456 J. Paleont., 78(3), 2004, pp. 456–483 Copyright  2004, The Paleontological Society 0022-3360/04/0078-456$03.00 A PROPOSED CLASSIFICATION OF ARCHEOPYLE TYPES IN CALCAREOUS DINOFLAGELLATE CYSTS MICHAEL STRENG, 1 TANIA HILDEBRAND-HABEL, 2 AND HELMUT WILLEMS 3 1 Uppsala Universitet, Institutionen fo ¨r geovetenskaper, Paleobiologi, Norbyva ¨gen 22, 75236 Uppsala, Sweden, michael.streng@geo.uu.se, 2 Universitetet i Oslo, Institutt for geologi, Postboks 1047 Blindern, 0316 Oslo, Norway, and 3 Universita ¨t Bremen, FB-5 Geowissenschaften, Postfach 330 440, 28334 Bremen, Germany ABSTRACT—The phylogenetic significance of archeopyles in calcareous dinoflagellates cysts has been evaluated, and a classification model is developed that focuses on the archeopyle categories and types established for organic-walled dinoflagellates by Evitt (1967, 1985). Several of Evitt’s archeopyle categories are presently recognized within the calcareous dinoflagellate cysts: apical, intercalary, and combination archeopyles, which are here subdivided into eight archeopyle types and several variations. Archeopyles that cannot be assigned to a distinct type, and those with outlines that do not allow an accurate interpretation, are together placed in a separate category: miscellaneous archeopyles. The stratigraphic distribution of the different archeopyle types reveals a phylogenetic trend char- acterized by an increase of the number of plates involved in archeopyle formation. The first calcareous dinoflagellate cysts to appear in the late Triassic have a monoplacoid apical archeopyle. The first taxa that show an archeopyle involving more than one plate are from the Early Cretaceous, with the first triplacoid apical archeopyle appearing at the Berriasian/Valanginian boundary. This is followed by the first combination archeopyle, which includes six plates, in the middle Aptian. Epitractal archeopyles originated no earlier than the early Oligocene. At the beginning of the Paleogene, species with a combination archeopyle increased in abundance, progressively replacing species possessing an apical archeopyle that dominated during the Mesozoic. Newly described species are: Calciodinellum clamosum, accommodating the two subspecies Calciodinellum clamosum subsp. clamosum Autonym, and Calciodinellum clamosum subsp. latum; Calciodinellum kerguelense; Fuettererella belliata; and Pernambugia? patata. New combinations are: Cervisiella oper- culata (Bramlette and Martini, 1964); Praecalcigonellum sulcatum (Keupp, 1979a); and Praecalcigonellum dolium (Keupp, 1979b). Because of the new interpretation of their archeopyles we emend the following genera: Cervisiella Hildebrand-Habel, Willems, and Versteegh, 1999; Echinodinella Keupp, 1980; Fuettererella Kohring, 1993a; and Pernambugia Janofske and Karwath in Karwath (2000). The species Orthopithonella? minuta and Pirumella johnstonei, which have been previously synonymized with Fuettererella deflandrei, are retained as independent taxa. INTRODUCTION T HE CLASSIFICATION of fossil calcareous dinoflagellate cysts has been based mainly on the ultrastructure of the calcareous wall. Four wall types are distinguished on the orientation of the crystallographic c-axes of the wall-forming crystallites: a radial, a tangential, an oblique, and a pithonelloid type (summarized in Young et al., 1997). Many authors assumed these wall types to be the most significant feature in classification (e.g., Keupp, 1987; Kohring, 1993a; Janofske, 1996). One particularly useful but of- ten overlooked structure is the excystment aperture or archeopyle. The term archeopyle was introduced by Evitt (1961) to describe openings in fossil dinoflagellate cysts resulting from the release of plates (operculum) along predetermined lines. This definition has been modified by subsequent authors (e.g., Evitt, 1967; Lentin and Williams, 1976) by including any opening in a dinoflagellate cyst that serves for excystment (see also Fensome et al., 1996 for details). The archeopyle, which is often the only feature that re- flects tabulation (cryptotabulation), is a feature of considerable significance in the taxonomy of organic-walled dinoflagellate cysts, but has not been used significantly for calcareous cysts. As a consequence, calcareous cysts with different archeopyles were unified within the same genus (see Streng et al., 2002 and Ap- pendix 1 herein). Moreover, Fensome et al. (1993) regarded the wall structure of calcareous dinoflagellate cysts to be of unproven value for phylogenetic considerations. They considered the tab- ulation of a cyst to be the major feature for classification. Because the tabulation patterns where present on calcareous dinoflagellate cysts can always be derived from a peridiniacean tabulation, Fen- some et al. (1993) accommodated all taxa possessing a peridini- acean tabulation and calcareous cysts in the subfamily Calciodi- nelloideae of the family Peridiniaceae. However, besides the var- ious forms of peridiniacean tabulation developed in the Calcio- dinelloideae, which includes, e.g., holotabulated, intratabulated, and cingulotabulated taxa, more than 50 percent of the species included in this subfamily are cryptotabulate (Streng, personal observations). This means that most calcareous dinoflagellate cysts have no tabulation patterns, except those reflected by the archeopyle. Therefore, Streng et al. (2002) proposed that the em- phasis in classification within the Calciodinelloideae should be on the archeopyle type, taking into account outline, number and pat- tern of plates involved, and features of the operculum. As such details have been overlooked by many authors, no distinct ar- cheopyle types have been defined, unlike the studies of organic- walled cysts (e.g., Evitt, 1967). The descriptive terminology for archeopyles in calcareous dinoflagellate cysts is based entirely on that for organic-walled dinoflagellate cysts. This terminology is often appropriate for calcareous cysts, but some of the character- istics of the archeopyle and operculum are different. The aim of our study is to review and to describe in detail the known ar- cheopyle types and characteristics of calcareous dinoflagellate cysts, while creating a common descriptive terminology for future studies. Terms for the description of archeopyles used herein are sum- marized in Williams et al. (1978, 2000) and Fensome et al. (1996). Additionally, we propose new terms for some particular charac- teristics of calcareous cysts. MORPHOLOGICAL TERMINOLOGY The new terms archeopyle zone and mesoepicystal archeopyle are proposed. An archeopyle zone is a defined area on the epicyst of a cal- careous dinoflagellate cyst within which excystment occurs. This zone is distinguished from adjacent regions by a different crys- tallographic surface pattern, although a gradual transition between both is possible. The archeopyle zone generally reflects tabulation as it contains the archeopyle, which may partly do so. Species with an archeopyle zone do not release a predetermined opercu- lum comparable to those of other Calciodinelloideae. Rather they lose part of the archeopyle zone. A mesoepicystal archeopyle is characterized by the loss of three