Back
Geonettia waltonensis
Geonettia waltonensis Head, 2000, p.819,821-825, fig.2; fig.3 (part), fig.4 (part), fig.6, nos.1-2; fig.7, nos.1-4; fig.8, nos.1-4; fig.9,nos.1-8; fig.10, nos.1-2.
Holotype: Head, 2000, fig.6, nos.1-2; fig.7, nos.1-3.
Age: late late Pliocene.
Original description (Head, 2000):
Diagnosis
A species of Geonettia characterized by a thin, smooth wall, a single unsculptured ovoidal preapical plate, a narrow first apical plate (1′) whose contact with 4′ is concave and whose contact is broadest with plates 1″ and 4′, and a posterior sulcal plate (ps) that is smaller than either fundital (1p or 1'′) and whose margins with 6' and 1p are approximately parallel. The posterior margin of ps is dithigmate geniculate, having a longer contact with 1'′ than 5'.
Description
Cysts proximate and spherical to subspherical, having a thin (less than 0.3 μm) wall, smooth surface, and no visible stratification under light microscopy. Tabulation expressed only by pre-formed lines of weakness, these being visible as pale narrow lines on undehisced specimens, which control the discrete separation of plates in dehisced specimens. These lines of weakness are incompletely developed on the sulcus but are otherwise fully developed on both epicyst and hypocyst. They facilitate an almost complete dissociation of the cyst into constituent plates, as follows: 1pr, 1′–4′, 1″–6″, 1c–6c, 2'–6', ps, 1p, 1'′, and a single plate (spc) representing the remaining sulcal plates (presumably: as, ras, rs, ls, and 1') from which 1' occasionally shows some separation. These plates form a holocystal archeopyle, with no apparent preferential order of plate loss.
The apical pore complex is represented by a single ovoidal preapical plate (pr) which is smooth and has no apparent markings. The first apical plate (1′) is insert, narrowly contacting the preapical plate. It also narrowly contacts the sulcal series. A characteristic feature of the first apical plate (1′) is its broad contacts with 4′ and 1″ and particularly its concave and slightly angular contact with 4′. Apical plates 2′, 3′, and 4′ are of approximately equal size, and 2′ and 4′ make the longest contact with the preapical plate.
Of the precingular series, 6″ is notably the smallest, the others being of somewhat similar size. Plates 3″ and 6″ have dithigmate geniculate anterior margins and are relatively symmetrical, whereas 5″ is dithigmate geniculate and strongly asymmetrical owing to a long contact with 4′ and short contact with 3′. Plates 1″, 2″ and 4″ have rectilinear monothigmate anterior margins, and 1″ is triangular owing to broad, strongly converging margins that contact 1′ and 2″. Plate 3″ is approximately mid-dorsal in position. The posterior (adcingular), dorsally-directed angles of plates 1″, 2″, 4″, 5″ and 6″ are drawn into minute acute projections, the corresponding ventrally-directed angles of these plates being minutely rounded. Both posterior angles in plate 3″ are minutely rounded. The adcingular margins of precingular plates are generally straight, but subtly expressed obtuse points may occur at the contact with boundaries between cingular plates.
The cingulum has a width of about 4.5–5.5 μm and is laevorotatory, with about one cingular width of displacement. The cingular plates, of which there are 6, have a rectangular shape that may be modified slightly in two ways: 1) plate boundaries separating 1c/2c, 3c/4c and 4c/5c and 5c/6c are weakly reverse-sigmoidal, whereas that separating 2c/3c is uncertain but may be nearly straight (see Fig. 8); and 2) anterior and posterior margins may show subtly expressed obtuse points where they contact plate junctions of the precingular and postcingular series, although this feature is often not visible.
The sulcus separates into two units, the posterior sulcal plate (ps) and the sulcal plate complex (spc). The sulcal plate complex consists of the anterior sulcal plate (as), right accessory sulcal plate (ras), right sulcal plate (rs), left sulcal plate (ls), and first postcingular plate (1'). These plates are usually completely fused, but the first postcingular plate (1') was seen on a few specimens to be partially separated, revealing a narrow, anteriorly tapering, subrectangular plate that just touches the first cingular plate (1' and 1c apparently share a triple junction in the one specimen where the anterior margin of 1' was clearly discernible, Fig. 9.5, 9.6, and see Fig. 2 for reconstruction). An angular notch on the posterior margin of the sulcal plate complex, always to the right of center, delimits the shared contact of the left and right sulcal plates, and corresponds to a projection on the contacting margin of the posterior sulcal plate (which fully detaches). The left and right sulcal plates are occasionally partly delimited by lines of weakness. The posterior sulcal plate (ps), which anteriorly abuts the sulcal plate complex, is the same width as the ls and rs combined; its lateral margins (with 1p and 6') run parallel. Its posterior margin is geniculate and dithigmate, contacting 1'′ on the longer side and 5' on the shorter side: it is the smallest individual plate routinely released from the hypocyst during dehiscence. The sulcus is posteriorly inclined to the right on the hypocyst.
There are five additional postcingular plates of which 2' and 6' are relatively small, 6' being approximately triangular. Plates 3', 4', and 5' are larger. Plate 4' has a monothigmate rectilinear posterior margin and is middorsally positioned. Plates 3' and 5' are similar in size and both have dithigmate geniculate posterior margins with characteristic arcuate indentations at their posterioventral corners where they contact plates lp and ps respectively. Plate 5' always has a smaller indentation than 3'. As with the precingular series, plates of the postcingular series have dorsally-directed angles drawn into minute acute projections along their anterior (adcingular) margins, with the corresponding ventrally-directed angles of these plates being minutely rounded. This feature occurs on 2', 3', 5' and 6', with both anterior angles in plate 4' being minutely rounded. The adcingular margins of postcingular plates are generally straight, but subtly expressed obtuse points may occur at the contact with boundaries between cingular plates.
There are two fundital plates, 1p and 1'′. The posterior intercalary (1p) is relatively small and comparable in size to 2' and 6'. The antapical plate (1'′) is quinqueform and larger than 1p.
Etymology
Named after Walton-on-the-Naze, Essex, eastern England, from where an important specimen (Head, 1998c, fig. 4d; Fig. 9.1–9.3) gave critical insights into the ventral tabulation of this species.
Type
Holotype, Figures 6.1, 6.2, 7.1–7.3. DSDP Hole 603C, New Jersey lower continental rise. Sample 603C-8-5, 100–102 cm, slide 1, England Finder reference R37/3. ROM 53654.
Other material examined
DSDP samples, 603C-7-2, 85–87 cm and 603C-8-5, 100–102 cm (Site 603 report, in van Hinte et al., 1987); Rockhall Wood samples CCS1–CCS4 and NQ2–NQ4 (Head, 1997); and Walton-on-the-Naze samples WC2 and WC4 (Head, 1998c).
Measurements
Holotype: cyst length 50 μm; cyst equatorial diameter 51 μm; width of cingulum 5.0–5.5 μm. Range: cyst length 41(46.9)57 μm; cyst equatorial diameter 41(48.4)56 μm; width of cingulum 4.5–5.5 μm. Ten specimens were measured.
Comparison
Geonettia clineae, the type species, differs from G. waltonensis in having a fibroreticulate luxuria, a more complexly structured apical pore complex, the presence of one or two Q plates, and in the geometries of several plates. In particular, the first apical plate is shorter, wider, and omegaform, and cingular plates have generally less straight and less parallel anterior and posterior margins. On the hypocyst, the posterior sulcal plate is much larger, widening posteriorly so that it is similar in size to the two funditals 1p and 1'′ (see Table 1).
Geonettia sp. (Figs. 11, 12; Table 1) from the upper Miocene differs from both G. waltonensis and G. clineae in having a granular surface; although its first apical plate, which is insert, appears closer in shape to G. waltonensis than to G. clineae. This species has other similarities with G. waltonensis (Table 1) but requires further study to allow detailed comparison.
Capisocysta lata has a hypocystal tabulation virtually identical to that of G. waltonensis, and the cyst may have a smooth wall. However, plates in Capisocysta lata only dissociate on the hypocyst (Table 1).
Type stratum and locality
Upper upper Pliocene, Chron C2r (=2.58–1.95 Ma), 66.42 m below sea floor, DSDP Hole 603C, New Jersey lower continental rise.
Occurrence
In DSDP Hole 603C, restricted to samples 603C-7-2, 85–87 cm and 603C-8-5, 100–102 cm, which occur in the upper upper Pliocene Chron C2r. The estimated age of Sample 603C-7-2, 85–87 cm is 2.1 Ma, and that of Sample 603C-8-5, 100–102 cm is 2.3 Ma. Geonettia waltonensis comprises less than 0.5 percent of the total dinoflagellates in these two samples. At Rockhall Wood in eastern England, it occurs in all but one of 8 samples taken from the Ramsholt and Sudbourne members of the Coralline Crag Formation, but does not exceed 1 percent of the total dinoflagellates in these samples (Head, 1997). The Ramsholt Member is middle lower or upper lower Pliocene, being tentatively dated at 3.8–3.6 Ma using nannofossils and no older than 4.6 Ma based on foraminifers (see discussions in Head, 1997; 1998b). The Sudbourne Member is probably lower upper Pliocene (Head, 1997; 1998b). At Walton-on-the-Naze, Geonettia waltonensis occurs in one of three samples analyzed from the basal horizon of the Walton Crag and in a sample from 2 m above the base. It comprises less than 1 percent of the total dinoflagellates in each of these two samples (Head, 1998c). The Walton Crag is lower upper Pliocene, between 3.0 Ma and a little older than 2.6 Ma, and younger than the Coralline Crag Formation (Head, 1998c). In Belgium, Geonettia waltonensis is known from a single specimen reported in the Lillo Formation (middle or upper Pliocene) of the Kalmthout Borehole (as Geonettia? sp. Head 1997 in Louwye and Laga, 1998).
Stratigraphic range
Lower Pliocene of eastern England (tentatively dated at 3.8–3.6 Ma, and no older than 4.6 Ma) through upper upper Pliocene (ca. 2.1 Ma, just below the Olduvai subchron) in DSDP Hole 603C, New Jersey lower continental rise, western North Atlantic.
Autecology
Precise interpretation of the ecology of Geonettia waltonensis is hindered by its scarcity in the samples examined. Occurring always with a frequency of less than 1 percent, Geonettia waltonensis might be near the limits of its environmental tolerance, particularly given that some pyrodinioideans form blooms in favorable circumstances.
Notwithstanding these limitations, the pattern of occurrence of Geonettia waltonensis in eastern England suggests an intolerance of cold, inequable conditions. It inhabited the North Sea when conditions were warm temperate (Coralline Crag) to mild temperate (Walton Crag), and its apparent absence from younger Pliocene deposits in eastern England may be in response to cooling at 2.54 Ma and possible increased seasonality thereafter.
DSDP Site 603 is oceanic in setting but contains abundant neritic dinoflagellates transported from the shelf. Specimens of Geonettia waltonensis may have a similar neritic origin. Geonettia waltonensis co-occurs in sample 603C-7-2, 85–87 cm with the temperate to cool-water taxa Bitectatodinium tepikiense (5 percent), Filisphaera spp. (12 percent), and Habibacysta tectata (4 percent); and the warm-water taxa Lingulodinium machaerophorum (5 percent), Polysphaeridium zoharyi (2 percent) and Tectatodinium pellitum (1 percent). In sample 603C-8-5, 100–102 cm, Geonettia waltonensis occurs with the temperate to cool-water taxa Bitectatodinium tepikiense (4 percent), Filisphaera spp. (12 percent), and Habibacysta tectata (0.5 percent); and the warm-water taxa Lingulodinium machaerophorum (1 percent), Polysphaeridium zoharyi (12 percent) and Tectatodinium pellitum (3 percent). On face value, these associations imply temperate climatic conditions, although it is not certain that the dinoflagellates all come from the same climatic zone: they may well have been mixed by long distance transport to Site 603, thereby complicating their ecological interpretation.
During a palynological study of Pliocene carbonate periplatform deposits from the Bahamas (Head and Westphal, 1999), specimens of Geonettia waltonensis were looked for. None were found, despite abundance of the other pyrodinioideans, Polysphaeridium zoharyi and Capisocysta lata. This absence allows conjecture that Geonettia waltonensis is not primarily a tropical/subtropical species, at least of carbonate periplatform environments.
In summary, Geonettia waltonensis occurs in temperate, neritic environments and seems to be intolerant of cold, perhaps inequable conditions. It is possibly a temperate rather than a tropical/subtropical species, but more records are needed to confirm this assessment. The evidence nevertheless seems consistent with the general observation that pyrodinioideans are warm-water dinoflagellates (Fensome et al., 1993; Head, 1998a).
Discussion
Regarding the morphology of Geonettia waltonensis, the presence of acute projections on the adcingular, dorsally-directed angles of plates in both precingular and postcingular series follows the expected imbrication style for gonyaulacaleans (Fensome et al., 1993, 1996; de Verteuil and Norris, 1996a; Head, 1998a), with plates 3″ and 4' lacking these projections and so identified as keystones. An analogous situation occurs within the cingular series, the sutures that separate these plates being reverse-sigmoidal, except possibly the 2c/3c boundary which seems to be nearly straight on at least some specimens (e.g., Fig. 8). This reverse-sigmoidal configuration presumably corresponds in some way to plate overlap on the cingular series.
The generic assignment of Geonettia waltonensis requires discussion because this species differs from the type, Geonettia clineae, in several details of epi- and hypotabulation (see Comparative Morphology of Geonettia waltonensis). In particular, Q plates (one or two) described for Geonettia clineae are not seen either in Geonettia waltonensis or in Geonettia sp. These Q plates are notable in Geonettia clineae because they are otherwise more or less restricted to the sexiform dinoflagellates, Suborder Gonyaulacineae (de Verteuil and Norris, 1996a).
On the other hand, the hypotabulation in Geonettia waltonensis is almost identical to that of Capisocysta lata, raising the question as to whether Geonettia waltonensis should be placed in Capisocysta. However, the archeopyle in Capisocysta is restricted to the hypocyst, and its epitabulation is not yet known. Furthermore, the hypotabulation of the type, Capisocysta wallii, is not known in detail (see Head, 1998a). Hence it would be premature at this stage to assign Geonettia waltonensis to Capisocysta solely because its hypotabulation is similar to that of Capisocysta lata.
Holotype: Head, 2000, fig.6, nos.1-2; fig.7, nos.1-3.
Age: late late Pliocene.
Original description (Head, 2000):
Diagnosis
A species of Geonettia characterized by a thin, smooth wall, a single unsculptured ovoidal preapical plate, a narrow first apical plate (1′) whose contact with 4′ is concave and whose contact is broadest with plates 1″ and 4′, and a posterior sulcal plate (ps) that is smaller than either fundital (1p or 1'′) and whose margins with 6' and 1p are approximately parallel. The posterior margin of ps is dithigmate geniculate, having a longer contact with 1'′ than 5'.
Description
Cysts proximate and spherical to subspherical, having a thin (less than 0.3 μm) wall, smooth surface, and no visible stratification under light microscopy. Tabulation expressed only by pre-formed lines of weakness, these being visible as pale narrow lines on undehisced specimens, which control the discrete separation of plates in dehisced specimens. These lines of weakness are incompletely developed on the sulcus but are otherwise fully developed on both epicyst and hypocyst. They facilitate an almost complete dissociation of the cyst into constituent plates, as follows: 1pr, 1′–4′, 1″–6″, 1c–6c, 2'–6', ps, 1p, 1'′, and a single plate (spc) representing the remaining sulcal plates (presumably: as, ras, rs, ls, and 1') from which 1' occasionally shows some separation. These plates form a holocystal archeopyle, with no apparent preferential order of plate loss.
The apical pore complex is represented by a single ovoidal preapical plate (pr) which is smooth and has no apparent markings. The first apical plate (1′) is insert, narrowly contacting the preapical plate. It also narrowly contacts the sulcal series. A characteristic feature of the first apical plate (1′) is its broad contacts with 4′ and 1″ and particularly its concave and slightly angular contact with 4′. Apical plates 2′, 3′, and 4′ are of approximately equal size, and 2′ and 4′ make the longest contact with the preapical plate.
Of the precingular series, 6″ is notably the smallest, the others being of somewhat similar size. Plates 3″ and 6″ have dithigmate geniculate anterior margins and are relatively symmetrical, whereas 5″ is dithigmate geniculate and strongly asymmetrical owing to a long contact with 4′ and short contact with 3′. Plates 1″, 2″ and 4″ have rectilinear monothigmate anterior margins, and 1″ is triangular owing to broad, strongly converging margins that contact 1′ and 2″. Plate 3″ is approximately mid-dorsal in position. The posterior (adcingular), dorsally-directed angles of plates 1″, 2″, 4″, 5″ and 6″ are drawn into minute acute projections, the corresponding ventrally-directed angles of these plates being minutely rounded. Both posterior angles in plate 3″ are minutely rounded. The adcingular margins of precingular plates are generally straight, but subtly expressed obtuse points may occur at the contact with boundaries between cingular plates.
The cingulum has a width of about 4.5–5.5 μm and is laevorotatory, with about one cingular width of displacement. The cingular plates, of which there are 6, have a rectangular shape that may be modified slightly in two ways: 1) plate boundaries separating 1c/2c, 3c/4c and 4c/5c and 5c/6c are weakly reverse-sigmoidal, whereas that separating 2c/3c is uncertain but may be nearly straight (see Fig. 8); and 2) anterior and posterior margins may show subtly expressed obtuse points where they contact plate junctions of the precingular and postcingular series, although this feature is often not visible.
The sulcus separates into two units, the posterior sulcal plate (ps) and the sulcal plate complex (spc). The sulcal plate complex consists of the anterior sulcal plate (as), right accessory sulcal plate (ras), right sulcal plate (rs), left sulcal plate (ls), and first postcingular plate (1'). These plates are usually completely fused, but the first postcingular plate (1') was seen on a few specimens to be partially separated, revealing a narrow, anteriorly tapering, subrectangular plate that just touches the first cingular plate (1' and 1c apparently share a triple junction in the one specimen where the anterior margin of 1' was clearly discernible, Fig. 9.5, 9.6, and see Fig. 2 for reconstruction). An angular notch on the posterior margin of the sulcal plate complex, always to the right of center, delimits the shared contact of the left and right sulcal plates, and corresponds to a projection on the contacting margin of the posterior sulcal plate (which fully detaches). The left and right sulcal plates are occasionally partly delimited by lines of weakness. The posterior sulcal plate (ps), which anteriorly abuts the sulcal plate complex, is the same width as the ls and rs combined; its lateral margins (with 1p and 6') run parallel. Its posterior margin is geniculate and dithigmate, contacting 1'′ on the longer side and 5' on the shorter side: it is the smallest individual plate routinely released from the hypocyst during dehiscence. The sulcus is posteriorly inclined to the right on the hypocyst.
There are five additional postcingular plates of which 2' and 6' are relatively small, 6' being approximately triangular. Plates 3', 4', and 5' are larger. Plate 4' has a monothigmate rectilinear posterior margin and is middorsally positioned. Plates 3' and 5' are similar in size and both have dithigmate geniculate posterior margins with characteristic arcuate indentations at their posterioventral corners where they contact plates lp and ps respectively. Plate 5' always has a smaller indentation than 3'. As with the precingular series, plates of the postcingular series have dorsally-directed angles drawn into minute acute projections along their anterior (adcingular) margins, with the corresponding ventrally-directed angles of these plates being minutely rounded. This feature occurs on 2', 3', 5' and 6', with both anterior angles in plate 4' being minutely rounded. The adcingular margins of postcingular plates are generally straight, but subtly expressed obtuse points may occur at the contact with boundaries between cingular plates.
There are two fundital plates, 1p and 1'′. The posterior intercalary (1p) is relatively small and comparable in size to 2' and 6'. The antapical plate (1'′) is quinqueform and larger than 1p.
Etymology
Named after Walton-on-the-Naze, Essex, eastern England, from where an important specimen (Head, 1998c, fig. 4d; Fig. 9.1–9.3) gave critical insights into the ventral tabulation of this species.
Type
Holotype, Figures 6.1, 6.2, 7.1–7.3. DSDP Hole 603C, New Jersey lower continental rise. Sample 603C-8-5, 100–102 cm, slide 1, England Finder reference R37/3. ROM 53654.
Other material examined
DSDP samples, 603C-7-2, 85–87 cm and 603C-8-5, 100–102 cm (Site 603 report, in van Hinte et al., 1987); Rockhall Wood samples CCS1–CCS4 and NQ2–NQ4 (Head, 1997); and Walton-on-the-Naze samples WC2 and WC4 (Head, 1998c).
Measurements
Holotype: cyst length 50 μm; cyst equatorial diameter 51 μm; width of cingulum 5.0–5.5 μm. Range: cyst length 41(46.9)57 μm; cyst equatorial diameter 41(48.4)56 μm; width of cingulum 4.5–5.5 μm. Ten specimens were measured.
Comparison
Geonettia clineae, the type species, differs from G. waltonensis in having a fibroreticulate luxuria, a more complexly structured apical pore complex, the presence of one or two Q plates, and in the geometries of several plates. In particular, the first apical plate is shorter, wider, and omegaform, and cingular plates have generally less straight and less parallel anterior and posterior margins. On the hypocyst, the posterior sulcal plate is much larger, widening posteriorly so that it is similar in size to the two funditals 1p and 1'′ (see Table 1).
Geonettia sp. (Figs. 11, 12; Table 1) from the upper Miocene differs from both G. waltonensis and G. clineae in having a granular surface; although its first apical plate, which is insert, appears closer in shape to G. waltonensis than to G. clineae. This species has other similarities with G. waltonensis (Table 1) but requires further study to allow detailed comparison.
Capisocysta lata has a hypocystal tabulation virtually identical to that of G. waltonensis, and the cyst may have a smooth wall. However, plates in Capisocysta lata only dissociate on the hypocyst (Table 1).
Type stratum and locality
Upper upper Pliocene, Chron C2r (=2.58–1.95 Ma), 66.42 m below sea floor, DSDP Hole 603C, New Jersey lower continental rise.
Occurrence
In DSDP Hole 603C, restricted to samples 603C-7-2, 85–87 cm and 603C-8-5, 100–102 cm, which occur in the upper upper Pliocene Chron C2r. The estimated age of Sample 603C-7-2, 85–87 cm is 2.1 Ma, and that of Sample 603C-8-5, 100–102 cm is 2.3 Ma. Geonettia waltonensis comprises less than 0.5 percent of the total dinoflagellates in these two samples. At Rockhall Wood in eastern England, it occurs in all but one of 8 samples taken from the Ramsholt and Sudbourne members of the Coralline Crag Formation, but does not exceed 1 percent of the total dinoflagellates in these samples (Head, 1997). The Ramsholt Member is middle lower or upper lower Pliocene, being tentatively dated at 3.8–3.6 Ma using nannofossils and no older than 4.6 Ma based on foraminifers (see discussions in Head, 1997; 1998b). The Sudbourne Member is probably lower upper Pliocene (Head, 1997; 1998b). At Walton-on-the-Naze, Geonettia waltonensis occurs in one of three samples analyzed from the basal horizon of the Walton Crag and in a sample from 2 m above the base. It comprises less than 1 percent of the total dinoflagellates in each of these two samples (Head, 1998c). The Walton Crag is lower upper Pliocene, between 3.0 Ma and a little older than 2.6 Ma, and younger than the Coralline Crag Formation (Head, 1998c). In Belgium, Geonettia waltonensis is known from a single specimen reported in the Lillo Formation (middle or upper Pliocene) of the Kalmthout Borehole (as Geonettia? sp. Head 1997 in Louwye and Laga, 1998).
Stratigraphic range
Lower Pliocene of eastern England (tentatively dated at 3.8–3.6 Ma, and no older than 4.6 Ma) through upper upper Pliocene (ca. 2.1 Ma, just below the Olduvai subchron) in DSDP Hole 603C, New Jersey lower continental rise, western North Atlantic.
Autecology
Precise interpretation of the ecology of Geonettia waltonensis is hindered by its scarcity in the samples examined. Occurring always with a frequency of less than 1 percent, Geonettia waltonensis might be near the limits of its environmental tolerance, particularly given that some pyrodinioideans form blooms in favorable circumstances.
Notwithstanding these limitations, the pattern of occurrence of Geonettia waltonensis in eastern England suggests an intolerance of cold, inequable conditions. It inhabited the North Sea when conditions were warm temperate (Coralline Crag) to mild temperate (Walton Crag), and its apparent absence from younger Pliocene deposits in eastern England may be in response to cooling at 2.54 Ma and possible increased seasonality thereafter.
DSDP Site 603 is oceanic in setting but contains abundant neritic dinoflagellates transported from the shelf. Specimens of Geonettia waltonensis may have a similar neritic origin. Geonettia waltonensis co-occurs in sample 603C-7-2, 85–87 cm with the temperate to cool-water taxa Bitectatodinium tepikiense (5 percent), Filisphaera spp. (12 percent), and Habibacysta tectata (4 percent); and the warm-water taxa Lingulodinium machaerophorum (5 percent), Polysphaeridium zoharyi (2 percent) and Tectatodinium pellitum (1 percent). In sample 603C-8-5, 100–102 cm, Geonettia waltonensis occurs with the temperate to cool-water taxa Bitectatodinium tepikiense (4 percent), Filisphaera spp. (12 percent), and Habibacysta tectata (0.5 percent); and the warm-water taxa Lingulodinium machaerophorum (1 percent), Polysphaeridium zoharyi (12 percent) and Tectatodinium pellitum (3 percent). On face value, these associations imply temperate climatic conditions, although it is not certain that the dinoflagellates all come from the same climatic zone: they may well have been mixed by long distance transport to Site 603, thereby complicating their ecological interpretation.
During a palynological study of Pliocene carbonate periplatform deposits from the Bahamas (Head and Westphal, 1999), specimens of Geonettia waltonensis were looked for. None were found, despite abundance of the other pyrodinioideans, Polysphaeridium zoharyi and Capisocysta lata. This absence allows conjecture that Geonettia waltonensis is not primarily a tropical/subtropical species, at least of carbonate periplatform environments.
In summary, Geonettia waltonensis occurs in temperate, neritic environments and seems to be intolerant of cold, perhaps inequable conditions. It is possibly a temperate rather than a tropical/subtropical species, but more records are needed to confirm this assessment. The evidence nevertheless seems consistent with the general observation that pyrodinioideans are warm-water dinoflagellates (Fensome et al., 1993; Head, 1998a).
Discussion
Regarding the morphology of Geonettia waltonensis, the presence of acute projections on the adcingular, dorsally-directed angles of plates in both precingular and postcingular series follows the expected imbrication style for gonyaulacaleans (Fensome et al., 1993, 1996; de Verteuil and Norris, 1996a; Head, 1998a), with plates 3″ and 4' lacking these projections and so identified as keystones. An analogous situation occurs within the cingular series, the sutures that separate these plates being reverse-sigmoidal, except possibly the 2c/3c boundary which seems to be nearly straight on at least some specimens (e.g., Fig. 8). This reverse-sigmoidal configuration presumably corresponds in some way to plate overlap on the cingular series.
The generic assignment of Geonettia waltonensis requires discussion because this species differs from the type, Geonettia clineae, in several details of epi- and hypotabulation (see Comparative Morphology of Geonettia waltonensis). In particular, Q plates (one or two) described for Geonettia clineae are not seen either in Geonettia waltonensis or in Geonettia sp. These Q plates are notable in Geonettia clineae because they are otherwise more or less restricted to the sexiform dinoflagellates, Suborder Gonyaulacineae (de Verteuil and Norris, 1996a).
On the other hand, the hypotabulation in Geonettia waltonensis is almost identical to that of Capisocysta lata, raising the question as to whether Geonettia waltonensis should be placed in Capisocysta. However, the archeopyle in Capisocysta is restricted to the hypocyst, and its epitabulation is not yet known. Furthermore, the hypotabulation of the type, Capisocysta wallii, is not known in detail (see Head, 1998a). Hence it would be premature at this stage to assign Geonettia waltonensis to Capisocysta solely because its hypotabulation is similar to that of Capisocysta lata.