Rhiostoma? americana Hanna, 1920

Tongkerd, Piyoros, Tumpeesuwan, Sakboworn, Inkhavilay, Khamla, Prasankok, Pongpun, Jeratthitikul, Ekgachai, Panha, Somsak & Sutcharit, Chirasak, 2023, Systematic revision of the snorkel snail genus Rhiostoma Benson, 1860 (Gastropoda, Caenogastropoda, Cyclophoridae) with descriptions of new species, ZooKeys 1142, pp. 1-144 : 1

Rhiostoma? americana Hanna, 1920

Fig. 55A

Rhiostoma americana Hanna, 1920: 5, 6, pl. 1, fig. 4a, b. Type locality: John Day Basin (Oligocene), Oregon. Roth 1986: 261.

Type material.

Holotype UO26890 by monotypy. The author states, "Only the type specimen has been found", implying that the original description was based on the illustrated specimen. This specimen is accepted as the holotype by monotypy (ICZN 1999: Art. 73.1.2).

Remarks.

The shell size is relatively small, with a major shell diameter of 8.5 mm and shell height of 7 mm. The author also stated the strange character of the palatal and columellar teeth; however, these teeth have never been described in recent species. Hanna (1920) also stated that the unique shell shape resembles the Southeast Asian genus Rhiostoma at first glance, but that complete specimens, when available, may show that these species belong to another genus. We totally agree with Hanna’s (1920) point of view, as the detached whorl (or detached last whorl), whether it is turning upward or downward, has evolved multiple times in terrestrial snails and it should not constitute a shared derived character (i.e., Páll-Gergely and Neubauer 2020; Chen et al. 2022). Nonetheless, we refrain from formally reclassifying this species, pending further paleontological research. However, the record of Rhiostoma in the Nearctic Realm (John Day Formation in Oregon (middle Eocene to late Miocene; William et al. 2015)) is far outside of the known range of the genus. Thus, further study is needed to relocate this species into its correct genus, which probably is the genus Aperostoma Troschel, 1847 ( Neocyclotidae ; see the revision of the genus in Torre et al. 1942).

Figures

Figure 55. A Rhiostoma? Rhiostoma americana, original figure (after Hanna 1920: pl. 1, fig. 4 a, b) B, C Rhiostoma? Rhiostoma amarapuraense sp. nov. B holotype SMF 130536 / 1 from Amarapura, Burma and C paratype SMF 130536 / 2 - 8 from type locality D-F Rhiostoma sp. D specimens CUMZ 10050 from Khau Nam Nua, Vieng Xai, Houaphanh and E, F specimens CUMZ 10051 from Ban Zox, Xaysetha, Attapeu.