Limacina sp. 2

Cotton, Laura J., Janssen, Arie W., Pearson, Paul N. & Driel, Rens van, 2017, Pteropoda (Mollusca, Gastropoda, Thecosomata) from the Eocene / Oligocene boundary interval of three cored boreholes in southern coastal Tanzania and their response to the global cooling event, Palaeontologia Electronica 20 (3), pp. 1-21 : 13-15

Limacina sp. 2

Figure 12.1-4

Material examined. One specimen from TDP 12 ( Table 2); 4 poorly preserved and presumably juvenile specimens in pyritic internal mould preservation, from TDP 17 ( Table 3).

Description. Limacinid of very low conical shape with almost flat, slightly raised, or slightly concave apical plane. Width of illustrated specimen 1.20 mm, height 0.86 mm. Whorls 3.75, regularly increasing in diameter. Aperture semicircular, occupying c. 80% or more of total shell height, reaching to far beyond base of preceding whorl. Apertural structures absent or not preserved. The apparently present groove along the apertural margin, as vsible in Figure 13.2-3 , is considered to be caused by damage of the mould. Base perforated by umbilicus of 1/5th to 1/7th of shell diameter.

Discussion. Several limacinids with an almost planorboid shell shape are known from the Eocene-Oligocene interval in Europe, Asia and the USA. Some of these are characterised by having a slightly concave apical plane, or, in other cases, by an irregular development of early whorls. In the present specimens, however, the whorls are in a regular spiral and the apical plane is a bit raised, with the first 1.5 whorls flattened ( Figure 12.3 ).

Very similar is a species from the early Oligocene of Japan, described as Limacina karasawai Ando (2011, p. 248 , figures 3.1-2. This species was said to have three quarters of a whorl more than the Tanzanian specimen illustrated here, but we fail to see that from Ando’s photographs. Also closely similar is Limacina canadaensis Hodgkinson (in Hodgkinson et al., 1992, p. 16, pl. 2, figures 4-6), but its last whorl seems to be relatively lower. This species was collected from downhole contaminated cutting samples and could be anything between early Eocene and early Oligocene. An occurrence of similar age (earliest Oligocene) was described from the North Sea Basin and is also known from contemporaneous rocks (base of Viborg Formation) in Jylland, Denmark, as Limacina mariae Janssen (1989, p. 111 , pl. 4, figures 2-5), but that species always has a concave apical spiral, has a somewhat wider umbilicus and reaches to over 2 mm shell width. Finally, as yet unpublished similar material is available from the Eocene - Oligocene interval in the NE United States and from the eastern part of Germany.

A reliable interpretation of all these forms depends on a larger material for comparisons and therefore the present specimens, apart from the illustrated specimen in poor condition, are left in open nomenclature.

Figures

FIGURE 12. Limacina sp. 2, RGM 777373,; 1: apertural, 2: apical, 3: oblique apical, and 4: umbilical views. Bar equals 100 μm.

FIGURE 13. Bovicornu aff. eocenense Meyer, 1886; 1: RGM 777370, apertural view; 2: RGM 1007784, basal view. Bar equals 100 μm.