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Mendicodinium echinatum
Mendicodinium echinatum Riding and Helby, 2001a, p.5,7, figs.4A-L.
Taxonomic junior synonym: Mendicodinium spinosum Helby (name not validly published), according to Riding and Helby (2001a, p.5).
Holotype: Riding and Helby, 2001a, fig.4H.
Age: Toarcian.
Mendicodinium echinatum sp.nov. (Figs 4A-L)
Previous Australian usage:
Mendicodinium spinosum – Helby.
Hemicystodinium sp. – Morgan.
Description: A proximochorate Mendicodinium of intermediate size, subcircular to ellipsoidal in dorsoventral outline. Hypocyst equal in size to, or slightly larger than, the epicyst. The hypocyst is frerquently subangular in the apical area (paraplate 1’’’’), and is often somewhat flattened. The cyst is normally slightly wider than long. The autophragm is moderately thick, scabrate to regulate, bearing up to 150 nontabular simple, short, slender, solid, straight to recurved, distally-pointed pines. In the rarer, regulate forms, the low-relief ornamentation is often present as short, smooth, randomly oriented ridges and scattered grana.
Dimensions (µm, n=31): Min.(Mean)Max.
Length of cyst body (excl. spines): 47(60)75
Width of cyst body (excl. spines): 54(65)78
Length of spines: 3(6)12
The measured specimens are from sidewall core samples in Skua-5 (2646.00m) and Skua-6 (2385.00m and 2391.50m)
Comments: The wide range in the measured cyst body length of this genus is due to the compactional style, which varies markedly. Where the epicyst and the hypocyst are attached in open-lid mode, the measured height of the cyst is exaggerated. In contrast, the epicyst may be compressed into the hypocyst, giving a relatively low cyst length.
Comparison: Mendicodinium echinatum is similar to the early Toarcian Mendicodinium spinosum Bucefalo Palliani et al., 1997 in being proximochorate, bearing solid, distally-pointed, nontabular spines. No other validly described species of Mendicodinium are spine-bearing. Mendicodinium spinosum ranges from 16 µm to 27 µm in cyst body length (Bucefalo Palliani et al., 1997b) and therefore is considerably smaller than M. echinatum. The length of the cyst body in M. echinatum varies from 47 µm to 75 µm, hence the size difference between the two species is mutually exclusive and there appears to be no possibility of an overlap. Furthermore, the autophragm of M. echinatum is of average thickness (c. 1µm) and is regulate to scabrate. The wall of M. spinosum is markedly thicker and, in M. spinosum ssp. Spinosum is psilate. The autophagm of M. spinosum ssp. Perforatum is perforate. Mendicodinium echinatum also has a denser cover of spines than M. spinosum.
Derivation of name: From the Latin echinatus, meaning prickly.
Holotype and type locality: Fig. 4H, CPC 35125 Skua-6 well, sidewall core sample at 2391.50m.
Stratigraphical distribution: See Appendix 1 and Fig. 12.
Taxonomic junior synonym: Mendicodinium spinosum Helby (name not validly published), according to Riding and Helby (2001a, p.5).
Holotype: Riding and Helby, 2001a, fig.4H.
Age: Toarcian.
Mendicodinium echinatum sp.nov. (Figs 4A-L)
Previous Australian usage:
Mendicodinium spinosum – Helby.
Hemicystodinium sp. – Morgan.
Description: A proximochorate Mendicodinium of intermediate size, subcircular to ellipsoidal in dorsoventral outline. Hypocyst equal in size to, or slightly larger than, the epicyst. The hypocyst is frerquently subangular in the apical area (paraplate 1’’’’), and is often somewhat flattened. The cyst is normally slightly wider than long. The autophragm is moderately thick, scabrate to regulate, bearing up to 150 nontabular simple, short, slender, solid, straight to recurved, distally-pointed pines. In the rarer, regulate forms, the low-relief ornamentation is often present as short, smooth, randomly oriented ridges and scattered grana.
Dimensions (µm, n=31): Min.(Mean)Max.
Length of cyst body (excl. spines): 47(60)75
Width of cyst body (excl. spines): 54(65)78
Length of spines: 3(6)12
The measured specimens are from sidewall core samples in Skua-5 (2646.00m) and Skua-6 (2385.00m and 2391.50m)
Comments: The wide range in the measured cyst body length of this genus is due to the compactional style, which varies markedly. Where the epicyst and the hypocyst are attached in open-lid mode, the measured height of the cyst is exaggerated. In contrast, the epicyst may be compressed into the hypocyst, giving a relatively low cyst length.
Comparison: Mendicodinium echinatum is similar to the early Toarcian Mendicodinium spinosum Bucefalo Palliani et al., 1997 in being proximochorate, bearing solid, distally-pointed, nontabular spines. No other validly described species of Mendicodinium are spine-bearing. Mendicodinium spinosum ranges from 16 µm to 27 µm in cyst body length (Bucefalo Palliani et al., 1997b) and therefore is considerably smaller than M. echinatum. The length of the cyst body in M. echinatum varies from 47 µm to 75 µm, hence the size difference between the two species is mutually exclusive and there appears to be no possibility of an overlap. Furthermore, the autophragm of M. echinatum is of average thickness (c. 1µm) and is regulate to scabrate. The wall of M. spinosum is markedly thicker and, in M. spinosum ssp. Spinosum is psilate. The autophagm of M. spinosum ssp. Perforatum is perforate. Mendicodinium echinatum also has a denser cover of spines than M. spinosum.
Derivation of name: From the Latin echinatus, meaning prickly.
Holotype and type locality: Fig. 4H, CPC 35125 Skua-6 well, sidewall core sample at 2391.50m.
Stratigraphical distribution: See Appendix 1 and Fig. 12.