Limacina sp. 1

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 : 14-15

Limacina sp. 1

Figures 11.1-6 , 12

Description. The few available specimens differ from Heliconoides mercinensis , as described above, by their planorboid shape with a somewhat lower height/width-ratio and a completely flat apical plane. Their whorls increase somewhat slower in diameter, but specimens of the same size as in H. mercinensis have the same number of whorls. The lower margin of the aperture reaches barely beyond the base of the penultimate whorl and the umbilicus is wider. In front view, the shape is similarly trapezoidal, but more depressed.

Material examined. Wilson Lake section, NP 9 and 10a ( Table 1); Cambridge-Dorchester section ( Table 4).

Discussion. We are hesitant to describe the present specimens as representing a new species, considering the close resemblance to H. mercinensis and its observed variability, as well as the poor preservation of all specimens, and thus we think it better to record this form in open nomenclature. As no indication of a reinforced apertural margin is present in these specimens, we include this species in the genus Limacina . These points of view may be reconsidered when better-preserved material becomes available.

Distribution. Only known from the specimens here recorded.

Figures

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.

FIGURE 12. Limacina sp. 1. Computed tomography (CT) scan of the specimen pictured in Figure 11.1-3. Resolution 4.9 micrometer/voxel, 145 kV, detector exposure timing 750 ms. For animated version, see online at palaeo-electronica.org/content/2016/1662- pteropoda-from-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.