The genus Harpagodes Gill (Gastropoda, Stromboidea) in the Lower Cretaceous of the Neuquén Basin, Argentina

CATALDO, CECILIA SOLEDAD; LAZO, DARÍO GUSTAVO

Neuquén

Congreso; XVIII Congreso Geológico Argentino; 2011

The genus Harpagodes was erected by Gill (1870). Harpagodes shells are obconic or ovate-conoid, with spire moderately elevated. The expanded external lip is divided into diverging spiniform digitations, one of which lies against the spire. The anterior canal is also digitiform, and curves towards the left. Surface sculpture is composed of spiral cords and larger carinae on last whorl produced into spiniform digitations. Type species is Pterocera desori Pictet & Campiche, 1863 originally described from the Valanginian of Sainte-Croix, Switzerland (see revision of this species in Kollmann, 2005: p. 130-131). Twenty two species of Harpagodes have been described so far, eight from Cretaceous deposits. The oldest species recorded is H. wrightii (Morris & Lycett), from the Bathonian (Middle Jurassic) of England, and the youngest is H. sachalinensis Yabe & Nagao, from the Santonian-Campanian (Zhonkier Formation) of Sakhalin Island, Russia. Suprageneric placement of Harpagodes has been much debated. Latest revision (Kollmann, 2009) places the genus in the subfamily Harpagodinae within the family Aporrhaidae, together with five other genera, on the basis of the assumed presence of a basal notch. Pchelintsev (1963) introduced the new family Harpagodesidae (corrected Harpagodidae) to include only Harpagodes. In spite of the discussion of its systematic affinities, there is consensus that Harpagodes belongs in the Stromboidea. Previous records of Harpagodes in the Neuquén Basin include: H. oceani (Brongniart, 1821) (Haupt, 1907), from the Tithonian of Cerro Lotena and H. cf. americanus Imlay (Damborenea et al., 1979), from the Late Hauterivian of Lomas Bayas. In addition, Palma et al., 2009 report the presence of Harpagodes sp. in the Middle Oxfordian of Mendoza. Mariñelarena (1959) indicates the presence of Harpagodes sp. in the Late Tithonian of southern Neuquén. Burckhardt (1900) points out records of Pterocera sp. in the "Neocomian" of Neuquén. The aims of this work are: (1) to describe newly collected specimens of Harpagodes, proceeding from 11 sections of the Agrio Formation in Neuquén and Mendoza; (2) to study the mode of occurrence of the specimens and the associated facies and paleoenvironments; and (3) to analyse the paleobiogeographic distribution of the genus during the Early Cretaceous. The material studied is characterized by a medium to large adult shell (minimum and maximum shell height of incomplete specimens: 8.5 mm and 154 mm) of subfusiform outline and mean spire angle of approximately 75º. Last whorl is large and moderately inflated, representing nearly three fourths of shell height. Spire is moderately elevated, with convex whorls. Aperture bears an expanded outer lip, divided into five spiniform digitations, of which the most adapical (fifth) seems to lie against the spire, and an anterior canal curved to the left. Surface sculpture consists of broad, flattened and gently rounded spiral cords in the spire whorls and in the last whorl five prominent, moderately broad, rounded carinae, separated by flat to slightly convex interspaces which bear minor spiral cords of varying number (from adapical to abapical): one between the fifth and fourth carinae, one or two between the fourth and third, two or three between the third and second, two between the second and first, and several between the first carina and the anterior canal. These five carinae produce spiniform digitations towards the margin of the extended outer lip. The fifth carina is less well developed on the surface of last whorl, and produces a digitation that rests against the right side of the spire, thus running posteriorly.  Most of the specimens are preserved as internal or compound moulds. However, some specimens preserve patches of recrystallized shell. Although with a high degree of corrasion, these make possible a detailed description of the sculpture. Most specimens have both apex and margin of the aperture chipped, which precludes full observation of the morphology of digitations and anterior canal. Specimens are frequently encrusted external and/or internally. Oysters are the most conspicuous encrusters, followed by serpulids. The overall degree of taphonomic modification showed by the studied specimens accounts for reworking and substantial exposure time of the shells on the sea floor, but not necessarily a high degree of lateral transport. The studied specimens very much resemble Harpagodes americanus Imlay, 1940 (p. 158, pl. 14, fig. 9), from the Early Hauterivian of Mexico, although it has four carinae on the last whorl. Photographs of the holotype UMMP-20413 (University of Michigan Museum of Paleontology, Ann Arbor, MI, USA) have been carefully analysed, and seems very probable that, due to the poor state of preservation of the holotype, the fifth carina was not identified by Imlay. Thus, the studied material is here regarded as Harpagodes cf. americanus Imlay. Based on ammonoid zonations (Aguirre-Urreta et al., 2007) the material ranges from the Late Valanginian (Chacantuceras ornatum Subzone) to latest Hauterivian (Paraspiticeras groeberi Zone). The record of H. oceani in Haupt (1907) is not figured. Moreover, the description given is too general, conforming to the diagnosis of the genus. Five carinae are mentioned, with one spiral cord in each interspace. This could be a consequence of the preservation. Specimen of H. cf. americanus figured in Damborenea et al. (1979) (MLP-14931; Colección de la División Paleozoología de Invertebrados, Museo de La Plata, Argentina) was revised and found to conform in morphology with the specimens described here. The studied specimens differ from H. desori (Pictet and Campiche), from the Valanginian-Hauterivian of several peri-Tethyan localities, in that the third or median carina does not form a prominent shoulder, which in H. desori delimits a broad and almost horizontal sutural ramp. Also from the Tethyan platforms, H. pelagi (Brongniart) (Hauterivian-Aptian) can be distinguished due to its shorter spire, the obconical shape of the last whorl, the presence of a shoulder and a sutural ramp and the morphology of the outer lip. H. beaumontianus (d´Orbigny), from the Late Barremian of France, is very similar to H. pelagi.  H. nodosus (J. de C. Sowerby), from the Cenomanian of several peri-Tethyan and North-Temperate localities, shows a very depressed spire, a high number of spiral cords and a moderately developed outer lip. H. sachalinensis Yabe and Nagao has four carinae on the last whorl, with no mention of a fifth carina resting against the spire. In the literature, Harpagodes specimens have been found in both carbonatic and mixed carbonatic-siliciclastic facies, deposited in open or protected internal ramps or platforms (e.g., Vullo et al., 2003; Palma et al., 2009; and others). Paleoenvironments include warm, shallow-water settings within the upper subtidal zone, both in low-energy lagoonal waters and in more agitated peri-reefal settings. Facies relationships of the Harpagodes-bearing levels were evaluated at four of the eleven studied sections: Bajada del Agrio, Cerro Birrete, Cerro Marucho and Lomas Bayas. All have yielded shallow-water marine settings above storm weather wave base. Associated palaeoenvironments are interpreted as inner shelf and inner carbonate ramp settings deposited under the influence storm currents and waves. They occur associated with a highly diverse and abundant macrofossil assemblage, including ammonoids, nautilids, epifaunal and infaunal bivalves, other gastropods, echinoids, corals and serpulids. During the Early Cretaceous Harpagodes was a widely distributed genus, recorded in peri-Tethyan tropical platforms (France, Switzerland, Bulgaria, Crimea, Caucasus-), in the Mexican-Texan tropical platform (Coahuila, Sonora and Puebla, México; Texas, USA), and in west-central Argentina. Its specific diversity in this interval was much higher in tropical than in temperate latitudes.