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Operculodinium eirikianum var. crebrum
Operculodinium eirikianum var. crebrum De Schepper and Head, 2008, p.103–104,106, pl.1, figs.1–20.
Holotype: De Schepper and Head, 2008, pl.1, figs.1–4.
Age: Piacenzian.
Original description (De Schepper & Head, 2008):
Operculodinium? eirikianum var. crebrum varietas nov. (Plate 1)
TYPE. Holotype, sample DSDP 610A-17-5, 109-114 cm (160.12 mbsf), slide 1, England Finder reference O21/4; ROMP 57991; Pl. 1, figs 1-4. Age: ca. 3.27Ma, Mammoth Subchron (C2An.2r), early Piacenzian (early Middle Pliocene).
DIAGNOSIS. A new variety of Operculodinium? eirikianum in which the central body wall comprises a thin (<0.3 µm) pedium and thicker (ca. 2.0 µm or more) luxuria, consisting of radiating, non-tabular septa that form a microreticulum.
OCCURRENCE. Recorded only from the Piacenzian (Middle Pliocene) of Hole 610A (Fig. 2), mostly between samples 610A-18-1, 81-86 cm (ca. 3.33 Ma) and 610A-17-2, 108- 104 cm (ca. 3.15 Ma) near and within the Mammoth Subchron (C2An.2r), in calcareous nannofossil zone NN16, and planktonic foraminifer zone PL3-6. A highest abundance of 13% is reached within this interval. However, specimens do occur higher in the hole and are considered in place, with a highest occurrence in sample 610A-15-4, 111-117 cm (ca. 2.82 Ma).
DESCRIPTION. Central body wall comprises thin solid pedium (less than 0.3 µm, only visible as dark line) and much thicker luxuria forming microreticulum of erect, sinuous, undulating, non-tabular muri of ca. 0.2-0.3 µm wide. Luxuria at least 2.0µm thick, with walls up to 3.0µm thick often observed. Muri enclose polygonal lumina of up to 0.7 µm in diameter. Microreticulum appears radially striate in optical section. Solid, granule-bearing processes arise directly from surface of microreticulate layer, with little if any structural modification of microreticulation beneath process bases. Processes arise mostly from circular bases. Precingular archeopyle (by loss of plate 3") is large, with well-defined angles.
DIMENSIONS. Holotype: central body length, 32 µm; central body width, 29µm; wall thickness, 3.0µm; archeopyle length, 20µm; archeopyle maximum width, 14µm; maximum process length, 10µm. Range (minimum, average and maximum measurements are given): central body maximum diameter, 28 (35.9) 42µm; wall thickness, 2.0 (2.4) 3.0µm; average process length, 9 (10.3) 12. Nine specimens measured.
ETYMOLOGY. Latin crebrum, thick, pressed together; with reference to the thick wall of this variety.
REMARKS. The greater thickness of the central body wall alone distinguishes this variety from Operculodinium? eirikianum var. eirikianum (autonym): the luxuria on Operculodinium? eirikianum var. eirikianum is between about 0.5 and 1.0µm (Head 1997), although thicknesses up to 1.5 µm have also been recorded (Head et al. 1989b; this study). In addition, the average process length (9-12 µm) appears slightly longer than for Operculodinium? eirikianum var. eirikianum (5-10µm in Head et al. 1989b; 4-9.5µm in Head 1997). Operculodinium? eirikianum var. crebrum differs from Filisphaera filifera Bujak, 1984 emend. Head, 1994 in possessing processes.
AUTECOLOGY. Operculodinium? eirikianum is generally considered a cold-intolerant species found in middle to high latitudes (Head 1993, 1997). Operculodinium? eirikianum var. crebrum may represent a morphological adaptation to relatively warm and stable climates, as revealed by the low amplitude of oscillations in the oxygen isotope record, following the well-expressed Marine Isotope Stage M2 (Lisiecki & Raymo 2005) around the Mammoth Subchron in the eastern North Atlantic. Multivariate analysis on a detailed dataset from a glacial-interglacial cycle around 3.30Ma (Marine Isotope Stage M2) in Hole 610A suggests that this variety has an affinity for warmer waters (De Schepper 2006), although confirmation from other datasets is needed.
Holotype: De Schepper and Head, 2008, pl.1, figs.1–4.
Age: Piacenzian.
Original description (De Schepper & Head, 2008):
Operculodinium? eirikianum var. crebrum varietas nov. (Plate 1)
TYPE. Holotype, sample DSDP 610A-17-5, 109-114 cm (160.12 mbsf), slide 1, England Finder reference O21/4; ROMP 57991; Pl. 1, figs 1-4. Age: ca. 3.27Ma, Mammoth Subchron (C2An.2r), early Piacenzian (early Middle Pliocene).
DIAGNOSIS. A new variety of Operculodinium? eirikianum in which the central body wall comprises a thin (<0.3 µm) pedium and thicker (ca. 2.0 µm or more) luxuria, consisting of radiating, non-tabular septa that form a microreticulum.
OCCURRENCE. Recorded only from the Piacenzian (Middle Pliocene) of Hole 610A (Fig. 2), mostly between samples 610A-18-1, 81-86 cm (ca. 3.33 Ma) and 610A-17-2, 108- 104 cm (ca. 3.15 Ma) near and within the Mammoth Subchron (C2An.2r), in calcareous nannofossil zone NN16, and planktonic foraminifer zone PL3-6. A highest abundance of 13% is reached within this interval. However, specimens do occur higher in the hole and are considered in place, with a highest occurrence in sample 610A-15-4, 111-117 cm (ca. 2.82 Ma).
DESCRIPTION. Central body wall comprises thin solid pedium (less than 0.3 µm, only visible as dark line) and much thicker luxuria forming microreticulum of erect, sinuous, undulating, non-tabular muri of ca. 0.2-0.3 µm wide. Luxuria at least 2.0µm thick, with walls up to 3.0µm thick often observed. Muri enclose polygonal lumina of up to 0.7 µm in diameter. Microreticulum appears radially striate in optical section. Solid, granule-bearing processes arise directly from surface of microreticulate layer, with little if any structural modification of microreticulation beneath process bases. Processes arise mostly from circular bases. Precingular archeopyle (by loss of plate 3") is large, with well-defined angles.
DIMENSIONS. Holotype: central body length, 32 µm; central body width, 29µm; wall thickness, 3.0µm; archeopyle length, 20µm; archeopyle maximum width, 14µm; maximum process length, 10µm. Range (minimum, average and maximum measurements are given): central body maximum diameter, 28 (35.9) 42µm; wall thickness, 2.0 (2.4) 3.0µm; average process length, 9 (10.3) 12. Nine specimens measured.
ETYMOLOGY. Latin crebrum, thick, pressed together; with reference to the thick wall of this variety.
REMARKS. The greater thickness of the central body wall alone distinguishes this variety from Operculodinium? eirikianum var. eirikianum (autonym): the luxuria on Operculodinium? eirikianum var. eirikianum is between about 0.5 and 1.0µm (Head 1997), although thicknesses up to 1.5 µm have also been recorded (Head et al. 1989b; this study). In addition, the average process length (9-12 µm) appears slightly longer than for Operculodinium? eirikianum var. eirikianum (5-10µm in Head et al. 1989b; 4-9.5µm in Head 1997). Operculodinium? eirikianum var. crebrum differs from Filisphaera filifera Bujak, 1984 emend. Head, 1994 in possessing processes.
AUTECOLOGY. Operculodinium? eirikianum is generally considered a cold-intolerant species found in middle to high latitudes (Head 1993, 1997). Operculodinium? eirikianum var. crebrum may represent a morphological adaptation to relatively warm and stable climates, as revealed by the low amplitude of oscillations in the oxygen isotope record, following the well-expressed Marine Isotope Stage M2 (Lisiecki & Raymo 2005) around the Mammoth Subchron in the eastern North Atlantic. Multivariate analysis on a detailed dataset from a glacial-interglacial cycle around 3.30Ma (Marine Isotope Stage M2) in Hole 610A suggests that this variety has an affinity for warmer waters (De Schepper 2006), although confirmation from other datasets is needed.