Limacina aegis Hodgkinson, 1992

Janssen, Arie, Sessa, Jocelyn & Thomas, Ellen, 2016, Pteropoda (Mollusca, Gastropoda, Thecosomata) from the Paleocene-Eocene Thermal Maximum (United States Atlantic Coastal Plain), Palaeontologia Electronica (Athens, Greece) 19 (3), pp. 1-26 : 13

Limacina aegis Hodgkinson in Hodgkinson, Garvie and Bé, 1992

Figures 7.1-11, 8

*1992 Limacina aegis Hodgkinson , in Hodgkinson et al., p. 15, pl. 1, figs. 10-15.

2013 Limacina aegis Hodgkinson ; Janssen et al., p. 31.

Type material. Holotype USNM 180485 , paratype USNM 180486 and 5 additional specimens.

Type locality. Amoco-Imperialis #A-1 Gannet O-54 well, 2,940 ft; 45°10’N 52°30’W, Nova Scotian shelf, offhore eastern Canada (early Eocene) .

Description. See Hodgkinson, in Hodgkinson et al. (1992).

Discussion. Limacina aegis , characterized by its depressed, lenticular shell (apical angle c. 133°) with a distinctly carinated periphery, seems to be related to Limacina yasdii Janssen , in Janssen et al., 2013 (p. 31, figures 13-14), introduced from an unnamed unit of latest Ypresian to earliest Lutetian age in the Soh area, Isfahan province, Iran. That species differs from the present one in being relative higher with somewhat more convex, less strongly carinated whorls and a much narrower umbilicus.

Material examined. Wilson Lake section, NP 9 and 10a ( Table 1), Bass River section, NP 10a ( Table 2), Clayton section, NP 9b and 10a ( Table 3), Cambridge-Dorchester section ( Table 4).

Distribution. Limacina aegis thus far was only known by its type material, the age of which could only roughly be indicated as ‘early Eocene? NP10- 13’ because of downhole contamination ( Hodgkinson et al., 1992). In the present material, this species is already present in the lowermost sample (NP 9) of the cored (and therefore contaminationfree) Wilson Lake section, and continues through zones NP 9 and NP 10a. Together with Altaspiratella elongatoidea , Limacina aegis appears 7.61 ky ( Table 1) after the onset of the CIE, and therefore is one of the oldest known pteropods, with the exception of a single Campanian unnamed Heliconoides specimen ( Janssen and Goedert, 2016) and Heliconoides mercinensis , which first appeared in the late Paleocene ( Janssen, 2010). The equally related species L. helicos Hodgkinson , in Hodgkinson et al. (1992, p. 17, pl. 3, figures 1-5), differing from L. aegis by a much higher shell with an apical angle of just 63° and completely flat whorls, is only known by its type specimens and was said to occur during the NP 10 and NP 11 zones.

Figures

FIGURE 7. Limacina aegis Hodgkinson in Hodgkinson, Garvie and Bé, 1992. 1, Clayton section, sample 19, depth 94.55-94.58 m, RGM 777319a; apertural view. 2, Clayton section, sample 21, depth 95.07-95.10 m, RGM 777 323; apertural view. 3-4, Wilson Lake section, sample 34, depth 101.80-101.86 m, RGM 777 236a; 3: apical view, 4: apertural view. 5-6, Wilson Lake section, sample 52, depth 07.08-107.11 m, RGM 777 264a; 5: apertural view, 6: umbilical view. 7-9, Wilson Lake section, sample 63, depth 108.20-108.26 m, RGM 777 283 (specimen lost); 7: apical view, 8: apertural view; 9: umbilical view. 10-11, Cambridge-Dorchester section, sample 15, depth 220.22-220.28 m, RGM 777 338; 10: apical view, 11: apertural view.

FIGURE 8. Limacina aegis Hodgkinson in Hodgkinson, Garvie and Bé, 1992. Computed tomography (CT) scan of specimen RGM 777.264b, which has the same locality data as the specimen in Figure 7.5-6. Resolution 2.0 micrometer/voxel, 165 kV, detector exposure timing 750 ms. For animated version, see online at palaeo-electronica.org/content/2016/1662-pteropodafrom-the-usa-petm. By clicking on the image, the interactive 3D model is activated, and the reader can use the mouse to rotate the specimen and change magnification.

FIGURE 9. Limacina novacaesarea Janssen and Sessa sp. nov. 1-3, Holotype, Wilson Lake section, sample 24, depth 98.76-98.82 m, RGM 777 219; 1: apical; view, 2: apertural view, 3: umbilical view. 4-6, Paratype 1, Wilson Lake section, sample 22. Depth 98.15-98.21 m, RGM 777 215a; 4: apical view, 5: apertural view, 6: oblique apical view to show lowering of initial whorl. 7, Paratype 2, Wilson Lake section, sample 27, depth 99.67-99.73 m, RGM 777 225; apertural view of poorly preserved, slightly depressed specimen.

FIGURE 10. Limacina novacaesarea Janssen and Sessa sp. nov. Computed tomography (CT) scan of the specimen pictured in Figure 9.1-3. Resolution 4.0 micrometer/voxel, 185 kV, detector exposure timing 750 ms. For animated version, see online at palaeo-electronica.org/content/2016/1662-pteropoda-from-the-usapetm. By clicking on the image, the interactive 3D model is activated, and the reader can use the mouse to rotate the specimen and change magnification.

FIGURE 11. Limacina sp. 1. 1-3, Wilson Lake section, sample 37, depth 102.72-102.78 m, RGM 777 241; 1: apical view, 2: apertural view, 3: umbilical view. 4-5, Wilson Lake section, sample 50, depth 106.68-106.74 m, RGM 777259a; 4: apical view, 5: apertural view. 6, Wilson Lake section, sample 40, depth 103.02-103.08 m, RGM 777248; apical view.