Eduardo Pedro Tonni

Predominance of arid climates indicated by mammals in the pampas of Argentina during the Late Pleistocene and Holocene

Abstract South American climates during the Pleistocene and Holocene are poorly known, especially in the lowlands east of the Andean ranges. However, there is relatively good biostratigraphic and climatic information for the late Lujanian and Platan local ages (latest Pleistocene to Holocene) of the Pampas region. Most of the climatic data from the Pampean area are based on land mammal assemblages, pollen records, and geology. Contrasting with North American faunal associations, there are many non-analogue assemblages in the Holocene of the Pampean area. The climate was arid and relatively cold during most of the early Lujanian. However, several relatively wet and warm events are suggested by the presence of subtropical mammals and evidence of tchernozoid paleosoils. The late Lujanian is well represented by the fossiliferous sediments of the La Chumbiada and Guerrero members of the Lujan Formation (ca. 30,000 and 21,000–13,000 yr B.P., respectively), cropping out in stream valleys in the Buenos Aires Province. Mammal assemblages suggest a change from a temperate to a cool climate from the La Chumbiada Member to the Guerrero Member, the latter representing the last glacial maximum. The lower portion of the eolian La Postrera Formation has been dated at 11,000–8000 yr B.P. This section was deposited on basin divides under dry and cool conditions. The mammal association was similar to that of the Gueffero Member and includes the last megaherbivore mammals in South America (e.g. Megatherium americanum). Mammals occurring in Platan beds usually indicate dry but not cool conditions. Warm and probably wetter conditions were detected in continental beds deposited synchronously with a mid-Holocene marginal marine ingression (ca. 7000–6000 yr B.P.). A Holocene non-analogue assemblage of subtropical mammals and dry/cold indicators occurred at latitude 38°30′S at ca. 2000 yr B.P. Subtropical mammals that require wetter conditions have been migrating southwards in the Pampean region since 1500–1000 yr B.P. Eolian sediments in the La Postrera Formation dated ca. 440 yr B.P. yielded an arid to semiarid mammal assemblage which could relate to the Little Ice Age period. Remarkably, most of the present-day manual biocenoses and climatic conditions in the Pampas seem to have been recently established. Paleogeographic maps for the last glacial maximum and the mid-Holocene are also provided.

Chronostratigraphy and "land-mammal ages" in the Cenozoic of southern South America; principles, practices, and the "Uquian" problem

is proposed. This stage is based on a biostratigraphic scheme. The stratotypes of the stage and biozones are located in the fossiliferous southeastern Buenos Aires Province marine cliffs. The lower boundary stratotype is proposed. The stage-age is probably correlated with the Gauss Chron and the lower Matuyama Chron. Additionally, some major units of Ameghino are validated and a different timing for the arrival of North American mammals to southern South America is presented.

The Broken Zig-Zag: Late Cenozoic large mammal and tortoise extinction in South America

During the latest Pleistocene-earliest Holocene, South American terrestrial vertebrate faunas suffered one of the largest (and probably the youngest) extinction in the world for this lapse. Megamammals, most of the large mammals and a giant terrestrial tortoise became extinct in the continent, and several complete ecological guilds and their predators disappeared. This mammal extinction had been attributed mainly to overkill, climatic change or a combination of both. We agree with the idea that human overhunting was the main cause of the extinction in South America. However, according to our interpretation, the slaughtering of mammals was accomplished in a particular climatic, ecological and biogeographical frame. During most of the middle and late Pleistocene, dry and cold climate and open areas predominated in South America. Nearly all of those megamammals and large mammals that became extinct were adapted to this kind of environments. The periodic, though relatively short, interglacial increases in temperature and humidity may have provoked the dramatic shrinking of open areas and extreme reduction of the biomass (albeit not in diversity) of mammals adapted to open habitats. Many populations were surely close to a minimum level of population viability. During the longer glacial periods, mammals populations recovered. This alternation of low and high biomass of mammals from open and closed areas is what we refer to as the Zig-Zag. During the present interglacial, humans entered South America and broke the Zig-Zag when killed all the megamammals and almost all the large mammals during their less favourable periodic lapse.

Changes of mammal assemblages in the pampean region (Argentina) and their relation with the Plio-Pleistocene boundary

Abstract A gradual impoverishment of autochthonous mammals in the southeastern pampean region (central eastern Argentina) during the Late Cenozoic is detected. It is partially coeval to the gradual immigration of holarctic mammals and to important climatic changes. We propose that the Plio-Pleistocene boundary (at around 2.5 Ma) in the area is located between the Vorohue “Formations” and the base of Miramar “Formations”.

The Ensenada and Buenos Aires formations (Pleistocene) in a quarry near La Plata, Argentina

Abstract An interdisciplinary study of a section in a quarry near La Plata sheds new light on the geologic, climatic, and biologic evolution of the northeastern Pampean area of Argentina. The stratigraphic succession is composed of seven mainly eolian levels, each one including a soil and separated by a disconformity. Two geosols defined in northern Buenos Aires Province (Hisisa and El Tala) are identified in the Ensenada Formation. The boundary between the Ensenada and Buenos Aires formations is defined by a conspicuous disconformity which overlies the El Tala Geosol. In the section, the boundary between the Tolypeutes pampaeus (Ensenadan) and Megatherium americanum (lower Lujanian) biozones coincides with that of Ensenada and Buenos Aires formations. The boundary between the Matuyama and Brunhes zones of polarity occurs in the upper part of the Ensenada Formation above the Hisisa Geosol. According to geologic evidence, most of the succession was deposited under semiarid to arid climate.

Did humans cause the Late Pleistocene-Early Holocene mammalian extinctions in South America in a context of shrinking open areas?

Capitulo VII de Haynes, Gary (ed). American megafaunal extinctions at the end of the Pleistocene. Springer, Vertebrate Paleobiology and Paleoanthropology Series, 2009, 202 p. + 32 il. ISBN 978-1-4020-8793-6 (e-book).

Reassesment of the Pliocene-Pleistocene continental time scale of Southern South America. Correlation of the type Chapadmalalan with Bolivian sections

Abstract Recently MacFadden et al. (1993) described undiferentiated Montehermosan-Chapadmalalan sections at Inchasi in Bolivia. The supposed failure to correlate more accurately the Inchasi sections with the standard time scale points up the drawbacks of “Land-mammal ages” view as discussed by Cione and Tonni (in press). Ongoing research in the pampean region permits the proposal of a relatively reliable local biostratigraphic scale for the Pliocene-Pleistocene. The previous Montehermosan faunal lists (e.g. Marshall et al. , 1983) actually are a mix of Montehermosan and Chapadmalalan taxa. In this paper, Montehermosan and Chapadmalalan are confirmed as distinct stages-ages, supported by diagnostic guide fossils. The Inchasi sections are correlated with the Chapadmalalan on the basis of mammal evidence. The magnetostratigraphic studies at Inchasi confirm that at least part of the Chapadmalalan is older than previously supposed. Our studies show that Chapadmalalan rocks appear to be more widespread than previously envisioned.

Environmental changes in the Pampean area of Argentina at the Matuyama–Brunhes (C1r–C1n) Chrons boundary

Abstract The so-called Pampean sediments are the uppermost sedimentary levels of the Chacopampean basin. They record the sequence of geologic changes that occurred during the late Cenozoic.In the Pampean area, the most important environmental and faunistic changes occurred close to the end of the Matuyama Chron and beginning of the Brunhes Chron. The relatively wet climate recorded in the region at the end of Matuyama Chron turned to drier and colder conditions during the beginning of the Brunhes Chron, which also seems to be characterized by an important volcanic activity in the Cordillera de los Andes. An important faunistic turnover (between Tolypeutes pampaeus and Megatherium americanum biozones) coincides with lithostratigraphic changes (between the Ensenada and Buenos Aires formations). The change of polarity Matuyama–Brunhes is observed below this boundary.

Paleoclimatic implications of the presence of Clyomys (Rodentia, Echimyidae) in the Pleistocene of central Argentina

The presence of the Echimyidae Eumysopinae Clyomys in Ensenadan (Early-Middle Pleistocene) sediments cropping out northeast of Mar del Plata and south of Necochea (costal region of Buenos Aires province, Argentina) is reported. This is the first citation of Clyomys in Argentina and the first post-Pliocene record of echymids at this latitude. These remains represent a species closely related to the extant representatives of the genus, which live at least 1600 km north of the new localities. These fossils suggest that a southward expansion of the Brazilian fauna occurred during a warm and humid pulse in the Pleistocene.

The fossil record of the desert-adapted South American rodent Tympanoctomys (Rodentia, Octodontidae). Paleoenvironmental and biogeographic significance

Abstract New mandibular remains of one specimen of the desert-adapted South American rodent Tympanoctomys from the Pleistocene of central Argentina are described. These materials, together with a hemimandible previously analyzed by Ameghino, represent the limited paleontological record of the genus, and the first undoubted record of a living genus of the Octodontinae subfamily. The taxonomic, paleoenvironmental and biogeographic significance of these materials is discussed. The sediments yielding the new remains, at the coastal region of Mar del Plata, represent one of the glacial events that deposited the Pichileufu Drift in the Cordillera region of Argentina (between isotopic stages 23 and 25). We propose that, as a consequence of the glaciation, favorable habitats for populations of Tympanoctomys would have extended eastwards. Consequently, the present distributional pattern of the genus may be considered a relict.