The First Scenario: Farola Monte Hermoso
The first scenario of this history is represented by Miocene–Pleistocene sediments that outcrop at the western end of the area considered in this article. This site, named Farola Monte Hermoso (FMH; Fig. 2), presents marine cliffs that reach 12 meters in height, along a stretch of more than three kilometers. At this site, one of the most important sequences for understanding the geologic evolution of the Pampean region during the Upper Miocene–Pliocene and Pleistocene crops out. The interpretation of the stratigraphy of the site has changed over time according to different authors, Zavala’s (1993) explanation being the most accepted one (Fig. 3). According to Zavala, the stratigraphic sequence begins with fine sediments of the Monte Hermoso formation deposited on a fluvial environment of muddy and meandering rivers that flowed from east–northeast to west–southwest and that were not connected to the history of Sauce Grande River.
These sediments contain vertebrate remains, particularly of Miocene–Pliocene mammals, members of the native fauna that lived in South America before the Great American Biotic Interchange. Among them, the marsupials were represented by some groups including Borhyaenids, which make their last appearance in the stratigraphic record. Other important representatives were the Xenarthra, both armadillos (Dasypodidae: i.e., Macroeuphractus and Chorobates; and Glyptodontidae: i.e., Plohophorus and Nopachthus) and ground sloths (Mylodontidae: i.e., Proscelidodon and Lestodon). Among the native ungulates, the remains of the Order Notoungulata are frequent, those of the Hegetotheridae, Mesotheriidae, and Toxodontidae families. Members of the two families of the Litopterna order, Protherotheriidae and Macraucheniidae (Promacrauchenia), are also found. There is also a varied representation of rodents corresponding to the second faunistic stratum of South America (Old Island Hoppers, Simpson 1950). Among them, species of Caviidae, Octodontidae, and Hydrochoeridae families are found. Freshwater fish, amphibious, reptile, and bird rests are present as well.
The mammal association of this site is extremely significative, and it is characteristic of a specific evolutionary stage called Montehermosan Stage/Age (Marshall et al. 1983). Actually, this site is the archetype of this biostratigraphic unit. Although most of the fossils preserved here are bones, which have helped define the Stage/Age, there are also medium-sized footprints, which belong to ground sloths, characteristic members of the South American native fauna (Aramayo and de Manera Bianco 1996).
After a long period during which there was no sedimentary record, the Monte Hermoso Formation was topped with aeolian sandstones at the beginning of the Upper Pleistocene. These sediments compose the Puerto Belgrano Formation, formed in a dune and interdune environment (Zavala 1993). On top of this sandstone level, breccias and fluvial conglomerates were deposited, forming the Lower and Medium Members of the Punta Tejada Formation. These deposits are connected to the configuration of the fluvial valley related to the present Sauce Grande River. At that moment, the direction of the regional drainage changed, establishing it from Ventania to the Atlantic. The old Sauce Grande River flowed near the east end of the modern FMH cliff (Bayón and Zavala 1997).
The Upper Member of Punta Tejada Formation was assigned by Zavala (1993) as Lower/Middle Holocene. It is in this sandy deposit scattered with pebbles that quartzite flakes, cores and tools are found; Ameghino (1934 ) assigned those remains to an exaggerated old tertiary age. Nowadays, we know it was a workshop where the fluvial pebbles of the Early Holocene mentioned above were used. Since the raw material was of medium quality for flaking, the Pampean hunter–gatherers invested little work on their manufacture (Bayón and Zavala 1997). The artifacts resemble rather crude-looking tools, and some of them are simply poorly flaked cores. This slight modification and its crudeness made Ameghino consider them to be “primitive” technology. On top of this sequence, the littoral dunes were set up, circa 7,000 years before present (BP), fixed today by vegetation typical of this landscape.
The Second Scenario: Playa Del Barco
The Playa del Barco site is located on the beach one kilometer to the west of Pehuen Co town. There, conglomerates and sandstones are found (Fig. 4). They are related to an old fluvial course of the Sauce Grande River, which by that time had migrated about 11 kilometers to the east of the Monte Hermoso cliff (Zavala and Quattrocchio 2001). These Late Pleistocene sediments contain mammal fossils of a more modern evolutionary stage than those from the Monte Hermoso Formation. So far, we are calculating with one radiocarbon measurement from macro-vegetable remains, which gives an age of 16.440 ± 320 years BP (Aramayo and de Manera Bianco 2000).
This faunal association contains members of the native fauna of either South and North America, composing a mixture post-Great American Biotic Interchange taxa. Representative species of the native fauna include some of the groups which were present at FMH but appearing as different, more modern genera: among the Xenarthra, the Dasypodidade with the genus Eutatus, and the Glyptodontidae with Glyptodon, Doedicurus, Panochthus, and others; the giant ground sloths with members of two families, Mylodontidae (Mylodon, Scelidotherium, Glossotherium) and Megatheridae (Megatherium). The endemic ungulates decreased notably in biodiversity and are only represented by one genus of the Notoungulata (Toxodon) and one of the Litopterna (Macrauchenia). The mammals of North American origin include Cervidae (Morenelaphus), Camelidae (Lama and Paleolama), Equidae, Proboscidea (Stegomastodon), and Carnivora (Smilodon; Aramayo and Manera de Bianco 1989). The studies carried out on this faunal group show that it was affected by taphonomic processes. The most remarkable process was the transportation of bones by a dense medium (debris flow), which affected the integrity of the group and produced a mixture of remains of different chronology and environments (Tomassini et al. 2010).
In addition to numerous bones, some footprints assigned to sloths and Macrauchenia were found in Playa del Barco (Fig. 5). Both of them are conspicuous members of the faunistic association of the Late Pleistocene. The layers where the ichnites were imprinted are clayey mudstones disposed on fluvial conglomerates.
The Third Scenario: Pehuen Co Palaeoichnological Site
By the end of the Pleistocene, about 12,000 radiocarbon years ago (14C), the south Pampean landscape was noticeably different from the present one. The Atlantic coast was located many hundred kilometers east due to a 100-meter lower sea level, a situation that caused an enlarging of the continental area. Climate conditions were extremely dry, cold, and windy (Quattrocchio et al. 2008). This produced an important change in the drainage system of the area. The rivers that nowadays drain their water from the hills of Ventania toward the Atlantic Ocean were not active. As a counterpart, an alternative drainage system was established with ephemerous watercourses that replaced the main fluvial system (Quattrocchio et al. 2009; Zavala et al. 2005). The water drained to the lower areas, forming temporary ponds. Those were the conditions present when the Pehuen Co palaeoichnological site was formed (Fig. 6), which in the local stratigraphy would correspond to the Middle Member of the Agua Blanca Sequence (Manera de Bianco et al. 2008). On the shores of these temporary ponds of the Pampean Late Pleistocene, an exceptional record of animal footprints and tracks crops out on platforms along 5,000 meters of the modern coast. The process was repeated quite regularly, each after a short period of time, generating overlapping layers.
The palaeoichnological record shows more than 100 trackways and hundreds of isolated footprints of different species. There are mammal ichnofossils of large animals, most of them extinct, and of medium-sized animals as well as of different birds that wandered along the pond shorelines. These footprints show that the represented fauna was very diverse and that it included the most conspicuous mammals of the South American Pleistocene fauna. This mammal association is rather similar to the one of Playa del Barco. The ichnogeneres described have been assigned to Megatheridae, Mylodontidae, Glyptodontidae, Macrauchenia, Paleolama, Lama, Equidae, Stegomastodon, Carnivora (Ursidae, Canidae, Felidae) and Caviidae (Fig. 7).
Up to the present, there is only one 14,000 BP dating, based on organic matter recovered in the middle levels of the sedimentary sequence that contains the footprints of the Pleistocene fauna. This dating produced an age of 12.000 ± 110 years BP, in coincidence with the chronologic estimations made as a result of the stratigraphic position of the faunistic group (Aramayo and de Manera Bianco 1996). Nevertheless, this age is still tentative due to the nature and uniqueness of the sample.
In 2005, new findings were made which also suggest the presence of human beings on the shores of these temporary ponds. So far, two types of signs of human presence have been found; namely, two isolated footprints and a trackway. The two isolated footprints were found on large pieces of clayey sediment separated from the layer by the tide. Each of them has a human footprint, in both cases of the left foot. In the sample PCO-05-1, there is a 23 centimeter-long footprint; the impression of the toes is indicated by a change of color in the sediment, and the big toe is better marked, with a depth of 0.7 centimeters. The second sample, PCO-05-01, exhibits a 23.5 centimeter-long trace with its lateral edges slightly deformed and with well-marked toes. These pieces contain lithologic characteristics similar to the middle levels of the site, where numerous Megatherium, Macrauchenia, American horse, and guanaco footprints were found (Fig. 8).
The other evidence is a track of 13 consecutive footprints, with alternated traces of the right and left foot, as in a walking situation, and they correspond to only one individual (Fig. 9). The length of each footprint ranges between 33 and 35 centimeters, the maximum width between 13 and 15 centimeters, the depth between one and two centimeters, and the inter-step distance varying between 54 and 59 centimeters. None of the footprints have marks of toes. This situation could be the product of the use of some kind of footwear or because the sample is an undertrack. The latter happens when the thinner pelitic level where the footprint was produced has been eroded and only the weight deformation on an inferior layer is present. The assignment of this trace to human beings is based particularly on the angle between the footprints, which is 160º. This trace is located on one of the middle strata of the sedimentary sequence. As shown in the picture, in the same layer, there is a parallel Megatherium trackway. In addition, it contains Macrauchenia, Artiodactyla and flamingo trackways that cross the human one in different directions.
The evidence found in Pehuen Co allows us to consider two aspects. One of them is related to the activities people were carrying out in the site, the other is connected to the relation between this site and other South American sites assigned to the latest Pleistocene. First, both the trackway and the loose footprints on isolated pieces show the transit of adult individuals, possibly in small groups, on the shores of freshwater bodies visited regularly by large mammals. This scenario would be compatible with hunting events, in which the few hunters, usually men, scout around places where there are better chances of finding prey.
Second, these human traces are chronologically related to older evidence of the first settlers of the Pampean region of the Buenos Aires province. The archeological evidence of human presence of about 12.000 14C years BP are few and weak, not only in the region but in all America. The human sign of continuous occupation of the continent appears only at circa 12,500 years BP in the Monte Verde site in Chile (Dillehay 1997) and in Brazil (the recent dating of a human tooth in Toca Do Gordo do Garrincho that gave an age of 12.210 ± 40; Peyre et al. 2009). In the Pampean region, there is a human signal, still feeble, at Arroyo Seco Site (130 kilometers northeast from Pehuen Co). At this site, some Pleistocene mammals were dated. Megatherium bones had marks of human action and were associated with stone tools, at circa 12,150 years BP (Steele and Politis 2009; Politis et al. in press, 2011). Another dating of Toxodon bone was estimated at 11,750 14C years BP, although in this case no human action was verified. Finally, a third event was dated at 14C years BP, which involves two equids present in the Pampean Latest Pleistocene: Equus and Hippidion. This event also exhibits evidence of human agency. It is important to highlight that the fauna of Arroyo Seco 2 dated between 12.150 and 11.200 BP also includes other genera, such as Glossoteherium and Paleolama; all are represented in the Pehuen Co ichnites, thus suggesting that both sites show similar faunal composition but in different kind of records.
In the Pampean region, 16 sites dated during the Pleistocene–Holocene transition have been excavated in different settings. Pehuen Co human footprints would be included in this group. Ten of these sites are in caves, rock shelters, or open-air sites in the Tandilia hill range (Flegenheimer 2003; Mazzanti 2003; Mazzanti et al. 2010). The other six are located on plains near ponds or rivers (Flegenheimer et al. 2010; Martínez 2006; Politis and Messineo 2008). Probably both the sites in the Tandilia hill ranges and those in the surrounding plains are related. The majority of the occupations (12 sites) have a chronology between 11.000 and 9.000 14C years BP (Fig. 1). Although each site presents specific characteristics, the early hunter–gatherers share technological decisions as well as the good quality raw material selection strategies (Flegenheimer 2003). At some six of the sites, particular projectile points called “fishtail points” were found. This point is also present in other places in South America, also dated at the end of the Pleistocene (Flegenheimer et al. 2003; Nami 2007).
The settlements in the plains are characterized by being open-air sites and by the presence of Pleistocene mega-mammals remains. For example, in Arroyo Seco 2 (Fig. 1) giant ground sloths and extinct horses were processed and consumed, while in Paso Otero 5 (Fig. 1) the hunter–gatherers took advantage of Hemiauchenia and Megatherium and might even have used the bones as fuel (Martínez 2006). On the other hand, Pehuen Co site is indicative of the wandering of one or many hunters around a place with a variety of Pleistocene fauna. The presence of these large animals, represented either by their bones or footprints, suggests some kind of cooperative hunting strategy. It probably implies that there was an important number of individuals nearby who would be the potential consumers, either the hunters’ families or the rest of the band (Politis 2008).
At the end of the Pleistocene, there was a particularly important event: the massive extinction of most of the mega-mammals whose footprints are found at Pehuen Co. The archeological and paleontological evidence of the region indicates that until circa 10,000 years BP, the Pampean grasslands supported an enormous biomass of large herbivores, which disappeared in a few thousand years. During the Late Pleistocene, 80% of mammals heavier than 44 kg and 100% of those heavier than 1000 kg were extinct (Owen-Smith 1987). Nevertheless, Pleistocene faunal remains (Megatherium and Doedicurus) whose radiocarbon dates indicate surprisingly recent ages were found in the Pampean plains in the La Moderna and Campo Laborde sites. These ages ranged between 7,500 and 8,000 14C years BP. Both sites (Fig. 1) have been interpreted as hunting/scavenging and primary butchering sites of very late mega-mammals on the shores of old swamps (Politis and Messineo 2008). These animals survived in the Pampean region for a few millennia within the Holocene (Gutiérrez and Martínez 2007; Politis and Messineo 2008).
What brought about the fast disappearance of the large and medium-sized mammal community? This topic has been extensively debated by different authors, and the answers vary from models that give human beings a central role in overkilling during the early stages of settlement (Cione et al. 2003; Martin 1984) to others who argue that the climatic and environmental changes were the main cause of the extinction (see summary in Haynes 2009). Some intermediate models propose either a minor role for human beings in the South American Pleistocene fauna extinction since the killing of these animals would have been occasional and opportunistic (Borrero 2009), or that the hunting would only have intensively affected the immigrant genera such as Equus and Hippidion and the Gomphoteridae Family (Politis et al. 1995). However, the evidence at Pehuen Co indicates that when the first human beings arrived in the Pampean region, most of the large- and medium-sized mammals were still alive. The evidence is too weak at this site to clarify the impact of these human beings on the Pampean animal community.
The Fourth Scenario: Monte Hermoso 1–La Olla (1, 2, 4)
At the beginning of the Middle Holocene, between 8,000 and 6,000 years BP, a new climatic change transformed the environment of the Pampean southwest. The cold and dry conditions, predominant during most of the final Pleistocene gave way to more moderate and mild conditions with more stability, characteristic of the beginning of the Holocene (Quattrocchio et al. 2008). Meanwhile, the sea level rose and the coast got closer to its present position (Fig. 2).
The archeological sites of Monte Hermoso 1 (MH1)–La Olla (with three discrete sectors LO 1, 2, and 4) are located on the Atlantic coast, 15 kilometers east of Pehuen Co palaeoichnological site, near the town of Monte Hermoso. It extends 1,200 meters along the beach and is composed of stratigraphic deposits of overlaying levels of clay and sand formed in shallow water bodies with a certain degree of marine influence. The deposits present a variety of evidence of human occupation of the coast. The archeological site La Olla shows abundant faunal remains, as well as artifacts made on stone, bone and wood. On the other hand, in MH1, the archeological record is formed by numerous human, mammal, and bird footprints.
MH1 was located near an extinct lagoon between the dunes near the ocean surrounded by the typical humid interdune vegetation with slight marine influence (Zavala et al. 1992). The ostracode species are characteristic of environments of continental brackish water (Quattrocchio et al. 2008). On the other hand, the site LO1-2-4 is in a coastal marsh (Blasi MS 2010), with a predominantly halophyte vegetation. The ostracode species are typical of marine waters (Fontana 2005; Martínez et al. 2010; Quattrocchio et al. 2008). The vertebrate species present in these sites are those typical of the continental and marine Holocene fauna of the Pampean Region. There are 18 AMS radiocarbon dates from the site and two conventional ones which indicate that the marsh lasted between 7,920 and 6,600 years 14C BP. Some of the dates come from marine mammals which have some reservoir effect not calculated yet.
Monte Hermoso 1
This site was located at the shore of the ancient interdune pond where hunter–gatherers carried out different activities (Bayón and Politis 1996). The several overlaying layers with footprints indicate that the place was periodically visited for hundreds of years. On the eastern sector of the site, there are hundreds of footprints (circa 472) of children, youths and women, who wandered around the pond shore while they gathered (probably plants and bird eggs; Fig. 10). Almost no artifacts have been found in this sector. In contrast, in the western sector of the site other activities are recorded. There, faunal remains and lithic and wooden artifacts were found scattered in very low density. This context suggests an area outside a camp. The footprints found in this sector belong to adults (Fig. 11) with a predominantly SE–NO direction, suggesting a transit path. Also, human remains of two adult individuals (skull fragments and parts of limb bones) dated at 6,60614C years BP and 7,886 14C years BP were found redeposited on the surface at the back of the beach. The stable isotope analysis (δ13C coll; mean = −13.4‰) of the human remains show remarkably enriched values, which indicates that marine foods were the basis of their diet (Politis et al. 2009). These findings were of great importance because they represent one of the few human remains of the Early–Middle Holocene from the coastal area of Argentina.
Is composed of three small outcrops (sectors 1-2-4) along 150 meters at the low tide line. They are usually covered with sand and have only been exposed, never simultaneously, five times in the last 20 years (Fig. 12). The archeological record is formed by abundant faunal remains and bone, wooden and lithic artifacts. Bones from two species of pinniped (Arctocephalus australis and Otaria byronia), guanaco (Lama guanicoe), deer (Ozotoceros bezoarticus), ostrich (Rhea americana), and fish were found. The two species of pinniped are the more abundant taxa present in the siteFootnote 1. Some elements present cut marks and combustion traces, while others were used to make expeditive tools (Johnson et al. 2000). The three sectors are interpreted as places where people finished the processing of the pinnipeds caught at the immediate marine coast. The stone tools are scarce, the raw material used in these assemblages was of poor quality, and they were available among the Sauce Grande River pebbles. Most of them are marginal or unifacial artifacts, which were shaped with little investment. Wood was also processed, especially as raw material to make artifacts. The wooden artifacts are varied and comprise spears straightened and hardened with fire, a stone ax hafted with a wooden handle, and several other shaped pieces whose function is difficult to assess. The most surprising artifact is a long wooden spatula, decorated on both sides with zigzag engravings and red paint (Fig. 13). It is not possible to know about its use, but according to the type of instrument and decoration, it might have been connected to some kind of ritual activity.Footnote 2
Both Monte Hermoso and the 1-La Olla site form a unique record in America, which also illustrates exceptionally the hunter–gatherers’ way of life when the Atlantic coast drew closer, in the Pampean region between circa 7,600 and 6,600 14C years ago. On the coast of Monte Hermoso, the first signs of exploitation of marine resources were established. Also, these sites would be part of a nomadic circuit between the coast and the inland, which started at least in the Early Holocene and probably lasted until the very Late Holocene (Bonomo 2005).