Back
Wilsonidium pechoricum
Wilsonidinium "pechoricum" Iakovleva and Heilmann-Clausen, 2007, p.1024,1025,1027–1031, fig.2, nos.1–5; fig.3, nos.1–12; fig.4, nos.1–6; fig.5, nos.1–4; fig.6, nos.1–4.
Holotype: Iakovleva and Heilmann-Clausen, 2007, fig.2, nos.4–5, fig.3, no.1.
Now Epelidinium. Originally Wilsonidium, subsequently (and now) Epelidinium.
Age: earliest Eocene.
Original description (Iakovleva and Heilmann-Clausen, 2007)
Diagnosis
A thin-walled wetzelielloideaen cyst with a well-developed right antapical horn and a reduced left antapical horn. A mesophragm is sometimes visible. Paratabulation partially outlined by fine parasutural spines or coni. A quadra-type intercalary archeopyle with four straight sides is present. The operculum is completely detached.
Description
A thin-walled, medium-sized to small species of the subfamily Wetzelielloideae. The cyst is dorsoventrally compressed. The outline in dorsoventral view is pentagonal to almost rhombic. Apical and lateral horns are of variable length. The antapical horns are of unequal length; the right antapical horn is well developed, while the left antapical horn is always shorter than the right horn and may be absent. The cyst is circumcavate and the outline of the thin-walled endocyst is subcircular. In well-preserved specimens, a separate mesophragm is sometimes present between endophragm and periphragm. When present the mesophragm can only be traced with certainty over part of the cyst, and is most clearly visible beneath the horns. The mesophragm forms a well-developed apical horn inside the pericyst. The periphragm is smooth and ornamented with a few short, thread-like spines or fine granules. The ornament shows a mainly parasutural alignment, which is best developed along the paracingulum and lateral cyst margins. The partially reflected tabulation suggests a normal wetzelielloidean paratabulation (Fig. 2). Separate endo- and periarcheopyles can only rarely be distinguished (Fig. 6.4). The periarcheopyle is distinctly four-sided, with all four sides straight, representing a quadra-type second anterior intercalary paraplate. The operculum is completely detached.
Etymology
Wilsonidium pechoricum n. sp. was first reported and illustrated in camera lucida drawings from borehole No. 228 in the Pechora Depression, Russia (Fig. 1), by Iakovleva (2000).
Holotype
Figs. 2.4–2.5, 3.1. Russia, Pechora Basin, Borehole 228. Sample 281.5 m, slide 1; England Finder reference O41/2. MGUH 27800.
Paratype 1
Fig. 3.2. Russia, western Siberia, Ust'-Manya 19 Borehole, slide 2493a-1; England Finder reference N52/3, MGUH 27801.
Paratype 2
Figs. 2.1–2.3, 3.11. Austria, Anthering section, Outcrop J. Sample 2352, slide 2352-F1, England Finder reference V29. MGUH 27802.
Type stratum and locality
Uppermost part of Kirshorskaya Formation, Pechora Basin, Borehole No. 228, 281.5 m, earliest Eocene, Russia.
Measurements
Holotype: total length, 101 μm; total width, 97 μm. Paratype 1: total length, 81 μm; total width, 77 μm. Paratype 2: total length, 80 μm; total width, 74 μm. Range: total length, 73–102 μm (mean value: 87 μm); total width, 61–114 μm (mean value: 89 μm). Eighteen specimens were measured.
Discussion
The quadra-type intercalary archeopyle is a unifying character shared by all Wetzelielloideae (Lentin and Williams, 1976; Fensome et al., 1993) and only occurs in this subfamily. Among the several genera included in the Wetzelielloideae, Wilsonidium is diagnosed by its parasutural distribution of the ornament. The unquestionable presence of a quadra-type intercalary archeopyle in our material (Figs. 2, 3, 4, 5, 6) and the mainly parasutural distribution of the ornament confirm assignment to Wilsonidium.
In order to determine which of the antapical horns was reduced in the new species, 24 specimens were studied in light microscope at 1,000× magnification (5 specimens from the Pechora Depression, 3 from western Siberia, 4 from Kazakhstan and 12 from Austria). For this purpose, the dorso-ventral orientation of the cyst in the slide must be known. Although the cysts are dorsoventrally compressed, the orientation of all specimens, except for one Austrian cyst, could be determined with certainty. Each of the 23 oriented specimens had reduced, or entirely absent, left antapical horns. Several of these specimens are shown in Figures 3 and 4. Fifteen more specimens from Kazakhstan and Austria were studied in scanning electron microscope and all showed reduced left antapical horns. Eight of them are shown in Figures 5 and 6. On basis of this unambiguous material it may be concluded that reduction of the left antapical horn is diagnostic for the new species.
Dinoflagellate cysts with two unequal antapical horns of which the right horn is longer than the left horn, as in W. pechoricum, seem to be very rare in the fossil record. According to Taylor (1987) and Fensome et al. (1993, p. 5) dinoflagellates show a general tendency to asymmetry, with features on the left side, such as antapical horns, being larger than on the right side. In the subfamily Wetzellielloideae there may be two unequal horns at the antapex, of which the larger horn is to the left, or there may be only a single horn offset to the left, according to Lentin and Williams (1976) and Fensome et al. (1993, p. 137).
However, the dominance of the left antapical horn in dinoflagellates is not universal. Thecae of some modern peridinoid dinoflagellate species have unequal antapical horns. Among these, according to Evitt (1985, p. 86), the larger horn is almost always the right one; i.e., the situation in these modern asymmetrical dinoflagellates is opposite to that seen in the fossil record. Dodge (1985) included scanning electron microscope images of thecae of several Recent peridinoid species with larger right antapical horns, lobes or spines, for example Protoperidinium claudicans (Paulsen, 1907) Balech, 1974 and Protoperidinium steinii (Jörgensen, 1900) Balech, 1974. None of the peridinoid thecae figured in Dodge (1985) show a strong asymmetry, and most of the illustrated species have almost symmetrical antapical regions.
The contrast between modern and extinct asymmetrical peridinoids appears not to have been previously discussed. Its ecological significance (if any) is unknown. Cysts with reduced left antapical horns here studied are from widely separated former seas and the left-reduction is almost certainly genetically controlled. The modern peridinoid dinoflagellates are not closely related to the extinct Wetzellielloideae, with their distinctive paratabulation of the dorsal epicyst. The reduction of the left antapical horn in W. pechoricum and in some modern peridinoids is, therefore, clearly due to convergent evolution in different groups.
Monteil (1991) described an Early Cretaceous dinoflagellate species Cometodinium habibii Monteil, 1991 in which the asymmetrical paraplate arrangement, i.e., the entire cyst organization, is a mirror image, an enantiomorph, of the normal situation. Apart from this Cretaceous species, according to Monteil (1991), there are no well documented examples of enantiomorphy among dinoflagellates. It is therefore of interest to ascertain if W. pechoricum is an enantiomorph or shows a normal paraplate arrangement. However, like in many other peridinoids, the paraplate pattern is almost bilateral symmetric (Fig. 2), and for this reason it was not possible to determine if the arrangement is normal or mirror reflected. The paracingulum shows practically no helicoidality in most of the specimens examined (best seen in Figs. 5, 6), but in the holotype, the paracingulum is slightly helicoidal (Fig. 2.5), and shows right-handed displacement, i.e., with the right end of the paracingulum more anterior than the left end. In modern dinoflagellates left-handed displacement of the cingulum is much more common than right-handed, the latter being present only in a few species of Protoperidinium Bergh, 1881, according to Taylor (1987). Judging from the holotype with the rare direction of displacement, W. pechoricum could be an enantiomorph. However, the displacement is not clearly seen in other specimens, so the evidence is not conclusive. For the identification of paraplates (Fig. 2), we have assumed a normal coiling direction, i.e., no enantiomorphy.
The presence of a mesophragm is likewise a rare feature in dinoflagellate cysts. In peridinoid cysts, a mesophragm is known in the Early Cretaceous Cepadinium variabile Duxbury, 1983. Also the Cretaceous-Paleocene species Palaeoperidinium pyrophorum (Ehrenberg, 1838) Sarjeant, 1967 can have a middle wall layer (Evitt et al., 1998) although perhaps not homologous with that of W. pechoricum. In W. pechoricum the walls are thin and the mesophragm could only be identified in well-preserved specimens and could only be traced along part of the cyst margin.
Wilsonidium tabulatum (Wilson, 1967) Lentin and Williams, 1976, the type of the genus, and other species of this genus are all of younger (Early to Late Eocene) ages than W. pechoricum. These younger species are larger and more robust than W. pechoricum. A similar relationship exists between the genus Apectodinium and its possible descendent genera Wetzeliella Eisenack, 1938, Dracodinium Gocht, 1955, and Charlesdowniea Lentin and Vozzhennikova, 1989.
Dinoflagellate cysts from Kazakhstan of approximately the same age as Wilsonidium pechoricum were previously tentatively referred to Wilsonidium (as Wilsonidium? sp. A and Wilsonidium? sp. B) by Iakovleva et al. (2001). These cysts are reexamined in the present study and differ markedly from Wilsonidium pechoricum. They show the typical dinoflagellate asymmetry, i.e., with antapical horns of equal length or with the left antapical horn longer. There are two hyaline cyst walls, endophragm and periphragm, which are smoother than the walls of W. pechoricum, and there is no trace of a mesophragm. Furthermore, the cysts are less compressed dorsoventrally than W. pechoricum. A wetzelielloidean dinoflagellate cyst, Wetzeliella? in Edwards (1989, Plate 2, fig. 6) from the uppermost Paleocene in Virginia is superficially similar to Wilsonidium pechoricum, but differs by having the right antapical horn reduced, i.e., a normal asymmetry.
stratigraphic occurrence
Wilsonidium pechoricum has been recorded in a number of sections spanning the uppermost Paleocene-lowermost Eocene in the Northern Hemisphere (Fig. 1). The known occurrence of W. pechoricum n. sp. is shown in Figure 7.
Stratigraphical occurrence in Austria, this study
Wilsonidium pechoricum is recorded in the section at Anthering previously described by Egger (1995) and Egger et al. (2000, 2003). The section exposes alternating grey to black, hemipelagic clays and distal turbidites of the Anthering Formation, which is a part of the Rhenodanubian flysch. The section consists of a succession of outcrops lettered A (top of section) to N (base of section). The section is expanded and closely sampled. It spans most of the calcareous nannoplankton zones NP9 and NP10 and the lower part of NP11 and isotope analyses have shown the position of the CIE (Egger et al., 2000). The lowest occurrence of W. pechoricum is in Outcrop L (sample 2396). This part is referred to the upper part of NP9, but below the CIE (Egger et al., 2000), and the sample therefore belongs in the uppermost Paleocene. The highest occurrence is in Outcrop Ja (sample 2365), also referred to the upper part of NP9 by Egger et al. (2000). This sample is within the upper part of the CIE and is associated with an acme of Apectodinium spp., including A. augustum (Harland, 1979) Lentin and Williams, 1981 (Egger et al., 2000; Crouch et al., 2001), and therefore belongs in the lowermost Eocene. W. pechoricum attains a maximum abundance of 2% in the middle of Outcrop J, coinciding with the peak of the CIE and the maximum abundance (66%) of Apectodinium spp., i.e. at the Paleocene-Eocene boundary.
Stratigraphic occurrence in Kazakhstan, this study
Wilsonidium pechoricum is recorded in a single sample (sample 2264) examined from the Kaurtakapy section, Mangyschlak. The Kaurtakapy section was previously described by Bolle et al. (2000). The sample derives from a 10 cm thick sapropelic clay layer occurring within a thick succession of marl and chalks. The clay layer represents planktonic foraminifera biozone P5b and the IETM, and also marks the base of the Givmra Formation (Bolle et al., 2000). W. pechoricum is common in the sample and occurs in a typical IETM-assemblage, strongly dominated by Apectodinium spp., including Apectodinium cf. augustum.
Stratigraphic occurrence in western Siberia, this study
Wilsonidium pechoricum is recorded from the Ust'-Manya 19 Borehole, which recovered a succession of clays and siliceous clays. Here it occurs only in a sample at 122.25 m. This sample is rich in dinoflagellates, including Apectodinium spp. [A. homomorphum (Deflandre and Cookson, 1955) Lentin and Williams, 1977, A. quinquelatum (Williams and Downie, 1966) Lentin and Williams, 1977, A. hyperacanthum (Cookson and Eisenack, 1965) Lentin and Williams, 1977, A. paniculatum (Costa and Downie, 1976) Lentin and Williams, 1977, and A. augustum]. Apectodinium spp. account for 7% of the total dinoflagelate cysts, and the sample can be correlated to the IETM interval based on the presence of A. augustum. The underlying sample at 127.0 m yielded only a few dinoflagellate cysts, including Apectodinium spp. (without A. augustum), and may belong in the IETM interval or the Upper Paleocene. The overlying sample at 105.0 m includes Dracodinium simile (Eisenack, 1954) Costa and Downie, 1979 and is thus probably age-equivalent to the early part of the NP11 calcareous nannoplankton Zone, according to zonal calibrations in Luterbacher et al. (2004).
Stratigraphic occurrence in the Pechora Depression, this study
Wilsonidium pechoricum is recorded in two samples from the interval 282.5 m to 281.5 m in Borehole No. 228. This is a clayey and silty interval, belonging to the uppermost part of the Kirshorskaya Formation (Iakovleva et al., 2000). The dinoflagellate assemblage is diverse, with more than 60 taxa, and is characterized by abundant (50%) Apectodinium spp., including A. augustum. The two samples can be correlated to the IETM interval based on the presence of A. augustum. Two underlying samples (287.0 m and 286.0 m) include A. homomorphum and A. quinquelatum, but A. augustum is absent, and the samples are probably of Late Paleocene age. Overlying samples at 274.8 m, 271.4 m, and 269.0 m yielded no age-diagnostic dinoflagellate cysts, but may represent the Glaphyrocysta ordinata Interval Biozone (Gor Biozone) of Powell (1992). The first occurrence of Wetzeliella meckelfeldensis Gocht, 1969, at 267.1 m indicates the W. meckelfeldensis Zone of Costa and Downie (1976) and this level may therefore be calibrated to the upper part of the NP 10 nannoplankton Zone, according to Powell (1992) and Luterbacher et al. (2004).
Previous records
Wilsonidium pechoricum was recorded and illustrated (as Rhombodinium sp.) from a sapropel occurring at the Medany outcrop section (western Georgia, approximate coordinates 43°10′N, 42°2′E) and at the Torangly outcrop section (western Kopetdag, Turkmenistan, approximate coordinates 39°8′N, 55°39′E) by Akhmetiev and Zaporozhetz (1996, Plate III, fig. 4). In both sections the sapropel was referred to the upper Thanetian by Akhmetiev and Zaporozhetz (1996) and, according to these authors, contained an abundance of Apectodinium spp., including A. augustum, A. parvum (Alberti, 1961) Lentin and Williams, 1977, A. summissum (Harland, 1979) Lentin and Williams, 1981, A. paniculatum, A. quinquelatum and A. homomorphum. The sapropel can therefore be correlated with the IETM interval. This is in agreement with Stupin and Muzylev (2001) and Muzilev (personal commun.) who referred the sapropels to the uppermost Thanetian PETM-interval based on the calcareous nannoplankton biozone CP8a/CP8b boundary assemblage. The PETM-interval is now placed in the initial Eocene (and renamed as IETM) after the international formalization of the Paleocene-Eocene boundary at the base of the CIE (Aubry and Ouda, 2003).
Wilsonidium pechoricum was recorded and illustrated by Crouch et al. (2003, Plate 7, figs. 1–4) from the Aktumsuk outcrop section (Uzbekistan, coordinates 43°85′N, 57°17′E) as Rhombodinium spp. The section was previously described by Bolle et al. (2000). According to Crouch et al. (2003), Rhombodinium spp. is common in an approximately 1.5 m thick interval spanning the 1 m thick, organic-rich IETM interval and the lowermost part of the overlying chalk. The illustrated specimens of W. pechoricum were all from the IETM interval. The record from the chalk about 30 cm above the IETM interval was not illustrated and the presence of W. pechoricum at this level is therefore uncertain. The section is highly condensed and this sample was assigned to the upper? NP10 or basal NP11 interval. A further uncertainty of this post-IETM record of W. pechoricum is due to possible upwards contamination, which might be caused by bioturbation or reworking from the IETM interval, as the top of the IETM interval is a burrowed surface (Bolle et al., 2000). According to Crouch et al. (2003), various morphotypes of Apectodinium are very common (up to 35%) in the IETM interval at Aktumsuk, although the extreme morphotype A. augustum was not recorded. Samples below and above this interval were barren of dinoflagellates.
It may be concluded that W. pechoricum first occurs in the uppermost Paleocene shortly below the IETM, based on the Anthering Section in Austria. In records from Georgia, Turkmenistan, Uzbekistan, and Kazakhstan, W. pechoricum is first recorded in a sapropel within the IETM level. However, samples were not taken (or were barren) from below this level in these regions. Consequently, the first occurrence of W. pechoricum in this part of the Peri-Tethys region remains questionable. In contrast, in the sections from western Siberia and the Pechora Depression, dinoflagellates were studied in the whole Thanetian interval. Here W. pechoricum was not recorded below the IETM interval.
The last occurrence is close to the top of the IETM in Austria and this seems to be the case too in the Pechora Depression. At Aktumsuk the last occurrence is uncertain and may be younger, as discussed above. In the remaining sections no samples were studied, or the spacing between samples was too wide to yield information about possible occurrences of Wilsonidium pechoricum above the IETM interval.
Holotype: Iakovleva and Heilmann-Clausen, 2007, fig.2, nos.4–5, fig.3, no.1.
Now Epelidinium. Originally Wilsonidium, subsequently (and now) Epelidinium.
Age: earliest Eocene.
Original description (Iakovleva and Heilmann-Clausen, 2007)
Diagnosis
A thin-walled wetzelielloideaen cyst with a well-developed right antapical horn and a reduced left antapical horn. A mesophragm is sometimes visible. Paratabulation partially outlined by fine parasutural spines or coni. A quadra-type intercalary archeopyle with four straight sides is present. The operculum is completely detached.
Description
A thin-walled, medium-sized to small species of the subfamily Wetzelielloideae. The cyst is dorsoventrally compressed. The outline in dorsoventral view is pentagonal to almost rhombic. Apical and lateral horns are of variable length. The antapical horns are of unequal length; the right antapical horn is well developed, while the left antapical horn is always shorter than the right horn and may be absent. The cyst is circumcavate and the outline of the thin-walled endocyst is subcircular. In well-preserved specimens, a separate mesophragm is sometimes present between endophragm and periphragm. When present the mesophragm can only be traced with certainty over part of the cyst, and is most clearly visible beneath the horns. The mesophragm forms a well-developed apical horn inside the pericyst. The periphragm is smooth and ornamented with a few short, thread-like spines or fine granules. The ornament shows a mainly parasutural alignment, which is best developed along the paracingulum and lateral cyst margins. The partially reflected tabulation suggests a normal wetzelielloidean paratabulation (Fig. 2). Separate endo- and periarcheopyles can only rarely be distinguished (Fig. 6.4). The periarcheopyle is distinctly four-sided, with all four sides straight, representing a quadra-type second anterior intercalary paraplate. The operculum is completely detached.
Etymology
Wilsonidium pechoricum n. sp. was first reported and illustrated in camera lucida drawings from borehole No. 228 in the Pechora Depression, Russia (Fig. 1), by Iakovleva (2000).
Holotype
Figs. 2.4–2.5, 3.1. Russia, Pechora Basin, Borehole 228. Sample 281.5 m, slide 1; England Finder reference O41/2. MGUH 27800.
Paratype 1
Fig. 3.2. Russia, western Siberia, Ust'-Manya 19 Borehole, slide 2493a-1; England Finder reference N52/3, MGUH 27801.
Paratype 2
Figs. 2.1–2.3, 3.11. Austria, Anthering section, Outcrop J. Sample 2352, slide 2352-F1, England Finder reference V29. MGUH 27802.
Type stratum and locality
Uppermost part of Kirshorskaya Formation, Pechora Basin, Borehole No. 228, 281.5 m, earliest Eocene, Russia.
Measurements
Holotype: total length, 101 μm; total width, 97 μm. Paratype 1: total length, 81 μm; total width, 77 μm. Paratype 2: total length, 80 μm; total width, 74 μm. Range: total length, 73–102 μm (mean value: 87 μm); total width, 61–114 μm (mean value: 89 μm). Eighteen specimens were measured.
Discussion
The quadra-type intercalary archeopyle is a unifying character shared by all Wetzelielloideae (Lentin and Williams, 1976; Fensome et al., 1993) and only occurs in this subfamily. Among the several genera included in the Wetzelielloideae, Wilsonidium is diagnosed by its parasutural distribution of the ornament. The unquestionable presence of a quadra-type intercalary archeopyle in our material (Figs. 2, 3, 4, 5, 6) and the mainly parasutural distribution of the ornament confirm assignment to Wilsonidium.
In order to determine which of the antapical horns was reduced in the new species, 24 specimens were studied in light microscope at 1,000× magnification (5 specimens from the Pechora Depression, 3 from western Siberia, 4 from Kazakhstan and 12 from Austria). For this purpose, the dorso-ventral orientation of the cyst in the slide must be known. Although the cysts are dorsoventrally compressed, the orientation of all specimens, except for one Austrian cyst, could be determined with certainty. Each of the 23 oriented specimens had reduced, or entirely absent, left antapical horns. Several of these specimens are shown in Figures 3 and 4. Fifteen more specimens from Kazakhstan and Austria were studied in scanning electron microscope and all showed reduced left antapical horns. Eight of them are shown in Figures 5 and 6. On basis of this unambiguous material it may be concluded that reduction of the left antapical horn is diagnostic for the new species.
Dinoflagellate cysts with two unequal antapical horns of which the right horn is longer than the left horn, as in W. pechoricum, seem to be very rare in the fossil record. According to Taylor (1987) and Fensome et al. (1993, p. 5) dinoflagellates show a general tendency to asymmetry, with features on the left side, such as antapical horns, being larger than on the right side. In the subfamily Wetzellielloideae there may be two unequal horns at the antapex, of which the larger horn is to the left, or there may be only a single horn offset to the left, according to Lentin and Williams (1976) and Fensome et al. (1993, p. 137).
However, the dominance of the left antapical horn in dinoflagellates is not universal. Thecae of some modern peridinoid dinoflagellate species have unequal antapical horns. Among these, according to Evitt (1985, p. 86), the larger horn is almost always the right one; i.e., the situation in these modern asymmetrical dinoflagellates is opposite to that seen in the fossil record. Dodge (1985) included scanning electron microscope images of thecae of several Recent peridinoid species with larger right antapical horns, lobes or spines, for example Protoperidinium claudicans (Paulsen, 1907) Balech, 1974 and Protoperidinium steinii (Jörgensen, 1900) Balech, 1974. None of the peridinoid thecae figured in Dodge (1985) show a strong asymmetry, and most of the illustrated species have almost symmetrical antapical regions.
The contrast between modern and extinct asymmetrical peridinoids appears not to have been previously discussed. Its ecological significance (if any) is unknown. Cysts with reduced left antapical horns here studied are from widely separated former seas and the left-reduction is almost certainly genetically controlled. The modern peridinoid dinoflagellates are not closely related to the extinct Wetzellielloideae, with their distinctive paratabulation of the dorsal epicyst. The reduction of the left antapical horn in W. pechoricum and in some modern peridinoids is, therefore, clearly due to convergent evolution in different groups.
Monteil (1991) described an Early Cretaceous dinoflagellate species Cometodinium habibii Monteil, 1991 in which the asymmetrical paraplate arrangement, i.e., the entire cyst organization, is a mirror image, an enantiomorph, of the normal situation. Apart from this Cretaceous species, according to Monteil (1991), there are no well documented examples of enantiomorphy among dinoflagellates. It is therefore of interest to ascertain if W. pechoricum is an enantiomorph or shows a normal paraplate arrangement. However, like in many other peridinoids, the paraplate pattern is almost bilateral symmetric (Fig. 2), and for this reason it was not possible to determine if the arrangement is normal or mirror reflected. The paracingulum shows practically no helicoidality in most of the specimens examined (best seen in Figs. 5, 6), but in the holotype, the paracingulum is slightly helicoidal (Fig. 2.5), and shows right-handed displacement, i.e., with the right end of the paracingulum more anterior than the left end. In modern dinoflagellates left-handed displacement of the cingulum is much more common than right-handed, the latter being present only in a few species of Protoperidinium Bergh, 1881, according to Taylor (1987). Judging from the holotype with the rare direction of displacement, W. pechoricum could be an enantiomorph. However, the displacement is not clearly seen in other specimens, so the evidence is not conclusive. For the identification of paraplates (Fig. 2), we have assumed a normal coiling direction, i.e., no enantiomorphy.
The presence of a mesophragm is likewise a rare feature in dinoflagellate cysts. In peridinoid cysts, a mesophragm is known in the Early Cretaceous Cepadinium variabile Duxbury, 1983. Also the Cretaceous-Paleocene species Palaeoperidinium pyrophorum (Ehrenberg, 1838) Sarjeant, 1967 can have a middle wall layer (Evitt et al., 1998) although perhaps not homologous with that of W. pechoricum. In W. pechoricum the walls are thin and the mesophragm could only be identified in well-preserved specimens and could only be traced along part of the cyst margin.
Wilsonidium tabulatum (Wilson, 1967) Lentin and Williams, 1976, the type of the genus, and other species of this genus are all of younger (Early to Late Eocene) ages than W. pechoricum. These younger species are larger and more robust than W. pechoricum. A similar relationship exists between the genus Apectodinium and its possible descendent genera Wetzeliella Eisenack, 1938, Dracodinium Gocht, 1955, and Charlesdowniea Lentin and Vozzhennikova, 1989.
Dinoflagellate cysts from Kazakhstan of approximately the same age as Wilsonidium pechoricum were previously tentatively referred to Wilsonidium (as Wilsonidium? sp. A and Wilsonidium? sp. B) by Iakovleva et al. (2001). These cysts are reexamined in the present study and differ markedly from Wilsonidium pechoricum. They show the typical dinoflagellate asymmetry, i.e., with antapical horns of equal length or with the left antapical horn longer. There are two hyaline cyst walls, endophragm and periphragm, which are smoother than the walls of W. pechoricum, and there is no trace of a mesophragm. Furthermore, the cysts are less compressed dorsoventrally than W. pechoricum. A wetzelielloidean dinoflagellate cyst, Wetzeliella? in Edwards (1989, Plate 2, fig. 6) from the uppermost Paleocene in Virginia is superficially similar to Wilsonidium pechoricum, but differs by having the right antapical horn reduced, i.e., a normal asymmetry.
stratigraphic occurrence
Wilsonidium pechoricum has been recorded in a number of sections spanning the uppermost Paleocene-lowermost Eocene in the Northern Hemisphere (Fig. 1). The known occurrence of W. pechoricum n. sp. is shown in Figure 7.
Stratigraphical occurrence in Austria, this study
Wilsonidium pechoricum is recorded in the section at Anthering previously described by Egger (1995) and Egger et al. (2000, 2003). The section exposes alternating grey to black, hemipelagic clays and distal turbidites of the Anthering Formation, which is a part of the Rhenodanubian flysch. The section consists of a succession of outcrops lettered A (top of section) to N (base of section). The section is expanded and closely sampled. It spans most of the calcareous nannoplankton zones NP9 and NP10 and the lower part of NP11 and isotope analyses have shown the position of the CIE (Egger et al., 2000). The lowest occurrence of W. pechoricum is in Outcrop L (sample 2396). This part is referred to the upper part of NP9, but below the CIE (Egger et al., 2000), and the sample therefore belongs in the uppermost Paleocene. The highest occurrence is in Outcrop Ja (sample 2365), also referred to the upper part of NP9 by Egger et al. (2000). This sample is within the upper part of the CIE and is associated with an acme of Apectodinium spp., including A. augustum (Harland, 1979) Lentin and Williams, 1981 (Egger et al., 2000; Crouch et al., 2001), and therefore belongs in the lowermost Eocene. W. pechoricum attains a maximum abundance of 2% in the middle of Outcrop J, coinciding with the peak of the CIE and the maximum abundance (66%) of Apectodinium spp., i.e. at the Paleocene-Eocene boundary.
Stratigraphic occurrence in Kazakhstan, this study
Wilsonidium pechoricum is recorded in a single sample (sample 2264) examined from the Kaurtakapy section, Mangyschlak. The Kaurtakapy section was previously described by Bolle et al. (2000). The sample derives from a 10 cm thick sapropelic clay layer occurring within a thick succession of marl and chalks. The clay layer represents planktonic foraminifera biozone P5b and the IETM, and also marks the base of the Givmra Formation (Bolle et al., 2000). W. pechoricum is common in the sample and occurs in a typical IETM-assemblage, strongly dominated by Apectodinium spp., including Apectodinium cf. augustum.
Stratigraphic occurrence in western Siberia, this study
Wilsonidium pechoricum is recorded from the Ust'-Manya 19 Borehole, which recovered a succession of clays and siliceous clays. Here it occurs only in a sample at 122.25 m. This sample is rich in dinoflagellates, including Apectodinium spp. [A. homomorphum (Deflandre and Cookson, 1955) Lentin and Williams, 1977, A. quinquelatum (Williams and Downie, 1966) Lentin and Williams, 1977, A. hyperacanthum (Cookson and Eisenack, 1965) Lentin and Williams, 1977, A. paniculatum (Costa and Downie, 1976) Lentin and Williams, 1977, and A. augustum]. Apectodinium spp. account for 7% of the total dinoflagelate cysts, and the sample can be correlated to the IETM interval based on the presence of A. augustum. The underlying sample at 127.0 m yielded only a few dinoflagellate cysts, including Apectodinium spp. (without A. augustum), and may belong in the IETM interval or the Upper Paleocene. The overlying sample at 105.0 m includes Dracodinium simile (Eisenack, 1954) Costa and Downie, 1979 and is thus probably age-equivalent to the early part of the NP11 calcareous nannoplankton Zone, according to zonal calibrations in Luterbacher et al. (2004).
Stratigraphic occurrence in the Pechora Depression, this study
Wilsonidium pechoricum is recorded in two samples from the interval 282.5 m to 281.5 m in Borehole No. 228. This is a clayey and silty interval, belonging to the uppermost part of the Kirshorskaya Formation (Iakovleva et al., 2000). The dinoflagellate assemblage is diverse, with more than 60 taxa, and is characterized by abundant (50%) Apectodinium spp., including A. augustum. The two samples can be correlated to the IETM interval based on the presence of A. augustum. Two underlying samples (287.0 m and 286.0 m) include A. homomorphum and A. quinquelatum, but A. augustum is absent, and the samples are probably of Late Paleocene age. Overlying samples at 274.8 m, 271.4 m, and 269.0 m yielded no age-diagnostic dinoflagellate cysts, but may represent the Glaphyrocysta ordinata Interval Biozone (Gor Biozone) of Powell (1992). The first occurrence of Wetzeliella meckelfeldensis Gocht, 1969, at 267.1 m indicates the W. meckelfeldensis Zone of Costa and Downie (1976) and this level may therefore be calibrated to the upper part of the NP 10 nannoplankton Zone, according to Powell (1992) and Luterbacher et al. (2004).
Previous records
Wilsonidium pechoricum was recorded and illustrated (as Rhombodinium sp.) from a sapropel occurring at the Medany outcrop section (western Georgia, approximate coordinates 43°10′N, 42°2′E) and at the Torangly outcrop section (western Kopetdag, Turkmenistan, approximate coordinates 39°8′N, 55°39′E) by Akhmetiev and Zaporozhetz (1996, Plate III, fig. 4). In both sections the sapropel was referred to the upper Thanetian by Akhmetiev and Zaporozhetz (1996) and, according to these authors, contained an abundance of Apectodinium spp., including A. augustum, A. parvum (Alberti, 1961) Lentin and Williams, 1977, A. summissum (Harland, 1979) Lentin and Williams, 1981, A. paniculatum, A. quinquelatum and A. homomorphum. The sapropel can therefore be correlated with the IETM interval. This is in agreement with Stupin and Muzylev (2001) and Muzilev (personal commun.) who referred the sapropels to the uppermost Thanetian PETM-interval based on the calcareous nannoplankton biozone CP8a/CP8b boundary assemblage. The PETM-interval is now placed in the initial Eocene (and renamed as IETM) after the international formalization of the Paleocene-Eocene boundary at the base of the CIE (Aubry and Ouda, 2003).
Wilsonidium pechoricum was recorded and illustrated by Crouch et al. (2003, Plate 7, figs. 1–4) from the Aktumsuk outcrop section (Uzbekistan, coordinates 43°85′N, 57°17′E) as Rhombodinium spp. The section was previously described by Bolle et al. (2000). According to Crouch et al. (2003), Rhombodinium spp. is common in an approximately 1.5 m thick interval spanning the 1 m thick, organic-rich IETM interval and the lowermost part of the overlying chalk. The illustrated specimens of W. pechoricum were all from the IETM interval. The record from the chalk about 30 cm above the IETM interval was not illustrated and the presence of W. pechoricum at this level is therefore uncertain. The section is highly condensed and this sample was assigned to the upper? NP10 or basal NP11 interval. A further uncertainty of this post-IETM record of W. pechoricum is due to possible upwards contamination, which might be caused by bioturbation or reworking from the IETM interval, as the top of the IETM interval is a burrowed surface (Bolle et al., 2000). According to Crouch et al. (2003), various morphotypes of Apectodinium are very common (up to 35%) in the IETM interval at Aktumsuk, although the extreme morphotype A. augustum was not recorded. Samples below and above this interval were barren of dinoflagellates.
It may be concluded that W. pechoricum first occurs in the uppermost Paleocene shortly below the IETM, based on the Anthering Section in Austria. In records from Georgia, Turkmenistan, Uzbekistan, and Kazakhstan, W. pechoricum is first recorded in a sapropel within the IETM level. However, samples were not taken (or were barren) from below this level in these regions. Consequently, the first occurrence of W. pechoricum in this part of the Peri-Tethys region remains questionable. In contrast, in the sections from western Siberia and the Pechora Depression, dinoflagellates were studied in the whole Thanetian interval. Here W. pechoricum was not recorded below the IETM interval.
The last occurrence is close to the top of the IETM in Austria and this seems to be the case too in the Pechora Depression. At Aktumsuk the last occurrence is uncertain and may be younger, as discussed above. In the remaining sections no samples were studied, or the spacing between samples was too wide to yield information about possible occurrences of Wilsonidium pechoricum above the IETM interval.