Protist, Vol. 164, 583–597, xx 2013 http://www.elsevier.de/protis Published online date xxx ORIGINAL PAPER Waking the Dead: Morphological and Molecular Characterization of Extant †Posoniella tricarinelloides (Thoracosphaeraceae, Dinophyceae) Haifeng Gu a , Monika Kirsch b , Carmen Zinssmeister c,d , Sylvia Soehner c,d , K.J. Sebastian Meier e , Tingting Liu a , and Marc Gottschling c,1 a Third Institute of Oceanography, State Oceanic Administration, 361005 Xiamen, China b Universität Bremen, Fachbereich Geowissenschaften – Fachrichtung Historische Geologie/Paläontologie, Klagenfurter Straße, D – 28359 Bremen, Germany c Department Biologie, Systematische Botanik und Mykologie, GeoBio-Center, Ludwig-Maximilians-Universität München, Menzinger Str. 67, D – 80638 München, Germany d Freie Universität Berlin, Fachbereich Geologische Wissenschaften, Fachrichtung Paläontologie, Malteserstraße 74-100, D – 12249 Berlin, Germany e Institut für Geowissenschaften, Christian-Albrechts-Universität zu Kiel, Ludewig-Meyn-Str. 10, D – 24118 Kiel, Germany Submitted January 22, 2013; Accepted June 1, 2013 Monitoring Editor: Mona Hoppenrath The Thoracosphaeraceae are dinophytes that produce calcareous shells during their life history, whose optical crystallography has been the basis for the division into subfamilies. To evaluate the validity of the classification (mainly applied by palaeontologists), living material of phylogenetic key species is necessary albeit frequently difficult to access for contemporary morphological and molecular analy- ses. We isolated and established five living strains of the rare fossil-taxon †Posoniella tricarinelloides from different sediment samples collected in the South China Sea, Yellow Sea and in the Mediterranean Sea (west coast off Italy). Here, we provide detailed descriptions of its morphology and conducted phylogenetic analyses based on hundreds of accessions and thousands of informative sites on con- catenated rRNA datasets. Within the monophyletic Peridiniales, †P. tricarinelloides was reliably nested in the Thoracosphaeraceae and exhibited two distinct morphological types of coccoid cells. The two morphologies of coccoid cells would have been assigned to different taxa at the subfamily level if found separately in fossil samples. Our results thus challenge previous classification concepts within the dinophytes and underline the importance of comparative morphological and molecular studies to better understand the complex biology of unicellular organisms such as †P. tricarinelloides. © 2013 Elsevier GmbH. All rights reserved. Key words: Calcareous dinoflagellates; cyst; distribution; molecular systematics; theca; ultrastructure. 1 Corresponding Author; e-mail gottschling@biologie.uni-muenchen.de (M. Gottschling). © 2013 Elsevier GmbH. All rights reserved. http://dx.doi.org/10.1016/j.protis.2013.06.001