New calcareous dinoflagellates from the Palaeogene of the South Atlantic Ocean (DSDP Site 357, Rio Grande Rise)

The new calcareous dinoflagellate taxa Bitorus truncus n.sp., Calcigonellum ansatum n.sp. and Fuettererella fungiforma n.sp. are formally described on the basis of Eocene and Oligocene samples from DSDP Site 357 (western South Atlantic Ocean).


INTRODUCTION
Calcareous dinoflagellates occur in significant quantities in many Mesozoic and Cenozoic marine sediments. However, for two main reasons the organisms have generally been overlooked in the past. As a result of their average size of 20-25pm, they appear to be too small for foraminifera1 studies and too large to be included in nannoplankton studies. Furthermore, they are dissolved during the preparation process for studies on organicwalled dinoflagellates.
Most information on fossil calcareous dinoflagellates focuses on Cretaceous material; few data on Palaeogene representatives are available. As a result of the interest in the Cretaceous/ Tertiary (K/T) boundary, the majority of studies on Palaeogene calcareous dinoflagellates deal with Palaeocene taxa exclusively (e.g. Fiitterer, 1990;Kienel, 1994;Willems, 1996). Eocene and Oligocene forms have been subject to only a few studies (e.g. Weiler, 1990;Kohring, 1993;Keupp & Kohring, 1994), and Fiitterer (1977and Fiitterer ( , 1984 alone has examined more extended time intervals, covering the entire Cenozoic. This study concentrates on the description of new taxa in the Palaeogene of DSDP Site 357. It forms part of an extensive evaluation of the latest Cretaceous to early Neogene calcareous dinoflagellate associations within two mid-latitude DSDP cores of the South Atlantic Ocean: Sites 356 and 357 (Hildebrand-Habel et al., in press; Hildebrand-Habel & Willems, in press). The investigations focus particularly on the Palaeogene. This interval is exceptionally interesting, as it not only includes the recovery following the K/T boundary, but also represents an intermediate stage between the warm Cretaceous and the colder Late Tertiary climates. It is marked by a general decline in temperature and shows significant palaeoceanographic changes (Miller et al., 1987;Crowley & North, 1991;Berger & Wefer, 1996). The associations of calcareous dinoflagellates and the morphology of the tests are influenced by external factors (e.g. nutrients, insolation, temperature, salinity), and the organisms thus represent sensitive tools for palaeoreconstructions (e.g. Zonneveld et al., in press). By describing the three new species, the present study gives additional information about the poorly known Palaeogene assemblages of the South Atlantic Ocean. The systematic inventory may provide the background for subsequent analyses of the palaeoenvironmental signal of individual calcareous dinoflagellate taxa, as shown in Zonneveld et al. (in press).

MATERIAL
The study is based on core material recovered during DSDP Leg 39 (Shipboard Scientific Party, 1977) at Site 357 (3O0O0.25'S, 35"33.59'W) in the western South Atlantic Ocean (Fig. I). The site was drilled in a water depth of 2086 m on the northern flank of the Rio Grande Rise. The cored section of 796.5m mainly consists of pelagic, biogenic carbonates. Terrigenous components occur only sporadically and in insignificant quantities; siliceous organisms are important only in the lower to middle Eocene interval.
Inferred hiatuses occur in the Campanian, at the K/T boundary, the Palaeocene/Eocene boundary, the Eocene/Oligocene boundary, and in the middle Miocene and Plio-Pleistocene (Shipboard Scientific Party, 1977).
A complete evaluation of the early Maastrichtian to early Miocene calcareous dinoflagellate assemblages of Site 357 is currently in preparation (Hildebrand-Habel & Willems, in press). The present study concentrates on the introduction of new species occurring in this time interval. Samples of Site 357 which contain new species are listed in Table I .

METHODS
The samples were processed as described by Willems (1996). They were treated with saturated Glauber's salt solution  Martini (1971 Original diagnosis (Keupp, 1992: 500) 'The bicarinate orthopithonelloid cysts have a single-layered calcareous wall. The two -.
prominent circular bulges are equivalent to the precingular and postcingular plates. The apical, conelike operculum ( = archae-(Na2S04 * 10H20) and repeatedly frozen and defrosted to opyle of exhausted cysts) includes the apical paraplates 2 ' 4 ' . ' disintegrate the material. Afterwards, the samples were washed through a 20pm sieve and the residual fraction dried at 70°C.
Bitorus truncus nsp. The specimens were then picked using a binocular microscope at (Plate 1, figs 1-7) a magnification of x256 and stuck on scanning electron microscopy (SEM) stubs covered with adhesive stickers. The Derivation of name. Truncus (Latin) = torso. With reference to stubs were coated with gold and the morphotypes examined the shape of the cyst, which resembles a torso. using a CamScan-44-SEM. Subsequently, some specimens were Diagnosis. A species of Bitorus with two cingulum-parallel broken with a scalpel to allow examination of the wall structure. arranged rings of intratabular lobes: seven precingular and five To investigate the crystallographic orientation of the wall-postcingular. Wide depressions reflect the cingulum and sulcus. forming crystallites, several specimens of Calcigonellum ansatum The single-layered wall consists of crystallites with radially n.sp. and Fuettererella fungiforma n.sp. were additionally oriented long axes. prepared as described by Janofske (1996). They were mounted Holotype. Cyst 116/36, SEM micrographs 116/6/3-6, 116/7/1-6, on the gelatinous surface of a transparent piece of developed 130/7/2-6, 131/1/1-5, 136/4/4-5, 136/5/1-5 (Plate I , figs 2-6). photographic film by wetting with water; they were then coated Type locality. Rio Grande Rise, western South Atlantic, DSDP with gold and examined using the scanning electron microscope. Leg 39, Site 357. The film strip was then embedded in low viscosity resin and cut Type stratum. Late Eocene zone NP19; sample 39-357-20-2, 79into 3 p m thin sections using a rotation microtome. After embedding in Canada balsam, these thin sections were examined using a Zeiss Axioplan light microscope. Only the examination in polarized light enabled the exact determination of the crystallographic orientation of the wall-forming calcite crystals.
The original material is stored in the Division of Historical Geology and Palaeontology of the University of Bremen.
Comparison. The elongation of crystal-rows stresses the close affinity of Calcigonellum ansatum to the other two species of Calcigonellum, i.e. C. injiula Deflandre, 1948 andC. granulata Kohring, 1993. The species show different expressions of the peridinoid paratabulation pattern, however. Additionally, the shape of the Calcigonellum species vary. The cysts of C. infula are notably elongated, whereas the cysts of C. granulata are almost spherical. Occurrence. Rare in the late Eocene of the Rio Grande Rise, sample 39-357-20-2, 79-80 cm.
Genus Fuettererella Kohring, 1993 Original diagnosis. (Kohring, 1993: 88) fig. 7). The thick calcareous wall is single-layered and consists of mushroom-shaped crystals (Plate 2, figs 10 and 11). Under the light microscope in crossed nicols and with gypsum plate, thin sections exhibit clear interference colours and have a positive optic sign. The species is thus assigned to the tangential wall type after Young e t al. (1997). Each individual crystal is proximally based on a solid, polygonal plate with distinct edges (Plate 2, figs 11 and 12). The crystals taper distally and continue as relatively slim stems until they widen again and constitute flat, distal plates (Plate 2, fig. 10). These undulated plates may be oval-shaped (Plate 2, figs 7 and 8) or polygonal, and commonly fuse, producing a homogenous surface (Plate 2, fig. 9). In the studied material, no archaeopyle has been observed. Dimensions. Holotype: length 36.5 pm, width 36.5 pm, thickness of wall 5.2pm. Other specimens: length 31.543.5pm, width 31.643.5 pm, thickness of wall 4.9-5.7pm.

SUMMARY
From the Eocene and Oligocene of DSDP Site 357, three new species of calcareous dinoflagellates are described. We thereby add information to the poorly known Palaeogene calcareous dinoflagellate assemblages of the western South Atlantic Ocean. By using both scanning electron and polarized light microscopy, tangential wall types have been proved for Calcigonellum ansatum n s p . and Fuettererella ,fungiforma nsp. The crystallographic orientations of the c-axes within Bitorus truncus, however. remain unverified.