Amberleya dilleri 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 : 682-683

Amberleya dilleri Stanton, 1895

Fig. 4A–C .

1895 Amberleya dilleri sp. nov.; Stanton 1895: 68, pl. 12: 7–9.

Material.— The three specimens figured by Stanton ( USNM 23074 ) from Paskenta (site 2); one specimen each from LACMIP loc. 15912 (Paskenta float blocks) and LACMIP loc. 15922 (Paskenta field number PS1−2; in Campbell 1995). Stanton’s (1895) specimen with the intact aperture (our Fig. 4C ) is herein designated as lectotype .

Description.—High−spired turbiniform shell of at least four or five whorls with incised suture and slightly oblique growth lines; whorls with broad and smooth subsutural ramp and prominent, tuberculate keel at center of whorl, which marks its widest point, about 15 tubercles per whorl; basal margin gently convex, marked by tuberculate spiral cord; at least four additional, finely beaded spiral cords on base, which decrease in strength and spacing towards columella; aperture almost round except for its pointed apical end.

Discussion.— Stanton (1895) emphasized similarities between his specimens and those assigned by d’Orbigny (1850 –60) to Purpurina d’Orbigny, 1850 . It was subsequently shown that Amberleya has nacreous shell and therefore belongs to the Trochoidea ( Hickman and McLean 1990); whereas Purpurina is a cenogastropod due to its orthostrophic, multiwhorled protoconch (Kaim 2004). One of Stanton’s specimens ( Fig. 4C ) shows that the aperture has a rounded base typical of juvenile eucyclins. In contrast, Purpurina has at least a small apertural notch. Thus, Stanton’s (1895) assignment of this species to Amberleya seems most appropriate. Two very similar species from the Great Oolite of southern England are A. pagodiformis ( Hudleston 1887 –1896: pl. 22: 9), which differs from A. dilleri only by the presence of a fine tuberculate row below the suture, and A. orbignyana ( Hudleston 1887 –1896: pl. 22: 7), which has slightly higher whorls, allowing an additional tuberculate row just above the lower suture. Observation by the senior author on Amberleya spp. from these English outcrops stored in the British Natural History Museum in London suggest that these species generally have a more distinctive spiral sculpture than the Californian ones concerned here.

Amberleya dilleri and A. cf. dilleri both differ from the Early Cretaceous (Valanginian) species A. morganensis from Rocky Creek (site 7) by lacking a tuberculate subsutural cord. Amberleya dilleri also differs from A. morganensis by being higher spired and having a weaker spiral cord below the main spiral cord that forms the whorl’s shoulder.

Stratigraphic and geographic range.—Tithonian (Upper Jurassic) at Paskenta (site 2), Great Valley Group, California, USA.

Figures

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.