The lower premolars have a relativelyhigh and massive main cusp a and low anterior and pos-terior cusps, which look like small cingular denticles.The anterior crest of the main cusp is poorly developed.The posterior longitudinal crest (corresponding to thecrest a–d of molars) is well developed, sharp. In addi-tion, the main cusp has a hardly discernible posterolin-gual rib, which corresponds in position to the crest a–cof molars. The lingual cingulid is distinct, complete.The labial cingulid is rather strong anteriorly and pos-

M2

M1

DP6

P5

P4

C1

Ix

(b)

0 1 mm

P3

P2

P1

DP5

(‡)

Fig. 4. Sibirotherium rossicum Maschenko, Lopatin etVoronkevich, 2003: (a) holotype PM TGU, no. 16/5-22, leftDP5, DP6, and M1 ; (b) specimen PM TGU, no. 120/9-34,right IX, C1, P1–P5, DP6, M1–M2; Shestakovo 1 locality;Lower Cretaceous, Ilek Formation.

458


LOPATIN et al.

teriorly; however, it is very narrow and hardly discern-ible at the level of the apex of the main cusp.

The isolated premolar, with a damaged anterior part(specimen PM TGU, no. 120/6-Sh1-8; Pl. 13, fig. 4),corresponds in structure and size to P2 of specimen PMTGU, no. 120/9-34. This tooth has a distinct wear faceton the anterolabial slope near the central apex. Sincethe tooth is fragmentary, it is possible that it could havebelonged to the deciduous generation or the upper jaw.

The penultimate permanent premolar (ê5) is consid-erably longer, wider, and higher than DP5, which is pre-served in the holotype (see Maschenko et al., 2003). Inaddition, Dê5 differs from P5 in the presence of lowerand weaker differentiated anterior and posterior cingu-lar cusps and in the better developed lingual cingulid(Fig. 4).

DP6 of specimen PM TGU, no. 120/9-34 is similarin structure to this tooth of the holotype (Fig. 4), differ-ing in somewhat larger dimensions. Like other knownultimate deciduous premolars of docodonts (see Averi-anov, 2004), the DP6 described differs from molars inthe absence of the cusp g. The holotype has a weak cresta–g, which is absent in specimen PM TGU, no. 120/9-34.The main cusp a is connected to the anterior cusp b andposterior cusp d by sharp crests. The cusp b has a dis-tinct anterior rib. A large cusp c and a strong transversecrest a–c are present. The anterior basin is narrow andshallow, while the posterior basin is larger and deeper.The cusps d and dd are hardly discernible, connected bya long transverse crest d–dd. The lingual cingulid isstrong in the anterior and posterior regions and narrowand weak at the level of the cusp c. A short labial cin-gulid is only well developed at the base of the cusp b.Wear facets are observed on the apices of the cusps aand b and crest d–dd.

DP5 and DP6 of specimen PM TGU, no. 16/7-2 aresimilar in structure to the specimens described above,

although worn to a greater extent (Figs. 5d, 5e). DP5 hasan extensive posterolabial wear facet on the cusp a; inDP6, distinct wear facets occupy most of the labial sideof the cusp b, the anterior part of the lingual cingulid,the posterolabial slope of the cusp a along the crest a–d, the posterior side of the crown along the crest d–dd,the crest a–c, and the upper part of the posterior side ofthe cusp c.

The molars å1 and å2 are similar in structure, dif-fering somewhat in particularities. The height ratios ofthe cusps of M1 are as follows: a � Ò > b > bb > g � d > dd;in å2, the ratios are almost the same, but g > bb. Themain cusp a is conical, with a concave lingual side andconvex labial side. Four crests extend from the apex ofthe cusp a, two are longitudinal (a–b and a–d) and twoare subtransverse (anterolingual a–g and posterolinguala–c). The angle between the crests a–g and a–c isapproximately 40°; in M1, a–g is much shorter andlower than a–c; in M2, the crest a–g is reduced, incom-plete. Other crests are sharp, without wear facets. Thecusps a, c, and g have apical wear facets. The bases ofthe cusps c and g of M1 are separated by a small space,whereas, in M2, they are fused for approximately half-height of the cusp c. The cusp b is located just anteriorto the cusp a. The wear facet on the anterolabial slopeof this cusp is poorly pronounced. The cusp b adjoinsanterolingually the crest b–bb. The cusp bb is some-what lower and much less massive than the cusp b. Themesiolingual cingular cusp e is indistinct, although itsposition is marked by a small thickening of the lingualcingulid. The crest bb–g is well developed in M1 andcompletely absent from M2. In M1, the crest bb–g isconnected to the base of the cusp g and obliquelydivides the anterior basin into two. No crenulations areobserved inside the basin. The cusps d and dd are small,connected by a long crest d–dd, which extends alongthe distal margin of the occlusal surface of the crown. Arelatively deep rounded posterior basin is bordered by

0 1 mm

(‡)

(b) (c)

(d)

(e)

Fig. 5. Sibirotherium rossicum Maschenko, Lopatin et Voronkevich, 2003: (a–c) specimen PM TGU, no. 120/5-Sh1-6, isolated leftDC1: (a) occlusal, (b) labial, and (c) lingual views; (d, e) specimen PM TGU, no. 16/7-2, worn left DP5 and DP6: (d) occlusal and(e) labial views; Shestakovo 1 locality; Lower Cretaceous, Ilek Formation.



the crests a–c, d–dd, and a–d. M1 has a short, narrow,and very low crest dd–c, which adjoins the posteriorbase of the cusp c but does not ascend on its wall. M2lacks this crest. The lingual cingulid is distinct over theentire extent, except for the base of the cusp c, the labialcingulid is absent. The posterior margin of the crown ofM1 is distinctly mesiolingually oblique, while that ofM2 is almost straight.

M1 of specimen PM TGU, no. 120/9-34 is closelysimilar to M1 of the holotype (Fig. 4), but is slightlylarger and has a more distinct lingual cingulid and alonger crest bb–g, which adjoins the base of the cusp g.In M2 of the specimen described, the lingual cusps gand c are fused at the base, as in specimen PM TGU,no. 16/5-14, which was preliminary determined as “M2(?)”(Maschenko et al., 2003); the first differs from the sec-ond in the incomplete crest a–g and the absence of acrest dd–c. The two characters have been recorded inthe tooth identified as “M3(?)”. in specimen PM TGU,no. 16/5-14 (Maschenko et al., 2003). These charactersare probably changeable.

The damaged molars in the right dentary fragmentof specimen PM TGU, no. 120/9-33 (Figs. 6a–6d) aredetermined based on their dimensions as M4? and M5?.An isolated lower molar (specimen PM TGU, no. 16/7-23,Pl. 12, fig. 5; Fig. 6e), with an incomplete crest a–g, ispresumably identified as M4.

M e a s u r e m e n t s, mm. Specimen PM TGU,no. 120/7-Sh1-3, upper molar: L, 2.1; W, 2.3; width ofthe lingual projection, 1.0. Specimen PM TGU; no.120/5-Sh1-5: penultimate upper molar: L, 1.7; W, 2.6;width of the lingual projection, 1.0; ultimate uppermolar: L, 1.2; W, 2.25; width of the lingual projection,0.9; total length of two molars, 2.9.

Specimen PM TGU, no. 120/5-Sh1-6, DC1 : L, 1.6;W, 0.65.

Specimen PM TGU, no. 120/6-Sh1-8, P2?: W, 0.6;H, 1.15.

Specimen PM TGU, no. 120/9-34: C1 : L, 1.4; W,0.75; P1 : L, 1.1; W, 0.5; H, 0.6 (tooth is heavily wornapically); P2 : L, 1.25; W, 0.55; H, 1.05; P3 : L, 1.55; W,0.65; H ~1.5; P4 : L, 1.8; W, 0.9; H, 1.65; P5 : L, 1.8; W,0.85; H, 1.55 (tooth is worn apically); DP6 : L, 1.9; W,1.25; H, 1.55 (tooth is heavily worn apically); M1 : L,2.0; W, 1.4; H, 1.85; M2 : L, 1.85; W, 1.4; H, 1.5; alve-olus of IX : length, 0.6; width, 0.5; h: under P3, 2.3;under P4, 2.5; under P5, 2.7; under DP6, 3.0; under M1,3.1; under M2, 3.0.

Specimen PM TGU; no. 16/7-2: DP5 : L, ~1.4; W,~0.8; DP6 : L, 1.55; W, 1.0; h under DP6, 2.6.

Specimen PM TGU, no. 16/7-23: M4? : L, 1.6; W,1.0.

Specimen PM TGU; no. 120/9-33: M4? : L, 1.6; W,1.1; M5? : length of the base, 1.35.

For the measurements of other specimens, seeMaschenko et al. (2003, p. 79).

R e m a r k s. The teeth of the dentary fragment ofspecimen PM TGU, no. 120/9-34 are larger (DP6 isapproximately 20% longer; the molars are longer by 5–15%) than other lower teeth assigned to S. rossicum(Fig. 4). At present, we are inclined to regard the differ-ences in size as individual or sexual variation within thesame species.

M a t e r i a l. In addition to the holotype, the collec-tion of PM TGU contains ten specimens from the typelocality, including a left maxillary fragment with thepenultimate and ultimate molars (no. 120/5-Sh1-5); anisolated left upper molar (no. 120/7-Sh1-3); right den-tary fragment with C1, P1–P5, DP6, M1, M2, and thealveolus of the ultimate incisor (no. 120/9-34); an iso-lated left DC1 (no. 120/5-Sh1-6); left dentary fragmentwith DP5 and DP6 (no. 16/7-2); isolated fragmentaryright P2? (no. 120/6-Sh1-8); right dentary fragment withM2 –M3 (no. 16/5-14; Maschenko et al., 2003, text-figs.2, 4 D–F); left dentary fragment with M4–M5, and frag-mentary M3 and M6 (no. 16/5-2; Maschenko et al.,2003, text-figs. 3, 4 G–I); right dentary fragment with

(‡)

(b)

(c)

(d)

(e)

0 1 mm

Fig. 6. Sibirotherium rossicum Maschenko, Lopatin etVoronkevich, 2003: (a–d) specimen PM TGU, no. 120/9-33, right dentary fragment with M4? and the base of M5?:(a) labial, (b) lingual, and (c) anterior views; (d) M4?,occlusal view; (e) specimen PM TGU, no. 16/7-23, left M4?,occlusal view; Shestakovo 1 locality; Lower Cretaceous,Ilek Formation.

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Plate 12

1a

1b4

5‡

2‡

5b

2b

3

6‡

6b

6c

6d



M4? and the base of M5? (no. 120/9-33); and an isolatedleft M4? (no. 16/7-23).

DISCUSSIONThe upper molars of Sibirotherium are strikingly

similar to the molars of tribosphenic mammals and pro-vide an example of convergent evolution. The labial(morphologically central) cusps A and C correspond tothe paracone and metacone, while the lingual cusps X,Z, and Y correspond to the protocone, paraconule, andmetaconule, respectively. The cusps E and B corre-spond to the preparastyle and parastyle; the cusp D, tothe metastyle; the crests A–C, A–B, and C–D, to thecentrocrista, preparacrista, and postmetacrista, respec-tively; and the lingual crests, to the protocrista andwings of conules. The assignment to Docodonta is sup-ported by the following characters: (1) the presence ofa transverse crest that extends lingually from the apexof the cusp A (a homologue of the crest A–X of otherdocodonts); (2) the wear facets of lingual cusps on thelabial rather than lingual side; (3) the presence of a dis-tinctive narrow depression at the base of the anteriorside of the cusp A; and (4) the presence of a small (lon-gitudinally narrow) lingual root, which is approxi-mately half as long as the lingual projection of thecrown.

The “pseudotribosphenic” upper molars with theanalogues of the paracone, metacone, protocone, andconules are also characteristic of Middle Jurassic doc-odonts of the family Tegotheriidae (see Sigogneau-Russell, 2003; Luo, 2007; Luo et al., 2007; Averianovet al., 2009); however, in Sibirotherium, this similarityis particularly well pronounced because of the stronglywidened lingual projection.

It is interesting that, in the Shestakovo Fauna, tri-bosphenic mammals have not been recorded. Theabsence of competition with tribosphenic mammals isprobably responsible for the changes in the uppermolars of Sibirotherium towards the tribosphenic pat-tern and for the development of an analogue of the pro-tocone in the upper molars of the tinodontid “symmet-rodont” Yermakia domitor Lopatin et al., 2005 from thesame fauna (Averianov and Lopatin, 2008).

Judging from a recent phylogenetic study, which isbased on the dental characters (Averianov et al., 2009),Sibirotherium is the most advanced member of theorder Docodonta, which, along with Tegotherium, Kru-satodon, and a new genus from the Middle Jurassic of

Western Siberia (see Averianov et al., 2009), areassigned to the family Tegotheriidae. This family dis-plays a distinctive structure of the pseudotalonid, thewall of which includes the additional cusp bb. Of alltegotheriids, Sibirotherium is most similar to Tegothe-rium; both have a lingual cingulid on the lower molars.Sibirotherium is relatively progressive in the lower jawstructure, with a short mandibular symphysis andreduced Meckel’s groove. However, the condition ofthese characters in other Asian docodonts is not known.Sibirotherium is unique (among Docodonta) in thepresence of six lower premolars (to date, the greatestnumber of the lower premolars recorded in docodontshas been five: in Castorocauda Ji et al., 2006 from theMiddle Jurassic of China, see Ji et al., 2006). Only twomammals with six lower premolars have previouslybeen described, i.e., the “obtuse-angled symmetrodont”Kuehneotherium Kermack et al., 1968 from the UpperTriassic–Lower Jurassic of Europe and Greenland andthe ausktribosphenid Bishops Rich et al., 2001 from theLower Cretaceous of Australia (Kermack et al., 1968;Rich et al., 2001; Gill, 2006).

Sibirotherium from the Aptian–Albian of Siberia isthe latest known docodont; in addition, Cifelli et al.(1997) indicated the presence of unpublished finds ofdocodonts in the Albian–Cenomanian of Utah, UnitedStates. An indication that docodonts occurred in theAptian–Albian Höövör (Khoboor) locality of Mongolia(Agadjanian, 1999) was in error; according to the revi-sion performed by A.V. Lopatin of the specimens storedat Borissiak Paleontological Institute of the RussianAcademy of Sciences (PIN), the isolated tooth previ-ously identified as “Docodonta indet.” is in fact anupper molar of the eutriconodont Gobiconodon boris-siaki Trofimov, 1978.

ACKNOWLEDGMENTS

A.O. Averianov is grateful to D. Sigogneau-Russell(National Museum of Natural History, Paris),J.J. Hooker (Natural History Museum, London),S.K. Bell (American Museum of Natural History, NewYork), W. Joyce (Peabody Museum of Yale University,New Haven), and H.-D. Sues (Museum of Natural His-tory, Smithsonian Institute, Washington) for the oppor-tunity to examine specimens of docodonts and otherMesozoic mammals.

This study was supported by the Russian Founda-tion for Basic Research (project no. 07-04-00393);

E x p l a n a t i o n o f P l a t e 1 2Figs. 1–6. Sibirotherium rossicum Maschenko, Lopatin et Voronkevich, 2003: (1) specimen PM TGU, no. 120/7-Sh1-3, isolated leftupper molar, occlusal view: (1a, 1b) stereopair; (2) specimen PM TGU, no. 120/5-Sh1-5, left maxillary fragment with penultimateand ultimate molars, occlusal view: (2a, 2b) stereopair; (3) specimen PM TGU, no. 120/5-Sh1-6, isolated left DC1, lingual view;(4) specimen PM TGU, no. 120/6-Sh1-8, isolated fragmentary right P2?, occlusal view; (5) specimen PM TGU, no. 16/7-23, leftM4?, occlusal view: (5a, 5b) stereopair; (6) specimen PM TGU, no. 120/9-34, right dentary fragment with C1, P1–P5, DP6, M1,M2, and root of the ultimate incisor: (6a) labial view, (6b, 6d) stereopair, occlusal view, (6c) lingual view; (1–5) ×18; (6) ×8.5;Shestakovo 1 locality; Lower Cretaceous, Ilek Formation.

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United States Civilian Research and DevelopmentFoundation, no. RUG1-2571-ST-04; the Board of thePresident of the Russian Federation (MD-255.2003.4and MD-3050.2007.4), the Russian State Program forSupport of Leading Scientific Schools (projectno. NSh-6228.2006.4), and by the National GeographicSociety (project no. 8268-07).

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