Aporrhais cf. gracilis Koenen, 1885

Hryniewicz, Krzysztof, Amano, Kazutaka, Bitner, Maria Aleksandra, Hagström, Jonas, Kiel, Steffen, Klompmaker, Adiël A., Mörs, Thomas, Robins, Cristina M. & Kaim, Andrzej, 2019, A late Paleocene fauna from shallow-water chemosynthesis-based ecosystems, Spitsbergen, Svalbard, Acta Palaeontologica Polonica 64 (1), pp. 101-141 : 106-107

Aporrhais cf. gracilis Koenen, 1885

Fig. 4A–C .

1925 Nassa spec. ; Hägg 1925: pl. 3: 5, 5a.

1927 Chenopus (Arrhoges) sp .; Gripp 1927: 32, pl. 6: 8–9.

1970 Chenopus gracilis ( Koenen, 1885) ; Vonderbank 1970: 87, pl. 8: 8–9.

2016 Aporrhaidae sp.; Hryniewicz et al. 2016: fig. 12H.

Material.— Two specimens ( GPIBo 116 , 117 ), incomplete adolescent shells from the upper Paleocene , Basilika Formation , locality 500 m west of Trigonometric point 25, Hollendarbukta, Spitsbergen, Svalbard .

Measurements.— GPIBo 117: H, 27 mm; W, 14 mm.

Description. —Shell elongate, largest specimen ( GPIBo 117) consists of seven whorls. Whorl flanks moderately inflated with neither keel nor knobs. Ornamentation consists of opisthocline, well-expressed but blunt axial ribs, which disappear at demarcation between lateral flank and base. Demarcation continuous with no sharp angulation. Both lateral flank and base ornamented by numerous delicate spiral ribs. Aperture not preserved, only specimen with piece of single straight digit of outer lip preserved has been illustrated by Gripp (1927: pl. 6: 8, 9), but this specimen was poorly preserved and collection did not survive 2 nd World War (Ulrich Kotthoff, personal communication 2015). No trace of ascending whorl has been observed.

Remarks.—The adolescent aporrhaid shells from Isfjorden display characters observed in three Paleogene species, but due to the lack of well-preserved diagnostic characters of the adult expanded outer lip, it is difficult to attribute it to any of these species with confidence. The three species in question are: Aporrhais gracilis Koenen, 1885 ; A. bowerbankii ( Morris, 1852) , and A. triangulata Gardner, 1884 . The former two are from the Paleocene, while the latter is early Eocene in age. All three have similar early whorls; a tall, elongate teleoconch without keel or knobs and without an ascending last whorl; and differ mostly in the morphology of the terminal whorl and its outer lip, characters not preserved in the aporrhaid from Isfjorden. Vonderbank (1970) attributed the specimens from the Paleocene of Spitsbergen to A. gracilis , which is known to occur in the Selandian of Denmark ( Koenen 1885; Schnetler and Nielsen 2018) under the generic name Chenopus Philippi, 1836 , which is a junior objective synonym of Aporrhais . It seems indeed that A. gracilis is the most similar species, but due to its fragmentary preservation we prefer to leave the species from the Paleocene of Spitsbergen in open nomenclature. Another similar but poorly preserved aporrhaid-like specimen from Fossildalen has been illustrated and decribed by Hägg (1925) as the neogastropod Nassa sp. This specimen ( NRM-PZ Mo149182; Fig. 4C herein) is an inner mould with traces of spiral ornamentation resembling the one of A. cf. gracilis , although it seems to be wider and slightly less elongate. We refrain from any further identification of this specimen and provisionally we leave it as a doubtful aporrhaid Aporrhais cf. gracilis .

Figures

Fig.4.Aporrhaid and scaphandrid gastropods from the upper Paleocene, Basilika Formation, locality 500 m west from Trigonometric point 25, Hollendarbukta (A, B, D) and Fossildalen (C), Spitsbergen, Svalbard. A, B. Aporrhais cf. gracilis Koenen, 1885, GPIBo 117 (A) and GPIBo 116 (B) in lateral (A1, B1) and apical (A2, B2) views.C.?Aporrhais cf.gracilis Koenen, 1885, identified by Hägg (1925) as Nassa sp., NRM-PZ Mo 149182 in apical (C1), lateral C2) and latero-apertural (C3) views. D. Ellipsoscapha sp. GPIBo 115 in apertural (D1), lateral (D2), apical (D3), and abapical (D4) views.