Early Cretaceous choristoderes (Diapsida, Choristodera) from Siberia, Russia

Accepted Manuscript Early Cretaceous choristoderes (Diapsida, Choristodera) from Siberia, Russia Pavel P. Skutschas, Dmitriy D. Vitenko PII:

S0195-6671(17)30070-8

DOI:

10.1016/j.cretres.2017.05.004

Reference:

YCRES 3601

To appear in:

Cretaceous Research

Received Date: 11 February 2017 Revised Date:

15 April 2017

Accepted Date: 2 May 2017

Please cite this article as: Skutschas, P.P., Vitenko, D.D., Early Cretaceous choristoderes (Diapsida, Choristodera) from Siberia, Russia, Cretaceous Research (2017), doi: 10.1016/j.cretres.2017.05.004. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

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Early Cretaceous choristoderes (Diapsida, Choristodera) from Siberia, Russia

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Pavel P. Skutschas a,b, *, Dmitriy D. Vitenko a

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a

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Saint Petersburg, Russia;

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b

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634050, Tomsk, Russia

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4 Vertebrate Zoology Department, Saint Petersburg State University, Universitetskaya Nab. 7/9, 199034

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Laboratory of Mesozoic and Cenozoic Continental Ecosystems, Tomsk State University, Pr. Lenina 36,

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* Corresponding author. Vertebrate Zoology Department, Saint Petersburg State University,

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Universitetskaya Nab. 7/9, 199034 Saint Petersburg, Russia.

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E-mail address: [email protected]; [email protected]

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ABSTRACT

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There are ten known Lower Cretaceous localities for skeletal remains of choristoderes in Siberia (Russia).

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Choristoderan remains at all these localities are represented by isolated bones, usually by isolated

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vertebrae of Choristodera indet. Three choristoderan taxa in two geological units were identified: the

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non-neochoristodere Khurendukhosaurus sp. (possibly closely related to the long-necked Sino-Japanese

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hyphalosaurids) from the Murtoi Formation, Transbaikalia; cf. Khurendukhosaurus sp. and the

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“Shestakovo choristodere” with possible neochoristoderan affinities from the Ilek Formation, Western

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Siberia. All these three choristoderan taxa had a microanatomical organization of vertebrae similar to

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that of in advanced large neochoristoderes (vertebral centra with tight spongiosa). The Siberian fossil

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record includes the westernmost (Shestakovo locality, Ilek Formation) and the northernmost (Teete

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locality, the Sangarian Group) occurrences of the Early Cretaceous choristoderes in Asia. Like in other

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regions of Asia, Siberian localities are characterized by the absence of neosuchian crocodyliforms.

3 Keywords: Choristodera, Siberia, Russia, Cretaceous

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1. Introduction

Choristodera is a clade of freshwater diapsid reptiles known from the Middle Jurassic to

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Miocene of Laurasia and characterized by a unique combination of characters (e.g., dorsoventrally

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depressed cordiform skull, prefrontals meeting along the midline, conical subthecodont marginal teeth,

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neurovascular foramina in the lateral surface of the dentary opening in longitudinal grooves; midline

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crest on the dorsal surface of the vertebral centrum, figure-of-eight-shaped articular surface of the

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synapophysis of the dorsal vertebrae, articular surface of the synapophysis passes through the suture

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between the neural arch and vertebral centrum ) (Gao, Fox, 1998; Evans, Klembara, 2005; Averianov et

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al., 2006; Matsumoto, Evans, 2010, 2015). Choristoderes were most taxonomically and morphologically

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diverse during the Early Cretaceous: seven of the eleven unquestionably valid choristoderan genera

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(Hyphalosaurus Gao, Tang et Wang, 1999, Shokawa Evans, Manabe, 1999, Khurendukhosaurus

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Sigogneau–Russell, Efimov, 1984, Monjurosuchus Endo, 1940, Philydrosaurus Gao, Fox, 2005, Tchoiria

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Efimov, 1975 and Ikechosaurus Sigogneau-Russell, 1981b) and all choristoderan morphotypes (short-

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necked longirostrine, short-necked brevirostrine and long-necked brevirostrine) are known from this

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interval of time (Matsumoto et al., 2007; 2013, 2015; Matsumoto, Evans, 2010, 2015). The Early

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Cretaceous choristoderes were represented by members of the most derived choristoderan clade

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Neochoristodera (sensu Evans, Hecht, 1993) and by diverse more basal taxa (non-neochoristoderes). The

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Early Cretaceous neochoristoderans are known from the Asia (Tchoiria and Ikechosaurus) and North

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America (Neochoristodera indet.), while all Early Cretaceous non-neochoristoderans (namely

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Hyphalosaurus, Shokawa, Khurendukhosaurus, Monjurosuchus and Philydrosaurus) are known only from

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Asia (Britt et al., 2006; Matsumoto, Evans, 2010, 2015).

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During last two decades our knowledge of the Early Cretaceous vertebrate life of Siberia (huge territory of the Asiatic part of Russia, see Fig. 1) has greatly increased due to the discovery of several rich

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localities containing diverse terrestrial vertebrate assemblages (e.g., Leshchinsky et al., 2003; Averianov

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et al., 2006, 2015; Averianov, Lopatin, 2015). Remains of choristoderes were found in all those localities

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and now choristoderes can be considered as a common component of Early Cretaceous Asiatic

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vertebrate faunas. However, the choristoderan remains from the Lower Cretaceous of Siberia have only

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been described and figured from two localities – Mogoito in Transbaikalia (south part of Eastern Siberia)

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(Efimov, 1996; Skutschas, 2008) and Shestakovo in Western Siberia (Skutschas, Vitenko, 2015). To fill

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this gap, we provide here updated information on the records of Early Cretaceous choristoderes from

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Siberia (focusing on new findings and previously undescribed material) and discuss some aspects of

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phylogeny, anatomy (bone microstructure), and palaeobiogeography of the Early Cretaceous

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choristoderes from this region.

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Institutional Abbreviations. LMCCE, Laboratory of Mesozoic and Cenozoic Continental Ecosystems,

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Tomsk State University, Tomsk, Russia; PIN, A.A. Borissyak Paleontological Institute, Russian Academy of

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Sciences, Moscow, Russia; ZIN PH, Paleoherpetological collection, Zoological Institute of the Russian

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Academy of Sciences, Saint Petersburg, Russia.

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2. Review of choristoderan localities in Siberia Siberian choristoderes are known from several localities in four Lower Cretaceous geological

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units: the Ilek Formation in Western Siberia, Murtoi and Khilok formations in Transbaikalia (south part of

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Eastern Siberia) and the Sangarian Group in Yakutia (north part of Eastern Siberia)(Fig. 1). The

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choristoderan localities of the Ilek Formation of Western Siberia are concentrated in two regions, the

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Kiya River Basin in Kemerovo Province (Shestakovo, Smolenskii Yar, Ust’-Kolba localities) and Kemchug 3

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River Basin in Krasnoyarsk Territory (Bol’shoi Kemchug 3, Bol’shoi Kemchug 4, Bol’shaya Terekhtul’ 2 and

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Bol’shaya Terekhtul’ 4 localities) )(Fig. 1).

3 2.1. Shestakovo

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Geographic position: Natural outcrop on the right bank of the Kiya River (GPS coordinates: N 55°54.6', E

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87°56.9'), 1.5 km downstream from Shestakovo village, in Chebula District, Kemerovo Province, Western

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Siberia, Russia.

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Stratigraphy: Ilek Formation.

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Age: Aptian–Albian (Averianov et al., 2006).

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Associated vertebrate assemblage: Palaeonisciform and amiiform actinopterygians, stem and crown

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salamanders, “macrobaenid” turtles, diverse lizards, “protosuchian” and shartegosuchid crocodyliforms,

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various ornithischian and saurischian dinosaurs, pterosaurs, birds, tritylodontids and diverse mammals

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(see Averianov et al., 2006 and references therein).

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Previously referred material. ?Neochoristodera indet. (“Shestakovo choristodere” in this paper):

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fragmentary dentaries LMCCE 2-3/4, cervical vertebra LMCCE 4/4, dorsal vertebrae LMCCE 5-8/4,

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proximal rib fragments LMCCE 9-10/4 (Skutschas, Vitenko, 2015).

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Newly referred material. ?Neochoristodera indet. (“Shestakovo choristodere” in this paper): dorsal

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vertebra LMCCE 11/4; Fig. 2A-F.

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Comments. The material previously referred to Choristodera indet. from the Shestakovo locality was

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described in and figured by Skutschas and Vitenko (2015). The choristodere from the Shestakovo locality

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has a suite of vertebral characters that are widely distributed among non-neochoristoderes such as

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closed vertebral notochordal canals, the absence of small spinous processes below the presacral

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postzygapophyses and the ventromedial crest on dorsal vertebrae, closed neurocentral sutures,

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elongated amphiplatycoelus vertebral centra, the absence of a dorsal expansion of the neural arch (with 4

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a rugose dorsal area) of the dorsal vertebrae, and pronounced ventral longitudinal keels on the cervical

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vertebrae (Skutschas, Vitenko, 2015; Fig.2). One of the most diagnostic characters of the choristodere

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from Shestakovo (that distinguishes this form from other Siberian choristoderes) is the presence of slit-

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like depressions between the zygapophyses (Skutschas, Vitenko, 2015; Fig.2). Among choristoderes with

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described vertebral morphology, similar slit-like depressions between the zygapophyses are present in

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the advanced neochoristodere Simoedosaurus Gervais, 1877 (Sigogneau-Russell, 1981a; pers. obs.).

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Despite the presence of the above-mentioned diagnostic features, we avoid naming a new taxon

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pending the discovery of additional and more informative material.

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Vertebrae that are morphologically identical to those of “Shestakovo choristodere” were found

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in some other localities of the Ilek Formation, namely Bol’shoi Kemchug 3 and Bol’shaya Terekhtul’ 2

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(see below).

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12 2.2. Smolenskii Yar

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Geographic position: Natural outcrop on the right bank of Serta River (GPS coordinates: N 55°58’23.9’’, E

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88°05’35.9’’), 1 km downstream from the Kursk-Smolenka, in Chebula District, Kemerovo Province,

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Western Siberia, Russia.

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Stratigraphy: Ilek Formation.

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Age: Early Cretaceous (Averianov et al., 2015).

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Associated vertebrate assemblage: Palaeonisciform and amiiform actinopterygians, crown salamanders,

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indeterminate turtles and lizards, “protosuchian” and shartegosuchid crocodyliforms, various

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ornithischian and saurischian dinosaurs, pterosaurs, tritylodontids and mammals (see Averianov et al.,

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2015).

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Material. Choristodera indet.: centrum of cervical vertebra LMCCE 12/4; Fig.3A-D.

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Description. The centrum is amphiplatyan. In lateral view, the centrum is longer than wide (maximum

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centrum width is 6 mm, ventral midline length is about 9.3 mm; ratio of maximum centrum

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width:ventral midline length about 0.65). The centrum has a notochordal pit but no open notochordal

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canal. In anterior and posterior views, the centrum is almost round. The ventral surface of the centrum

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bears a pair of low midventral keels (in the central part). As in cervical vertebrae of other choristoderes,

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the transverse processes are represented by separated diapophyses (on the border of the centrum and

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the neural arch) and parapophyses (ventral to the diapophysis). There is a rugose area anteroventral to

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the diapophysis. There are two neurovascular foramina posterior to the diapophysis (the more dorsal

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foramen is larger than ventral). The neurocentral sutures are open. The floor of the neural canal bears a

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well-developed medial ridge.

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Comments. The cervical centrum LMCCE 12/4 (Fig.3A-D) demonstrates a combination of characteristic

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choristoderan features (e.g., amphiplatyan centrum, midline crest on the dorsal surface of the vertebral

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centrum), but differs from other choristoderes in having a pair of low midventral keels (versus one well-

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pronounced midventral keel). Additionally, cervical centrum LMCCE 12/4 differs from all Early

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Cretaceous choristoderes from Siberia (“Shestakovo choristodere”; Khurendukhosaurus sp.) in the

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presence of two relatively large neurovascular foramina posterior to the diapophysis. A similar pair of

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relatively large neurovascular foramina posterior to the diapophysis is present in Simoedosaurus

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(Sigogneau-Russell, 1981a), but the pattern of distribution of this character among choristoderes and its

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phylogenetic significance if any is unclear. On the basis of the differences between the cervical centrum

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LMCCE 12/4 and that of other Early Cretaceous Siberian choristoderes, we suggest that it might belong

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to a new choristoderan taxon.

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2.3. Ust’-Kolba

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Geographic position: Small quarry on the left bank of the Serta River near the Ust’-Kolba settlement

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(GPS coordinates: N 55°52’36.2’’, E 88°19’07.6’’), ~2 km upstream from the mouth of the river, in Tisul’

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District, Kemerovo Province, Western Siberia, Russia.

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Stratigraphy: Ilek Formation.

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Age: Early Cretaceous (Averianov et al., 2015).

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Associated vertebrate assemblage: Palaeonisciform and amiiform actinopterygians, indeterminate

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turtles and lizards, “protosuchian” and shartegosuchid crocodyliforms, ornithischian and saurischian

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dinosaurs, pterosaurs and mammals (see Averianov et al., 2015).

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Material. Choristodera indet.: fragment of vertebral centrum LMCCE 13/4; Fig.3E-F.

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Description. The surface of the anterior or posterior cotyle is flat. The floor of the neural canal bears a

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well-developed medial ridge (like in other choristoderes).

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Comments. The fragmentary nature of material from Ust’-Kolba permits identification of specimen

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LMCCE 13/4 only as Choristodera indet.

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2.4. Bol’shoi Kemchug 3

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Geographic position: Natural outcrop on the right bank of the Bol’shoi Kemchug River (GPS coordinates:

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N 56°31.566', E 91°48.604'), Chulym River Basin, Krasnoyarsk Territory, Western Siberia, Russia.

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Stratigraphy: Ilek Formation.

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Age: Hauterivian–Barremian (Leshchinsky et al., 2003); Early Cretaceous (Averianov et al., 2005).

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Associated vertebrate assemblage: Palaeonisciform and amiiform actynopterygians, crown salamanders,

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“macrobaenid” turtles, diverse lizards, “protosuchian” and shartegosuchid crocodyliforms, various

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ornithischian and saurischian dinosaurs, pterosaurs, tritylodontids and diverse mammals (Leshchinsky et

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al., 2003; Averianov et al., 2005).

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Material. ?Neochoristodera indet. (“Shestakovo choristodere” in this paper): neural arch of dorsal

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vertebrae LMCCE 14/4; cf. Khurendukhosaurus sp.: dorsal vertebra LMCCE 15/4, sacral vertebra LMCCE

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21/4; Choristodera indet.: centrum of dorsal vertebra LMCCE 17/4; centrum of anterior caudal vertebra

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LMCCE 20/4; centrum of posterior caudal vertebra LMCCE 16/4; fragmentary clavicles LMCCE 18-19/4;

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Fig.4.

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Description. ?Neochoristodera indet. (“Shestakovo choristodere” in this paper): The neural spine of the

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neural arch LMCCE 14/4 (Fig.4A-D) is low and narrow and has no a transversally expanded and rugose

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distal tip. The zygapophyses are inclined medially at about 45°. There are slit-like depressions between

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the zygapophyses.

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cf. Khurendukhosaurus sp.: The centrum of the dorsal vertebra LMCCE 15/4 (Fig.4E-H) is amphiplatyan

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and elongated (maximum centrum width is 5 mm, ventral midline length is about 10.1 mm; ratio of

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maximum centrum width:ventral midline length about 0.5), with a slight midventral constriction. In end

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view, the centrum is nearly circular. The midventral keel is absent. The notochordal canal is closed. The

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neurocentral sutures are closed and the neural arch is fused with the centrum without any trace of a

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suture. The neural spine has a transversally expanded and rugose distal tip. The zygapophyses lie at

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about 45° to the horizontal. The rib facets are fused into a single surface (synapophysis).

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The posterior (third?) sacral vertebra LMCCE 21/4 (Fig.4I-L) is amphiplatyan, longer than wide

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(maximum centrum width is 7.8 mm, ventral midline length is about 11 mm; ratio of maximum centrum

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width:ventral midline length about 0.71). The preserved right rib is fused with the centrum, but the

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suture is well defined. As in dorsal vertebrae LMCCE 15/4, the notochordal canal and the neurocentral

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sutures are closed, the neural spine has a transversally expanded and rugose distal tip and the

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zygapophyses lie at about 45° to the horizontal. The preserved rib is slender proximally and wide

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distally. It is dorsoventrally compressed midshaft and thickens distally to form a rugose contact surface

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for the ilium. The distal end has an oblique anterior facet for the contact with the more anterior

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(second?) sacral rib.

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Choristodera indet.: The centrum of the dorsal vertebra LMCCE 17/4 (Fig.4M-O) is amphiplatyan, slightly

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longer than wide (maximum centrum width is about 4.5 mm, ventral midline length is about 5.6 mm;

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ratio of maximum centrum width:ventral midline length about 0.8). The notochordal canal is open. The

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lateral surface of the centrum bears two (right side) or three (left side) small foramina. The centrum of

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the anterior caudal vertebra LMCCE 20/4 (Fig.4P-R) is elongated (maximum centrum width is 5.3 mm,

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ventral midline length is about 7.2 mm; ratio of maximum centrum width:ventral midline length about

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0.74), without bilateral compression. Small notochordal pits are present. A wide midventral groove is

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flanked by raised ridges. The centrum of the posterior caudal vertebra LMCCE 16/4 (Fig.4S-U) is

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elongated (maximum centrum width is about 6 mm, ventral midline length is about 13 mm; ratio of

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maximum centrum width:ventral midline length about 0.46) and slightly bilaterally compressed. A deep

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and relatively narrow midventral groove is flanked by raised ridges. The fragmentary clavicles LMCCE 18-

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19/4 (Fig.4V-X) have an extended medial portion with a large dorsally exposed interclavicular facet. The

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shaft bears a sharp posterodorsal edge.

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Comments. The neural arch LMCCE 14/4 (Fig.4A-D) is morphologically identical to those of the

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“Shestakovo choristodere” and is referred to this formally undescribed taxon. The dorsal vertebrae

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LMCCE 15/4 and sacral vertebra LMCCE 21/4 (Fig.4E-L) are similar to those of Khurendukhosaurus (see

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Skutschas, 2008, Matsumoto et al., 2009 for comparisons) in having elongated amphiplatyan vertebral

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centra with closed vertebral notochordal canals, closed neurocentral sutures, and neural spines with

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transversally expanded and rugose distal tips. Taking in account that the sacral rib is fused with the

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sacral vertebra in LMCCE 21/4 (vs. unfused sacral ribs in Mongolian Khurendukhosaurus orlovi;

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Matsumoto et al., 2009), we tentatively assign the dorsal vertebrae LMCCE 15/4 and sacral vertebra

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LMCCE 21/4 to Khurendukhosaurus.

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The centra of the dorsal vertebrae LMCCE 17/4 and 20/4, the centrum of the posterior caudal

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vertebra LMCCE 16/4, and the fragmentary clavicles LMCCE 18-19/4 (Fig.4M-X) show typical

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choristoderan morphology and cannot be referred with confidence either to the “Shestakovo

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choristodere” or cf. Khurendukhosaurus sp. 9

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1 2.5. Bol’shoi Kemchug 4

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Geographic position: Natural outcrop on the right bank of the Bol’shoi Kemchug River (GPS coordinates:

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N 56°33.945', E 91°50.712'), Chulym River Basin, Krasnoyarsk Territory, Western Siberia, Russia.

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Stratigraphy: Ilek Formation.

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Age: Early Cretaceous.

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Associated vertebrate assemblage: Palaeonisciform and amiiform actinopterygians, crown salamanders,

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“macrobaenid” turtles, diverse lizards, “protosuchian” and shartegosuchid crocodyliforms, various

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ornithischian and saurischian dinosaurs, pterosaurs, tritylodontids and diverse mammals (pers. obs.).

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Material. Choristodera indet.: posterior fragment of maxilla LMCCE 22/4; centrum of cervical vertebra

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LMCCE 23/4; centra of dorsal vertebrae LMCCE 24-25/4; Fig.5A-I.

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Description. Small fragment of a posterior process of the maxilla LMCCE 22/4 (Fig.5A-B) bears

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subthecodont conical teeth with mediolaterally elongated bases. The teeth have no enamel infoldings at

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their bases.

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The centrum of the cervical vertebra LMCCE 23/4 (Fig.5C-E) is very small and short (maximum

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centrum width is 2.4 mm, ventral midline length is about 3.1 mm; ratio of maximum centrum

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width:ventral midline length about 0.77). The centrum is amphiplatyan, slightly depressed

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dorsoventrally and has an open notochordal canal. The diapophysis and the small, tubercle-like

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parapophysis are not widely separated and both are situated close to the anterior border of the

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centrum. The ventromedial crest is very low. The neurocentral sutures are open.

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Like the cervical centrum LMCCE 23/4, the centrum of dorsal vertebra LMCCE 24/4 (Fig.5H-I) is

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amphiplatyan, relatively short and small (maximum centrum width is 3 mm, ventral midline length is

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about 3.9 mm; ratio of maximum centrum width:ventral midline length about 0.77) and has an open

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notochordal canal and open neurocentral sutures. There is no a ventromedial crest on the ventral 10

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surface of the centrum. The parapophysis was situated close to the anterior border of the centrum. The

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larger centrum of the dorsal vertebra LMCCE 25/4 (Fig.5F-G) differs from LMCCE 24/4 in having

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notochordal pits.

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Comments. The maxillary fragment LMCCE 22/4 (Fig.5A-B) has a primitive choristoderan tooth

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morphology (the absence of enamel infoldings at the tooth bases). On the basis of small size, vertebrae

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LMCCE 23-25/4 (Fig.5C-I), probably, belonged to juvenile individuals.

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7 2.6. Bol’shaya Terekhtul’ 2

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Geographic position: Natural outcrop on the bank of the Bol'shoi Terekhtyul’ River (GPS coordinates N

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56°38.15', E 91°59.21'), the east tributary of the Bol'shoi Kemchug River, Krasnoyarsk Territory, Western

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Siberia, Russia.

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Stratigraphy: Ilek Formation.

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Age: Early Cretaceous (Averianov, Lopatin, 2015).

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Associated vertebrate assemblage: Palaeonisciform and amiiform actinopterygians, crown salamanders,

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indeterminate turtles, “protosuchian” and shartegosuchid crocodyliforms, ornithischian and saurischian

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dinosaurs, tritylodontids and mammals (Averianov, Lopatin, 2015).

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Material. ?Neochoristodera indet. (“Shestakovo choristodere” in this paper): dorsal vertebra LMCCE

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26/4; Choristodera indet.: centrum of cervical vertebra LMCCE 27/4; centrum of dorsal vertebra LMCCE

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28/4; Fig.5J-T).

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Description. ?Neochoristodera indet.: The dorsal vertebra LMCCE 26/4 (Fig.5J-O) has an elongated

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amphiplatyan centrum (maximum centrum width is about 5.5 mm, ventral midline length is about 7.9

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mm; ratio of maximum centrum width:ventral midline length about 0.7), without a midventral

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constriction. In end view, the centrum is nearly circular. A notochordal pit is present. A pronounced

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midventral keel is absent, but there is a rugosity on the ventral surface of the centrum. The neurocentral 11

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sutures are closed and the neural arch is fused with the centrum without any trace of a suture. The

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neural spine has a weak dorsal expansion but without the formation of a wide rugose dorsal area. The

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zygapophyses lie at about 45° to the horizontal. The rib facets are fused into a single surface

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(synapophysis).

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Choristodera indet.: The small centra of cervical vertebra LMCCE 27/4 (Fig.5P-R) and dorsal vertebra

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LMCCE 28/4 (Fig.5S-T) are amphiplatyan and elongated (LMCCE 27/4: maximum centrum width is about

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4.5 mm, ventral midline length is about 5.5 mm; ratio of maximum centrum width:ventral midline length

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about 0.8; LMCCE 28/4: maximum centrum width is about 4 mm, ventral midline length is about 5.8

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mm; ratio of maximum centrum width:ventral midline length about 0.69). Both centra are without open

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notochordal canals or notochordal pits. The neurocentral sutures were open. The midventral keel on the

11

centrum of cervical vertebra LMCCE 27/4 is weakly developed.

12

Comments. The dorsal vertebra LMCCE 26/4 (Fig.5J-O) demonstrates a morphology that is characteristic

13

for the “Shestakovo choristodere” (e.g., elongated amphiplatycoelus vertebral centra, closed

14

neurocentral sutures, the absence of a wide dorsal expansion of the neural arch (with a rugose dorsal

15

area) and the presence of slit-like depressions between the zygapophyses; Skutschas, Vitenko, 2015).

16

The small centra of cervical vertebra LMCCE 27/4 and dorsal vertebra LMCCE 28/4 (Fig.5P-T) have a

17

general choristoderan morphology and can be identified only as Choristodera indet.

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2.7. Bol’shaya Terekhtul’ 4

20

Geographic position: Natural outcrop on the bank of the Bol'shoi Terekhtyul’ River (GPS coordinates: N

21

56°37.33', E 91°59.22'), the east tributary of the Bol'shoi Kemchug River, Krasnoyarsk Territory, Western

22

Siberia, Russia.

23

Stratigraphy: Ilek Formation.

24

Age: Early Cretaceous (Averianov, Lopatin, 2015). 12

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Associated vertebrate assemblage: Palaeonisciform and amiiform actinopterygians, indeterminate

2

turtles, “protosuchian” and shartegosuchid crocodyliforms, ornithischian and saurischian dinosaurs,

3

tritylodontids and mammals (Averianov, Lopatin, 2015).

4

Material. Choristodera indet.: centrum of posterior caudal vertebra LMCCE 29/4; Fig.5U-X.

5

Description. The centrum of posterior caudal vertebra LMCCE 29/4 (Fig.5U-X) is amphiplatyan. In lateral

6

view, the centrum is longer than wide (maximum centrum width is 3.6 mm, ventral midline length is

7

about 8.5 mm; ratio of maximum centrum width:ventral midline length about 0.42). In anterior and

8

posterior views, the centrum is almost round and has deep notochordal pits. The ventral surface of the

9

centrum bears a deep midventral groove that is flanked by raised ridges. There are two foramina in the

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deepest part of the midventral groove.

11

Comments. The fragmentary nature of the material from Bol’shoi Terekhtul’ 4 allows specimen LMCCE

12

29/4 to be identified only as Choristodera indet.

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2.8. Mogoito

15

Geographic position: A series of natural outcrops in shallow ravines on the western bank of Lake

16

Gusinoe between villages of Gusinoe Ozero and Baraty (GPS coordinates: N 51°12’03’’, E 106°17’06’’ for

17

MRT-102 site), in Selenga District, the Republic Buryatia, Eastern Siberia, Russia.

18

Stratigraphy: Murtoi Formation.

19

Age: Barremian?–Aptian (Averianov et al., 2003).

20

Associated vertebrate assemblage: Palaeonisciform, acipenceriform and teleostean actinopterygians,

21

“macrobaenid” turtles, lizards, various ornithischian and saurischian dinosaurs, pterosaurs, birds, and

22

eutherian mammals (Averianov, Skutschas, 2009).

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Previously referred material. Khurendukhosaurus sp.: basioccipitals ZIN PH 1-3/25; left exoccipital ZIN PH

2

4/25; anterior fragment of left maxilla ZIN PH 22/25; right dentary fragment ZIN PH 23/25; left dentary

3

fragment ZIN PH 24/25; cervical vertebra ZIN PH 8/25; dorsal vertebra ZIN PH 17/25; sacral centrum ZIN

4

PH 18/25; caudal vertebrae ZIN PH 19-20/25; dorsal ribs PIN 2234/201b, ZIN PH 13/25, ZIN PH 16/25;

5

right scapulocoracoid PIN 2234/201a; medial fragments of right clavicles ZIN PH 14-15/25; anterior part

6

of interclavicle ZIN PH 8/25; right humerus ZIN PH 9/25; distal fragment of left humerus ZIN PH 10/25;

7

right radius ZIN PH 12/25; ulna ZIN PH 11/25; fibula ZIN PH 21/25; fragment of gastralia ZIN PH 25/25;

8

(Efimov, 1996; Skutschas, 2008).

9

Newly referred material. Khurendukhosaurus sp.: fused postorbital and postfrontal (= postorbitofrontal)

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ZIN PH 26/25; postorbital ZIN PH 27/25; Fig.6A-E.

11

Description of newly referred material. Specimen ZIN PH 26/25 (Fig.6A-C) is a single bone element (=

12

right postorbitofrontal), but the suture between the postfrontal and the postorbital is clearly defined on

13

the outer surface. Both bones form the posterior margin of the orbit and only the postorbital forms the

14

anterior border of the small (much smaller than the orbit) upper temporal fenestra. The postorbital

15

consists of two distinct plates (dorsal and lateral) that are situated at an angle of about 100° to each

16

other (Fig.6C). There is a facet for the jugal along the ventral edge and on the inner surface of the bone

17

and a narrow facet for the squamosal along the posterior edge of the lateral plate of the postorbital. The

18

anterior edges of both plates contribute to the posterior margin of the orbit. The posterior, free

19

unfacetted posterior margin of the dorsal plate of the postorbital borders the upper temporal fenestra.

20

The posteromedial part of the dorsal plate bears a wide parietal facet on its inner surface.

21

The postfrontal is triangular and its medial edge has a deep complex facet for a contact with a frontal

22

(anteriorly) and, probably, with a parietal (posteriorly). The absence of a free unfacetted margin on the

23

posterior and ventral regions of the postorbitofrontal suggests the complete closure of the lower

24

temporal fenestra. The sculpture of the postorbitofrontal consists of two strong tuberculated rims, high

25

and low tubercles, short narrow grooves and small pits. The first strong tuberculated rim is situated

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along the orbital edge and the second anteroposteriorly oriented rim is situated along the border

2

between the dorsal and lateral plates of the postorbital. The isolated left postfrontal ZIN PH 27/25 (Fig.6D-E) is comparable in size and morphology with

4

the “postfrontal” part of the postorbitofrontal ZIN PH 26/25. On the basis of the fact that a postfrontal

5

of similar size could be a separate element or fused with the postorbital, as well as the presence of the

6

suture between the postfrontal and the postorbital in ZIN PH 26/25, we suggest that fusion of the

7

postorbital and the postfrontal took place rather late in ontogeny of Khurendukhosaurus sp. and that

8

the timing of this ontogenetic event was variable among different individuals.

9

Comments. The material previously referred to Khurendukhosaurus sp. from the Mogoito locality was

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described in detail and figured by Skutschas (2008). Khurendukhosaurus sp. shares with

11

Khurendukhosaurus orlovi (type species of the genus) from the Lower Cretaceous of Mongolia such

12

characters as closed vertebral notochordal canals, the presence of small spinous processes below the

13

presacral postzygapophyses, the absence of the ventromedial crest on dorsal vertebrae, closed

14

neurocentral sutures; anteroposteriorly elongated neural spines with transversally expanded and rugose

15

distal tips on dorsal vertebrae, elongated amphiplatyan vertebral centra, pronounced ventral

16

longitudinal keels on the cervical vertebrae, co-ossified scapula and coracoid, clavicular facets on the

17

interclavicle that are continuous across the midline (Skutschas, 2008; Fig.6F-I). Additionally,

18

Khurendukhosaurus sp. is characterized by mediolaterally elongated marginal tooth bases, moderately

19

laterally expanded basal tubera on the basioccipital (Skutschas, 2008), small upper temporal fenestra

20

that is much smaller than the orbit and a closed lower temporal fenestra (this study).

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2.9. Krasniy Yar

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Geographic position: Natural outcrop on the right bank of the Khilok River (GPS coordinates: N

24

50°40’33’’, E 107°54’55’’) approximately 1-2 km downstream from the confluence with the Shibertui

25

River, near Ust’-Zagan village, in Bichura District, the Republic Buryatia, Eastern Siberia, Russia. 15

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Stratigraphy: Khilok Formation.

2

Age: Barremian–Aptian (Gordienko et al., 1999).

3

Associated vertebrate assemblage: Hybodont sharks, palaeonisciform, amiiform and teleostean

4

actinopterygians, frogs, “macrobaenid” turtles, lizards, various ornithischian and saurischian dinosaurs

5

and pterosaurs (Averianov, Skutschas, 2009).

6

Material. Choristodera indet.: centrum of posterior caudal vertebra ZIN PH 28/25; proximal fragment of

7

dorsal rib ZIN PH 29/25; Fig.6J-O.

8

Description. The centrum of caudal vertebra ZIN PH 28/25 (Fig.6J-L) is amphiplatyan. In lateral view, the

9

centrum is longer than wide (maximum centrum width is about 3.1 mm, ventral midline length is about

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7.2 mm; ratio of maximum centrum width:ventral midline length about 0.43). The centrum has a

11

notochordal pit but no open notochordal canal. In anterior and posterior views, the centrum is almost

12

round. The ventral surface of the centrum bears a deep midventral groove that is flanked by raised

13

ridges. The transverse processes are damaged, so the structure of the contact between them and caudal

14

ribs is unknown.

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The proximal part of the dorsal rib ZIN PH 29/25 (Fig.6M-O) is anteroposteriorly compressed with a figure-of-eight-shaped articular surface.

17

Comments: The fragmentary nature of the material from Krasniy Yar allows specimens ZIN PH 28-29/25

18

to be identified only as Choristodera indet.

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2.10 Teete

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Geographic position: Natural outcrops along the Teete Creek, the left tributary of the Botomoiu River,

22

Kempendyai Depression, Vilyui River Basin, in Yakutia, Eastern Siberia, Russia.

23

Stratigraphy: the Sangarian Group. 16

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Age: Early Cretaceous (“Neocomian” in Kolosov et al., 2009).

2

Associated vertebrate assemblage: palaeonisciform and chondrostean actinopterygians, frogs,

3

salamanders, lizards, various ornithischian and saurischian dinosaurs and tritylodontids (Kurzanov et al.,

4

2003; Lopatin, Agadjanian, 2008; Kolosov et al., 2009).

5

Comments: Choristoderes from Teete were identified as Khurendukhosaurus sp. by Kolosov et al. (2009),

6

but a choristoderan material from this locality was not figured or described. Later, Skutschas and Efimov

7

(2015) noted that the Teete choristodere is characterized by primitive vertebral features (small size,

8

elongated vertebral centra, the absence of small spinous processes below the presacral

9

postzygapophyses, closed neurocentral sutures, the presence of a notochordal pit) and differs from

10

Khurendukhosaurus in lacking the transversally expanded and rugose distal tips on dorsal vertebral

11

spines. Pending to new findings, the Teete choristodere can be identified only as Choristodera indet.

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3. Discussion

14

3.1. General discussion

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The taxonomic affinities of the most of the Early Cretaceous Siberian choristoderes are unclear due to the rarity and fragmentary nature and scarcity of the material. The exceptions are

17

Khurendukhosaurus sp. from the Mogoito locality (Murtoi Formation, Transbaikalia), cf.

18

Khurendukhosaurus sp. from the Bol’shoi Kemchug 3 locality (Ilek Formation, Western Siberia) and the

19

“Shestakovo choristodere” from the Shestakovo, the Bol’shoi Kemchug 3 and the Bol’shaya Terekhtul’ 2

20

localities (Ilek Formation, Western Siberia).

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Khurendukhosaurus sp. from the Mogoito locality (Murtoi Formation, Transbaikalia) is the best

22

known Siberian choristodere (Skutschas, 2008; Matsumoto et al., 2009; this paper). Despite this, its

23

phylogenetic position is unclear (Skutschas, 2008; Matsumoto et al., 2009). Previous phylogenetic

24

analyses placed Khurendukhosaurus in an unresolved polytomy with Philydrosaurus, neochoristoderes, 17

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hyphalosaurids, Monjurosuchus and Lazarussuchus Hecht, 1992 (Skutschas, 2008; see strict consensus

2

tree obtained by PAUP in Matsumoto et al., 2013); as the sister taxon to the Sino-Japanese

3

Hyphalosauridae (Shokawa, Hyphalosaurus) (Matsumoto et al., 2009) or in an unresolved polytomy with

4

neochoristoderes and all other non-neochoristoderes (see bootstrap consensus tree in Matsumoto et

5

al., 2013). In order to elucidate the relationships of Khurendukhosaurus within Choristodera, a

6

phylogenetic analysis was conducted using NONA v. 2.0 (Goloboff 1999), run with the WINCLADA v.

7

1.00.08 interface (Nixon 1999). We used the most recent matrix of Matsumoto et al. (2013), with the

8

addition of one new character state for Khurendukhosaurus (closed lower temporal fenestra; character

9

34 (0)) and correction of one character state (character 103, bone structure of the rib: cancelous (0),

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having reduced medullary cavity (1); pachyostotic in adult (2); character state (0) for Khurendukhosaurus

11

was changed to (2); see “Bone Microanatomy of the Early Cretaceous Siberian choristoderes” section

12

below). Multistate characters were treated as unordered. One thousand repetitions of the parsimony

13

ratchet (island hopper) algorithm recovered 12 most parsimonious trees (tree length 311; consistency

14

index 0.53; retention index 0.74). The addition and correction of the character states for

15

Khurendukhosaurus sp. from the Mogoito locality into the matrix of Matsumoto et al. (2013) did not

16

change the topography of the resulting strict consensus tree. As in the strict consensus tree obtained by

17

PAUP in the analysis by Matsumoto et al. (2013), a monophyletic Khurendukhosaurus is placed in an

18

unresolved polytomy with Philydrosaurus, neochoristoderes, hyphalosaurids, Monjurosuchus and

19

Lazarussuchus. In the fifty per cent majority-rule consensus tree Khurendukhosaurus is placed as the

20

sister taxon to the Sino-Japanese Hyphalosauridae (Shokawa, Hyphalosaurus), as in the analysis by

21

Matsumoto et al. (2009) (Fig. 7). In order to test the hypothesis of the sister taxon relationships of long-

22

necked hyphalosaurids and Khurendukhosaurus, new material (e.g., an articulated cervical region of

23

vertebral column) is strongly needed.

24

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The vertebral material (dorsal vertebrae LMCCE 15/4 and sacral vertebra LMCCE 21/4; Fig.4E-L)

25

morphologically similar to that of Khurendukhosaurus (see Skutschas, 2008, Matsumoto et al., 2009; Fig.

26

6F-I for comparisons) was found at the the Bol’shoi Kemchug 3 locality (Ilek Formation, Western Siberia).

18

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If the assignment of this material to Khurendukhosaurus is correct then it suggests a wide distribution of

2

choristoderes of this genus in the Early Cretaceous of Asia.

3

The formally undescribed “Shestakovo choristodere” (Ilek Formation, Western Siberia) shares one apomorphic character with neochoristoderes – the presence of enamel infoldings on the marginal

5

tooth bases (Skutschas, Vitenko, 2015). However, the absence of any other characters that are

6

synapomorphic for neochoristoderes (e.g., open cervical neurocentral sutures, short dorsal vertebrae;

7

Evans and Hecht, 1993; Matsumoto et al., 2007) precludes definitive referral of this taxon to

8

Neochoristodera. The discovery of the “Shestakovo choristodere” in different localities of the Ilek

9

Formation suggests that this taxon also had a wide distribution in the territory of Western Siberia in the

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Early Cretaceous.

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The vertebral specimens of unidentified choristoderes from different Early Cretaceous Siberian

12

localities demonstrate generally primitive vertebral morphology (e.g., elongated amphiplatyan centra)

13

and could belong to non-neochoristoderes.

The revealed taxonomic diversity of choristoderes in the Early Cretaceous of Siberia is relatively

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low and the most of localities yield only one taxon, or rarely two. Among geological units, only the Ilek

16

Formation (Western Siberia) contains two or three choristoderan taxa, while other (Murtoi and Khilok

17

formations in Transbaikalia and the Sangarian Group in Yakutia) have only one taxon. In contrast, the

18

Lower Cretaceous geological units of China (Jiufotang Formation) and Japan (the Tetori Group) yield

19

greater taxonomic diversity (up to three taxa) (Matsumoto et al., 2015). The lower taxonomic diversity

20

of choristoderes in the most of Lower Cretaceous fossil deposits of Siberia could be a result of sampling

21

artefact (and more choristoderan taxa may be found in future) or may reflect a real feature of faunal

22

compositions.

23 24

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Despite the relatively low taxonomic diversity, choristoderes were widely distributed across Siberia in the Early Cretaceous. The Siberian fossil record includes the westernmost (Shestakovo, Ust’-

19

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Kolba and Smolenskii Yar localities, Ilek Formation) and the northernmost (Teete, the Sangarian Group)

2

occurrences of the Early Cretaceous choristoderes in Asia.

3

As in other Asiatic Early Cretaceous choristoderan localities, the Siberian localities are notable for the absence of neosuchian crocodyliforms (Matsumoto, Evans, 2010; Matsumoto et al., 2015). This is

5

in accordance with the hypothesis that a climatic barrier separated the distribution of choristoderes

6

(which were tolerant of cold temperatures, see Amiot et al., 2011) and thermophilic neosuchian

7

crocodyliforms in Asia during the Early Cretaceous (Matsumoto et al., 2015). The supposed tolerance to

8

cold temperatures explains the presence of choristoderes in the Teete locality (the Sangarian Group,

9

Yakutia) which was formed under polar conditions.

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3.2. Bone Microanatomy of the Early Cretaceous Siberian choristoderes Bone microanatomy of aquatic amniotes generally differs from that of terrestrial forms in one of two opposing specializations: an increase in bone mass (typical for poorly active subsurface swimmers)

14

and a spongious organization (in active swimmers) (Houssaye, 2009; Houssaye et al., 2016). There are

15

many intermediate patterns between the two extreme specializations found in different

16

aquatic/semiaquatic taxa, and there may be variation in combination of different patterns within a

17

single skeleton (Houssaye et al., 2016).

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2010; Matsumoto et al., 2015) and this adaptation is reflected in their bone microanatomy. The

20

advanced neochoristodere Champsosaurus Cope, 1876 has vertebrae with an extremely tight spongiosa

21

in adults and long bones (femora) with a very thick cortex and a very small medullary cavity in juveniles

22

and a relatively thick cortex and no open medullary cavity in adults (de Buffrenil et al., 1990; Katsura,

23

2010; Houssaye et al., 2016). Thus adult champsosaur femora demonstrate a pachyosteosclerotic

24

(hyperplasy of periosteal cortices associated with compactness of the inner structure of the bones sensu

25

Wiffen et al., 1995) structural specializations of the bone. Another large advanced aquatic 20

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neochoristodere Simoedosaurus has structural specializations of the bone similar to those of

2

Champsosaurus (de Buffrenil et al., 1990; Katsura, 2010; Houssaye et al., 2016). For the

3

neochoristoderes Ikechosaurus and Tchoiria, and the non- neochoristodere Hyphalosaurus a

4

“pachyostotic sensu lato” structural specialization of the ribs was noted (Houssaye, 2009 and references

5

therein).

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Skutschas (2008) suggested a relatively terrestrial lifestyle for Khurendukhosaurus sp. from the

7

Mogoito locality on the basis of the closed neurocentral sutures, the lack of pachyostosis in the dorsal

8

ribs and the co-ossified scapula and coracoid. Later, Matsumoto et al. (2009) noted that the presence of

9

unfused sacral ribs, tall caudal neural spines (indicative of a deep swimming tail), weak development of

10

the distal humerus, and the dorsoventrally compressed body profile (all those features are common for

11

other choristoderes) supported an aquatic lifestyle of Khurendukhosaurus.

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To test these competing hypotheses (terrestrial vs. aquatic lifestyle) we studied the histology of the humerus ZIN PH 10/25, and of rib and gastralium fragments (specimens without number from the

14

collection ZIN PH 25) of large and, presumably, adult individuals of Khurendukhosaurus sp. from

15

Mogoito. The humerus ZIN PH 10/25 has a rather thick cortex that consists of thick nearly avascular

16

primary parallel-fibred bone and a defined medullary cavity that is infilled by secondary endosteal

17

cancellous bone (Fig. 8A-D). The secondary endosteal cancellous bone constitutes short and thick bone

18

trabeculae. The bone trabeculae consist of lamellar bone. There are several secondary osteons and

19

erosion bays in the transition between the periosteal cortex and the cancellous bone. The density of the

20

endosteal cancellous bone is rather high but less than in adult champsosaurs (see Katsura, 2010:fig. 2).

21

The thick, nearly avascular cortex and the dense endosteal cancellous bone in the medullary cavity

22

suggest both pachyostotic and pachyosteosclerotic structural specializations in the humeral

23

microstructure of Khurendukhosaurus sp. from the Mogoito locality. The rib of Khurendukhosaurus sp.

24

from Mogoito (Fig. 8E), like its humerus, has a relatively thick cortex and defined medullary cavity that is

25

infilled by a secondary endosteal cancellous bone and also demonstrates the pachyosteosclerotic

26

structural specialization. The gastralium (Fig. 8F) is relatively thin and lightly built, but its

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microanatomical structure is pachyostotic. The pachyosteosclerotic structural specializations indicate

2

the aquatic adaptation of Khurendukhosaurus sp., as suggested by Matsumoto et al. (2009). In

3

Khurendukhosaurus sp., the pachyosteosclerotic humera, ribs and gastralia are combined with a

4

relatively tight spongious microanatomical organization (with a weak increase in compactness of the

5

periosteal cortex) of the vertebral centra and a spongious organization of the neural arch (Fig.9A-B). A

6

similar combination (pachyosteosclerotic limb bones and vertebral centra with tight spongiosa) was

7

reported earlier for the large advanced large neochoristodere Champsosaurus (de Buffrenil et al., 1990;

8

Katsura, 2010; Houssaye et al., 2016). The discovery of a similar microanatomical combination in the

9

non-neochoristodere Khurendukhosaurus and advanced neochoristoderes suggests that it might be a

11

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widely distributed microanatomical bone adaptation among choristoderes.

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Except Khurendukhosaurus sp. from the Mogoito locality, other Siberian choristoderes are represented mostly by isolated vertebrae and the microanatomical organization of their limb bones is

13

unknown. The sacral vertebra of cf. Khurendukhosaurus sp. from the Bol’shoi Kemchug 3 locality (Ilek

14

Formation, Western Siberia) demonstrates a relatively tight spongious microanatomical organization of

15

the centrum (similar to that of Khurendukhosaurus sp. from the Mogoito locality) and a more spongious

16

sacral rib (Fig.9C-D). The “Shestakovo choristodere” from the Shestakovo, Bol’shoi Kemchug 3 and

17

Bol’shaya Terekhtul’ 2 localities (Ilek Formation, Western Siberia) also had a tightly spongious

18

microanatomical organization of the centra and spongious neural arches (Fig.9E-J). A characteristic

19

microanatomical feature of the vertebrae of the “Shestakovo choristodere” is the retention of a large

20

portion of a notochordal canal inside the central part of the centra.

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5. Conclusion Lower Cretaceous terrestrial deposits containing diverse vertebrate assemblages are widely distributed in Siberia and four geological units of this age contain choristoderes: the Ilek Formation in

22

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Western Siberia, Murtoi and Khilok formations in Transbaikalia (south part of Eastern Siberia) and the

2

Sangarian Group in Yakutia (north part of Eastern Siberia).

3

The revealed taxonomic diversity of choristoderes in the Early Cretaceous of Siberia is relatively low. Most of choristoderan specimens from the Siberian localities are fragmentary vertebrae that

5

demonstrate a generally primitive choristoderan morphology and could be identified only as

6

Choristodera indet. On the basis of more complete material, three Siberian Early Cretaceous

7

choristoderan taxa were identified: (1) the non-neochoristodere Khurendukhosaurus sp. (possibly the

8

sister taxon to the Sino-Japanese Hyphalosauridae) from the Mogoito locality (Murtoi Formation,

9

Transbaikalia), (2) cf. Khurendukhosaurus sp. from the Bol’shoi Kemchug 3 locality (Ilek Formation,

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Western Siberia), and (3) the “Shestakovo choristodere” with possible neochoristoderan affinities from

11

the Shestakovo, the Bol’shoi Kemchug 3 and the Bol’shaya Terekhtul’ 2 localities (Ilek Formation,

12

Western Siberia).

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As in advanced large neochoristoderes, Khurendukhosaurus sp. from the Mogoito locality had pachyosteosclerotic limb bones and vertebral centra with tight spongiosa. The similar microanatomical

15

organization of the vertebrae was found in cf. Khurendukhosaurus sp. and the “Shestakovo

16

choristodere” from the Ilek Formation.

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The Siberian fossil record includes the westernmost (Ilek Formation) and the northernmost (the

18

Sangarian Group) occurrences of the Early Cretaceous choristoderes in Asia. The absence of neosuchian

19

crocodyliforms in any of the Early Cretaceous Siberian vertebrate assemblages containing choristoderes

20

is in accordance with the previously published pattern of choristodere distribution in the Early

21

Cretaceous of Asia: during this interval of time choristoderes were distributed in northern continental

22

ecosystems under the cool climatic conditions while thermophilic neosuchian crocodyliforms lived in

23

warmer southern ecosystems (see Matsumoto et al., 2015).

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Acknowledgements 23

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We thank all the members of expeditions in Siberia for their help. We thank Ryoko Matsumoto

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(Kanagawa Prefectural Museum of Natural History, Odawara, Japan) for providing photos of vertebrae

3

of Simoedosaurus. We are very grateful to the staff of the ‘Geomodel’ Research Center at the Saint

4

Petersburg State University (Saint Petersburg, Russia), with special consideration to Alexander M. Kulkov

5

for his help with using Leica 2500P microscope and in preparing images of the sections and to Sergey

6

Nilov for CT scanning of specimens. We are grateful to two anonymous reviewers for providing helpful

7

comments that improved the quality of the manuscript. The work was supported by the Ministry of

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Education and Science of the Russian Federation (Project 5.4217.2017/PCH) and by Saint Petersburg

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State University (grants 0.38.292.2015 and 1.42.1097.2016).

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Figure captions

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Figure 1. Map showing Early Cretaceous choristoderan localities in Siberia. Inset map of central and

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northern portions of the Asian continent depicting the territory of Siberia in dark grey. 1 – Shestakovo, 2

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- Smolenskii Yar, 3 - Ust’-Kolba, 4 - Bol’shoi Kemchug 3, 5 - Bol’shoi Kemchug 4, 6 - Bol’shaya Terekhtul’ 28

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2, 7 - Bol’shaya Terekhtul’ 4, (Ilek Formation; Lower Cretaceous), Western Siberia; 8 – Mogoito (Murtoi

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Formation; Barremian?–Aptian), 9 - Krasniy Yar (Khilok Formation; Barremian–Aptian), Transbaikalia; 10

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– Teete (Sangarian Group), Yakutia.

4 Figure 2. Dorsal vertebrae of ?Neochoristodera indet. (“Shestakovo choristodere”) from the Shestakovo

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locality (Ilek Formation, Western Siberia). A-F, LMCCE 11/4; A, anterior view; B, posterior view; C, left

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lateral view; D, dorsal view; E, anterolateral view (digital restoration); F, posterolateral view (digital

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restoration). G-H, LMCCE 7/4; G, anterolateral view (digital restoration); H, posterolateral view (digital

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restoration). Scale bar represents 5 mm. Abbreviations: poz – postzygapophyses; prz –

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prezygapophyses; sd – slit-like depression; sph – synapophyses.

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Figure 3. Choristoderan remains from the localities Smolenskii Yar (A-D) and Ust’-Kolba (E-F) (Ilek

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Formation, Western Siberia). A-D, LMCCE 11/4, centrum of cervical vertebrae of Choristodera indet.; A,

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dorsal view; B, left lateral view; C, ventral view; D, anterior view. E-F, LMCCE 13/4, fragment of vertebral

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centrum of Choristodera indet.; E, dorsal view; F, anterior or posterior view. Scale bar represents 5 mm.

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Abbreviations: dph – diapophyses; mk – midventral keels; nv – neurovascular foramina; pph –

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parapophyses.

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Figure 4. Remains of ?Neochoristodera indet. (“Shestakovo choristodere”) (A-D), cf. Khurendukhosaurus

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sp. (E-L) and Choristodera indet. (M-X) from the Bol’shoi Kemchug 3 locality (Ilek Formation, Western

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Siberia). A-D, LMCCE 14/4, neural arch of dorsal vertebra of ?Neochoristodera indet. (“Shestakovo

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choristodere”); A, left lateral view; B, anterior view; C, posterior view; D, dorsal view. E-H, LMCCE 15/4,

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dorsal vertebra of cf. Khurendukhosaurus sp.; E, right lateral view; F, dorsal view; G, anterior view; H,

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posterior view. I-L, LMCCE 21/4, posterior (third?) sacral vertebra of cf. Khurendukhosaurus sp.; I,

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anterior view; J, posterior view; K, dorsal view; L, ventral view. M-O, LMCCE 17/4, centrum of dorsal 29

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vertebra of Choristodera indet.; M, dorsal view; N, ventral view; O, anterior view. P-R, LMCCE 20/4,

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centrum of anterior caudal vertebra of Choristodera indet.; P, dorsal view; Q, ventral view; R, anterior

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view. S-U, LMCCE 20/4, centrum of posterior caudal vertebra of Choristodera indet.; S, dorsal view; T,

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ventral view; R, lateral view. V-X, LMCCE 18/4, fragment of clavicle of Choristodera indet.; V, vental

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view; W, dorsal view; X, anterior view. Scale bar represents 5 mm.

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Figure 5. Choristoderan remains from the Bol’shoi Kemchug 4 (A-I), Bol’shaya Terekhtul’ 2 (J-T) and

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Bol’shaya Terekhtul’ 4 (U-X) localities (Ilek Formation, Western Siberia). A-B, LMCCE 20/4, fragment of

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maxilla of Choristodera indet.; A, lateral or medial view; B, ventral view. C-E, LMCCE 23/4, centrum of

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cervical vertebra of Choristodera indet.; C, dorsal view; D, left lateral view; E, anterior view. F-G, LMCCE

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25/4, centrum of dorsal vertebra of Choristodera indet.; F, dorsal view; G, anterior or posterior view. H-I,

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LMCCE 24/4, centrum of dorsal vertebra of Choristodera indet.; H, dorsal view; I, anterior or posterior

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view. J-O, LMCCE 26/4, dorsal vertebra of ?Neochoristodera indet. (“Shestakovo choristodere”); J, left

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lateral view; K, anterior view; L, posterior view; M, dorsal view; N, anterolateral view (digital

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restoration); O, posterolateral view (digital restoration). P-R, LMCCE 27/4, centrum of cervical vertebra

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of Choristodera indet.; P, right lateral view; Q, dorsal view; R, anterior view. S-T, LMCCE 28/4, centrum

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of dorsal vertebra of Choristodera indet.; S, dorsal view; T, posterior view. U-X, LMCCE 29/4, centrum of

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posterior caudal vertebra of Choristodera indet.; U, lateral view; V, dorsal view; W, ventral view; X,

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anterior or posterior view. Scale bar represents 5 mm. Abbreviations: np – notochordal pit; sd – slit-like

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depression.

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Figure 6. Remains of Khurendukhosaurus sp. (A-I) from the Mogoito locality (Murtoi Formation,

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Transbaikalia) and Choristodera indet. (J-O) from the Krasniy Yar locality (Khilok Formation,

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Transbaikalia). A-C, ZIN PH 26/25, fused right postorbital and postfrontal (= postorbitofrontal); A, dorsal

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view; B. medial view; C, posterior view. D-E, ZIN PH 27/25, left postfrontal; D, lateral view; E, medial

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view. F-G, ZIN PH 17/25, dorsal vertebra; F, anterolateral view (digital restoration); G, posterolateral

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view (digital restoration). H-I, ZIN PH 18/25, centrum of sacral vertebra; H, anterolateral view (digital

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restoration); I, posterolateral view (digital restoration). J-L, ZIN PH 28/25, centrum of posterior caudal

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vertebra; J, left lateral view; K, dorsal view; L, anterior view. M-O, ZIN PH 29/25, fragment of dorsal rib;

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M, anterior view; N, posterior view; O, proximal view. Scale bar represents 5 mm. Abbreviations: frf –

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frontal facet; jf – jugal facet; orb – orbit; pf – postfrontal; parf – parietal facet; po – postorbital; sqf –

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squamosal facet; utf – upper temporal fenestra.

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Figure 7. Choristoderan part (without outgroups) of the 50 per cent majority-rule consensus tree of twelve most parsimonious trees obtained in NONA v. 2.0 with using the parsimony ratchet (island

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hopper) algorithm showing the position of Khurendukhosaurus sp. from the Mogoito locality (Murtoi

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Formation, Transbaikalia) within non-neochoristoderes. Khurendukhosaurus sp. is asterisked.

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Figure 8. Histological transverse sections of the humerus (A-D), the rib (E) and the gastralium (F) of

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Khurendukhosaurus sp. from the Mogoito locality (Murtoi Formation, Transbaikalia). A-B, humerus,

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microanatomical overview; A, under polarized light with lambda waveplate; B, under polarized light

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without lambda waveplate. C, close-up of the humeral cortex under polarized light with lambda

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waveplate. Note almost avascular primary parallel-fibred bone; D, close-up of the secondary endosteal

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cancellous bone in the medullary cavity. Note the short and thick trabeculae. E, rib, microanatomical

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overview under polarized light with lambda waveplate. F, gastralium, microanatomical overview under

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polarized light with lambda waveplate.

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Figure 9. Vertebral microanatomy of Khurendukhosaurus sp. (A-B) from the Mogoito locality (Murtoi

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Formation, Transbaikalia), cf. Khurendukhosaurus sp. (C-D) from the Bol’shoi Kemchug 3 locality (Ilek

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Formation, Western Siberia) and ?Neochoristodera indet. (“Shestakovo choristodere”) from the 31

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Shestakovo (E-H) and Bol’shaya Terekhtul’ 2 (I-J) localities (Ilek Formation, Western Siberia). A-B, ZIN PH

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17/25, dorsal vertebrae; A, transverse digital section; B, longitudinal digital section. C-D, LMCCE 12/4,

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sacral vertebra; C, transverse digital section; D, longitudinal digital section. E-F, LMCCE 11/4, dorsal

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vertebrae; E, longitudinal digital section; F, transverse digital section. G-H, LMCCE 7/4, dorsal vertebrae;

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G, longitudinal digital section; H, transverse digital section. I-J, LMCCE 26/4, dorsal vertebrae; I,

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longitudinal digital section; J, transverse digital section. Scale bar represents 5 mm. Abbreviations: na –

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neural arch.

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Table 1. Summary of Early Cretaceous choristoderan occurrences in Siberia. Unit

Age

Identity (this paper)

Previous identity

Affinities

Material

Main references

Shestakovo

Ilek Fm.

Aptian-Albian

?Neochoristodera indet. (“Shestakovo choristodere”)

Choristod era indet.

possible neochoristodere

fragmentary dentaries, cervical vertebra, dorsal vertebrae, proximal rib fragments

Skutschas, Vitenko 2015; this paper

Smolenskii Yar

Ilek Fm.

Early Cretaceous

Choristodera indet.

none

-

Ust’-Kolba

Ilek Fm.

Choristodera indet.

none

-

Bol’shoi Kemchug 3

Ilek Fm.

Early Cretaceous Early Cretaceous

?Neochoristodera indet. (“Shestakovo choristodere”); cf. Khurendukhosaurus sp.; Choristodera indet.

none

Bol’shoi Kemchug 4

Ilek Fm.

Choristodera indet.

none

Bol’shaya Terekhtul’ 2

Ilek Fm.

Early Cretaceous Early Cretaceous

“Shestakovo choristodere”: possible neochoristodere; cf. Khurendukhosaurus sp.: nonneochoristodere -

none

Bol’shaya Terekhtul’ 4

Ilek Fm.

?Neochoristodera indet. (“Shestakovo choristodere”); Choristodera indet. Choristodera indet .

Mogoito

Murtoi Fm.

Krasniy Yar

Khilok Fm.

Teete

Sangarian Group

fragment of vertebral centrum

this paper

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this paper

this paper

this paper

“Shestakovo choristodere”: possible neochoristodere -

posterior fragment of maxilla, centrum of cervical vertebra, centrum of dorsal vertebra “Shestakovo choristodere”: dorsal vertebrae; Choristodera indet.: centrum of cervical vertebra; centrum of dorsal vertebra centrum of posterior caudal vertebra

this paper

non-neochoristodere

isolated cranial and postcranial bones

Efimov, 1996; Skutschas 2008; this paper

-

centrum of caudal vertebra, proximal fragment of dorsal rib isolated postcranial bones

this paper

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“Shestakovo choristodere”: neural arch of the dorsal vertebrae; cf. Khurendukhosaurus sp.: dorsal vertebra, sacral vertebra; Choristodera indet.: centrum of dorsal vertebra; centrum of anterior caudal vertebra; centrum of posterior caudal vertebra; fragmentary clavicles

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Khurendukhosaurus sp.

centrum of cervical vertebra

none

Khurendu khosaurus bajkalensi s, Khurendu khosaurus sp. none

this paper

Barremian– Aptian Early Cretaceous

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Early Cretaceous Barremian?– Aptian

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Locality

Choristodera indet. Choristodera indet.

Khurendu khosaurus sp.

?nonneochoristodere

Kolosov et al., 2009; Skutschas, Efimov, 2015; this paper

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Highlights • • •

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We provide updated information on the record of Early Cretaceous choristoderes from Siberia (4 geological units, 10 localities). We discuss the taxonomic affinities, phylogenetic positions and distribution of the Early Cretaceous Siberian choristoderes. The hypothesis that a climatic barrier separated the distribution of choristoderes and neosuchian crocodyliforms in Asia during the Early Cretaceous is supported. We analyze the bone microstructure of the Early Cretaceous Siberian choristoderes.