Amberleya, Morris and Lycett, 1850

Ferrari, S. Mariel, 2009, Cosmopolitan Early Jurassic marine gastropods from west-central Patagonia, Argentina, Acta Palaeontologica Polonica 54 (3), pp. 449-461 : 450-451

Amberleya ? espinosa sp. nov.

Fig. 2A .

Etymology: Referring to the strongly spinose ornament.

Type material: Holotype, MPEF−PI 1882 , well preserved teleoconch; paratype, MPEF−PI 1874 , poorly preserved teleoconch.

Type horizon: Osta Arena Formation, Lower Toarcian, Jurassic .

Type locality: PA 06 site, “ El Córdoba ” fossiliferous locality .

Material.— Holotype and one paratype.

Diagnosis.—Anomphalous, conical shell, trochiform to slightly littoriniform, with peripheral carina. Well incised suture bordered by row of small nodes. Ornament strongly spinose, with poorly developed spiral and axial elements. Base with three strong spiral ribs.

Description.—Dextral, medium−sized and trochiform to slightly littoriniform shell, anomphalous and conical. Protoconch is not preserved and teleoconch consists of seven whorls. The adapical portion of each whorl is slightly concave and angular near the periphery of the shell. Sutures are incised, and the depth of the incision increases abapically. On the last three whorls of the teleoconch, a small row of nodes borders the suture.

The ornament is strongly spinose with weakly developed spiral and axial elements. The spiral elements consists of two ribs; the adapical one is weak and borders the suture, and the abapical one, more conspicuous and developed as a carina at the periphery. Over both spiral ribs, rows of regularly spaced and strong spines are developed; there are about 7 spines per whorl on the juvenile portion of the shell and 10 or more on the adult teleoconch whorls. The spines on the adapical rib are much smaller than the ones on the abapical carina. The axial ribs are well developed on the early teleoconch whorls becoming gradually weaker and weaker during ontogeny. Fine prosocline growth lines appear on the last teleoconch whorl. The base is convex and ornamented by three conspicuous, slightly nodose spiral ribs. The aperture is incompletely preserved but a narrow collumelar lip is visible.

Dimensions.—MPEF−PI 1882: maximum height 20.6 mm; spire height 16.8 mm; maximum width 14.4 mm. MPEF−PI 1874: maximum height 19.9 mm; maximum width 8.2 mm.

Remarks.—The littoriniform shell, strongly spinose ornament, weak spiral ribs, and lack of an umbilicus suggest assignment to Amberleya (see Knight et al. 1960; Gründel 2003). As the specimen in hand does not display all of the diagnostic characters for the genus, and Amberleya itself is of rather disputable identity (see e.g., Kaim et al. 2004), the new species is assigned questionably to Amberleya .

Two species of Amberleya were previously reported from the Early Jurassic of South America. Möricke (1894) described Amberleya americana Möricke, 1894 and Gründel (2001) provided description of Amberleya ? sp. These species were reported from north−central Chile and Mendoza, Neuquén, and Chubut provinces in Argentina. A. americana was subsequently reported by several authors ( Weaver 1931; Wahnish 1942; Damborenea and Lanés 2007) from the Early Jurassic of Argentina, and Wahnish (1942) mentioned this species from the Early Jurassic of Chubut province. However, the descriptions and illustrations available in the literature ( Möricke 1894: 29, pl. 4: 8a, b) show that A. americana differs significantly from A. bathonica , which is the type species of the genus. Amberleya bathonica has higher−spired littoriniform shell than A. americana . Moreover it has an acute apex, six teleoconch whorls, greatest whorl width situated directly above the abapical suture, and three weak and narrow spiral ribs at its base. A. americana has only four convex teleoconch whorls, and has no ornament at its base. Hence, it is disputable whether A. americana can be classified as Amberleya . Amberleya ? espinosa sp. nov. is seemingly the first occurrence of the genus in the Early Jurassic of Argentina.

Amberleya ? sp. of Gründel (2001: 50, pl. 3: 1, 2), from Lower Sinemurian of Chile, is comparable with Amberleya ? espinosa sp. nov. Both have a similar gross shell morphology, with weakly developed spiral elements limited to nodose ribs or carinae. However, Gründel’s Amberleya ? sp. has a thinner shell, nodes instead of spines, and better developed axial ornament.

Amberleya ? espinosa sp. nov. is similar to the type species, Amberleya bathonica , named by Cox and Arkell (1950) as a replacement name for Amberleya nodosa of Morris and Lycett (1850: 55, pl. 5: 19), from the Bathonian of England. Both share the same shell morphology, with strong nodose or spinose elements and weak spiral ribs; however, A. bathonica has slightly convex whorls, and strong, rounded nodes rather than spines. Moreover, the British species has three weak basal spiral ribs.

Amberleya ? espinosa sp. nov. is closely related to Amberleya torosa Marwick, 1953 from the Hettangian (Lower Jurassic) of New Zealand ( Marwick 1953: 113, pl. 15: 3). Marwick’s species, however, has a more conical shell, nodes instead of spines, three prominent keels, and the base bears four or five smooth spiral cords. Amberleya ( Eucyclus ?) sp. ( Edwards 1980: 42, fig. 3a) from the Early Jurassic (according to Thompson and Turner 1986) of central Alexander Island ( Antarctica) is similar to the Argentinean species; however, Amberleya (E.?) sp. has a more prominent spiral ornament with three strong and deeply furrowed spiral ribs on each whorl, and five spiral threads on the base.

Amberleya (Eucyclus) capitanea (Münster, 1844) from the Toarcian of Europe ( Szabó 1982: 24, pl. 3: 1, 2) is similar to the Argentinean species. However, A. (Eucyclus) capitanea has more convex whorls, three conspicuous carinae with nodose rows on the last whorl, and fine prosocline growth lines.

Amberleya (Eucyclus) alpina (Stoliczka, 1861) from the Early Jurassic of Hungary ( Szabó 1982: 23, pl. 2: 11–13) is similar to A.? espinosa sp. nov., but the European species has more convex whorls, smaller nodes, and four spiral carinae on adult teleoconch whorls. Amberleya (Amberleya) aff. decorata Martin, 1859 from the Hettangian of Italy ( Gaetani 1970: 391, pl. 31: 18) differs from A.? espinosa sp. nov. in having weaker spines but better developed spiral elements.

Amberleya elegans (Münster, 1844) and Amberleya ornata (Sowerby, 1819) from the Early and Middle Jurassic of England ( Hickman and McLean 1990: 76, fig. 38) are similar to A.? espinosa sp. nov. A. ornata , however, is larger and has no spines; and A. elegans is smaller, has weaker spines and better developed axial and spiral elements.

Stratigraphic and geographic range.— PA−06 site from “El Córdoba ” fossiliferous locality, Chubut province, Argentina. Osta Arena Formation, Lower Toarcian , Lower Jurassic .

Figures

Fig. 2. Early Jurassic gastropods from the “El Córdoba” fossiliferous locality, Chubut Province, Argentina. A. Amberleya? espinosa sp. nov. MPEF−PI 1882, holotype, latex cast of external mold in lateral view (A1, A2), juvenile teleoconch in lateral view (A3), last whorl in lateral view (A4), basal and apertural view (A5), ornament pattern and sutural detail (A6). B. Colpomphalus? sp. MPEF−PI 1863, latex cast of external mold of incomplete teleoconch in apical and lateral view (B1–B3), last whorl ornament detail showing the spiral furrow (B4). C. Striatoconulus sp. MPEF−PI 1867b, latex cast of external mold in apical view (C1), apical detail (C2), first teleconch whorls lateral view (C3). Scale bars 2 mm unless otherwise indicated.