Sulcoactaeon Cossmann, 1895
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publication ID |
https://doi.org/ 10.4202/app.2009.0042 |
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persistent identifier |
https://treatment.plazi.org/id/03FBF625-FFC1-212A-7EBD-93B4FCB627DF |
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treatment provided by |
Felipe |
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scientific name |
Sulcoactaeon Cossmann, 1895 |
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Genus Sulcoactaeon Cossmann, 1895 View in CoL
Type species: Actaeonina striato−sulcata Zittel and Goubert, 1861 ; Glos , Normandy, France;? Oxfordian (Late Jurassic); original designation .
Fig. 12A View Fig .
Material.—Single incomplete shell ( UMUT MM30185) from Yasukawa.
Dimensions.—The shell UMUT MM30185 is 2.96 mm high and 1.41 mm wide.
Description.—The shell is broadly spindle−like, 1.41 times as high as broad. Early whorls are poorly preserved. Teleoconch whorls are ornamented by numerous spiral furrows. The last whorl possesses a narrow subsutural ramp and two furrows just below the ramp. All other furrows (approx. 12) are present in the abapical part of the whorl. The density of the furrows increases abapically. The aperture is D−shaped. The inner lip is narrow. A narrow umbilical chink is present. Neither teeth nor other apertural elaborations are visible.
Discussion.— Sulcoactaeon is a genus known so far from Bajocian to Valanginian times ( Kaim 2004) although it is considered by some authors ( Bandel et al. 2000) as a subgenus of the Recent Bullina (family Bullinidae ). The most important shell character of Bullinidae is the absence of teeth or columellar plications on the inner lip. Such features are not observed at the specimen under consideration. Nevertheless, due to generally poor preservation, there is a possibility that these features are not preserved in our specimen and that is why we classified it as Sulcoactaeon with some hesitation. Sohl (1964) described some species of Acteon and Troostella from Upper Cretaceous shallow water sediments of the US Gulf Coast which have an oblique low fold on the columella with which later merges—and is not expressed on the inner lip. Our specimen may also be related to this group. The understanding of relations between fossil groups of acteonelloids is still rather limited and out of scope of this paper. Kiel (2006) described similar species from Oligocene cold seep carbonates of Washington State as Acteon sp. He reported a presence of “...weak plication at base” (Kiel 2006: 129) in his specimen.
Naticiform gastropod
Fig. 13 View Fig .
Material.— 24 specimens from Omagari site: 23 specimens at UMUT and one at NMM.
Dimensions.—The best preserved shell UMUT MM30190 ( Fig. 13B View Fig ) is 1.96 mm high and 2.04 mm wide.
Description.—The shell is small, globose, smooth apart from prosocline growth lines. Early whorls are poorly preserved but the protoconch is apparently orthostrophic. Teleoconch consists of 3–3.5 inflated whorls with incised suture. Aperture is circular and tangential. No aperture elaborations are visible. Umbilicus is absent.
Discussion.—The shells under consideration are difficult to classify due to their poor preservation. The small size and tangentially located circular aperture may suggest that the shells belong to neomphalid Retiskenea . The specimens from the Omagari site are similar in gross morphology to the Recent Retiskenea diploura Warén and Bouchet, 2001 living in the Japan Trench ( Okutani and Fujikura 2002). The latter species, however, possesses slit−like umbilicus, a character not observed in our specimens. Similar is also Retiskenea statura ( Goedert and Benham, 1999) from Eocene and Oligocene cold seep carbonates of Western Washington ( Goedert and Benham 1999, Kiel 2006, Campbell et al. 2008). The state of the umbilicus in R. statura is not reported by any of the abovementioned authors and the published illustrations are not conclusive on that matter. It seems, however, that there is no umbilicus in the holotype of Goedert and Benham (1999: fig. 2A) and the specimen described by Kiel (2006) (Steffen Kiel, personal communication 2009). Campbell et al. (2008) reported a species Retiskenea ? kieli Campbell, Peterson, and Alfaro, 2008 from Lower Cretaceous seep carbonates of California. The shells from the Omagari site are higher spired than the latter species. The most important shell character of Retiskenea is a protoconch with reticulate pattern ( Warén and Bouchet 2001). Unfortunately the protoconch ornamentation is not preserved in any of our specimens; therefore, we decided to leave this gastropod unnamed—potentially it may represent an entirely different group of gastropods—pending collection of better preserved specimens.
Skeneiform gastropod
Fig. 14 View Fig .
Material.—44 poorly preserved shells from the Omagari site, all deposited at UMUT.
Dimensions.—The best preserved shell UMUT MM30193 ( Fig. 14A View Fig ) is 2.2 mm high and 1.14 mm wide.
Description.—The shell is small and skeneiform with 3–3.5 inflated whorls and the suture deeply incised. Initial whorl poorly preserved at all accessible specimens but apparently orthostrophic. The teleoconch is smooth. The umbilicus is wide open with no sculpture on the basal area. The aperture is drawn out peripherally and is cemented to the preceding whorl in the parietal part.
Discussion.—The shells under consideration are of very simple morphology observed in a number of gastropod groups including small trochids, skeneids, neomphalids, and heterobranchs. It seems that the initial whorl is orthostrophic and therefore affinities to the heterobranchs are less likely though some heterobranchs have the heterostrophy obscured. Among vent and seep taxa a small trochid Helicrenion reticulatum Warén and Bouchet, 1993 possesses very similar shell ( Warén and Bouchet 1993; Desbruyères et al. 2006). The latter species is characterized by large net−like pattern on its protoconch. This character unfortunately is not preserved on our specimens and that is why we decided to leave the species unnamed. Similar shells from Oligocene cold−seep carbonates of Washington State have been classified by Kiel (2006) as the peltospirid Depressigyra ? sp. This identification, however, remains uncertain as the protoconch ornament could not be observed.
Gastropoda indet. 1
Fig. 12B View Fig .
Material.—A single incomplete shell from Omagari.
Dimensions.—The shell UMUT MM30186 ( Fig. 12B View Fig ) is 10.9 mm high and 5.82 mm wide.
Description.—The shell is elongated with 2.5 whorls preserved. The whorls are weakly inflated and their surface is smooth. The suture is weakly incised. It remains unsure if this lack of ornamentation is original character or resulted from peeling off the outermost layer. The protoconch and aperture characters are unknown.
Discussion.—The only strongly elongated gastropod in the seep localities under consideration is Hokkaidoconcha hikidai . However, the latter species occurs exclusively in Yasukawa and its shell expansion is much weaker. The shell under consideration may belong to a weakly preserved neogastropod. Better preserved material is necessary to confirm this supposition.
Gastropoda indet. 2
Fig. 12C View Fig .
Material.—A single incomplete shell from Yasukawa. Dimensions.—The shell UMUT MM30187 ( Fig. 12C View Fig ) is 5.45 mm high and 4.62 mm wide (note that the shell is laterally compressed).
Description.—The shell is globose, low spired and it possesses an enveloping last whorl. The shell surface is smooth. The shell consists of two layers. Inner layer is cross−lamellar while outer layer is fibrous prismatic.
Discussion.—The taxonomic position of this cassid−like shell remains unresolved pending additional better preserved material.
| UMUT |
University Museum, University of Tokyo |
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