A new, relatively diverse gastropod fauna is reported from the Chubut province of west-central Patagonia. The gastropod association at the “El Córdoba” fossiliferous locality (Lower Toarcian of Osta Arena Formation) consists of three new species: the eucyclid Amberleya? espinosa sp. nov. and two procerithiids Cryptaulax damboreneae sp. nov. and Cryptaulax nulloi sp. nov. Other members of the association are the ataphrid Striatoconulus sp., discohelicid Colpomphalus? sp., and an undetermined zygopleurid. Knowledge on Early Jurassic gastropods from South America and other southern continents is reviewed to show that the taxonomic composition of the El Cordoba association strongly resembles other gastropod associations of this age (even those from Europe), suggesting a wide distribution of cosmopolitan genera.
Introduction
Early Jurassic gastropods from South America have been studied for a long time (Bayle and Coquand 1851; Gottsche 1878, 1925; Behrendsen 1891, 1922; Möricke 1894; Burckhardt 1900, 1902; Jaworski 1925, 1926a, b; Weaver 1931; Feruglio 1934; Piatnitzky 1936, 1946; Wahnish 1942), but their descriptions were usually appended to papers describing some other invertebrate groups, mostly cephalopods and bivalves. Gründel (2001) recently described some Early Jurassic gastropods from Chile and the Neuquén Basin in Argentina. There are, however, very few reports (Feruglio 1934; Piatnitzky 1936; Wahnish 1942) of Mesozoic gastropods from west-central Patagonia.
The following Early Jurassic gastropods from Chubut province have been reported so far: Natica catanlilensis by Weaver (1931), Amberleya cf. americana, Natica sp., and Natica cf. philipi by Möricke (1894), Pleurotomaria sp. and Cerithium cf. quinetteum by Piette (1856), Nerinea sp., Trochus sp., and Lithotrochus humboldtii by Buch (1839). All these reports need careful revision, including re-investigation of the fossil localities to collect new material with accurate geographical and stratigraphical data. This paper reports a new abundant and fairly diverse fauna from the marine deposits of the Osta Arena Formation that crop out over wide areas of Chubut province. In addition, knowledge of Argentinean and Chilean Early Jurassic gastropod faunas is summarized, and an attempt is made to compare these to coeval faunas known from other southern continents.
Geological settings
The Early Jurassic sediments in Chubut province are distributed along a NW-SE belt of outcrops between 42°30′ and 44°30′ S, and 69°30′ and 71° W (Riccardi 1983; Giacosa and Márquez 1999). They rest unconformably on Late Paleozoic rocks of the Tepuel Group. In the Pampa de Agnia area, the Jurassic sequence begins with ignimbrites and tuffites of the Puntado Alto Formation (Herbst 1966, 1968) which are of continental origin and have yielded plants of Early Jurassic age (Riccardi 1983). The Puntudo Alto Formation is overlain by the El Córdoba Formation, described by Robbiano (1971) and considered by Riccardi (1983) as part of the Lonco Trapial Group. The Early Jurassic marine deposits of the Osta Arena Formation (Herbst 1966; Nullo 1983) are overlain by the El Córdoba Formation which reaches a thickness of 190–340 m. The most extensive outcrops of this unit are on the western slope of Sierras de Lonco Trapial, Cajón de Ginebra, and Cerro Negro (Fig. 1). The “El Cordoba” fossiliferous locality is located west of the Sierra del Cerro Negro (Fig. 1) at the access to Quebrada El Córdoba where a stratigraphical section was measured by Robbiano (1971) and later also modified by Nullo (1983). The Osta Arena Formation consists of marine sandstones, tuffites and conglomerates. The formation is dated as Toarcian (Lower Jurassic) based on dactylioceratid and hildoceratid ammonoids (Musacchio and Riccardi 1971; Blasco et al. 1978). The gastropod-bearing locality yielded a single specimen of Dactylioceras (Orthodactylites) hoelderi Hillebrandt and Schmidt-Effing, 1981 that indicates a Lower Toarcian age for this association.
Material and methods
The material was collected in 2007 during field work and belongs to the MPEF collection. Latex casts of external molds were prepared by technical staff of the MPEF laboratory. The photographs were made by digital camera Nikon (Nikon E8800) at MPEF and by Scanning Electronic Microscopy (SEM) (Jeol JSM-6460LV scanner with retrodifussion detector) at ALUAR.
Systematic paleontology
The systematic classification follows Ponder and Lindberg (1997), Gründel (2000, 2001, 2003, 2007, 2008), and Bouchet and Rocroi (2005). The morphological terminology is based on Knight et al. 1960), Kaim (2004), and Gründel (2000, 2005).
Class Gastropoda Cuvier, 1797
Subclass Orthogastropoda Ponder and Linberg, 1996
Superorder Vetigastropoda Salvini-Plawén, 1980
Order Trochomorpha Naef, 1911
Superfamily Trochoidea Rafinesque, 1815
Family Eucyclidae Koken, 1897
Genus Amberleya Morris and Lycett, 1850
Type species: Amberleya bathonica Cox and Arkell, 1950 (= Amberleya nodosa Morris and Lycett, 1850), Middle Jurassic (Bathonian), England.
Stratigraphic and geographic range.—Middle Triassic-Middle Jurassic (according to Hickman and McLean 1990); Asia, Europe, New Zealand, Antarctica and America.
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 Lanes 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.
Superfamily Turbinoidea Rafinesque, 1815
Family Ataphridae Cossmann, 1915
Subfamily Ataphrinae Cossmann, 1915b
Tribe Homalopomatini Keen, 1960
Genus Striatoconulus Gründel, 2000
Type species: Striatoconulus latus Gründel, 2000, from the Callovian of Poland.
Stratigraphic and geographic range.—Toarcian (Lower Jurassic)—Callovian (Middle Jurassic); Poland and Argentina.
Striatoconulus sp.
Fig. 2C.
Material.—MPEF-PI 1867a and b (part and counterpart); poorly preserved teleoconch.
Description.—Medium-sized, oval and low-spired shell. Protoconch unknown. Dextral teleoconch with four convex whorls; last whorl is incomplete. The spire represents about 1/3 of teleoconch height and is characterized by the presence of a high expansion rate. Sutures are incised.
On first whorls of the teleoconch ornament is poorly developed. In the later whorls of the teleoconch ornament consists of regularly spaced spiral furrows. No axial ornament is present. Apertural and umbilical features are not preserved.
Dimensions.—MPEF-PI 1867b: maximum height 25.6 mm; spire height 12.7 mm.
Remarks.—The convex teleoconch whorls, low spired shape, incised sutures, and the spiral ornament suggest an assignment to Striatoconulus (compare Gründel 2000; Kaim 2004).
Striatoconulus sp. is the first ataphrid recorded in the Early Jurassic of South America. The type species, Striatoconulus latus Gründel, 2000 is very similar to the Argentinean species. S. latus has a base weakly convex, with rounded edge to the flanks of the whorls, aperture broadly oval, inner lip with a broad callus and a halfmoon-shaped cavity on the columella (Gründel 2000: 228, pl. 6: 11—15; Kaim 2004: 29, fig. 15). Basal, apertural and umbilical features are regrettably not preserved in Striatoconulus sp. and that is why the species is left in open nomenclature.
Superfamily Cirroidea Cossmann, 1916
Family Discohelicidae Schröder, 1995
Genus Colpomphalus Cossmann, 1915
Type species: Straparollus altus d'Orbigny, 1853, from the Bathonian of France.
Stratigraphic and geographic range.—Hettangian (Lower Jurassic)—Bathonian (Middle Jurassic); Europe and South America.
Colpomphalus? sp.
Fig. 2B.
Material.—MPEF-PI 1863, poorly preserved teleoconch.
Description.—Small-sized and almost planispiral shell. Protoconch is not preserved. Teleoconch consists of three rapidly expanding whorls with the last whorl incomplete. The adapical portion of the whorls is almost horizontal and slightly concave near the angular edge of the whorls. Sutures are well incised.
Ornament pattern with nodose, spiral and axial elements. The spiral elements consist of weak ribs and two strongly nodose carinae near the suture. The abapical carina forms the peripherical edge; it has fewer and stronger nodes than the adapical carina. Prosocline collabral ribs intersect the spiral elements. The collabral ribs on the lateral flank of the last whorl are orthocline and intersect a weak spiral furrow. Base is not preserved, and apertural and umbilical features are unknown.
Dimensions.—MPEF-PI 1963: maximum height 3.1 mm; maximum width 6.9 mm.
Remarks.—Although important diagnostic characters of aperture and base are not preserved in the present material, the almost planispiral shape, the angular whorls as well as the ornament strongly suggest that this is a species of Colpomphalus.
The South American species, Colpomphalus toarciensis Gründel (2001: 46, pl.1: 9–13) from the Lower Toarcian of Chile, is similar to Colpomphalus? sp. Both share a low spira, ornament pattern with spiral ribs and two nodose carinae, and spiral ribs on lateral side of the last whorl. However, C. toarciensis is larger than Colpomphalus? sp. and has no colabral elements. Moreover, Gründel's (2001) species has a third row of nodes surrounding the relatively narrow umbilicus. The basal, apertural and umbilical features are not preserved in Colpomphalus? sp.
Colpomphalus angulati (Quenstedt, 1856) from the Hettangian of Germany (Gründel 2003: 6, pl. 1: 9, 10) is similar to Colpomphalus? sp., but the latter differs from the European species in having a more rapidly increasing width and fewer nodes.
Superorder Caenogastropoda Cox, 1959
Order Ptenoglossa Gray, 1853
Family Zygopleuridae Wenz, 1938
Zygopleuridae? gen. et sp. indet.
Fig. 3A.
Material.—MPEF-PI 1886, poorly preserved teleoconch.
Description.—Dextral, small-sized and high-spired shell, anomphalous and conical. Protoconch is not preserved and teleoconch consists of six convex whorls. The last two teleoconch whorls are considerably more expanded than the early whorls. Sutures are distinct. Shell is either externally smooth or the sculpture is not preserved. Base is convex and the aperture oval.
Dimensions.—MPEF-PI 1886: maximum height 9.8 mm; spire height 6.1 mm; maximum width 4.6 mm.
Remarks.—The Argentinean species may probably be a Zygopleuridae of the Azyga group. Mesozoic zygopleurids are generally characterized by a high-spired teleoconch with axial ribs (Nützel and Senowbari-Dayran 1999; Nützel and Erwin 2004) but there are some zygopleurids of the Azyga group having a smooth teleoconch and axially pleated protoconch (Nützel 1998; Kaim 2004). Due to lack of protoconch in the specimen under consideration, crucial for a correct taxonomic assignment, this specimen is left in open nomenclature. The other possibilities are Pseudomelaniidae and Ampullinidae or even pyramidelloidean heterobranchs (compare Gründel and Kaim 2006; Hikuroa and Kaim 2007).
Order Sorbeoconcha Ponder and Lindberg, 1997
Suborder Cerithiomorpha Golikov and Starobogatov, 1975
Superfamily Cerithioidea Férussac, 1819
Family Procerithiidae Cossmann, 1906
Subfamily Cryptaulacinae Gründel, 1976
Genus Cryptaulax Tate, 1869 (= Xystrella Cossman, 1906)
Type species: Procerithium (Xystrella) protortile Cox, 1965 (pro Cerithium tortile Hébert and Eudes-Deslongchamps, 1860, non Eudes-Deslongchamps, 1842); from the Callovian of Normandy, France.
Stratigraphic and geographic range.—Upper Triassic-Early Cretaceous; Europe, Asia, Africa, New Zealand, and South America.
Cryptaulax damboreneae sp. nov.
Fig. 3B-D.
Etymology: Dedicated to Susana E. Damborenea.
Type material: Holotype, MPEF-PI 1878a and b (part and counterpart); well preserved teleoconch; paratypes, MPEF-PI 1877 and MPEF-PI 1872a and b; poorly preserved teleoconch fragments.
Type horizon: Osta Arena Formation, Lower Toarcian, Jurassic.
Type locality: PA 06 site, “El Córdoba” fossiliferous locality.
Other material.—MPEF-PI 1875, MPEF-PI 1868, MPEF-PI 1876, MPEF-PI 1880, MPEF-PI 1865, MPEF-PI 1884, and MPEF-PI 1864; poorly preserved teleoconchs.
Diagnosis.—Anomphalous and turriculate shell. Ornament predominantly axial, with spiral and nodose elements. Two weak spiral ribs of second order appear on adult teleoconch between three strong primary spiral ribs. Circular and holostomatous aperture.
Description.—Dextral, small-sized and high-spired shell, anomphalous, slender and turriculate. Protoconch is not preserved. Teleoconch comprises eight whorls with gradate outline. Deeply incised sutures. Ornament with nodose, spiral and axial elements. Axial elements are predominant and consist of orthocline and narrow axial ribs that run from suture to suture. Axial ribs regularly spaced, 8–9 per whorl. Axial ornament intercepts spiral elements. The latter comprises three strong spiral ribs; two of which are situated somewhat above and below the suture; the third spiral rib is median. On the sixth teleoconch whorl two weak secondary spiral ribs appear between the primary ribs. At the intersection of axial and spiral elements, strong conspicuous and acute nodes are present; nodes are strongest on primary spiral ribs. Base is strongly convex with five acute spiral ribs. The aperture is holostomatous and oval.
Dimensions.—See Table 1.
Remarks.—The present material shows the characters which are typical of Cryptaulax (compare Gründel 1999; Kaim, 2004). Most representatives of the genus are characterized by mode of occurrence of the weak secondary spiral ribs during ontogeny. The presence of two such spiral ribs on sixth teleoconch whorl separates Cryptaulax damboreneae sp. nov. from other species of the genus.
Cryptaulax variformatum Bandel, 1994 from the Early Jurassic of Peru (Bandel 1994: 140, pl. 3:16,17, 19), is similar to C. damboreneae sp. nov. but is more high-spired than the Argentinean species, has a stronger spiral ornament, and its aperture is quadrangular rather than oval. Cryptaulax? sp. of Gründel (2001: 54, pl. 3: 10) from the Bathonian of Chile resembles C. damboreneae sp. nov. in having a dominant axial ornament with at least one abapical spiral rib.
Cryptaulax sp. cf. protortile Cox, 1969 from the Early and Middle Jurassic of New Zealand (Bandel et al. 2000: 89, pl. 6: 9, 11–13), is very similar to C. damboreneae sp. nov. However, this species (a juvenile teleoconch) has stronger spiral ribs. Cryptaulax sp. 1 and Cryptaulax sp. 2, both described by Gründel (2003: 26, 27, pl. 6: 8–12, pl. 7: 1) from the Sinemurian of Germany, share some features with C. damboreneae sp. nov. However, Cryptaulax sp. 1 has weakly prosocline axial ribs and two basal spiral ribs, and Cryptaulax sp. 2 has ten opistocline axial ribs on the last whorl. Kaim (2004) described several species from the Middle Jurassic of Poland which resemble C. damboreneae sp. nov. Cryptaulax quenstedti (Walther, 1951) (see: Kaim 2004: 36, fig. 21; Gründel 1974: 842, pl. 2: 9–15; Gründel 1999: 18; pl. 4: 8–12 identified as Cryptaulax ex. gr. bellayensis sp. 3) differs from the Argentinean species only in having a weak median spiral rib or, a second order of one additional spiral rib between the strong median and adapical spiral ribs. Cryptaulax sp. 1 of Kaim (2004: 34, fig. 19) differs from C. damboreneae sp. nov. in having a weak fourth spiral rib on the sixth teleoconch whorl, and an additional fifth, weak spiral rib on the tenth teleoconch whorl. Cryptaulax sp. 2 of Kaim (2004: 34, fig. 19) is larger than the Argentinean species and has a second order of two weak spiral ribs on ninth teleoconch whorl. Cryptaulax shiptonensis (Cox and Arkell, 1950) is similar to C. damboreneae sp. nov. in shell morphology. However, this European species has a second order of two median spiral ribs on the fourth or fifth teleoconch whorls (Kaim 2004: 34, fig. 20). Cryptaulax armata (Goldfuss, 1844) differs from C. damboreneae sp. nov. in having a weaker median spiral rib on the mature teleoconch (Kaim 2004: 37, fig. 22). Cryptaulax muricata (Sowerby, 1825) is larger than C. damboreneae sp. nov., has four strong spiral ribs on the last teleoconch whorl, lacks secondary weak spiral ribs, and the nodes result from the intersection with weakly prosocline axial ribs (Kaim 2004: 32, fig. 17). Cryptaulax undulata (Eudes-Deslongchamps, 1842) is more elongated than the Argentinean species, has five strong spiral ribs intersecting the axial ribs, and the peristome has an anteriorly directed siphonal channel (Kaim 2004: 33, fig. 18). C. echinata (von Buch, 1831) differs from C. damboreneae nov. sp. in having two conspicuous and nodose ribs on all teleoconch whorls (Kaim 2004: 38, fig. 23). Cryptaulax sp. 3 of Kaim (2004: 43, fig. 27) and Cryptaulax? mutabilis (Gerasimov, 1955) are similar to the species here described, but the whorls of C. sp. 3 have a straighter outline and fewer axial ribs while C.? mutabilis has a more Bittium-like teleoconch (Kaim 2004: fig. 28).
Stratigraphic and geographic range.—PA-06 site, from “El Córdoba” fossiliferous locality, Chubut province, Argentina. Osta Arena Formation, Lower Toarcian, Lower Jurassic.
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 suturai 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.
Table 1.
Dimensions (in mm) of Cryptaulax damboreneae sp. nov and Cryptaulax nulloi sp. nov. Abbreviations: H, maximum height; h, spire height; W, maximum width.
Cryptaulax cf. damboreneae sp. nov.
Fig. 3E.
Material.—MLP 18742, MLP 18743, MLP 18744; poorly preserved teleoconch fragments from Cerro Puchenque locality (Mendoza) (Fig. 1), Neuquén Basin, Argentina. Dactylioceras hoelderi Zone, Lower Toarcian, Lower Jurassic.
Description.—Small-sized and high-spired shell, slender and turriculate. Protoconch is not preserved. Teleoconch comprises five whorls with a straight outline. Sutures are deeply incised. Ornament consists of spiral and axial ribs with conspicuous nodes at intersections. Axial ribs are predominant and consists of orthocline to slightly prosocline axial ribs running from suture to suture. Axial ribs are regularly spaced and show no marked change during ontogeny. Seven distinct regularly spaced spiral ribs appear on mature teleoconch whorls (fewer in juvenile growth stages). A second order of spiral ribs is not developed. Basal and apertural features are not preserved.
Remarks.—The present material strongly resembles Cryptaulax damboreneae sp. nov. However, Cryptaulax cf. damboreneae sp. nov. has larger shells, orthocline to slightly opistocline axial ribs, seven stronger spiral ribs intersecting the axial elements, and a weakly developed second order of spiral ribs. The present material probably represents a new Cryptaulax species but the specimens are too poorly preserved for such proposal.
Fig. 3.
Early Jurassic gastropods from Argentina A. Zygopleuridae? gen. et sp. indet., “El Córdoba” fossiliferous locality, Chubut Province. MPEF-PI 1886, latex cast of external mold in lateral view. B-D. Cryptaulax damborenae sp. nov., “El Córdoba” fossiliferous locality, Chubut Province. B. MPEF-PI 1878b, holotype, latex cast of external mold in lateral (B1, B2) and basal (B3) views, ornament detail (B4). C. MPEF-PI 1877, paratype, latex cast of external mold of incomplete teleoconch in apertural view. D. MPEF-PI 1872b, paratype, latex cast of external mold of incomplete teleoconch in lateral view. E. Cryptaulax cf. damboreneae sp. nov., Cerro Puchenque locality, Mendoza Province. MLP 18742, latex cast of external mold of incomplete teleoconch; adult teleoconch in lateral view. F-H. Cryptaulax nulloi sp. nov., “El Córdoba” fossiliferous locality, Chubut Province. F. MPEF-PI 1870, holotype, latex cast of external mold. Adult teleoconch in lateral view (F1), juvenile teleoconch in lateral view (F2). G. MPEF-PI 1861, paratype, latex cast of external mold in lateral and apertural view. H. MPEF-PI 1871, latex cast of external mold of incomplete teleoconch in lateral view (H1), and its ornament detail (H2). Scale bars 1 mm.
Table 2.
Early Jurassic gastropods described form Argentina and Chile. Original identifications and references are given. Note that in many cases the taxonomic relationships are outdated and the known age is just Early Jurassic. Indigenous taxa are indicated by asterisks.
Continued
Cryptaulax nulloi sp. nov.
Fig. 3F-H.
Etymology: Dedicated to Francisco E. Nullo.
Type material: Holotype, MPEF-PI 1870, well preserved teleoconch; paratype, MPEF-PI 1861, poorly preserved teleoconch.
Type horizon: Osta Arena Formation, Lower Toarcian, Jurassic.
Type locality: PA 06 site, “El Córdoba” fossiliferous locality.
Other material.—MPEF-PI 1862, MPEF-PI 1866, MPEF-PI 1873, MPEF-PI 1881, MPEF-PI 1869, MPEF-PI 1871, and MPEF-PI 1885, poorly preserved teleoconch fragments.
Diagnosis.—Anomphalous and turriculate shell. Juvenile teleoconch characterized by predominance of axial ornament and presence of weak nodes. Adult teleoconch with spiral ornament predominant, consisting of two strong ribs near the suture both in adapical and abapical parts of the flank, and weaker ribs between. On adult teleoconch whorls conspicuous nodes are present. Subcircular holostomatous aperture.
Description.—Dextral, small-sized, and high-spired shell, anomphalous, slender and turriculate. Protoconch is not preserved. Teleoconch comprises eight whorls which are straight in outline. Sutures are well delimited and bordered by two spiral ribs. Teleoconch ornament consists of spiral and axial ribs with nodose intersections. The early teleoconch whorls (juvenile stage) differ from the adult stage in having a strongly axial ornament pattern, with orthocline ribs intersecting two weak spiral ribs near the sutures. At the intersections of axial and spiral ribs, eight weak nodes are developed. The mature ornament of the teleoconch stabilizes on the fifth whorl; a strong spiral ornament appears and the axial ribs become weaker. The spiral elements comprise two strong ribs near the sutures; additionally, a number of much weaker ribs appears. The presence of strong spiral ribs near the suture gives the whorl face a concave appearance. Base is convex and ornamented with four spiral ribs. The aperture is holostomatous and subcircular.
Dimensions.—See Table 1.
Discussion.—The presence of strong axial and weak spiral ribs on juvenile teleoconch whorls allow this material to be identified as Cryptaulax. The adult teleoconch whorl with strong spiral ribs and conspicuous nodes separate C. nulloi sp. nov. from other species of the genus. Cryptaulax nulloi sp. nov., C. damboreneae sp. nov., and C. cf. damboreneae are the first cryptaulacins recorded from the Early Jurassic of Argentina. C. nulloi sp. nov. and C. damboreneae sp. nov. are similar in having a small-sized high-spired shell and axial and spiral ribs with nodose intersections. C. nulloi nov. sp., however, has weakly developed axial ribs and much better developed spiral ribs during ontogeny. C. damboreneae sp. nov. is slightly smaller than C. nulloi sp. nov., has fewer nodes and strong axial ornament pattern on all growth stages. C. cf. damboreneae differs from C. nulloi sp. nov. in having better developed axial ribs and more spiral ribs.
Stratigraphic and geographic range.–PA-06 site, from “El Córdoba” fossiliferous locality, Chubut province, Argentina. Osta Arena Formation, Lower Toarcian, Lower Jurassic.
Table 3.
Records of Early Jurassic marine gastropods from the southern hemisphere (exclusive of South America, see Table 2). Original identifications and references are given. Note that in most cases the taxonomic classification is updated.
continued.
Fig. 4.
Southern hemisphere map showing the paleobiogeographical distribution of the cosmopolitan Jurassic gastropod genere reported herein. Note that the occurrence of the new Lower Toarcian gastropod association in “El Córdoba” fossiliferous locality (*) (Chubut province, Argentina) extends the paleobiogeographical distribution of the group in South America.
Concluding remarks
The taxonomic classifications of most gastropod taxa described so far from the Early Jurassic of Argentina are outdated in comparison to those of gastropods from Antarctica (Edwards 1980; Thompson and Turner 1986), Africa (Cox 1965), and New Zealand (Bandel et al. 2000). There are only few illustrations of the species available in the published literature, and in many cases their taxonomic position is uncertain. More complete and recently reviewed data (Table 2) on Early Jurassic gastropods are available from Chile (Gründel 2001). A survey of other records of Early Jurassic gastropods from the southern hemisphere (Table 3) shows rather limited systematic knowledge of particular gastropod taxa in Argentina. A more detailed research (including the investigation of new fossiliferous localities in Argentina, the collection of new gastropod material with accurate geographical and stratigraphical data, and the revision of all described gastropod groups of Early Jurassic age) is currently in progress. Nevertheless, the new gastropod association of the “El Cordoba” fossiliferous locality provides new data on its taxonomic composition in the Jurassic of Argentina and South America. It clearly shows that some taxa (especially at generic level) were of cosmopolitan distribution in the southern hemisphere (Fig. 4) and other regions of the world (e.g., Europe). Nevertheless, these genera are usually represented by indigenous species known so far only from Chubut province and other localities in Argentina and Chile (Table 2).
Several taxa are recorded for the first time from the Early Jurassic of South America (Striatoconulus) or Argentina (Cryptaulax, and possibly Amberleya). The new findings reported here from just one locality, show that new localities of South American Jurassic gastropods may greatly extend our knowledge and stimulate further research in the future, facilitating appropriate interpretation of the palaeobiogeographical distribution of gastropods in the southern hemisphere in the Jurassic.
Acknowledgements
I want to thank Alejangra Pagani, Mariano Caffa, and Leandro Canessa (all at Museo Paleontologico “Egidio Feruglio”, Trelew, Argentina) for their help during fieldwork and, especially, Leandro Canessa for his laboratory work. I thank Jaime Groizard (ALUAR, Pto. Madryn, Argentina) for allowing the access to the SEM. I am grateful to Alberto Riccardi (División Paleontología Invertebrados of Museo de Ciencias Naturales de La Plata, Argentina) for identifying the Patagonian cephalopod, to Susana Damborenea (División Paleontología Invertebrados of Museo de Ciencias Naturales de La Plata, Argentina), Andrzej Kaim (Institute of Paleobiology PAS, Warsaw, Poland), Joachim Gründel (Freie Universität, Berlin, Germany), János Szabó (Hungarian Natural History Museum, Budapest, Hungary), Alexander Nützel (Bayerische Staatssammlung für Paläontologie und Geologie, Munich, Germany), and Stefano Monari (Università Degli Studi di Padova, Padua, Italy) for supplying important literature. I thank Susana Damborenea, Andrzej Kaim, and the referees Alexander Nützel, Joachim Gründel, and Alan Beu (GNS Science, Lower Hutt, New Zealand) for their valuable comments. I am grateful to Museo Paleontológico “Egidio Feruglio” and Ministerio de Educación de la Provincia del Chubut for financing the fieldwork. Special thanks are given to Paul D. Taylor (Natural History Museum, London, UK) for linguistic corrections.
