Limacina robusta ( Eames, 1952 )

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 : 7-8

Limacina robusta ( Eames, 1952)

Figures 5 , 6

*1952 Aplexa robusta Eames , p. 152, pl. 6, figure 149.

Type material. Holotype ( Figure 6 ) in the Natural History Museum , London, NHMUK BM 68457 , presented by the Burma Oil Co. Ltd., March 1950; Eames (1952) furthermore recorded 19 paratypes from the type locality.

Type locality. Rahki Nala ( Pakistan, western Punjab), 255’ above base, local zone 9. Lower Chocolate Clays; according to Afzal et al. (2009, p. 20) nowadays indicated as Kirthar Formation of late Lutetian - Priabonian age .

Material examined. TDP 11 (1 specimen), TDP 12 (1 specimen), TDP 17 (16 specimens, all from the Priabonian), see Tables 1-3 for specification.

Description. Most available specimens are juveniles and most in pyritic internal mould preservation. Largest and best preserved specimen ( Figure 5 ) higher than wide (H = 0.94, W = 0.72 mm) with four convex whorls rapidly increasing in diameter. Aperture large, oval, occupying more than half shell height. Base of shell regularly rounded with narrow umbilicus.

Discussion. Most of the available specimens are poorly preserved and juvenile, but the illustrated Tanzanian specimen ( Figure 5 ) has a shell height of 0.94 mm, whereas the holotype of Limacina robusta has a shell height of 0.75 mm. Some of the smaller specimens have a somewhat wider apical angle than the illustrated specimen. In spite of these small differences the Tanzanian specimens are thought to represent the same species as the Pakistanian L. robusta , which has a comparable age and was described from a pre-eastern-Paratethys locality under influence of the Indian Ocean. Initially the Tanzanian specimens were thought to represent Limacina conica ( von Koenen, 1892, p. 994, pl. 62, figures 5-6), a species described from the “early Oligocene” of two localities (Atzendorf, Unseburg) in the eastern part of Germany. However, specimens from Atzendorf (NP 21 interval), made available by Arnold Müller (Leipzig, Germany) differ in shape and reach far larger dimensions. Their apical angle is smaller, the whorls are more convex and the aperture remains smaller than half shell height.

Several species described by Hodgkinson et al. (1992), from the Paleogene of the United States also resemble the Tanzanian shells. Especially similar is Limacina smithvillensis Hodgkinson (in Hodgkinson et al., 1992, p. 19, pl. 3, figure 16) from the Lutetian of Texas. However, that species reaches a shell height of 1.5 mm, has less convex whorls, a wider apical angle, and its aperture occupies just half the shell height. Finally, that species is considerably older (Lutetian, NP 15) than the Priabonian (NP 21) specimens from Tanzania. At first glance also L. stenzeli Garvie (in Hodgkinson et al., 1992, p. 19, pl. 4, figure 1) from the NP13-14 interval (Ypresian) of Texas is similar. However, that species should be included in the genus Heliconoides , because of its reinforced apertural margin.

Figures

FIGURE 5. Limacina robusta (Eames, 195); RGM 777423b, apertural view. Bar equals 100 μm.

FIGURE 6. Limacina robusta (Eames, 1952), Holotype, Natural History Museum, London BM 68457. Rahki Nala (Pakistan, western Punjab); Kirthar Formation, Lower Chocolate Clays (late Lutetian – Priabonian); 1: apical, 2: apertural, 3: lateral view. Bar equals 100 μm.