The first plesiosaur from the Primorye krai

Abstract

Riabinin, 1915 wasfound by N.N. Tikhonovich in 1909 in the LowerSenonian (Coniacian–Santonian, 89–83 Ma) of Sakha-lin [3, 4]. This specimen is impossible to identify moreclosely than Plesiosauria indet. [1]. The teeth and ver-tebrae of the long-necked plesiosaurs Elasmosauridaewere found by L.A. Nessov in 1988 in the upper strataof the Koryak Formation (Upper Maastrichtian, 68–65 Ma) on the Pacific coast of the Chukotka Peninsula[4]. In the areas bordering on Russia (Korea, China, andJapan), plesiosaurs have not been recorded. In 2000,V.K. Popov discovered the fourth plesiosaur occurrencein the Far East; it was found during the geological sur-vey in southern Primorye in marine deposits of theKangauz Formation of the Upper Albian (103–98 Ma)on the right slope of the Sukhodol River (Kangauz),opposite the village of Novonezhino ( 43°12 ′ N, 132°35 ′ E), Primorye krai (Fig. 1). This is a caudal ver-tebra of a plesiosaur (specimen ZIN PH, no. 1/48, Pale-oherpetological collection of the Zoological Institute ofthe Russian Academy of Sciences). The bone beds arecomposed of dark greenish gray fine- and medium-grained tuffaceous sandstones, with carbonate material,containing abundant gastropod and bivalve shells, plantdetritus, and rare fragments of reptile bones. Thesesandstones are interbedded with siltstones containingplant remains. The presence of fragmentary reptilebones in these beds (Cape Sedlovidnyi) was first estab-lished in an unpublished report by N.A. Belyaevskiiand V.D. Prinada (1948). They found a relatively largefragment of a reptile bone (at least 45 cm long) incoarse-grained (gravellike) sandstones forming inter-beds in massive dense medium-grained sandstones inthe northern extremity of the Cape Sedlovidnyi (Khan-gan). This was the first finding of reptile bones in theNikanskaya Beds of southern Primorye. Subsequently,in the explanatory note to the geological map of Pri-morye, scale 1 : 500000 [5, p. 123], it was indicatedthat “…in siltstone deposits considered, vertebrates arerepresented by unidentifiable fish remains (scale andgill covers) and an imprint of a vertebral column, whilesandstone deposits have yielded large fragments of rep-tile bones.” Siltstones of these strata yielded conchost-racans, ostracodes, freshwater gastropods and bivalves,and plant imprints. From 2001 to 2003, V.S. Markevich,E.V. Bugdaeva (Institute of Biology and Soil Sciences,Far East Division, Russian Academy of Sciences), andYu.L. Bolotskii (AMURKNII) collected fish imprintsand unidentifiable fragments of reptile bones in out-crops on capes Palets and Sedlovidnyi.The strata described extend in northeast directionfrom Cape Sedlovidnyi (Andreev Bay) on the easterncoast of Ussuriiskii Gulf to the village of Novonezhino(Sukhodol River Basin) over a distance of more than20 km. Belyaevskii and Prinada (report, 1948)described bone-bearing beds in the green siltstonestrata dated Early Cretaceous. Subsequently, thesedeposits were assigned to the Cenomanian–Santonian(97–83 Ma) [5]. As a result of geological survey(1 : 200000), these strata were later assigned to theFrentsevka Formation dated Albian–Cenomanian(112–93 Ma) [6]. At present, these strata are recognizedas the separate Kangauz Formation dated Middle–LateAlbian (105–98 Ma) [7]. The Kangauz Formation iscomposed of siltstones, sandstones, tuffites, and con-glomerates and occurs on the eastern coast of Ussuri-iskii Gulf.The vertebra (Fig. 2) is platycelous, with circulararticular facets. The ventral side of the vertebral cen-trum is 23.5 mm long. The greatest height of the verte-bra, including the neural spine, is 48 mm. The posteriorarticular facet of the vertebral centrum is slightlyoblique. Its ventral margin, along with the articular fac-ets for the haemal arch, are broken off. The transverseprocess is stout, its base occupies the ventral area of theneural arch and the dorsal area of the vertebral centrum.The distal end of the transverse process expandsabruptly dorsoventrally. The transverse process is39 mm long. The vertebral foramen is relatively small.The prezygapophyses are broken off. The postzygapo-physes are shifted close to one another, located near themidheight of the neural arch. Anteroposteriorly, the