Orbitestella kieli, Chernyshev & Goedert, 2021

Chernyshev, Alexei V. & Goedert, James L., 2021, New species of latest Eocene / earliest Oligocene microgastropods (Heterobranchia Orbitestellidae and Omalogyridae) from the Gries Ranch Formation, Lewis County, Washington State, USA, Zootaxa 4981 (3), pp. 469-480 : 472-474

Orbitestella kieli sp. nov.

Figure 2A–F

Zoobank LSID. urn:lsid:zoobank.org:act:B792FEA1-5E06-4365-AC4C-639558F1DFB5

Material. Holotype: MIMB 41134 ; Paratypes: MIMB 41135 (11 specimens) , NRM PAL Mo 193050–193053 (4 specimens), SBMNH 467093 (1 specimen) , UWBM IP 114130–114135 (6 specimens) ; all from type locality.

Type locality. LACMIP locality 41621; Gries Ranch Formation , Lewis County, Washington State, USA; latest Eocene or earliest Oligocene , 33.5–34.7 Ma (latest Priabonian or earliest Rupelian; Prothero & Burns 2001) .

Etymology. Named for Steffen Kiel (NRM) in recognition of his many contributions to molluscan paleontology.

Diagnosis. Orbitestella with flattened spire and sunken apex, nucleus of protoconch with net-like ornamentation, faint axial riblets between low axial ribs on apical surface, broad and smooth upper carina, faint spiral threads more pronounced on apical surface near the periphery.

Description. Shell small for genus, depressed, with a flat spire; maximum shell diameter 0.63–0.93 mm; dimensions of the holotype 0.33 mm height × 0.93 mm diameter. Protoconch approximately 1.5 whorls, 0.08 mm in maximal diameter, nucleus with net-like ornamentation, visible in both apical and basal view. Teleoconch approximately 1.5–1.8 whorls, with radial ribs prominent only on the inner portion of each whorl, becoming obsolete on periphery so that the distinct cord on the basal, outer part of each whorl is smooth, lacking nodes; subapical margin of each whorl slightly wider that the subbasal margin, also delineated by a strong cord without nodes, radial ribs present but not prominent. Umbilicus broad and wide, second whorl of teleoconch partly covers first whorl. Apical surface of whorls smooth low arch to shoulder. Aperture polygonal.

Comparisons. Orbitestella kieli sp. nov. is similar to O. palaiopacifica with the nucleus of the protoconch clearly visible in both the basal and, to a lesser extent, apical view. Orbitestella kieli differs in lacking a distinct keellike border to the apical periphery of the whorls and lacks nodes on the basal periphery of the whorls. Orbitestella kieli also has only fine spiral lirae, except near the periphery in apical view where faint spiral threads produce a wrinkled texture on the last whorl of mature specimens ( Figs. 2A, F ). More pronounced spiral threads are visible on the shell of O. palaiopacifica , including on the keel-like upper carina as well as the basal carina (e.g., Figs. 1B–C ), and O. palaiopacifica has a more domed or humped apical surface of the whorls than O. kieli . Radial ribs are prominent on both the apical and basal surface of the shell of O. palaiopacifica and in most specimens, even extend to and are present on the apical carina, whereas in O. kieli they fade toward the periphery, especially on the basal surface, to smooth completely as they reach the periphery, and those of O. kieli are not prominent on the apical surface, although they are clearly present. Compared with O. palaiopacifica , the flattened basal surface of the whorls is slightly wider in O. kieli , with a distinct cord marking the outer border.

The only other fossil orbitestellid from western North America, O. dioi Hybertsen & Kiel, 2018 , from a hydrocarbon seep deposit in the Eocene deep water Humptulips Formation, differs in having more prominent spiral ornamentation, a rounded whorl profile, and an elevated spire.

A single living species, O. diegensis ( Bartsch, 1907) , inhabits the Pacific Coast of North America southward from central California. Orbitestella diegensis differs greatly from all the fossil species from Washington in having whorls with a strong peripheral keel and differs from both O. palaiopacifica and O. kieli in having a more domed apical surface of the whorls, with deeply incised apical suture. According to Bartsch (1907: 172, figs. 7a–b) and Myers (1979: fig.1) the base of the whorls of O. diegensis are rounded, whereas in O. palaiopacifica and O. kieli they are flattened.

Remarks. Orbitestella kieli is the third fossil species of the genus Orbitestella and family Orbitestellidae reported from western North America.

Figures

FIGURE 1. Orbitestella palaiopacifica Squires & Goedert (SEM). A, specimen MIMB 41133–1, apical view; B, specimen MIMB 41133–2, umbilical view; C, specimen MIMB 41133–3, apertural view. Scales: 100 µm.

FIGURE 2. Orbitestella kieli sp. nov. (SEM). A, B, F – holotype MIMB 41134, C, D, E—paratypes MIMB 41135–1 (C, E) and MIMB 41135–2 (D); A, apical view; B, apertural view; C, umbilical view; D, protoconch, apical view; E, protoconch, umbilical view; F, periphery, wrinkled sculpture formed by spiral threads, apical view. Scales: A-C, 100 µm; D-F, 20 µm.