Early Cretaceous mammals of Western Siberia: 2. Tegotheriidae

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    ISSN 0031-0301, Paleontological Journal, 2009, Vol. 43, No. 4, pp. 453462. 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. 92100.

    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

    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

    Sibirotherium rossicum

    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: alopat@paleo.ru

    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

    Sibirotherium rossicum

    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 Meckels 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.

  • 454

    PALEONTOLOGICAL JOURNAL

    Vol. 43

    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:

    ab, ag, ac, ad, bg, bbb, bbg,cd, ddd, ddc, AB, AC, CD, BE, XZ

    , and

    XY

    .

    We use the following abbreviations: (L) maximumlength 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

    AX

    ; lower molars with cusp

    bb

    and largepseudotalonid basin bordered by crests

    ab

    ,

    bbb, bbg

    ,

    and

    ag

    , without crest

    bg

    .

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

    AX

    on the upper molars, thepresence of the cusp

    bb

    and the formation of walls ofthe pseudotalonid basin by the crests

    ab

    ,

    bbb, bbg

    ,

    and

    ag

    combined with the absence of a crest

    bg

    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.

    Sibirotherium rossicum

    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

    bbb

    short, crest

    bbg

    incomplete, reduced, crest

    be

    or distinct cusp

    e

    absent,crest

    ddd

    well developed, crest

    ddc

    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

    bbb

    , reduced crest

    bbg

    , and well-developed crest

    ddd.

    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

    be

    and distinct cusp

    e

    , possible presence of crest

    ddc

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

    Sibirotherium rossicum

    Maschenko, Lopatin et Voronkevich, 2003

    Plate 12, figs. 16

    Sibirotherium rossicus

    [sic, incorrect ending of the species epi-thet]: Maschenko et al., 2003, p. 77, text-figs. 14.

    Sibirotherium rossicum

    : 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 (5554'12'' N, 8757'28'' 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. 26). 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 AB. 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 CD 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: (ae) specimen PM TGU, no. 120/7-Sh1-3, isolated leftupper molar: (a) occlusal, (b) anterior, (c) posterior, (d) labial, and (e) lingual views; (fh) 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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    PALEONTOLOGICAL JOURNAL Vol. 43 No. 4 2009

    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 AB and CD and the anterior portion ofthe crest AC 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 (XZ, XY, and theposterior portion of AC of three available molars, thelabial crest of the cusp A, and the crest BE 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...

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