Amberleya morganensis ( Stanton, 1895 )

Kiel, Steffen, Campbell, Kathleen A., Elder, William P. & Little, Crispin T. S., 2008, Jurassic and Cretaceous gastropods from hydrocarbon seeps in forearc basin and accretionary prism settings, California, Acta Palaeontologica Polonica 53 (4), pp. 679-703 : 683-684

Amberleya morganensis ( Stanton, 1895)

Fig. 3E–I .

1895 Turbo morganensis sp. nov.; Stanton 1895: 66–67, pl. 12: 13.

Material.— Stanton’s (1895) type material ( USNM 23071 and 23074) from the Rocky Creek site in Morgan Valley (site 7) and specimens collected from the same site by Dean A. Enderlin, James L. Goedert, and KAC. Most specimens examined were steinkerns, along with several external rubber molds extracted from the same indurated carbonates. Because Stanton (1895) did not designate a holotype, the specimen illustrated on Fig. 4E is here designated as lectotype of “ Turbo ” morganensis .

Description.—Shell moderately high−spired, turbiniform, with angulate whorls; medium sized, up to 30 mm in height, with at least 4–5 teleoconch whorls; sutures incised to grooved. Whorls sculptured with three nodular spiral cords, first cord below suture, either directly adjacent to suture or up to the cords’ widths away from suture; two lower cords variable in strength and position, they may be equally strong and greatest width of the whorls are at the lower cord, or the upper cord is stronger and also marks the greatest width of the whorl; number and position of tubercles in these three cords are not aligned, central row has slightly fewer tubercles per whorl than the other two. Succeeding whorl attached at or below lower cord. Basal margin convex, base sculptured by 5–7 tuberculate cords that decrease in strength and increase in number of finer nodes towards columella. Spiral cords not crossed by axial ribs, nodes remain distinct and separate between successive spiral ribs. Aperture subovate to subquadrate; prosocline to prosocyrt growth lines visible in some specimens.

Discussion.—The specimen illustrated by Stanton (1895) is a “gutta−percha cast of a natural mold” which actually has a deeper suture than shown on his illustration. Subsequently collected specimens often differ from Stanton’s by having less angular whorls resulting from a steeper subsutural ramp, and by having a strong middle spiral and a weak lower spiral, whereas in Stanton’s specimen the two spirals are almost equally strong. However, there is sufficient morphological flexibility in these characters among the examined material that we consider this as intra−specific variation.

Stanton (1895: 67) noted from his total collection of 4 internal molds and 2 external molds that Amberleya morganensis was somewhat similar to “ Turbo ” colusaensis (see Paskentana paskentaensis below), “which has the same general form and style of sculpture.” Our inspection of type material in the USNM and additional material from our own collection shows that the sculpture of A. morganensis and “ Turbo ” colusaensis is not at all similar. It is possible that confusion arose because many specimens are preserved as internal molds, which are generally moderately to coarsely cancellate in appearance. Amberleya morganensis is actually much more similar to Amberleya dilleri than to “ Turbo ” colusaensis .

Stratigraphic and geographic range.—Valanginian (Lower Cretaceous), Great Valley Group, in seep carbonates of Rocky Creek, Morgan Valley (site 7), California, USA.

Figures

Fig. 3. Hand sample and petrographic thin section (plane polarized light) occurrences of selected Mesozoic seep gastropod fossils, California, USA. A. Fossil coquina of Paskentana paskentensis from Bear Creek (site 4) carbonate pod. B. Thin section photomicrograph of Bathypurpurinopsis stantoni in clotted micrite with pore−filling fibrous cement, from Cold Fork of Cottonwood Creek (site 1). C. Longitudinal section through Paskentana paskentensis encased in early diagenetic (seafloor) anhedral yellow calcite cement (dark; cf. Campbell et al. 2002), with pores filled by lighter−colored fibrous cement, from Paskenta (site 2). D. Small individual of Paskentana paskentensis preserved in clotted micrite within serpulid worm tube; Bear Creek site (site 4).

Fig. 4. Late Jurassic and Early Cretaceous eucyclid Amberleya spp. from seep carbonates in California, USA. A–C. Amberleya dilleri Stanton, 1895, type specimens from Paskenta (site 2). A. Spire of juvenile specimen (USNM23074a). B. Spire of large specimen (USNM 23074b). C. Lectotype (USNM 23074c) showing the aperture.D. Amberleya cf. dilleri (UCMP 555090) from NW Berryessa (site 10). E–I. Amberleya morganensis (Stanton, 1895) from Rocky Creek (site 7). E. Lectotype (USNM 23071). F. Spire (CAS 70386) with two tuberculate spiral cords. G. View on flank and basal margin (CAS 70387). H. Spire (CAS 70388) with lower tuberculate spiral largely concealed by succeeding whorl. I. Specimen (CAS 70389) with narrow apical angle. F–I, rubber casts.