756
Journal of Paleontology 89(5):748–761
Comparative remarks.—This species of Charopidae is assigned to Zilchogyra on the basis of the following conchological characters: shell discoidal, umbilicus perspective, regular con- vex whorls; nepionic whorls smooth; adult whorls with axial ribs and delicate ribblets and aperture enlarged (Miquel and Cádiz Lorca 2008). Zilchogyra miocenica n. sp. agrees with the type species, Z. costellata (Recent, living in Buenos Aires province), essentially by shell characters, but that species differs in size and sculpture, having a larger shell, with fewer axial ribs (52 to 60 in the last whorl) (Hylton Scott, 1964). Z. matteriae Hylton Scott 1972 (Recent, from Tierra del Fuego, Argentina) has a shell with a lower spire and a greater number of axial ribs (165 in the last whorl). Patula michaelseni Strebel 1907 (recent, from Chilean-Argentinean Patagonia) (possibly belonging to Zilchogyra, according to Hylton Scott, 1970), differs by the number of axial ribs (42 on the first teleconch whorl, 76 on the last whorl; Hylton Scott 1970). Fossils described from the Eocene at Gran Barranca (Chubut, Argentina) (Bellosi et al. 2002) are much larger (Zilchogyra sp. 1: 5.6×3.1mm; Zilchogyra sp. 2: 5.3× 2.7mm), but they are too poorly preserved for detailed comparisons.
? Punctoidea ?Charopidae
Genus Patagocharopa new genus
Type species.—Patagocharopa enigmatica n. sp., by monotypy.
Diagnosis.—Shell lenticular, with numerous semi-internal bar- riers, mostly in the periphery (angular, parietal, and basal walls) but others on the columellar area.
Etymology.—Feminine. Conjunction of the names ‘Patagonia’ and ‘Charopa,’ name of a typical genus for austral mollusks.
Occurrence.—Southern tip of America. Miocene.
Description.—Shell lenticular, very small, with both proto- conch and teleoconch smooth; with numerous rounded semi- internal barriers: one angular, six parietal, one basal, three columellar, and one lamella and two additional teeth in the columellar wall.
Remarks.—Assignation of the new genus to Punctoidea and Charopidae is tentative, because of the fragmentary preservation of the only studied specimen. Internal or semi-internal barriers became visible due to the loss of the last part of the body whorl in the analyzed specimen (Fig. 7.1). Within Punctoidea, Payenia Rochebrune and Mabille, 1891, and Flammulina Martens, 1873, have a smooth shell, ornamented with growth lines only, but both genera are different from Patagocharopa because they include larger species (5 and 6mm, respectively), with less whorls (3.25 and 3 whorls), a very tight umbilicus, and lack apertural, semi-internal, or internal barriers. Only a few species of Charopidae (Lilloiconcha Weyrauch, 1965, and Zilchogyra) possess apertural and/or internal barriers, but they differ by their number and position within the shell (Weyrauch, 1965; Miquel et al. 2007; Miquel and Cádiz Lorca, 2008). This is the first time
that a specimen with the present features is described for South America.
Patagocharopa enigmatica new species Figure 7.1–7.3
Diagnosis.—Shell lenticular, with protoconch and teleoconch smooth, and with numerous rounded teeth, one angular, six parietal, one basal, three columellar and a lamella and two additional teeth in the columellar wall.
Etymology.—Feminine. Referring to its particular and amazing internal aperture structures (lamella and teeth).
Examined material.—MPM PI 3336: Holotype.
Geographical provenance.—‘Puesto de la Estancia La Costa’ (=Corriguen Aike; 51° 12’ 8.2” S and 69° 03’ 35.6” W), between the Coyle and Gallegos Inlets.
Stratigraphical provenance.—Santa Cruz Formation, late Bur- digalian/early Langhian, Early-Middle Miocene (between 18 and 15 Myr.). Fossiliferous Level 6 of Tauber (1997a), late Burdigalian, ~17.5 Myr (Fleagle et al. 2012).
Description.—Fragment of shell lenticular, very small, with more than three whorls, with smooth protoconch and tele- oconch; with numerous rounded semi-internal barriers—teeth and lamellae—on the inner surface of the whorl: one basal, six parietal and one angular lamellae, and on the columellar area lie an oblique lamella and two additional teeth; umbilicus covered by matrix.
Measurements.—Holotype: 1.6 ×0.7mm, approx. four whorls.
Discussion and paleoenvironmental interpretations The examined specimens are very well preserved, retaining the shape, embryonic and adult whorls, and sculpture. In general terms, the fragmented shells seem to respond to taphonomic processes and can be attributed to sediment compaction; how- ever, the fragmentary condition of the specimens can also be associated with our extraction procedures as evidence of abra- sion and post-mortem transport is missing. The volcanic ashes containing the land snail fossil assemblage within Level 6 of the Santa Cruz Formation also contain vertebrate remains and coprolites, excellently preserved and showing concordant taphonomic signatures (Rodriguez P. E., unpublished data). Therefore, this land snail fossil assemblage can be attributed to demic, parauthochtonous paleological entities and processes (Kidwell 1986), which preserved the shells and maintained them in situ (Fernández López, 1990: 75). Geographic ranges of land snails are usually explained to a
great extent by the prevailing relative environmental humidity. Other relevant factors such as atmospheric and soil temperature, leaf-litter depth, type of vegetation cover, and canopy closure affect the distribution of land snails. Extant representatives of the land snail families recognized in this fossil assemblage can
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