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Hystrichodinium pulchrum ssp. globosum
From Fensome et al., 2019:
Hystrichodinium pulchrum ssp. globosum (Deflandre, 1936a, p.183, pl.9, figs.1–2) Lentin and Williams, 1973, p.70. Holotype: Deflandre, 1936a, pl.9, fig.2. Originally Hystrichodinium pulchrum var. globosum, subsequently (and now) Hystrichodinium pulchrum subsp. globosum. Age: Senonian.
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Original description: [Deflandre, 1936a] (translated from French):
The transverse groove, helical and left-handed, divides the theca into two roughly equal parts: the epitheca, which is slightly conical with well-rounded sides, and the hypotheca, which is roughly hemispherical. I saw no trace of sutures, despite some appearances due to membrane folds, and these two parts therefore appear to be made of a single piece. The transverse groove, in some well-preserved individuals, shows fine transverse trabeculae, as seen in many dinoflagellates. These trabeculae unite two rows of puncta that are easily observed in most specimens.
The longitudinal groove, represented by a simple notch on the epitheca, extends over about half of the hypotheca, but, given the frequent folds, it is possible that it is even longer than I indicate, simply to be on the safe side. The entire surface of the theca is bristling with long, hollow, pointed horns, their bases widening to blend into the membrane without any trace of the point of insertion. These horns appear to be distributed rather irregularly, although they are roughly equidistant. They radiate all around the theca, including the edges of the grooves, and they must have been stiff in the living state. However, many individuals show them variously curved, which proves a certain flexibility, for they are rarely broken. Their length generally exceeds half the diameter of the theca and often reaches this diameter. The membrane itself is covered with heavy pits. In some individuals, these pits occupy the center of depressed, rounded polygonal areoles (figs. 7, 8, pl. VIII). The punctuations are arranged regularly in lines parallel to the transverse furrow, and less regularly along meridian lines. Most of the time, the membrane is tinged with a more or less dark brown, but this color is perhaps due to fossilization: some individuals, such as the first one I have depicted, are colorless and excessively transparent.
The dimensions of the typical forms are as follows: length (without horns), 45/55 μ; length with horns, 110/125 μ; width (without horns), 42/48 μ.
Besides these forms, all very similar to each other, I believe I can now distinguish two others, which I will place, at least for the moment, as varieties of the type.
The first, var. globosum n. var. (pl. IX, fig. 1, 2), is distinguished by its significantly smaller dimensions (length = width = approximately 28 to 30 μ; total length, including horns, approximately 70 μ) and by its more globular shape: the epitheca is as rounded as the hypotheca. This form has already been encountered several times (Holotype: AJ 82, from flint S. 32, pebble, Paris).
The second form, var. densispinum n. var. (pl. IX, fig. 3), of which I still only have one broken specimen, would be similar in size to the type (or slightly larger), but its horns are much more numerous and relatively somewhat shorter. They also appear to be arranged along meridian lines, which is made evident by their greater number (Holotype: AH 25, Silex S. 44. pebble, Paris).
Hystrichodinium pulchrum is not rare. I have found it in various flints from Paris and the surrounding area (Le Mée, Seine-et-Marne), from the peat bog on Wimereux beach, in fragments of a tabular flint from the Senonian chalk of Nouvelles (Belgium) which was communicated to me by Mr. Caveux, and finally in Cenomanian flints from Villers-sur-Mer.
The relationships between Hystrichodinium and the actual forms are quite difficult to establish. There are, in fact, only few Dinoflagellates with long horns distributed like this over their entire theca: we can only cite the genus Cladopyxis Stein (= Acanthodinium Kofoid), which seems to include three species (1): Cladopyris brachiolata Stein, Cladopycis caryophullum (Kofoid) Pavillard (Acanthodinium caryophyllum Kof.) and Cladopyxis spinosa (Kof.) Pav. Only the latter has hollow, pointed horns like those of Hystrichodinium. But a fundamental difference immediately separates the two genera: Cladopyxis has a theca made up of plates, and its tabulation - under the name of the genus Acanthodinium - was established by Kofoid (2). Furthermore, in Cladopyxis, the transverse groove divides the theca into two very unequal parts: the epitheca is very notably smaller than the hypotheca. The horns, much fewer in number, arise from the central part of well-defined plates, and there is no horn decorating the edge of the groove as in Hystrichodinium. This characteristic is very important: even if, subsequently, a tabulation were to be discovered in Hystrichodinium, it would be enough to separate the two genera.
The ornamentation of the membrane is also very different: The punctuations (perforations) of Cladopyzis are very irregularly arranged, unlike Hystrichodinium. Finally, the general appearance of the latter, its very clear longitudinal sulcus, its physiognomy in a word, do not suggest any direct connection between the two genera.
As for the position of Hystrichodinium in the general classification of Dinoflagellates, it is most enigmatic; I will return to this a little later, after having described the two following genera, Raphidodinium and Palæohystricophora, which seem to have common characteristics with Hystrichodinium.
Hystrichodinium pulchrum ssp. globosum (Deflandre, 1936a, p.183, pl.9, figs.1–2) Lentin and Williams, 1973, p.70. Holotype: Deflandre, 1936a, pl.9, fig.2. Originally Hystrichodinium pulchrum var. globosum, subsequently (and now) Hystrichodinium pulchrum subsp. globosum. Age: Senonian.
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Original description: [Deflandre, 1936a] (translated from French):
The transverse groove, helical and left-handed, divides the theca into two roughly equal parts: the epitheca, which is slightly conical with well-rounded sides, and the hypotheca, which is roughly hemispherical. I saw no trace of sutures, despite some appearances due to membrane folds, and these two parts therefore appear to be made of a single piece. The transverse groove, in some well-preserved individuals, shows fine transverse trabeculae, as seen in many dinoflagellates. These trabeculae unite two rows of puncta that are easily observed in most specimens.
The longitudinal groove, represented by a simple notch on the epitheca, extends over about half of the hypotheca, but, given the frequent folds, it is possible that it is even longer than I indicate, simply to be on the safe side. The entire surface of the theca is bristling with long, hollow, pointed horns, their bases widening to blend into the membrane without any trace of the point of insertion. These horns appear to be distributed rather irregularly, although they are roughly equidistant. They radiate all around the theca, including the edges of the grooves, and they must have been stiff in the living state. However, many individuals show them variously curved, which proves a certain flexibility, for they are rarely broken. Their length generally exceeds half the diameter of the theca and often reaches this diameter. The membrane itself is covered with heavy pits. In some individuals, these pits occupy the center of depressed, rounded polygonal areoles (figs. 7, 8, pl. VIII). The punctuations are arranged regularly in lines parallel to the transverse furrow, and less regularly along meridian lines. Most of the time, the membrane is tinged with a more or less dark brown, but this color is perhaps due to fossilization: some individuals, such as the first one I have depicted, are colorless and excessively transparent.
The dimensions of the typical forms are as follows: length (without horns), 45/55 μ; length with horns, 110/125 μ; width (without horns), 42/48 μ.
Besides these forms, all very similar to each other, I believe I can now distinguish two others, which I will place, at least for the moment, as varieties of the type.
The first, var. globosum n. var. (pl. IX, fig. 1, 2), is distinguished by its significantly smaller dimensions (length = width = approximately 28 to 30 μ; total length, including horns, approximately 70 μ) and by its more globular shape: the epitheca is as rounded as the hypotheca. This form has already been encountered several times (Holotype: AJ 82, from flint S. 32, pebble, Paris).
The second form, var. densispinum n. var. (pl. IX, fig. 3), of which I still only have one broken specimen, would be similar in size to the type (or slightly larger), but its horns are much more numerous and relatively somewhat shorter. They also appear to be arranged along meridian lines, which is made evident by their greater number (Holotype: AH 25, Silex S. 44. pebble, Paris).
Hystrichodinium pulchrum is not rare. I have found it in various flints from Paris and the surrounding area (Le Mée, Seine-et-Marne), from the peat bog on Wimereux beach, in fragments of a tabular flint from the Senonian chalk of Nouvelles (Belgium) which was communicated to me by Mr. Caveux, and finally in Cenomanian flints from Villers-sur-Mer.
The relationships between Hystrichodinium and the actual forms are quite difficult to establish. There are, in fact, only few Dinoflagellates with long horns distributed like this over their entire theca: we can only cite the genus Cladopyxis Stein (= Acanthodinium Kofoid), which seems to include three species (1): Cladopyris brachiolata Stein, Cladopycis caryophullum (Kofoid) Pavillard (Acanthodinium caryophyllum Kof.) and Cladopyxis spinosa (Kof.) Pav. Only the latter has hollow, pointed horns like those of Hystrichodinium. But a fundamental difference immediately separates the two genera: Cladopyxis has a theca made up of plates, and its tabulation - under the name of the genus Acanthodinium - was established by Kofoid (2). Furthermore, in Cladopyxis, the transverse groove divides the theca into two very unequal parts: the epitheca is very notably smaller than the hypotheca. The horns, much fewer in number, arise from the central part of well-defined plates, and there is no horn decorating the edge of the groove as in Hystrichodinium. This characteristic is very important: even if, subsequently, a tabulation were to be discovered in Hystrichodinium, it would be enough to separate the two genera.
The ornamentation of the membrane is also very different: The punctuations (perforations) of Cladopyzis are very irregularly arranged, unlike Hystrichodinium. Finally, the general appearance of the latter, its very clear longitudinal sulcus, its physiognomy in a word, do not suggest any direct connection between the two genera.
As for the position of Hystrichodinium in the general classification of Dinoflagellates, it is most enigmatic; I will return to this a little later, after having described the two following genera, Raphidodinium and Palæohystricophora, which seem to have common characteristics with Hystrichodinium.