J. Paleont., 81(5), 2007, pp. 1023–1033 Copyright  2007, The Paleontological Society 0022-3360/07/0081-1023$03.00 WILSONIDIUM PECHORICUM NEW SPECIES—A NEW DINOFLAGELLATE SPECIES WITH UNUSUAL ASYMMETRY FROM THE PALEOCENE/EOCENE TRANSITION ALINA I. IAKOVLEVA 1,2 AND CLAUS HEILMANN-CLAUSEN 1 1 Department of Earth Sciences, Aarhus University, 8000 A ˚ rhus C, Denmark, claus.heilmann@geo.au.dk, 2 Geological Institute, Russian Academy of Sciences, Pyzhevsky pereulok 7, 109017 Moscow, Russia, iakovl@yahoo.com ABSTRACT—Fossil dinoflagellates, when asymmetrical, almost always have features such as antapical horns on the right side reduced relative to features on the left side. A new species here described, Wilsonidium pechoricum, is therefore unusual in having a reduced left antapical horn. W. pechoricum seems to have originated in the northern Tethys in the latest Paleocene. It subsequently spread northwards and became widely distributed in the Peri-Tethys and parts of the Arctic region during the short interval known as the Initial Eocene Thermal Maximum (IETM). The new species was probably favored by extraordinary paleoecological conditions (high sea-surface temperatures and probably also high nutrient levels) prevailing in neritic waters of the IETM; a time during which aberrant morphotypes were also recorded among other planktonic protists. The apparent absence of W. pechoricum from the North Atlantic region suggests that the Turgay Strait may have functioned as a waterway between the Arctic and Peri-Tethys during the IETM. W. pechoricum is the oldest species of the genus Wilsonidium and possibly descended from the genus Apectodinium. Its early appearance points to a Late Paleocene radiation of the Wetzelielloideae before the well- known Early Eocene radiation in the subfamily, and its morphology is in accordance with a monophyletic origin of the group. INTRODUCTION D URING THE Late Paleocene-Early Eocene, latitudinal temper- ature gradients were reduced and levels of atmospheric CO 2 were elevated relative to the present day (Stott et al., 1990; Za- chos et al., 1994). Within this interval, a unique, short-lived (ap- proximately 220 Ky, Ro ¨hl et al., 2000, 2003), intense warming event is recognized and named the Initial Eocene Thermal Max- imum (IETM, previously known as the PETM [Paleocene-Eocene Thermal maximum]) (Wing et al., 2003). The IETM corresponds to a strong global negative carbon isotope excursion, the CIE (Zachos et al., 1993), which has recently been selected to define the base of the Eocene (Aubry and Ouda, 2003). During the last decade, this short warming event was conceived as an important turning point in Cenozoic reorganization of the biosphere, asso- ciated with an evolutionary turnover in terrestrial and marine bi- otas (Berggren et al., 1998). Among planktonic protists, calcare- ous nannoplankton species of unusual morphology (the Rhomboaster—Discoaster araneus association) proliferated in the Atlantic and Tethys Oceans (Aubry and Requirand, 2000; Kahn and Aubry, 2004). The dinoflagellate record indicates an inter- ruption of normal ecological conditions on continental shelves during this short period. According to Crouch et al. (2001), a strong increase of Apectodinium (Costa and Downie, 1976) Lentin and Williams, 1977 is precisely coincident with the onset of the CIE and appears to be synchronous on a global scale at the be- ginning of the IETM. Apectodinium dominated during the CIE but declined simultaneously with the normalization of the isotopic ratio. The aim of the present study is to describe an unusual species of the dinoflagellate genus Wilsonidium Lentin and Williams, 1976, which originated in the Late Paleocene and spread in certain parts of the Northern Hemisphere during the IETM, and to discuss its evolutionary, paleoecologic and paleobiogeographic signifi- cance. MATERIAL Samples spanning the Paleocene-Eocene transition, primarily representing the IETM interval, were studied from Austria, Ka- zakhstan, and two regions in Russia, western Siberia and the Pe- chora Depression (Fig. 1). The 56 samples from Austria were collected by Dr. Hans Egger, Geological Survey of Austria, Vi- enna, and by the second author (CH-C) in an exposed section at Anthering (map-sheet 63 Salzburg, coordinates 0426725/ 0305575). The single sample from Kazakhstan was collected in the Kaurtakapy section (coordinates 44°15'N, 52°19'E), Mangys- chlak, by Dr. C. King, Bridport, U.K. The twelve samples from the western Siberian Ust’-Manya 19 Borehole (coordinates 62°13'N, 60°42'E) were provided by the Geological Institute of the Russian Academy of Sciences, Moscow. The 48 samples from the Pechora Depression are from Borehole No. 228 (coordinates 66°18'N, 60°48'E) and were provided by Dr. T. A. Afanasieva, Geological Survey of Vorkuta, via the Geological Institute of the Russian Academy of Sciences, Moscow. Stratigraphic details of the studied sections are given below. METHODS The samples from Austria and Kazakhstan were processed by conventional palynological techniques. Carbonates and silicates were removed using HCl and HF. The acid-treated material was sieved on 20 m nylon filters and inspected. Samples rich in amorphous organic matter were oxidized using cold 65% HNO 3 for five or ten minutes. Samples with undissolved mineral grains were heavy-liquid separated using ZnCl 2 in dilute HCl with a specific gravity of 2.0. Finally, the organic residue was stained with safranin-O and mounted in gelatine-glycerine on microscope slides. Samples from western Siberia and the Pechora Depression were treated by standard palynological techniques used at the Paleo- floristic Laboratory of the Geological Institute, Russian Academy of Sciences, as described by Iakovleva et al. (2001). For scanning electron microscopy, single specimens were pick- ed from organic residue suspended in water, using a micropipette, as described in detail by Evitt (1984). Illustrations were made using a Leitz Orthoplan light micro- scope with plain transmitted light and an oil-immersion 100 objective, and with a CamScan MaXim 2040S scanning electron microscope. The optical system of the light microscope is con- ventional; i.e., it creates a rotated image, but not a mirror image. Drawings were made using a drawing apparatus connected to the light microscope. Due to the thin-walled nature of the cysts, lines representing endocyst or mesocyst margins, parasutures or folds could not always be distinguished, and were drawn as seen. A black and white film was employed for photomicrography, the images subsequently digitized. An England Finder reference fol- lows the slide number for each photomicrographed specimen. Repository.The holotype and paratypes are housed in the type collection of the Geological Museum, Copenhagen, under the catalog numbers MGUH 27800 to MGUH 27802. Other il- lustrated specimens are housed in the palynological collection of the Department of Earth Sciences, Aarhus University.