- Diconodinium lurense sp. nov., a late Maastrichtian to Danian dinoflagellate cyst from southwest Atlantic basins
Division DINOFLAGELLATA (Bütschli 1885) Fensome et al . 1993
Subdivision DINOKARYOTA Fensome et al . 1993
Class DINOPHYCEAE Pascher 1914
Subclass PERIDINIPHYCIDAE Fensome et al . 1993
Order PERIDINIALES Haeckel 1894
Suborder PERIDINIINEAE Autonym
Family PERIDINIACEAE Ehrenberg 1831
Genus Diconodinium Eisenack and Cookson 1960 emend . Morgan 1977
Type species. Diconodinium multispinum Deflandre and Cookson, 1955, pl.1, fig.5, as Palaeohystrichophora multispina .
1981 ? Lejeunia ? sp. Gamerro and Archangelsky: page 123, pl II, fig.6.
Holotype. Slide PL-1: 631(2), coordinates: 31/94.8 (England Finder reference: D 45/4) Figure 3.A, Figure 4.A.
Paratype. Slide PL-1: 632(1), coordinates: 41/109 (England Finder reference: T 35/0) Figures 4.D, G.
Repository. Palynological Collection, Departamento de Geología de la Universidad Nacional del Sur, Bahía Blanca, Argentina.
Type locality. Pedro Luro-1 Borehole, Buenos Aires Province, Argentina.
Type stratum. 1487-1488 m depth (core sample).
Derivation of name. After the Pedro Luro Formation, from which this species was first recorded.
Diagnosis. A species of Diconodinium characterized by relatively long apical and antapical horns and with an autophragm densely covered by short nontabular spines.
Description. Cyst fusiform in shape, consisting of a subtriangular epitract slightly larger than the rounded hypotract. The epitract extends into a 20 to 30 µm long apical horn with a rounded distal tip. The hypotract bears a single, pointed, 15 to 25 µm long antapical horn clearly offset to the left from the midline of the cyst. The cingulum is about 4 µm wide, and slightly offset at the sulcus. The autophragm is densely covered by thin, solid spines up to 1.5 µm high. The spines are all of the same type and size. They are distributed randomly except along the margin of the cingulum, where they are always aligned in rows (figures 3.A-C). The archeopyle is rarely discernible but, when visible, is intercalary normally type Ia but occasionally IPa. The former type includes the stenodeltaform plate 2a only with clear apical and lateral sutures (figures 3.B, 4.H). When having a type Ipa archeopyle the lateral sutures of plate 4´´ are partly open but never reach the cingulum (figures 3.A, 4.F, G). In both archeopyle types the operculum remains in situ and is always adnate posteriorly (figures 4.H, I, 5.H, I).
Dimensions . Overall length: 55 (80) 110 µm; width: 30 (41) 50 µm (53 specimens measured).
Stratigraphic distribution. Late Maastrichtian to Danian.
Comparison. Diconodinium lurense differs from most other described species of Diconodinium in having a fusiform shape and relatively long apical and antapical horns. Other species also bearing relatively long horns are Diconodinium martianum Srivastava and Diconodinium longicorne Olaru. The former resembles Diconodinium lurense closely in shape and size, but differs in having the antapical horn located close to the midline and in having a smooth autophragm. Diconodinium longicorne Olaru is characterized by having a subspherical to rhomboidal body, an epitract that is bigger than the hypotract, and an apical horn that is longer than the shorter antapical horn. In addition, its surface is finely granulate and shows tabulation on both epitract and hypotract. Diconodinium davidii Morgan is similar in size to Diconodinium lurense sp. nov. and also has a slightly offset antapical horn, but differs in having coarser, aligned spines that may reveal traces of tabulation. Diconodinium pusillum Singh also has an asymmetrically aligned antapical horn and an epitract that is longer than the hypotract. It differs from the new species by having a shorter, bifid apical horn and finer spines 0.5 µm long, and in being smaller. Diconodinium lurense also resembles Diconodinium multispinum (Deflandre and Cookson) Eisenack and Cookson emend . Morgan, however, the latter shows signs of paratabulation, especially on the epitract. Furthermore, Diconodinium multispinum has ornamentation which is slightly larger and adjacent ornamentation elements may be fused to form discontinuous parasutural ridges. Diconodinium vitricornu Roncaglia et al . differs from Diconodinium lurense in having a smooth autophragm, mostly thin but thickened at the apex to form a solid tip of the horn. Alterbidinium acutulum (Wilson) Lentin and Williams possesses a similar outline and comparable horns to Diconodinium lurense , and also has an epitract larger than the hypotract. However, Alterbidinium acutulum (Wilson) Lentin and Williams is characteristically circumcavate and both endophragm and periphragm are smooth.
Remarks. A few specimens of Diconodinium lurense appear to show cavation, especially below the apical horn (figures 4.F, H). A detailed examination of our specimens revealed that this is an optical effect, produced by folding of the thin autophragm at the anterior margin of the archeopyle, and does not indicate a second wall layer. According to Fensome et al. (1993, p. 131), Diconodinium is morphologically intermediate between the two subfamilies Palaeoperidinioideae and Deflandroideae, since the type species can have either a single plate intercalary archeopyle type 1Ia or a single plate intercalary plus single plate precingular archeopyle type 1I1Pa. Although the archeopyle sutures are not always clearly discernible, most of our specimens have an archeopyle type Ia and just a few of them seem to present an Ipa type. This variation reinforces the comments of Fensome et al . (1993).
The authors thank Rob Fensome (Geological Survey of Canada), Henrik Nøhr Hansen (Geological Survey of Denmark and Greenland) and Henk Brinkhuis (Utrecht University, The Netherlands) for critical comments which improved the previous version. To the journal referees Graham Williams (Geological Survey of Canada) and Poul Schiøler (Geological Survey of Denmark and Greenland) for the constructive criticism of the final version of the manuscript. Technical laboratory support by Bernie Crilley (Geological Survey of Canada) is gratefully thanked. Dann Saveanou (Universidad Nacional del Sur) provided helpful assistance regarding the translation from papers published in Romanian language. This contribution has been funded by Fondo para la Investigación Científica y Tecnológica (PICT Nº 07-09659/01), Consejo Nacional de Investigaciones Científicas y Técnicas (PIP 02040) and Secretaría General de Ciencia yTecnología de la Universidad Nacional del Sur.
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