Bovicornu aff. eocenense Meyer, 1886

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 : 15-16

Bovicornu aff. eocenense Meyer, 1886

Figures 13.1-2

cf 1886 Bovicornu eocenense Meyer , p. 79, pl. 3,

figure 12 (not figure 2). cf 1892 Meioceras eocenense (Meyer) – Dall, p.

302. cf 1912 Bovicornu eocaenense [sic] Meyer – Cossmann, p. 155. cf 1934 Bovicornu eocenense Meyer – Collins, p.

212, pl.9, figure 3, pl. 13, figure 5. cf 1959 Bovicornu eocenense O. Meyer – Zilch, p.

49, figure 164. cf 1992 Bovicornu eocenense Meyer – Hodgkinson et al., p. 24, pl. 7, figures 9-10 (with additional synonymy). Type material. Holotype (H = 2,8, W = 0, 7 mm) United States Natural Museum (Smithsonian Institution) nr. 644596. Type locality. Red Bluff, USGS locality 5264, Mississippi, USA (Red Bluff Clay; Oligocene, Rupelian, NP 21). Material examined. Only fragments were found, 2 from TDP 11, 9 from TDP 12, and 11 from TDP 17 (see Tables 1-3 for details). Description. Spatially spiralised tube with free volutions. Transverse section of tube circular, no surface ornamentation or growth lines visible. Only smaller fragments are available among which no complete aperture or protoconch. Diameter of tube doubles in about half a volution. Discussion. Two species of the genus Bovicornu are currently known and both are exclusively recorded from the USA. The older one of these, B. gracile Meyer, 1887 (p. 9, pl. 2, figure 17), of the Moodys Branch Formation, Texas, has an age of Bartonian (NP 17). The other species, B. eocenense Meyer, 1886 (p. 79, pl. 3, figure 12), from the Red Bluff Formation of Mississippi, USA occurred during the Priabonian and early Rupelian (NP19- 21) ( Hodgkinson et al., 1992, figure 3). In both species the shell is creseid, but instead of being straight or slightly curved, as in Creseis species, the tube demonstrates clear twisting in a wide spatial spiral that was said to be stronger in B. gracile . Hodgkinson et al. (1992), however, collected numerous specimens at both type localities and noted that in many specimens of both species the twisting is stronger and that the species cannot be distinguished on the degree of twisting. There is, however, a clear difference in protoconch morphology: an inflated bulb in B. eocenense and a more cylindrical shape in B. gracile . The holotypes of both species were re-illustrated in Hodgkinson et al. (1992, pl. 7, figures 9-10 and 11-12), reproduced herein as Figure 14.1-4 .

Although the few larger fragments from Tanzania seem to indicate a considerably stronger spirally twisted shell than in either of the holotypes it is preferred to indicate the Tanzanian species as related to the younger of the two American species and is indicated here as Bovicornu aff. eocenense . Once specimens preserving their protoconch become available this position might be revised .

Figures

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

FIGURE 14. Holotypes of Bovicornu eocenense Meyer, 1886 (1, 2) and B. gracile Meyer, 1887 (3, 4). Shell height of 1 = 2.8 mm, of 2 = 2.7 mm; 2 and 4 are magnifications of 1 and 3, respectively. Photographs after Hodgkinson et al. (1992, pl. 7, figures 9-12).