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Nummus apiculus
Original description: [Riding and Helby, 2001d]:
Nummus apiculus sp. nov. (Figs 21A-P)
Previous Australian usage
M.P.556 Helby.
Nummus sp. 556 - Helby.
Nummus ambitomegasimilis - Morgan.
Description. A species of Nummus ellipsoidal in dorsoventral view. Vesicle normally longer than broad. The vesicle wall is thick and robust, the
extemal surface is smooth and may be sparsely and irregularly microreticulate. Occasionally the wall is also microscabrate. The pylome is located in an apical position and frequently has a thickened rim. A detached plug is occasionally present within the pylome. The thin ventral wall is often entirely absent, however small fragments may be preserved. A thickened rim surrounds the ventral part of the vesicle, which has the thinned vesicle wall; this rim is ovoidal in outline.
Dimensions (μm; n=41): Min. (Mean) Max.
Length: 60 (81) 105
Width:52 (68) 92
Diameter of pylome: 10 (17) 28
The measured specimens are from conventional core samples in wells Undan-1 at 3063.18m, Undan-3 at 3057.00m and Undan-4 at 3135.24m.
Comments. The vesicle wall is frequently folded due to compaction, and extensive longitudinal and/or transverse folds occur, largely on the dorsal surface (Fig. 21). The species has a relatively wide size range and the pylome varies markedly in width (see Fig. 21 and Dimensions, above). The ovoidal area ofthe ventral surface, originally covered with a thin wall, is also variable in area. In some specimens, it occupies the majority of the ventral surface (Fig. 21A) and in others it accounts for around 50% of the venter (Fig. 21C). The sparse, irregular microreticulation may be a preservational artefact.
The thin, often broken or absent, ventral wall of this genus, together with the equatorial rim indicates a possible encrusting habit (Backhouse,
1988, p. 112). It is therefore similarto the Jurassic non-marine acritarch Truncatisphaeridium clevelandense Riding & Duxbury 1993, which is
primarily hemispherical. This species is believed to have had an adherent habit (Riding & Duxbury, 1993). Backhouse (1988, p. 113) noted that
Nummus similis (Cookson & Eisenack 1960) Burger 1980 is common in horizons which are relatively sparse in marine microplankton and speculated that the species may be indicative of marginal marine settings. Similarly, the proportion of Nummus apiculus and miospores increase as the levels of dinoflagellate cysts decrease. Therefore large proportions of Nummus apiculus are interpreted as being indicative of nearshore depositional regimes.
Comparison. The apical position ofthe pylome in this species and the lack of an equatorial fold distinguish Nummus apiculus from the other
validly described members of the genus. The genotype of Nummus is N. monoculatus Morgan 1975 from the Aptian of the Great Australian Basin. This species differs from N. apiculus in having a relatively large pylome, which is located between the apex and the equator, in an 'intercalary' position (Morgan, 1975, p. 163) and also being smaller. Nummus apiculus differs from N. similis (Cookson & Eisenack 1960) Burger 1980 in lacking a prominent equatorial ridge, which may represent a paracingulum. Jain & Garg (in Jain et al.,1984) described N. mallajoharensis from the Late Jurassic Spiti Shale of northem India; this form has a prominent equatorial ridge/paracingulum and appears to be a junior synonym of N. similis.
Nummus pentagonus Backhouse 1988 lacks a pylome and has five dorsal folds in the cell wall. The features which distinguish N. apiculus from
N. parvus Backhouse 1988 are the equatorial constrictions and the relatively small size of the latter (Backhouse, 1988, fig. 33). Sketches of the
four previously published Australian species of this genus were given by Backhouse (1988, fig. 33).
Derivation of name. From the Latin apiculus meaning apex or apical and referring to the anterior or apical position of the pylome.
Holotype and type locality. Fig. 21K, CPC 35451, Undan-4 well, conventional core sample at 3135.24m.
Stratigraphical distribution. Nummus apiculus has been recorded from the uppermost Bathonian to Callovian strata of the Timor Sea region. It
ranges from the upper Wanaea verrucosa Zone (7ciai) to the lower Rigaudella aemula Zone (7aiia) (Foster, this volume; Helby & Partridge, in prep.). The species is particularly prominent (i.e. has intermittent minor acmes) possibly in the Wanaea indotata Zone (7bii) and unequivocally in the Wanaea digitata Zone (7bi) and basal R. aemula Zone (7 aiib).
Nummus apiculus sp. nov. (Figs 21A-P)
Previous Australian usage
M.P.556 Helby.
Nummus sp. 556 - Helby.
Nummus ambitomegasimilis - Morgan.
Description. A species of Nummus ellipsoidal in dorsoventral view. Vesicle normally longer than broad. The vesicle wall is thick and robust, the
extemal surface is smooth and may be sparsely and irregularly microreticulate. Occasionally the wall is also microscabrate. The pylome is located in an apical position and frequently has a thickened rim. A detached plug is occasionally present within the pylome. The thin ventral wall is often entirely absent, however small fragments may be preserved. A thickened rim surrounds the ventral part of the vesicle, which has the thinned vesicle wall; this rim is ovoidal in outline.
Dimensions (μm; n=41): Min. (Mean) Max.
Length: 60 (81) 105
Width:52 (68) 92
Diameter of pylome: 10 (17) 28
The measured specimens are from conventional core samples in wells Undan-1 at 3063.18m, Undan-3 at 3057.00m and Undan-4 at 3135.24m.
Comments. The vesicle wall is frequently folded due to compaction, and extensive longitudinal and/or transverse folds occur, largely on the dorsal surface (Fig. 21). The species has a relatively wide size range and the pylome varies markedly in width (see Fig. 21 and Dimensions, above). The ovoidal area ofthe ventral surface, originally covered with a thin wall, is also variable in area. In some specimens, it occupies the majority of the ventral surface (Fig. 21A) and in others it accounts for around 50% of the venter (Fig. 21C). The sparse, irregular microreticulation may be a preservational artefact.
The thin, often broken or absent, ventral wall of this genus, together with the equatorial rim indicates a possible encrusting habit (Backhouse,
1988, p. 112). It is therefore similarto the Jurassic non-marine acritarch Truncatisphaeridium clevelandense Riding & Duxbury 1993, which is
primarily hemispherical. This species is believed to have had an adherent habit (Riding & Duxbury, 1993). Backhouse (1988, p. 113) noted that
Nummus similis (Cookson & Eisenack 1960) Burger 1980 is common in horizons which are relatively sparse in marine microplankton and speculated that the species may be indicative of marginal marine settings. Similarly, the proportion of Nummus apiculus and miospores increase as the levels of dinoflagellate cysts decrease. Therefore large proportions of Nummus apiculus are interpreted as being indicative of nearshore depositional regimes.
Comparison. The apical position ofthe pylome in this species and the lack of an equatorial fold distinguish Nummus apiculus from the other
validly described members of the genus. The genotype of Nummus is N. monoculatus Morgan 1975 from the Aptian of the Great Australian Basin. This species differs from N. apiculus in having a relatively large pylome, which is located between the apex and the equator, in an 'intercalary' position (Morgan, 1975, p. 163) and also being smaller. Nummus apiculus differs from N. similis (Cookson & Eisenack 1960) Burger 1980 in lacking a prominent equatorial ridge, which may represent a paracingulum. Jain & Garg (in Jain et al.,1984) described N. mallajoharensis from the Late Jurassic Spiti Shale of northem India; this form has a prominent equatorial ridge/paracingulum and appears to be a junior synonym of N. similis.
Nummus pentagonus Backhouse 1988 lacks a pylome and has five dorsal folds in the cell wall. The features which distinguish N. apiculus from
N. parvus Backhouse 1988 are the equatorial constrictions and the relatively small size of the latter (Backhouse, 1988, fig. 33). Sketches of the
four previously published Australian species of this genus were given by Backhouse (1988, fig. 33).
Derivation of name. From the Latin apiculus meaning apex or apical and referring to the anterior or apical position of the pylome.
Holotype and type locality. Fig. 21K, CPC 35451, Undan-4 well, conventional core sample at 3135.24m.
Stratigraphical distribution. Nummus apiculus has been recorded from the uppermost Bathonian to Callovian strata of the Timor Sea region. It
ranges from the upper Wanaea verrucosa Zone (7ciai) to the lower Rigaudella aemula Zone (7aiia) (Foster, this volume; Helby & Partridge, in prep.). The species is particularly prominent (i.e. has intermittent minor acmes) possibly in the Wanaea indotata Zone (7bii) and unequivocally in the Wanaea digitata Zone (7bi) and basal R. aemula Zone (7 aiib).