Santa Cruz Formation Research Articles

Two new fossil vertebrate localities in the Santa Cruz Formation (late early – early middle Miocene, Argentina), ∼51° South latitude

Two new fossil vertebrate localities are described from the Santa Cruz Formation (late early – early middle Miocene) of coastal Patagonia. They are noteworthy because they are the lowest stratigraphically of any precisely recorded in coastal Santa Cruz Province and they contain a rich fauna including many partially articulated skeletons undisturbed by collecting. Thus, they offer the potential for taphonomic analysis and paleocommunity reconstruction. The latter is particularly intriguing because the fauna document the Miocene Climatic Optimum at >51° South latitude. Together with several previously documented sites in this region, it offers a potential window into the nature of mammalian communities farther south than any other in the world during this time and documents the farthest south distribution of primates.

Eocardiids (Rodentia, Hystricognathi) from the Pinturas Formation, late early Miocene of Patagonia, Argentina

ABSTRACT Eocardiid rodents from the late early Miocene Pinturas Formation (Ameghino's “Piso Astrapothericulense”) of Patagonia are here studied. Three species are recognized: Luantus propheticus Ameghino and Luantus toldensis sp. nov. are recorded exclusively from the Pinturas Formation; Phanomys mixtus Ameghino is also known from the Santa Cruz Formation (Santacrucian SALMA). The dentition of Luantus propheticus has rudiments of accessory prisms in M3 and p4, and the cheek teeth exhibit a bilobated occlusal pattern with interlobular cement and discontinuous enamel covering in senile stages. Luantus toldensis has more hypsodont cheek teeth than L. propheticus, even more developed M3 and p4 accessory prisms, and a bilobated pattern that is attained in earlier stages of wear. Luantus propheticus, L. toldensis, and Phanomys mixtus represent an evolutionary series that leads to the hypselodont pattern of Eocardia; thus, the traditional dichotomy between “Luantinae” and “Eocardiinae” is not supported. Luantus propheticus occurs in the lower and middle sequences of the Pinturas Formation, whereas the more derived species are recorded in localities interpreted as corresponding to the upper sequence. This distribution agrees with that of other faunal components recorded at the Pinturas Formation.

The armadillos (Mammalia, Xenarthra, Dasypodidae) of the Santa Cruz Formation (early–middle Miocene): An approach to their paleobiology

The Santacrucian (early–middle Miocene) fauna is exceptional in its richness (over 70 species recorded in 21 stratigraphic levels). In this contribution we attempt a preliminary paleobiological characterization of the armadillos (Dasypodidae and Peltephilidae). Santacrucian Dasypodidae includes the genera Prozaedyus Ameghino, Proeutatus Ameghino, Stenotatus Ameghino, and Stegotherium Ameghino. The Peltephilidae include the genus Peltephilus Ameghino and others undergoing systematic revision. Body masses and locomotory habits are inferred from the sizes of the proximal and middle shaft dimensions of the limb bones from allometric equations and indices previously modeled in living dasypodids. Masticatory and dietary interpretations are based on the jaw mechanics, the analysis of the temporomandibular joint and mandibular symphysis, and the shape, arrangement and wear patterns of teeth. The body mass averages are 15.28 kg for Proeutatus, 3.72 kg for Stenotatus, 1.12 kg for Prozaedyus, 11.47 for Stegotherium, and 11.07 kg for Peltephilus. The diversity in limb bone proportions is not so marked as in the living species; forelimb indices reflect proportions similar to those of the living dasypodines and euphractines, all of whom are relatively good diggers. None of the Santacrucian taxa reach the degree of cursoriality of Tolypeutes Illiger, the extreme fossoriality of Priodontes F. Cuvier, or the subterranean habits of Chlamyphorus Harlan. Within a potentially generalized omnivory, some specializations can be easily detected within the Santacrucian armadillos: herbivory in Proeutatus and Peltephilus, and myrmecophagy in Stegotherium. This could help to explain the potential coexistence of so many genera in a single area, diminishing the degree of competition for resources.

Climatic and ecologic changes during Miocene surface uplift in the Southern Patagonian Andes

The up to ∼4 km high southern Patagonian Andes form a pronounced topographic barrier to atmospheric circulation in the southern hemisphere westerlies, and cause one of the most drastic orographic rain shadows on earth. Geologic data imply that this climatic pattern has been established or significantly enhanced during Miocene surface uplift of this Andean segment. We report evidence for important climatic and ecologic changes in the eastern foreland of the Patagonian Andes that appear to be the result of this uplift. To provide constraints on Miocene plant ecosystems and precipitation in the eastern (leeward) foreland of the Patagonian Andes, we determined carbon and oxygen isotope values of pedogenic carbonate nodules from a ∼500 m thick section of the continental Santa Cruz Formation. The age of these deposits was constrained by Ar/Ar dating of intercalated tuffs, which range from ∼22 to 14 Ma. At ∼16.5 Ma, the δ13C values increase by ∼3‰, the δ18O values decrease by >2‰, and the scatter in the oxygen isotope data increases significantly. We interpret these changes as the consequence of >1 km surface uplift in this Andean segment (from the δ18O values), and increased aridity to its east (from the δ13C values and the increased scatter in the δ18O values). Sediments overlying the Santa Cruz Formation are very limited in extent and volume, and dominated by coarse conglomerates related to Pleistocene and older glaciations. It thus seems that, by ∼14 Ma, deposition in the eastern foreland of the Southern Patagonian Andes had essentially ceased as the result of rain shadow formation.

Hystricognath rodents from the Pinturas Formation, Early–Middle Miocene of Patagonia, biostratigraphic and paleoenvironmental implications

The Pinturas Formation is a continental succession, characterized by eolian sediments (mainly tuffaceous) and paleosols, originated in an upland setting of west central Patagonia. The main intraformational erosive surfaces and lithological changes define three sequences. The Formation bears a rich mammalian association (Florentino Ameghino's Astrapothericulan fauna), whose age in relation to the Santacrucian SALMA (middle Miocene) is still controversial. Recent collections from the Pinturas Formation, performed with stratigraphic control, allow differentiation of two distinct hystricognath rodent associations. The lower and middle sequences bear a particular combination of Colhuehuapian (early Miocene) and Santacrucian genera, mostly represented by species exclusively known to the Pinturas Formation (‘Pinturan’ association). The upper sequence bears typical Santacrucian species, more derived than its Pinturan counterparts. According to the rodent record, the lower and middle sequences of the Pinturas Formation are older than the base of the Santa Cruz Formation exposed at Monte Observación and Monte León, and the upper sequence may be correlated with the lowermost levels of the Santa Cruz Formation and deposits exposed at Karaiken that bear Ameghino's ‘Notohippidian’ fauna. These correlations agree with more recent radiometric dates and other biostratigraphic evidence, supporting Ameghino's original hypothesis. The Pinturan rodent assemblage of the lower and middle sequences suggests the presence of humid forests, in accordance with other faunal components and palynological data. Sedimentologic, paleopedologic, and ichnologic evidence, however, suggest environments dominated by herbaceous vegetation. This seeming contradiction is interpreted as the result of a marked environmental gradient due to the paleotopography and/or climatic fluctuations. The mammal record corresponds to the more humid intervals, which have less representation in the sedimentary record. The Pinturas case reveals that more detailed analyses are necessary to understand the relationship between the stressed abiotic Patagonian scenarios and the evolution of their Tertiary vertebrate communities.

Abderitid marsupials from the Miocene of Patagonia: An assessment of form, function, and evolution

Abderitid marsupials are common in vertebrate-bearing deposits from the middle Miocene of Argentine Patagonia. Recent collections from the inland Pinturas Formation and slightly younger coastal Santa Cruz Formation have dramatically increased the number of abderitid specimens. These new collections permit a re-assessment of abderitid taxonomy as well as an investigation of the dietary habits of these unique small mammals. The vast majority of new specimens representAbderites meridionalis; Pithiculites minimusis rare. Patterns of macrowear on the double-bladed, plagiaulacoid shearing complex suggest that abderitids used these teeth to prepare a variety of resistant food items as do modern marsupials with double-bladed shearing systems. Data summarizing molar-shearing morphology and body size further suggest that A. meridionalis was a frugivore. The diet of the small P. minimus is equivocal, although it may represent a mixed feeder (frugivore/faunivore). A comparison of relative species richness and dietary adaptation between abderitids and palaeothentids (a closely related caenolestoid family that lacks the highly specialized shearing complex of abderitids) reveals distinct evolutionary patterns within the two lineages. Abderitids exhibit low species diversity. In contrast, palaeothentids are represented by 17 species, lack highly specialized shearing mechanisms, and typically exhibit molar morphologies that range from frugivory to faunivory and include mixed feeders. Both temporal and geographic variation are introduced as possible factors affecting differences in the relative abundance of abderitids and palaeothentids in the Pinturas and Santa Cruz Formations.

ABDERITID MARSUPIALS FROM THE MIOCENE OF PATAGONIA: AN ASSESSMENT OF FORM, FUNCTION, AND EVOLUTION

Abderitid marsupials are common in vertebrate-bearing deposits from the middle Miocene of Argentine Patagonia. Recent collections from the inland Pinturas Formation and slightly younger coastal Santa Cruz Formation have dramatically increased the number of abderitid specimens. These new collections permit a re-assessment of abderitid taxonomy as well as an investigation of the dietary habits of these unique small mammals. The vast majority of new specimens represent Abderites meridionalis; Pithiculites minimus is rare. Patterns of macrowear on the double-bladed, plagiaulacoid shearing complex suggest that abderitids used these teeth to prepare a variety of resistant food items as do modern marsupials with double-bladed shearing systems. Data summarizing molar-shearing morphology and body size further suggest that A. meridionalis was a frugivore. The diet of the small P. minimus is equivocal, although it may represent a mixed feeder (frugivore/faunivore). A comparison of relative species richness and dietary adaptation between abderitids and palaeothentids (a closely related caenolestoid family that lacks the highly specialized shearing complex of abderitids) reveals distinct evolutionary patterns within the two lineages. Abderitids exhibit low species diversity. In contrast, palaeothentids are represented by 17 species, lack highly specialized shearing mechanisms, and typically exhibit molar morphologies that range from frugivory to faunivory and include mixed feeders. Both temporal and geographic variation are introduced as possible factors affecting differences in the relative abundance of abderitids and palaeothentids in the Pinturas and Santa Cruz Formations.

Systematics, Biostratigraphy, and Dental Evolution of the Palaeothentidae, Later Oligocene to Early–Middle Miocene (Deseadan–Santacrucian) Caenolestoid Marsupials of South America

The family Palaeothentidae contains some of the dentally more specialized of the small-bodied marsupials of South America and was a clade almost equivalent with the Abderitidae in having been the most abundant caenolestoids. They were unquestionably the most diverse, containing two subfamilies, nine genera, and 19 species, with a distribution ranging from Colombia to Tierra del Fuego. The best and most continuous record of the Palaeothentidae is from Patagonian Argentina where eight genera and 17 species are recognized. There, the Palaeothentidae ranged in age from the Deseadan (later Oligocene) through the late Santacrucian (middle Miocene—the Santacrucian record lasting from about 19.4 m.y. to considerably less than 16.05 m.y. before the present). The family appears to have survived longer in Colombia. The palaeothentinePalaeothentes boliviensis(Bolivia) and the incertae sedis genus and speciesHondathentes cazador(Colombia) are the only taxa restricted to an extra-Argentine distribution.Two palaeothentid subfamilies are recognized. The subfamily Acdestinae is new and is erected to accommodate four genera and five species of herbivorous to frugivorous palaeothentids known from the Deseadan through the middle–late Santacrucian. Three of those genera are new (Acdestoides, Acdestodon, andTrelewthentes), as are three acdestine species placed in the generaAcdestodon, Trelewthentes, andAcdestis.The largely faunivorous Palaeothentinae includes four genera and 13 species; the generaPropalaeothentesandCarlothentesare new and new species are described for the generaPropalaeothentes(2) andPalaeothentes(3).Carlothentesis named for Ameghino's Deseadan speciesEpanorthus chubutensis, and Ameghino's genusPilcheniais resurrected to accommodate DeseadanP. lucina.New species include:Acdestodon bonapartei, Trelewthentes rothi, Acdestis lemairei, Palaeothentes marshalli, P. migueli, P. pascuali, andPropalaeothentes hatcheri.The Palaeothentinae contains more generalized palaeothentid species than does the Acdestinae, but also includes some very specialized forms. The most generalized known palaeothentid is the ColombianHondathentes cazador.Both the Acdestinae and Palaeothentinae have large- and small-bodied species;Palaeothentes arataewas the largest palaeothentid (about 550 g), andP. pascualin. sp. the smallest (about 50 g). The oldest known members of both subfamilies consist of five of the six largest palaeothentids.The evolutionary history of the Palaeothentidae is complicated by thick sequences containing no fossils, several lacunae in sequences that yield fossils, and a continent-wide distribution of localities. By far the densest and most continuous record of the family exists in the coastal Santa Cruz Formation of Patagonian Argentina. Three major clades exist within the Palaeothentidae: 1) the incertae sedis speciesHondathentes cazador; 2) the Acdestinae; and 3) the Palaeothentinae (including the new genusPropalaeothentes). The evolution of dental characters in these clades is documented with the aid of 719 new specimens (about 80% of the hypodigm of the family), most of which (about 90% of the new specimens) have precise stratigraphic data.Biostratigraphic study of the new samples was assisted by a new technique of temporal analysis of paleosols and by radiometric age determinations, the latter indicating that the upper part of the Pinturas Formation (16.6 Ma) is older than the lower part of the Santa Cruz Formation (16.4 Ma) and that the top of the marine Monte León Formation (Grupo Patagonica) is older than either (19.4 Ma).Fifty-two gnathic and dental characters were used to identify the taxonomy and to reconstruct the phylogeny of the Palaeothentidae. Analysis of sequencing of appearances of derived characters documents rampant convergences at all taxonomic levels and considerable phenotypic plasticity (variable percent representation of different mutable character morphs) in the organization of the palaeothentid dentition. Certain highly generalized character states survive for the duration of the family in some lineages, whereas others are phenotypically lost for a time and then reappear as a minor percentage of character variability. In general, replacement faunas of palaeothentids were morphologically more generalized than their antecedent forms. The high rate of character mutability and the survival and reappearance of generalized dental characters in the Palaeothentidae were probably related to massive events of pyroclastic deposition that periodically caused at least local extinctions of small mammal populations throughout the duration of the Patagonian middle Tertiary. Dental character regression indicates that palaeothentids arose prior to the Deseadan from a relatively large-bodied marsupial having generalized tribosphenic molars with more or less bunodont cusps; probably an unknown member of the Didelphidae.

Iron and manganese deposits of the Urucum District, Mato Grosso do Sul, Brazil

The Urucum district is located in the Brazilian state of Mato Grosso do Sul near the city of Corumba on the Brazilian-Bolivian border. The district comprises an area of 200 km 2 , where eight up to 1,000-m-high tabular mountains rise out of the swampy lowlands of the Pantanal. The tabular mountains are partly built by an up to 350-m-thick rock sequence of the Santa Cruz Formation. From bottom to top, the Santa Cruz Formation is composed of manganese ore horizon Mn 1, an 80-m-thick unit of ferruginous sandstones, and manganese ore horizon Mn 2, which is overlain by an up to 270-m-thick monotonous sequence of hematite-jaspilites. Forty meters above Mn 2 is manganese ore horizon Mn 3; another 40 m above Mn 3 is manganese ore horizon Mn 4. Both are intercalated with the hematite-jaspilites. The iron ore reserves of the Urucum district amount to approximately 36 billion tons. The calculation of the manganese ore reserves in the four horizons results in 608 million tons, comprised of 23.4 million tons of measured, 34.5 million tons of indicated, 465.1 million tons of inferred, and 86 million tons of hypothetical and speculative reserves. In the Urucum district predominantly layered, detritus-rich, and concretionary cryptomelane ores are found. The ores consist mainly of micro- to cryptocrystalline cryptomelane associated with hematite and quartz. Braunite, pyrolusite, and lithiophorite are minor constituents. Layered braunite ores mined are composed of braunite, cryptomelane, pyrolusite, hematite, and quartz. The average Mn contents of these four different types of manganese ores range from 25.63 percent in the detritus-rich cryptomelane ores to 49.51 percent in the concretionary ones. The fresh hematite-jaspilitic iron ores contain about 50 percent Fe, whereas in the weathered and quartz-depleted hematite-jaspilites the iron content increases to 67 percent.Most of the iron and manganese ores were precipitated as chemical sediments in a partly ice-covered, fjordlike basin. Field observations, Rb/Sr isotope studies, and leaching experiments show that large-scale solution, transport, and reprecipitation processes led to supergene concentrations of iron and manganese in the underlying sandstones at the bottom of the Santa Cruz Formation and in manganese ore horizon Mn 1.

Sedimentary paleoenvironments of fossil platyrrhine localities, Miocene Pinturas Formation, Santa Cruz Province, Argentina

Abstract The Pinturas Formation is a pyroclastic and epiclastic aeolian deposit of Miocene age lying discordantly upon Jurassic rocks in the elevated Andean precordillera of northwest Santa Cruz Province, Argentina. The history of development of the Pinturas Formation was significantly affected by the gradual, though sporadic, draping of this aeolian sediment across a profound, slowly filling paleotopography. The Pinturas depositional cycle consisted of: (1) minor aeolian deposition followed by soil formation, and (2) major aeolian deposition followed by intervals of regional erosion. Fluvial action seems to have been almost wholly confined to intraformational erosion, and two significant intraformational erosional unconformities divide the Pinturas Formation into three sequences. The lower sequence is dominated by pyroclastic mudrocks upon which were formed very mature, probably mollic, paleosols; the middle sequence is composed largely of epiclastic sand occurring as barchanoid paleodunes; and the upper sequence consists of massive, poorly bedded pyroclastic mudrocks. Many Pinturas lacunae were reconstructed on the basis of locally preserved strata, and a novel method of holostrome reconstruction using relative paleosol maturities places Pinturas sedimentation in a more accurate temporal light. It also indicates: (1) that the Pinturas sediment accumulation rate increased with time; (2) that regional erosive intervals are correlated directly with major influxes of pyroclastic material; and (3) that the introduction of the Pinturas platyrrhine primates occurred in the sequence: Carlocebus carmenensis, C. intermedius and Soriacebus ameghinorum. Soriacebus adrianae . Pinturas paleosols appear to have formed under moist conditions, and both mature and immature varieties yield a host of ichnofossils. These include the burrows and nests of bees, scarabeid beetles, termites, and at least two different kinds of colonial rodents, in addition to rhizoliths and the calcified boles and root systems of trees. A fossil nest of a nasutitermitine termite in the lower sequence of the formation indicates the presence of tropical forest. Climatic conditions may have been drier during deposition of the barchanoid paleodunes in early late Pinturas time, or the dunes might reflect drier source areas or have simply encroached on areas of highland forest. Fossil mammals are abundant in Pinturas sediments, and attritional concentrations of them were recognized in the upper parts of paleosols and on the floors of erosional scours. Radiometric dates indicate that the fossil mammals (including platyrrhine primates) occurring in the lower and middle parts of the formation may range in age from about 16·6 to younger than 13·3 Ma (million years ago) (Santacrucian and, almost certainly, Friasian land-mammal ages). This age range is somewhat younger than previous estimates, and suggests that the Pinturas faunas correlate broadly with those from the type Santa Cruz Formation, with the presumed position of the type Friasian, and with the base of the marine Gaiman Formation in the lower valley of the Rio Chubut.

Geochronology of Type Santacrucian (Middle Tertiary) Land Mammal Age, Patagonia, Argentina

Mammal-bearing lacustrine and tuffaceous sediments from three localities of the Santa Cruz Formation, type fauna of the Santacrucian Land Mammal Age, in Patagonia, southern Argentina, are calibrated by radioisotope dating with the aid of magnetostratigraphy. The strata range from about 17.6 Ma to perhaps 16.0 Ma, and are thus of late-early Miocene age. The Santacrucian Land Mammal Age ranges from about 18.0 Ma to about 15.0 Ma.

South American Geochronology: Radiometric Time Scale for Middle to Late Tertiary Mammal-Bearing Horizons in Patagonia

Radiometric (potassium-argon) age determinations for basalts and tuffs associated with middle to late Tertiary mammal-bearing horizons in Patagonia, southern Argentina, permit refinement of boundaries and hiatuses between beds of Deseadan (early Oligocene) through Friasian (middle to late Miocene) age. At two localities beds of Deseadan age are overlain by basalts, which gave dates of 33.6 and 35.4 million years ago; 34.0 million years ago is tentatively accepted as a terminal date for known Deseadan. At several localities beds of Colhuehuapian age are underlain by basalts, which gave dates ranging from 28.8 to 24.3 million years ago; 25.0 million years is tentatively taken as a basal age for known Colhuehuapian. The paleontological hiatus between known Deseadan and known Colhuehuapian is thus in the order of 9.0 million years. Two tuffs from the Santa Cruz Formation (Santacrucian) gave ages of 21.7 and 18.5 million years. Plagioclase and biotite concentrates of an ignimbrite from the Collón Curá Formation (Friasian) gave ages ranging from 15.4 to 14.0 million years.

II.—The Relationships of the ‘Sparassodonta.’

The Sparassodonta are an interesting group of extinct mammals found in the Tertiary formations of Patagonia. They appear to have taken the place of true Carnivora in South America during most of the Tertiary period, as the Carnivorous Marsupials do in the modern fauna of Australia. Their relationship has been in dispute since they were first made known to science by the distinguished South American paæontologist Florentino Ameghino. In dental formula and other characters of the teeth and jaws they agree with the Carnivorous Marsupials, and show a high degree of adaptive specialization for predaceous habits. Dr. Ameghino was able to determine, however, from specimens in his collection, that the replacement of the premolars, in certain genera at least, was more complete than in modern marsupials, and approached that of the placental carnivora. He regarded them, therefore, as an intermediate group, ancestral to the Creodonta and modern Carnivora. Mr. Tomes, in his interesting study of thb enamel structure of the Creodonta and allied groups, has recently shown that in one genus of Sparassodonts the enamel shows the more highly differentiated structure of the true Carnivora, instead of the more primitive structure of Marsupial Carnivores. The collections made by the Princeton and American Museums in the Santa Cruz formation of Patagonia contain a splendid series of Sparassodont remains, including skulls and skeletons of the principal genera, which have recently been exhaustively studied by Dr. Sinclair. These show that the Sparassodonts agree with Marsupials, and especially with Thylacinus in almost all the distinctive features of dentition, skull, and skeleton characters by which they are separated from the placental mammals.