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Arvalidinium cristatum
Arvalidinium cristatum, Lucas-Clark, 2006, p. 192, pl.1. figs.7–15; text-figs.4A–B.
Holotype: Lucas-Clark, 2006, pl.1, figs.7–10.
Occurrence: (Lucas-Clark, 2006) Lower Paleocene, lower Ellenton Formation, Savannah River Site, South Carolina.
Age: early Paleocene.
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Original description: [Lucas-Clark, 2006]:
Diagnosis:
Small to medium, pale peridinioid cyst with short, blunt apical horn and two short, unequal antapical horns. Round in overall outline with slight shoulders. Covered with crests and short, blunt spines that are penitabular and intratabular such that tabulation can be discerned. Small 2a archeopyle. Distinguished by crests, shoulders, and small archeopyle.
Description:
Small to medium, proximate peridiniacean, circumcavate or apically cavate in the epicyst and cornucavate in the hypocyst (exact nature of cavation uncertain), dorsoventrally compressed cysts with a short apical horn and one or two short antapical horns. Antapical horns unequal in length; right horn reduced. Endophragm smooth, about equal in thickness to periphragm, apparently not appressed to the periphragm. Endocyst circular to hexagonal in outline. Periphragm thin. Pericyst outline circular to rounded hexagonal, slightly peridinioid; lateral margins convex; antapical margin straight except for the horns. Faint to moderately prominent shoulders formed by a slightly inflated pericoel in the epicystal to apical region. Surface ornamented with irregular crests, spines and spinose crests. Crests often sutural, more frequently penitabular, and also occur irregularly and discontinuously in intratabular regions or cut across sutures. Spines occur along sutures, in penitabular rows, in intratabular regions, on the sides of crests, and along the top of crests. Spines that occur along the tops of crests are usually longer than other spines and are sometimes capitate.
Tabulation indicated by archeopyle, cingulum, and by crests and spines which may lie along sutures or more frequently are penitabular, lining up along either side of an inferred suture. Tabulation relatively clear in some specimens, obscured in others by crests that are not sutural. Tabulation formula 1pr, 4', 3a, 7'', ?c, 5''', 4s, 2'''' (Text-Figure 4).
Archeopyle type I (2a), periarcheopyle rounded hexagonal, eurydeltaform; may be adnate along the posterior margin. Endoarcheopyle not observed.
Cingulum indicated by somewhat discontinuous crests that are sometimes spinose, and by local indentation in the periphragm; offset in the sulcal region.
Sulcus indicated by break in cingulum and depression in the ventral hypocyst, as well as by crests and low spines that suggest the outlines of sulcal plates. Accumulation bodies sometimes present.
Dimensions:
Length, 60–75 µm; width, 55–60 µm (14 specimens measured).
Remarks:
This species fits the description of the genus by Lentin and Vozzhenikova (1990) in that it is similar to species of Chatangiella, but has a more peridinioid outline, reduced shoulders, and sutural features that indicate the tabulation. Tabulation is not as clearly expressed in this species as it is in other species of the genus, and the tabular elements are sometimes penitabular and intratabular, but they are present, in contrast to species of Chatangiella. It is necessary to make a careful analysis of crests and spines to detect tabulation, but it is usually possible to ascertain all plates of a typical peridinioid except in the sulcal region (Text-Figure 4).
Comparison. Arvalidinium cristatum sp. nov. bears a superficial resemblance to Spinidinium macmurdoense, but the latter has spines along sutures as in the genus Spinidinium; moreover, it has more spines closer together, and lacks the intratabular, non-spinose crests and the inflated shoulders of Arvalidinium cristatum sp. nov. Arvalidinium cristatumsp. nov. also resembles Spinidinium rallum, but the spinose crests of the latter are more clearly tabular; the spines are longer, and it lacks the intratabular crests and inflated shoulders of Arvalidinium cristatum sp. nov. Arvalidinium cristatum sp. nov. differs from other species of Arvalidinium in having a more round overall shape and unique surface ornamentation.
Holotype: Lucas-Clark, 2006, pl.1, figs.7–10.
Occurrence: (Lucas-Clark, 2006) Lower Paleocene, lower Ellenton Formation, Savannah River Site, South Carolina.
Age: early Paleocene.
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Original description: [Lucas-Clark, 2006]:
Diagnosis:
Small to medium, pale peridinioid cyst with short, blunt apical horn and two short, unequal antapical horns. Round in overall outline with slight shoulders. Covered with crests and short, blunt spines that are penitabular and intratabular such that tabulation can be discerned. Small 2a archeopyle. Distinguished by crests, shoulders, and small archeopyle.
Description:
Small to medium, proximate peridiniacean, circumcavate or apically cavate in the epicyst and cornucavate in the hypocyst (exact nature of cavation uncertain), dorsoventrally compressed cysts with a short apical horn and one or two short antapical horns. Antapical horns unequal in length; right horn reduced. Endophragm smooth, about equal in thickness to periphragm, apparently not appressed to the periphragm. Endocyst circular to hexagonal in outline. Periphragm thin. Pericyst outline circular to rounded hexagonal, slightly peridinioid; lateral margins convex; antapical margin straight except for the horns. Faint to moderately prominent shoulders formed by a slightly inflated pericoel in the epicystal to apical region. Surface ornamented with irregular crests, spines and spinose crests. Crests often sutural, more frequently penitabular, and also occur irregularly and discontinuously in intratabular regions or cut across sutures. Spines occur along sutures, in penitabular rows, in intratabular regions, on the sides of crests, and along the top of crests. Spines that occur along the tops of crests are usually longer than other spines and are sometimes capitate.
Tabulation indicated by archeopyle, cingulum, and by crests and spines which may lie along sutures or more frequently are penitabular, lining up along either side of an inferred suture. Tabulation relatively clear in some specimens, obscured in others by crests that are not sutural. Tabulation formula 1pr, 4', 3a, 7'', ?c, 5''', 4s, 2'''' (Text-Figure 4).
Archeopyle type I (2a), periarcheopyle rounded hexagonal, eurydeltaform; may be adnate along the posterior margin. Endoarcheopyle not observed.
Cingulum indicated by somewhat discontinuous crests that are sometimes spinose, and by local indentation in the periphragm; offset in the sulcal region.
Sulcus indicated by break in cingulum and depression in the ventral hypocyst, as well as by crests and low spines that suggest the outlines of sulcal plates. Accumulation bodies sometimes present.
Dimensions:
Length, 60–75 µm; width, 55–60 µm (14 specimens measured).
Remarks:
This species fits the description of the genus by Lentin and Vozzhenikova (1990) in that it is similar to species of Chatangiella, but has a more peridinioid outline, reduced shoulders, and sutural features that indicate the tabulation. Tabulation is not as clearly expressed in this species as it is in other species of the genus, and the tabular elements are sometimes penitabular and intratabular, but they are present, in contrast to species of Chatangiella. It is necessary to make a careful analysis of crests and spines to detect tabulation, but it is usually possible to ascertain all plates of a typical peridinioid except in the sulcal region (Text-Figure 4).
Comparison. Arvalidinium cristatum sp. nov. bears a superficial resemblance to Spinidinium macmurdoense, but the latter has spines along sutures as in the genus Spinidinium; moreover, it has more spines closer together, and lacks the intratabular, non-spinose crests and the inflated shoulders of Arvalidinium cristatum sp. nov. Arvalidinium cristatumsp. nov. also resembles Spinidinium rallum, but the spinose crests of the latter are more clearly tabular; the spines are longer, and it lacks the intratabular crests and inflated shoulders of Arvalidinium cristatum sp. nov. Arvalidinium cristatum sp. nov. differs from other species of Arvalidinium in having a more round overall shape and unique surface ornamentation.