Geographical and stratigraphical provenance of the fossil terrestrial shells

The outcrop where the material was collected emerges between the Coyle and Gallegos inlets, on the Atlantic coast of Patagonia in southern Santa Cruz, and belongs to the Rousseau Santa Cruz Formation, of continental origin. This formation is exposed at many places in the eastern portion of southern Extra-Andean Patagonia (Russo et al. Reference Russo, Flores and Di Benedetto1980; Fig. 1.1, 1.2) and was deposited during the late Burdigalian/early Langhian, as conceived in the International Stratigraphic Chart (between 18 and 15 Myr.; Marshall et al. Reference Marshall, Cifelli, Drake and Curtis1986). The Santa Cruz Formation can be divided into two members, i.e. the Estancia La Angelina Member and the underlying Estancia La Costa Member. The materials described herein were recovered from a stratum of volcanic ashes within the Estancia La Costa Member. This member is 120 m thick and characterized by predominantly greenish epiclastic claystones, chonites, and limolitic sandstones, with mature and immature paleosols and low-sinuosity paleochannels (Tauber, Reference Tauber1994). According to Tauber (Reference Tauber1999), there are 21 distinct fossiliferous levels. However, subsequent work by Fleagle et al. (Reference Fleagle, Perkins, Matthew, Heizler, Nash, Bown, Tauber, Dozo and Tejedor2012) and Perkins et al. (Reference Perkins, Fleagle, Heizler, Nash, Bown, Tauber and Dozo2012) suggests that the lowest seven levels identified by Tauber (Reference Tauber1997a) are a single sequence repeated at other northern coastal localities of the Santa Cruz Formation and that there is no temporal difference between them. Accordingly, a rectified age for PLC (paleontological site Puesto de la Estancia La Costa) sequence is estimated between 16.4 and 17.5 Myr. Yet, in this work the designation of the fossiliferous levels follows the nomenclature and the stratigraphical profile of Tauber (Reference Tauber1999; Fig. 2) for ease of identification.

Figure 1

Geographic distribution and location of extant and fossil specimens analysed in this study: (1) Distribution of extant Zilchogyra species extending down to the Colorado River (striped) in northern Patagonia, distribution of extant Gastrocopta species (dots), the square indicates the fossil record of Gastrocopta from the Pliocene of Mendoza province, the triangle indicates the record of Punctum from Juan Fernández Islands; (2) (inset), detail of the location of Corriguen Aike between Gallegos and Coyle rivers in Santa Cruz province.

Figure 2

Stratigraphic profile of Santa Cruz Formation (continental, late Burdigalian/early Langhian, between 18 and 15 Myr). Fossiliferous levels are indicated in arabic numerals. CA: outcropping fossiliferous levels at Corriguen Aike. The arrow indicates fossil-bearing Level 6 (after Tauber, Reference Tauber1999), from which land snail specimens were recovered. Modified from Tauber Reference Tauber1999.

The material studied was collected at the paleontological site ‘Puesto de la Estancia La Costa’ (=Corriguen Aike; PLC: 51° 12’ 8,2” S and 69° 03’ 35,6” W; Fig. 1.1, 1.2). The bed yielding the fossil material is Tauber’s fossiliferous Level 6 (Fig. 2), which consists of a brownish massive cineritic tuff, settled as a pyro-arenite layer. This level is distinguished by a significant amount of relatively complete skeletal remains of mammals and birds, and coprolites appearing at the exposed beds on the intertidal zone during low tide. The lithogenetic characteristics of the fossil-bearing rocks suggest low paleotopographic environments in proximity to floodplains, where the humidity input was high to moderate (Tauber, Reference Tauber1994).

Systematic paleontology

Superfamily Pupilloidea Turton, Reference Turton1831

Family Vertiginidae Fitzinger, Reference Fitzinger1833

Subfamily Gastrocoptinae Pilsbry, Reference Pilsbry1918

Genus Gastrocopta Wollaston, Reference Wollaston1878

Occurrence in Argentina

Modern records indicate its occurrence from temperate and tropical regions of the north and central areas of the country down to the area over northern Patagonia, approximately at 34°S (Hylton Scott, Reference Hylton Scott1963, Reference Hylton Scott1968). Doering (Reference Doering1881) reported it from the banks of the Colorado River, in northern Patagonia. The new species described herein is the oldest record of Gastrocopta from Argentina. The genus had been previously recorded from the Pliocene of Mendoza (Turazzini and Miquel, Reference Turazzini and Miquel2013, Reference Turazzini and Miquel2014) and the Holocene of Entre Ríos (Miquel and Aguirre, Reference Miquel and Aguirre2011). Miocene–Recent.

Gastrocopta patagonica new species

Figure 3.1–3.4

Figure 3

Gastrocopta patagonica n. sp; (1) MPM PI 3314, holotype, apertural view; (2–4) MPM PI 3315, paratype: (2) apertural view; (3) apertural view and detail of teeth; (4) detail of the pustulae. Scale bars represent 500 µm (1, 3), 200 µm (2), and 20 µm (4) 20 µm.

Occurrence

Brazil, Bolivia, Paraguay, Uruguay, and Argentina. Pleistocene–Recent (Miquel and Aguirre, Reference Miquel and Aguirre2011).

?Scolodonta sp.

Figure 4.1–4.3

Figure 4

Scolodonta sp: (1–3) MPM PI 3316; (1) apical view; (2) detail of apex; (3) detail of umbilicus. Scale bars represent 1000 µm (1, 3) and 500 µm (2).

Occurrence

Geographical and stratigraphical records of the genus include Europe, Africa, Australia, New Zealand, subantarctic islands and Juan Fernández Archipelago (Chile) (Odhner, Reference Odhner1922; Zilch, Reference Zilch1959; Solem, Reference Solem1983; Stuardo and Vega, Reference Stuardo and Vega1985; Miquel and Araya, 2015). Oligocene–Recent (Zilch, Reference Zilch1959). Particularly in Argentina, this genus has not been found living. This is the first mention of the genus for continental South America; its record is Miocene–Recent.

Punctum patagonicum new species

Figure 5.1–5.4

Figure 5

Punctum patagonicum n. sp.: (1) MPM PI 3317, holotype, apical view; (2) detail of the protoconch; (3) MPM PI 3318, paratype, apical view; (4) detail of the protoconch. Scale bar represents 500 µm (1, 2) and 200 µm (3, 4).

Occurrence

The current range of this genus covers tropical areas of South America, including at least Peru, Brazil, Chile and Argentina (Fernández, Reference Fernández1973; Flórez Bustamante, Reference Flórez Bustamante1968; Hylton Scott, Reference Hylton Scott1970; Miquel et al. Reference Miquel, Ramírez and Thomé2007; Miquel and Barker, Reference Miquel and Barker2009; Weyrauch, Reference Weyrauch1965). Middle Eocene–Recent (Miquel and Bellosi, Reference Miquel and Bellosi2010).

Zilchogyra miocenica new species

Figure 6.1–6.3

Figure 6

Zilchogyra miocenica n. sp.: (1) MPM PI 3319, holotype, apical view; (2) MPM PI 3320, paratype, detail of umbilicus and protoconch; (3) MPM PI 3321, paratype, umbilical view. Scale bars represent 500 µm (1, 3) and 100 µm (2).

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, Reference Rochebrune and Mabille1891, and Flammulina Martens, Reference von Martens1873, have a smooth shell, ornamented with growth lines only, but both genera are different from Patagocharopa because they include larger species (5 and 6 mm, 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, Reference Weyrauch1965, and Zilchogyra) possess apertural and/or internal barriers, but they differ by their number and position within the shell (Weyrauch, Reference Weyrauch1965; Miquel et al. Reference Miquel, Ramírez and Thomé2007; Miquel and Cádiz Lorca, Reference Miquel and Cádiz Lorca2008). This is the first time that a specimen with the present features is described for South America.

Figure 7

Patagocharopa enigmatica n. gen. n. sp.: (1–4) MPM PI 3336, holotype; (1) apertural view, arrows point the suture cicatrix; (2) detail of apertural teeth, arrows point the semi-internal barrier-teeth and lamellae; (3) apical view. Note: image (2) was digitally enhanced and modified to better visualize structures. Scale bars represent 500 µm (1, 3) and 200 µm (2).

Patagocharopa enigmatica new species

Figure 7.1–7.3

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; however, the fragmentary condition of the specimens can also be associated with our extraction procedures as evidence of abrasion 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 Reference Kidwell, Fuersich and Aigner1986), which preserved the shells and maintained them in situ (Fernández López, Reference Fernández López1990: 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 be currently found in environments ranging from subtropical to cold, and from very humid to semiarid. The Vertiginids of the northern hemisphere inhabit a wide range of environments from costal dunes, swamp forests or woody talus slopes of northern Europe and North America, to open deciduous forests, treeless marshlands, and wet meadows of warm lowland areas of central regions of the Holarctic continents (Vavrovà et al. Reference Vavrovà, Horsàk, Šteffek and Cejka2009). However, some North American Vertiginidae are limited to xeric herbaceous-dominated prairies and shrublands (Nekola and Coles, Reference Nekola and Coles2001). In addition, South American records of Gastrocopta nodosaria occur in temperate and tropical regions of the country down to the northern area of Patagonia (Miquel and Aguirre, Reference Miquel and Aguirre2011). South American charopids are primarily mesic forest dwellers, but extend to dry and open scrublands of Andean-Patagonian forests in northwestern Patagonia (Miquel and Barker, Reference Miquel and Barker2009; Miquel and Bellosi, Reference Miquel and Bellosi2010). Punctidae species inhabit humus, and can be found on fallen leaves, under mesophytic bushes and trees (Pilsbry, Reference Pilsbry1948); particularly, in Juan Fernández Archipelago Punctum sp. dwells on the soil, among leaves and on ferns (Odhner, Reference Odhner1922). The species of Scolodontidae represent an important stock of the soil biota, mainly in tropical rain forests, but they also can be found in dry forests, and a few in grasslands, being carnivorous (Ramírez, Reference Ramírez1993). Thus, the families represented in the Santa Cruz Formation are not particularly informative about the paleoenvironmental conditions prevailing in the area during the Miocene, if the biology of the individual species is not considered.

The fossil site is located, phytogeographically, in the Patagonian Province (Cabrera, Reference Cabrera1971), and zoogeographically it is located within the Patagonian Domain of the Andean-Patagonian Subregion (Ringuelet, Reference Ringuelet1961). Currently, the extensive xerophytic plateau of the Patagonian territory offers only few areas for the survival of land snails because of the dry and cold climate, with snowy winters, and strong winds and frosts during the whole year. This is reflected in the records of land snails from the region where collections mostly come from the mountain ranges and very few from the Patagonian plateau sensu stricto (Hylton Scott, Reference Hylton Scott1963, Reference Hylton Scott1968). The modern fauna is predominantly that of micromollusks of the family Charopidae with a much lower representation by species of Succineidae, Bulimulidae, Scolodontidae, Vertiginidae, and Macrocyclidae (Fernández, Reference Fernández1973).

To account for paleoenvironmental conditions prevailing in the area inhabited by the present fossil assemblage, a brief overview of the paleoclimatic progression and paleoenvironmental evolution that occurred throughout the Early Miocene is required.

According to Compagnucci (Reference Compagnucci2011) changes in land surface represent one of the processes that affect climate in time scales from millions to hundreds of thousands of years, and the climatic conditions in the region of Patagonia were strongly affected by the uplift of the Andes. The Andean orogeny is the final consequence of the Andean structural cycle that developed throughout the past ~190 Myr (Early Jurassic to present), which results from the subduction of the eastern plates of the Pacific Ocean (Farallon and Nazca) and the Antarctic plate under Gondwanan basement (South American plate) at the west margin of South America (Armijo et al., Reference Armijo, Lacassin, Coudurier-Curveur and Carrizo2015). As part of this Andean cycle, the uplift of the southern Patagonian Andes (approximately between 17 and 14 Myr) formed a large topographic barrier to atmospheric circulation in the Southern Hemisphere Westerlies and established a pronounced orographic rain shadow; such a barrier led to a strong aridification in the eastern foreland and increased precipitation rates on the windward western side of the mountains during the Miocene in Patagonia (Blisniuk et al., Reference Blisniuk, Stern, Chamberlain, Zeitler, Ramos, Haschke, Sobel, Strecker and Warkus2006, cited in Compagnucci, Reference Compagnucci2011).

Besides, ice had begun accumulating at the South Pole during the Late Eocene (approximately between 37.2 and 33.8 Myr) culminating in the Pleistocene ice age. Oligocene cooling occurred together with changes in continental distribution, plateau uplift and opening of oceanic gateways (Drake Passage and Tasmanian Gateway), implying the late Eocene establishment of the Antarctic Circumpolar Current and development of the Polar Frontal Zone, reducing poleward ocean heat transport and favoring glaciation over Antarctica. The Late Miocene Tortonian period (approximately between 11 and 7 Myr) was characterized by intensive Antarctic glaciations and the beginning of glaciations in the North Atlantic region (Compagnucci, Reference Compagnucci2011).

The Andes orogen interplayed with an increasingly drying climate driven by the global Cenozoic cooling, resulting in pronounced alterations of the paleoclimate conditions over Patagonia (Armijo et al., Reference Armijo, Lacassin, Coudurier-Curveur and Carrizo2015). During the Miocene paleoclimate settings along southern Patagonia were fluctuating, but showed an overall downward trend in temperature and upward trend in aridity (Blisniuk et al., Reference Blisniuk, Stern, Chamberlain, Idleman and Zeitler2005, Reference Blisniuk, Stern, Chamberlain, Zeitler, Ramos, Haschke, Sobel, Strecker and Warkus2006; Kay et al. Reference Kay, Vizcaíno and Bargo2012a). Animal and plant communities changed according to these oscillations. Toward the end of the Paleogene and the beginning of the Neogene (Late Oligocene–Early Miocene) warm climates allowed the development of rainforest elements in the flora of extra-Andean Patagonia, with dominance of gallery forests to the east and a subtle emergence of xerophytic assemblages in coastal salt environments and pockets of inland areas. Even though drier conditions would have prevailed in lowland areas during the late Early Miocene (Late Aquitanian), the latest Early Miocene (Burdigalian) was characterized by a predominance of aquatic herbs and hydrophyte vegetation (Palazzesi et al., Reference Palazzesi, Barreda and Prieto2003; Barreda and Palazzesi, Reference Barreda and Palazzesi2007, Reference Barreda and Palazzesi2010), suggesting plentiful water availability.

Moreover, Palazzesi et al. (Reference Palazzesi, Barreda and Prieto2003) suggested that the inferred paleofloristics is consistent with the paleovertebrate record, as deduced from the work of Patterson and Pascual (Reference Patterson and Pascual1972) and Pascual et al. (Reference Pascual, Carlini, Bond and Goin2002), who highlighted that the greatest diversification of hypsodont-grazer mammals in Patagonia occurred during the Oligocene and Early Miocene; this supports a paleoenvironmental change toward an increasing predominance of grassy vegetation. Several works on fossil vertebrates have assessed the paleoenvironmental shift that occurred in southern Patagonia throughout the Miocene. Among others, these studies addressed a diverse group of mammals including arboreal porcupines (Echimyidae, Erethizontidae), microbiotheres similar to the extant “monito del monte” (Microbiotheriidae), and new world primates (Platyrrhini), all of which have been regarded as indicators of warm and humid climates and of forest environments (Pascual and Ortiz-Jaureguizar, Reference Pascual and Ortiz-Jaureguizar1990; Tauber, Reference Tauber1997b, Reference Tauber1999; Vizcaíno et al. Reference Vizcaíno, Bargo, Kay and Milne2006; Kay et al., Reference Kay, Perry, Malinzak, Allen, Kirk, Plavcan and Fleagle2012b). Patterson and Pascual (Reference Patterson and Pascual1972) and Pascual et al. (Reference Pascual, Carlini, Bond and Goin2002). In this regard, Tauber (Reference Tauber1997b) also described a relative increase in the percentage of organisms with euhypsodont dentition—which are considered specialized grazers—against organisms with brachydont and brachydont-mesodont dentition toward the upper levels of Santa Cruz Formation. In addition, a general decrease in taxonomic and body size diversity of small-sized mammals and increasing taxonomic diversity and size of other mammals like glyptodonts and toxodonts was found. Overall, this points to a climate turnover throughout the Miocene from warm, moist, stable conditions to drier conditions with marked seasonality, and a shift from environments with predominant tree or shrubby vegetation to environments more open with predominantly grassy vegetation. Similar inferences resulting from analysis based on the diversity of the Santacrucian armadillos by Vizcaíno et al. (Reference Vizcaíno, Bargo, Kay and Milne2006) reinforced the climate-turnover hypothesis.

In addition, analyses of sedimentological, paleopedological, and geomorphological evidence (Tauber, Reference Tauber1994, Reference Tauber1996), support the aforementioned Miocene paleoenvironment progression. Immature and mature paleosols, the latter with a high degree of animal and plant bioturbation, have been found in the lower beds of the Santa Cruz Formation, suggesting humid and stable environmental conditions; while toward the upper levels occurrence of evaporite crystals, geomorphological structures (interpreted as desiccation cracks) and inferred seasonal variations in rivers paleo-flux, indicate increasing seasonality and a drying trend.

However, paleoclimatic approximations based on analyses of community structure (cenograms), which allowed to infer variables such as rainfall and vegetation structure (Croft Reference Croft2001), showed a less marked shift in paleoenvironmental conditions suggesting a slightly greater rainfall in the upper levels of Santa Cruz Formation that described in previous studies.

Following Kay et al. (Reference Kay, Vizcaíno and Bargo2012a), the Early Miocene (17.4–17.5 Myr) Santacrucian-age fauna thrived in a mosaic of open temperate humid and semi-arid forests, with paleoclimatic reconstructions indicating annual rainfalls greater than 1000 mm and a mean annual temperature above 14°C in a period characterized by wet winters and dry summers, and a marked seasonality in the length of daylight. In addition to the presence of lakes, seasonal flooding led to the formation of marshlands dominated by herbaceous (both grass and forb) vegetation. To this extent, various elements of the vertebrate associations of the Santa Cruz Formation, other than mammals, suggest a mosaic of closed canopy forest and open habitats during the late Early Miocene. Several studies have assessed groups of vertebrates such as frogs (Calyptocephalellidae and Leptodactylidae), teiids (Tupinambis sp.), colubrids, and iguanids, whose presence suggests warm and humid environmental conditions and the occurrence of permanent lowland lakes and quiet streams in forested areas (Fernicola and Albino Reference Fernicola and Albino2012). Other records such as birds like the falconid Thegornis (Noriega et al., 2011), and aquatic birds such as waterfowl, limpkins, spoonbills, and darters, indicated the presence of seasonally flooded open areas or permanent water bodies in more forested areas, whereas phorusrhacids and seriemids may have required open habitats because of their cursorial mode of locomotion (Degrange et al., Reference Degrange, Noriega and Areta2012).

Subsequently, the Late Miocene vegetation became similar to the modern pattern, with an expansion of steppe throughout extra-Andean Patagonia and withdrawal of forests to western areas where rainfall was still abundant (Barreda and Palazzesi, Reference Barreda and Palazzesi2007).

Even though some degree of aridity occurred throughout the Early Miocene, paleoenvironmental conditions shifted into a more humid context toward the latest Early Miocene (Burdigalian), when Neotropical floral elements become predominant again (Barreda and Palazzesi, Reference Barreda and Palazzesi2007, Reference Barreda and Palazzesi2010). The environment during this time slice, known as the mid-Miocene Climatic Optimum (MMOC, 17 to 15 Myr; Zachos et al., Reference Zachos, Pagani, Sloan, Thomas and Billups2001), can confidently be taken to be that which prevailed when the snail-bearing layer was deposited. According to Fleagle et al. (Reference Fleagle, Perkins, Matthew, Heizler, Nash, Bown, Tauber, Dozo and Tejedor2012), this brief geological period can also be reliably correlated with warm-humid tropical conditions. Thus, the wide range of suitable habitat types suggested by the land snail paleocommunities, as indicated by the terrestrial molluscan taxa of our study, support and reinforce the pre-Late Miocene warm and humid tropical paleoenvironmental and paleoclimate settings proposed previously worldwide and for Patagonia.

In summary, the studied land snail assemblage includes the following specimens and their related current habitats: (a) Gastrocopta patagonica n. sp. (Vertiginidae), oldest record of the genus for Argentina and the southernmost record for South America, inhabits temperate and tropical regions; (b) Scolodonta sp. (Scolodontidae), inhabits from tropical rain forests, to dry forests and grasslands; (c) Punctum patagonicum n. sp. (Punctidae), first mention of the genus for continental South America, inhabits ferns, under mesophytic bushes and trees; (d) Zilchogyra miocenica n. sp. (Charopidae), first record of the genus from Miocene, inhabits tropical areas of South America from mesic forests to dry and open scrublands; and (e) Patagocharopa enigmatica n. gen and sp. (?Charopidae), first record for South America.

The studied assemblage comprises taxa in part typical of austral regions such as the punctoidean genera Zilchogyra, and Punctum (previously known in South America only from extant specimens recorded in Juan Fernández Archipelago), together with the much more widely distributed Gastrocopta. Patagocharopa enigmatica has no clear affinities to any fossil or recent South American continental molluscan faunas.

Being this the first record of this type of assemblage for South America, and considering the high significance of land and fresh-water snails for paleoenvironmental reconstructions (Rousseau et al., Reference Rousseau, Limondin, Magnin and Puisségur1994; Rousseau and Puisségur, Reference Rousseau and Puisségur1999; Rousseau and Wu, Reference Rousseau and Wu1999; Del Río et al., Reference Del Río, Camacho, Aguirre, Caramés, Cuminsky, De Francesco, Farinati, Gordillo, Laprida, Miquel and Morton2007; Miquel and Bellosi, Reference Miquel and Bellosi2010), it is of major importance to emphasize the great potential of further studies of terrestrial mollusks from Patagonia to reveal other interesting elements of the assemblages and to enhance our full understanding of the Austral Miocene paleoenvironmental evolution.