Actinocyclus ingens var ovalis

From Frustule

SPECIES Actinocyclus ingens var ovalis
Gersonde (1990)
Taxonomic Information Jr. Synonymy: Actinocyclus ingens var. ovalis Gersonde, 1990, p. 792, pl. 1, fig. 7, pl. 3, figs. 1-3, pl. 5, figs. 4 and 7, pl. 6, figs. 1, 4-5; Gersonde and Burckle, 1990, pl. 5, figs. 4-5; Censarek and Gersonde, 2002, p. 350, p. 1, figs. 6 and 8; Bohaty et al., 2003, p. 16 (CD-ROM), pl. 1, figs. 2-3. Hemidiscus ovalis (Gersonde, 1990) Harwood and Maruyama, 1992, p. 703, pl. 11, fig. 2, pl. 12, figs. 1-3. Morphometry: Species Description: Mini Description: Differs from the nominate variety A. ingens var. ingens by slightly oval to slightly triangular valve outline. Original Name Actinocyclus var. ovalis Gersonde, 1990: Differs from the nominate variety A. ingens var. ingens by slightly oval to slightly triangular valve outline, length 22-52 µm, length/width ratio - 1. 1-1.2. Valve face generally flat.

Additional Remarks

• Remarks by Gersonde (1990):
  

Structure, shape, and arrangement of areolae on valve face and valve margin, and structure and location of the pseudonodulus are the same as in flat A. ingens var. ingens. Girdle bands were not observed. Because of the distinct age range of the described taxon, it was named as a variety.

A. ingens var. ovalis has some affinities to a form described by Ciesielski (1983) from Neogene deposits in the Subantarctic as Hemidiscus karstenü f. 1. However, based on the photographic documentation of H. karstenii f. 1, it can be deduced that this form has larger valve size, a more densely packed areolar pattern of the valve face and an easily observable pseudonodulus, compared to A. ingens var. ovalis. Ciesielski (1983) states that this form is common only in sediments of "Chronozone 5 to upper Chronozone 6", at -6.1-5.3 Ma, and thus is slightly younger than A. ingens var. ovalis. However, it is not clear how Ciesielski dated the range of his form based on the data he published, because no direct correlation with paleomagnetic data was available in his study.

• Remarks by Bohaty et al. (2003):
  

Harwood and Maruyama (1992, p. 703) transferred Actinocyclus ingens var. ovalis to the genus Hemidiscus and elevated the variety "ovalis" to species status. We retain the original designation, however, due to its close affinity with Actinocyclus ingens. In addition, Hemidiscus ovalis Lohman (1938, p. 91-92, pl. 22, fig. 9) maintains priority over Hemidiscus ovalis Harwood and Maruyama (1992). Specimens identified in the present study as Actinocyclus ingens var. ovalis are restricted to morphologies with a strongly oval to broadly rounded lanceolate valve outline (see Pl. 1, Figs. 2-3). Actinocyclus ingens var. ovalis is oval shaped and symmetrical in valve outline and possesses reduced areolation in the central area; these features distinguish it from Hemidiscus karstenii.

Useful References

• Gersonde, R., 1990. Taxonomy and morphostructure of Neogene diatoms from the Southern Ocean, ODP Leg 113. In Barker, P. F., Kennett, J. P., et al., Proceedings of the Ocean Drilling Program, Scientific Results, 113: College Station, TX (Ocean Drilling Program), 791-802.