Heliconoides mercinensis (Watelet and Lefèvre, 1885)

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 : 10-12

Heliconoides mercinensis (Watelet and Lefèvre, 1885)

Figures 5.1-3 , 6

1874 Planorbis ikke ulig Pl. vortex men maaske en Valvatina ; Mörch, p. 279.

*1885 Spirialis mercinensis Watelet and Lefèvre , p. 102, pl. 5 fig. 2a c.

1900 Valvatina raphistoma ; Stolley, p. 12, figs. 1a-c, 2.

1907 Valvatina raphistoma ; Ravn, p. 368.

1913 Valvatina merciniensis [sic] (Watelet and Lefèvre); Cossmann and Pissarro, caption of plate; pl. 60, Pteropodes 2-2.

v. 1965 Spiratella mercinensis (Watelet and Lefèvre, 1880) – Curry, p. 366, figs. 15a-b, 16.

1966 Spirialis mercinensis Watelet and Lefèvre, 1885 – Korobkov, pp. 73, 77, 78.

1967 Valvatina raphistoma – Hucke and Voigt, pp. 99, 104, pl. 45 fig. 1.

1980 Spiratella mercinensis (Watelet and Lefèvre, 1880) – Bristow, Ellison and Wood, p. 266, fig. 3.

1981 Spiratella mercinensis (Watelet and Lefèvre, 1880) – King, pp. 124, 125, 131, figs. 44, 45.

non 1982 SpiratelIa mercinensis (Watelet and Lefèvre) – Curry, 1982: 36, pl. 1 fig. la-b (= Heliconoides pyrenaica Cahuzac and Janssen, 2010 ).

v. 1984 Spiratella mercinensis (Watelet and Lefèvre, 1880) – King, pp. 142, 143, fig. 10.

? 1992 Limacina planidorsalis Hodgkinson – Hodgkinson et al., p. 19, pl. 3 figs. 11-13.

v. 2007 Heliconoides mercinensis (Watelet and Lefèvre) – Janssen et al., p. 163, figs. 7-8 (with extensive synonymy).

v. 2010 Heliconoides mercinensis (Watelet and Lefèvre, 1885) – Janssen, p. 165, fig. 2.

v. 2011 Heliconoides mercinensis (Watelet and Lefèvre, 1885) – Janssen et al., p. 76, figs. 13-16.

v. 2013 Heliconoides mercinensis (Watelet and Lefèvre, 1885) – Janssen et al., p. 29, fig. 9.

Type material. Six syntypes should be present in the Watelet collection, housed in the Musée de Soissons ( France), but could not be traced, according to Curry (1965, p. 366).

Type locality. Mercin (Aisne department, France) , ‘Sables inférieurs’ = Sables de Cuise = Montagne de Laon Group, Cuise Formation (Eocene, Ypresian, zone NP 10) (King, 2016, figure 51).

Material examined. Wilson Lake section, NP 9 and NP 10a ( Table 1). Bass River section, NP 10a ( Table 2), Clayton section, NP 9b and 10a ( Table 3), Cambridge-Dorchester section, ( Table 4). Mattawoman Creek-Billingsley Road section ( Table 5). Description. Shell sinistral, almost planispiral, c. two times wider than high when adult, with approximately 3½-3¾ rounded, but in larger specimens laterally somewhat flattened whorls in a regular spiral. Initial whorl hardly or not protruding, apical plane slightly concave or flat. Aperture rounded, with upper margin at the same height or only very slightly lower or higher than penultimate whorl. Basal part of the aperture lowered beyond base of foregoing whorl. Umbilicus wide and deep, occupying c. four tenths of total diameter.

Discussion. The largest available and best preserved specimen, illustrated Figure 5.1 (H 0.84, W 1.60 mm) deviates from the typical form of H. mercinensis as described from the Paris Basin ‘Cuisian’ by a slightly flattened side of the whorls, giving the shell a somewhat rounded trapezoidal shape in frontal view ( Figure 5.2 ). Also, the aperture of this specimen is less circular and more elliptical than in the type, and situated obliquely with respect to the shell’s axis. This feature, however, might have been caused by damage on the most abaxial part of the apertural margin. In the same specimen, a further difference is present in a faint indication of a preapertural reinforcement, visible as a weak furrow on the mold just anticipating the margin ( Figure 5.1 ), the result of a preapertural fold or internal thickening. This differs from typical H. mercinensis , in which the extreme apertural margin is just slightly widened and v-shaped laterally (see Curry, 1965, figure 16). In spite of these differences, observed in a single specimen only, we consider this to be intraspecific variability for the time being, and final conclusions on identification depend on well-preserved specimens. The same is true for Limacina planidorsalis Hodgkinson in Hodgkinson et al. (1992, p. 18, pl. 3, figure 11-13), the holotype of which has a diameter of 1.0 mm), and closely resembles immature H. mercinensis . Its indicated stratigraphical range (‘early to middle Eocene’) coincides with that of H. mercinensis .

Although a single, yet unnamed Heliconoides species is currently described ( Janssen and Goedert, 2016) from Cretaceous rocks (Campanian), H. mercinensis was long considered to be the oldest known pteropod species, with a stratigraphical range starting in the latest Paleocene (Tuscahoma Sand Formation, Bear Creek Marls of Alabama, USA, Zone NP 9; Janssen, 2010) and continuing until the earliest Lutetian. The upper Vincentown Formation in New Jersey is the age equivalent of the Tuscahoma Formation in Alabama ( Gibson et al., 1993, 2000). In the southern Salisbury Embayment, the upper portion of the Aquia Formation is age equivalent to the Tuscahoma Formation ( Gibson et al., 1993, 2000). In the sections studied here, H. mercinensis is the most commonly occurring pteropod species in both NP Zones 9 and 10a, starting immediately above the Paleocene/Eocene boundary, thus slightly younger than the Alabama occurrences.

Distribution. The oldest known occurrence of this species is from the uppermost Paleocene (Tuscahoma Sand Formation, Bear Creek Marls (NP 9 according to Siesser, 1983) of Alabama ( USA). This species likely was recorded as Limacina planidorsalis by Hodgkinson et al. (1992) in lower and middle Eocene cuttings of boreholes offshore eastern Canada. Apart from several occurrences in the Cuise Formation of the Paris Basin (see Curry, 1965) and the Ypresian Middle Blue Marl Formation of Pradelle-en-Val (Aude, France; Pirkenseer et al., 2013), this species is known in Europe from the Ypresian of Denmark (Fur Formation, Mo Clay Member and Lillebaelt Formation), from the Ypresian (London Clay Formation) in southern England, the Ypresian (Panisel and Flanders formations) in Belgium, and from contemporaneous rocks in the Netherlands (Opende borehole, Rotterdam E55 borehole). Finally, the species was recorded from the Alai Formation, Uzbekistan (Ypresian, Eocene, NP 13; Janssen et al., 2011) and an unnamed formation at the Ypresian/Lutetian transition in the Soh Area (Isfahan, Iran; Janssen et al., 2013). Most of these occurrences are documented in the Naturalis collection with c. 175 specimen lots. Curiously, this species is absent from the very rich pteropod assemblage of Gan, SW France; Ypresian, NP 12-13.

Figures

FIGURE 5. Heliconoides mercinensis (Watelet and Lefèvre, 1885). 1-3, Wilson Lake section, sample 27, depth 99.67-99.73 m, RGM 777 223a; 1: apical view, 2: apertural view, 3: umbilical view.

FIGURE 6. Heliconoides mercinensis (Watelet and Lefèvre, 1885). Computed tomography (CT) scan of the specimen pictured in Figure 5.1-3. Resolution 1.9 micrometer/voxel, 165 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.