Heliconoides nikkieae, 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 : 5-7

Heliconoides nikkieae sp. nov.

Figures 3.1-5, 4

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Type material. Holotype RGM 777415 a ( Figure 3.1 ); paratypes 1-2, RGM 777415 b-c ( Figure 3.3, 3.5 ) from the type locality. Kilwa Group, Pande Formation ( upper Eocene , Priabonian), Zones P 18 and NP 21; paratype 3, RGM 777 381 ( Figures 3 , 4 ), from TDP 17.21.1, 9-20 cm, Kilwa Group, Pande Formation (early Oligocene, Rupelian), biozones P18 and NP 21.

Additional specimens. TDP 11 (2 specimens), 12 (1 specimen) and 17 (49 specimens, some of which with a query because of poor preservation) ( Tables 1-3).

Type locality. Stakishari ( Tanzania, Kilwa region), cored borehole TDP 17, sample 17.36.1, 10-25 cm, 104.00- 104.10 m below surface, composite depth 116.00- 116.10 m.

Etymology. Named after Nikkie Elert, the author’s second granddaughter. At age six she is, in many respects, more up-grown than many grown-ups. Heliconoides gender masculine (ICZN 1992, art. 30.1.4.4).

Diagnosis. Very small limacinid of 2.5 whorls in low-conical spiral, about as high as wide, with large aperture, apertural margin externally thickened and internally doubled in some specimens ( Figure 3.2 ), basal margin with denticle. Apertural margin preceded by about 10 fine, margin parallel riblets.

Description. Strikingly small limacinid of 2.5 whorls, height/width-ratio variable between c. 1.19 (holotype H = 0.62, W = 0.52 mm, Figure 3.1 ) and 0.89 (e.g., H = 0.50, W = 0.56 mm, Figure 3.2 ), with depressed, low conical spire and distinct, incised suture. Shell surface smooth and shiny, growth lines invisible. Last whorl large, occupying almost entire shell height. Aperture relatively large, slightly higher than wide to almost circular, occupying 4/5th of entire shell height. Apertural margin externally thickened by narrow ridge, running all around margin, flexuous at base of shell, reaching umbilicus, internally ( Figure 3.2 ) in some specimens. Protruding denticle on basal part of margin ( Figure 3.3-4 ). Marginal thickening preceded by about 10 fine, margin parallel orthocline riblets. Shell base regularly rounded, umbilicus very narrow, c. 1/20th of shell diameter.

Discussion. In general shape the new species resembles somewhat the holotype of Limacina wechesensis Hodgkinson (1992, p. 21 , pl. 5, figures 4-6) from the Lutetian of Texas, USA, but that species has one whorl more and reaches double the size of H. nikkieae , does not have the apertural structures of that species and its umbilicus is considerably wider.

Size and apertural structures of the new species form a unique combination and cannot be compared to any limacinid currently known. The holotype was chosen from the sample with most specimens (13) of Priabonian age, but the species continues, in low numbers, well into the Rupelian part of the TDP 17 section.

Figures

FIGURE 3. Heliconoides nikkieae sp. nov.; 1: Holotype, RGM 777415a, apertural view; 2: apertural view, specimen lost; 3: paratype 1, RGM 777415b, umbilical view; 4: paratype 3, RGM 777 381, umbilical view; 5: paratype 2, RGM 777415c, apical view. Figure 3-1, -2, -3 and -5 from the type locality, TDP 17.36.1, 10-25 cm; Figure 3-4 from TDP 17.21.1, 9-20 cm. Bar equals 100 μm.

FIGURE 4. Heliconoides nikkieae sp. nov.; holotype, RGM 777415a, 3dPDF. For animation see: http:// palaeo-electronica.org/content/2017/2024-pteropodafrom-tanzania.

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