Limacina timi, Cotton & Janssen & Pearson & Driel, 2017

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 : 10-13

Limacina timi sp. nov.

Figures 9.1-4 , 10

zoobank.org/ E5DC43EC-4F67-4173-890D-0145DEBEC0B6

Type material. Holotype ( Figure 9.1 ), RGM 1007748 b; paratype 1 ( Figure 9.2 ), RGM777408 a, from TDP 17.34.1, 0-7 cm, 98.90-98.97 m below surface and composite depth; paratype 2 ( Figure 9.3 ), RGM 777408 b, TDP 17.34.1, 0-7 cm; paratype 3 ( Figure 9.4 ), RGM 777414m TDP 17.36.1, 0-5 cm. Kilwa Group, Pande Formation (Eocene, Priabonian); planktic foraminefera zone P 16-17, calcareous nannoplankton zone NP 19-20.

Additional specimens. Boreholes TDP 11, 12 and 17: 15 specimens (see Tables 1 and 3).

Type locality. South of Stakishari , Tanzania, Kilwa Region, cored borehole TDP 12, coordinates UTM 37L 560222-8981309, sample TDP 12, sample 27.1, 35-45 cm = 89.60-89.70 m below surface = 147.60-147.70 m composite depth .

Etymology. Named after Tim Janssen, the author’s six year old second grandson. For him life is nonstop fun. Keep it that way, my friend!

Diagnosis. Small limacinid of 3.75 whorls, slightly higher than wide. Whorls rounded angular in juveniles, more regularly rounded in adults. Aperture lunate, somewhat higher than half shell height. Base imperforate.

Description. Holotype (H = 1.04, W = 1.00 mm) a small, regularly coiled, conical limacinid of 3.75 slightly convex whorls, separated by incised suture. Last whorl occupying 80% of shell height, slightly angular at periphery. Shell surface smooth, growth lines invisible. Aperture large, lunate, 65% of shell height, no apertural reinforcements present. Columellar side of aperture smooth, slightly concave internally, straight externally. Base of shell regularly convex, umbilicus absent in holotype, extremely small or absent in juveniles. Angularity of whorls more clearly developed in juvenile specimens, resembling a rounded keel, but covered by following whorls attaching at the place of angularity, almost disappeared in adults. Juvenile specimens wider than high, in some of these the keel is rather strong and the apical angle wider (RGM 777425).

Discussion. The angularity of especially juvenile specimens is not very clearly visible in the SEM images given here in Figure 9.3-4 , but under light microscope, with illumination from left above it cannot be overlooked.

Limacina timi resembles somewhat the Ypresian species L. gormani ( Curry, 1982) described from the Marnes de Gan Formation, of Gan, SW France ( Curry, 1982; Cahuzac and Janssen, 2010). The same species or a closely related form was also described from the Stone City and Cook Mountain formations (Lutetian/Bartonian) of Texas, USA ( Hodgkinson et al., 1992, p. 19, pl. 3, figures 14-15) as Limacina pygmaea (non Lamarck, 1805), in which also the juvenile whorls are angular and the base imperforate. The adult shell, however, reaches one and a half times the size of L. timi , has a different, more spherical shape and a distinctly wider apical angle. Subsutural crests ( Cahuzac and Janssen, 2010, pl. 11, figure 2) as seen in L. gormani are not present in L. timi .

Adult specimens of Limacina timi also resemble Heliconoides nemoris ( Curry, 1965) , described from the Bartonian of the UK and also recorded from the Priabonian of Biarritz, SW France by Curry (1982) and Cahuzac and Janssen (2010). That species belongs to the genus Heliconoides because of its apertural reinforcements that are apparently absent in L. timi , but also its apex is flattened and juvenile specimens have no angular periphery. Limacina timi straddles the Eocene/Oligocene boundary.

Figures

FIGURE 9. Limacina timi sp. nov.; 1: holotype, RGM 1007748b; 2: paratype 1, RGM 777408a; 3: paratype 2, RGM 777408b; 4: paratype 3, RGM 777414 m;. Apertural views. Bar equals 100 μm.