Early Cretaceous mammals of Western Siberia: 2. Tegotheriidae

Embed Size (px)

Text of Early Cretaceous mammals of Western Siberia: 2. Tegotheriidae

  • 453

    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