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Ñuagapua (Chaco, Bolivia): Evidence for the latest occurrence of megafauna
in association with human remains in South America
Mauro Coltorti
a
,
*
, Jacopo Della Fazia
a
, Freddy Paredes Rios
b
,
1
, Giuseppe Tito
c
a
Dipartimento di Scienze della Terra, University of Siena, Via di Laterina 8, 53100 Siena, Italy
b
Museo National Paleontologico Arqueologico, Tarija, Bolivia
c
Dipartimento di Scienze della Terra, University of Florence, Via G. La Pira 4, 50121 Florence, Italy
article info
Article history:
Received 13 January 2010
Accepted 3 July 2011
Keywords:
Alluvial fan
Human impact
Megafauna
Last occurrence
Holocene
Chaco
Bolivia
abstract
Quebrada (stream) Ñuagapua, which is located in the Bolivian Chaco in the Andean foothill generates an
alluvial fan many kilometres in length. Three major lithostratigraphic units characterise the sedimentary
sequence in this region. The lower and upper parts are formed from predominantly sandy sediments that
demonstrate rapid growth of the alluvial fan, associated with an intense erosion of barren slopes. The
intermediate unit consists of forest soil that seals deep channels containing bones together with a forest
association. The remains of a wooden plank, dated 140 yr BP, were found at the top of this soil, which
laterally contains charcoals, ash layers and large charred trunks, sometimes in growth positions. Roots
localised in this layer also sustain a number of very large still living trees. These findings are evidence of
a recent phase of alluvial fan sedimentation resulting from slope erosion activated by forest clearing. The
Chaco has been intensively settled for agricultural and pastoral purposes since the 18th century. The
lower unit contains a hearth, scattered burnt bones, flint flakes and ceramic artefacts. Radiometric dating
indicates a middle Holocene human occupation, between ca. 7.79 and 6.65 ka cal yr BP. We suggest that
the sedimentary unit is associated with intense soil erosion processes triggered by early Neolithic
deforestation. A sandy layer of the lower unit, slightly above the archaeological remains, contains
transported bones of megafaunal elements that apparently represent the South American latest occur-
rence of some extinct taxa. The mammal association is highly heterogeneous, containing species living in
aquatic, forest, prairie and savannah environments from a very specific layer that represents the almost
simultaneous burial of animals killed slightly up-valley. This anomalous association is probably the result
of human impact as opening the forest favoured the introduction of open environment fauna that had
previously survived on the southernmost part of the continent. Therefore, humans may have played
a role in mammalian extinctions in this region, either directly, due to hunting, or due to changing the
paleoenvironmental conditions on a wider scale.
Ó2011 Elsevier Ltd. All rights reserved.
1. Introduction
At the end of the Late Pleistocene and the beginning of the
Holocene, the South American megafauna were subjected to a great
mass-extinction (Martin and Klein,1984; Marshall and Cifelli,1990;
MacPhee,1999; Hubbe et al., 2007; Barnoskyand Lindsey, 2010 and
ref. therein). A large debate has taken place regarding why this
extinction occurred, with many authors suggesting it was associated
with climatic and/or climatically-driven environmental changes
(Axelrod, 1967; Graham and Lundelius, 1984; Grayson, 1987, 1999;
Guthrie, 1984, 1990; Markgraf, 1985; Ochsenius, 1985; Falguères
et al., 1994; Núñez et al., 2001; De Vivo and Carmignotto, 2004),
anthropic influence (Martin, 1967, 1973, 1984, 1986; Mosimann and
Martin, 1975; Diamond, 1989; Alroy, 2001; Johnson, 2002; Burney
and Flannery, 2005; Steadman et al., 2005) or the interplay of
these factors (Van Der Hammen,1981; Owen-Smith, 1987; Kelly and
Todd,1988; Haynes,1991; Nami, 1996; Politis et al., 1995;Politis and
Gutiérrez, 1998; Cione et al., 2003; Ficcarelli et al., 2003; Barnosky
et al., 2004; Fiedel and Haynes, 2004; Guthrie, 2006; Wroe et al.,
2006; Barnosky and Lindsey, 2010). Recently, Firestone et al.
(2007) suggested the occurrence of a catastrophic extraterrestrial
impact that would explain both the origin of the Younger Dryas and
the megafaunal extinction. Although Early Holocene human settle-
ments are scarce, evidence that megafauna were a common element
in the local diet (sensu Martin, 1984), which would be expected if
*Corresponding author. Tel.: þ39 0577233814.
E-mail addresses: coltorti@unisi.it (M. Coltorti), dellafazia@unisi.it (J. Della
Fazia), museou@uajms.edu.bo (F. Paredes Rios), megaterio@leonardo.it (G. Tito).
1
Tel.: þ591 6636680.
Contents lists available at ScienceDirect
Journal of South American Earth Sciences
journal homepage: www.elsevier.com/locate/jsames
0895-9811/$ esee front matter Ó2011 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jsames.2011.07.003
Journal of South American Earth Sciences 33 (2012) 56e67
Author's personal copy
this extinction was directly linked to the arrival of humans is even
more so (Falguères et al., 1994; Grayson and Meltzer, 2003). Late
Pleistocene and Early Holocene human settlements are widespread
on the continent, although their number is quite limited (Barnosky
and Lindsey, 2010). Some sites have been recognised along the
western coast of Chile (Núñez et al., 1994), in the extreme southern
part of the continent (Massone,1996), in the central Andes (Dillehay
et al., 1992), in central-northern areas of Argentina (Nami, 1987;
Martinez, 1997; Borrero et al., 1998; Long et al., 1998; Politis and
Gutiérrez, 1998; Grosjean et al., 2005; Rodriguez, 2005; Yacobaccio
and Morales, 2005), in the northwestern (Correal and Van der
Hammen, 2003), northeastern (Guérin et al., 1993; Guidon et al.,
1994) and central-southern regions of Brazil (Miller, 1987;
Schmitz, 1987; Kipnis, 1998; Prous and Fogaça, 1999; Neves et al.,
2004; Araujo et al., 2005), in northern Venezuela (Ochsenius and
Gruhn,1979) and along theEcuadorian coast (Ficcarelli et al., 2003).
In Bolivia, the occurrence of megafauna is frequent, and the Tarija
basin, which is not far from the study area, contains some of the
richest faunal deposits on the continent. These were recently reat-
tributed to the final part of the Marine Isotope Stage (MIS) 4, to the
MIS 3 and to the earlier part of the MIS 2 (Coltorti et al., 2007,2010b),
when humans apparently still had not reached the continent. In
Tarija, no artefacts have been found associated with megafauna.
In the Bolivian Chaco, in the lower layers of Quebrada (stream)
Ñuagapua (referred to as Ñuapua in previous works), Hoffstetter
(1968) described the occurrence of three layers that lie uncon-
formably over Miocene bedrock. The lowest (layer 1) is formed of
alluvial sands and silts with minor gravels. Megafauna remains have
been discovered in the upper part of this layer (Table 1). Layer 2 has
been interpreted as a grey pyroclastic tuff that contained a modern
fauna (Table 1) attributed to a palustrine environment. The upper-
most layer 3 was described as a pink reworked tuff containing
modern fauna attributed to a palustrine environment. The upper-
most (layer 3) is described as a pink reworked tuff locally interca-
lated with grey unconsolidated fine volcanic ashes. MacFadden and
Wolff (1981) redescribed Hoffstetter’s layers, giving the sequence
the informal name Ñuapua Fm, and provided a list of species found
in layer 1 (Table 1). This layer was attributed to the Late Pleistocene,
in particular, to the Lujanian Mammal unit, although chronological
evidence was missing. Wolff et al. (in Lynch,1990) tried to utilise the
isoleucine epimerisation method but without results. In layer 2,
together with many mammalian bones, they found the remains of
Homo sapiens (Table 1). Radiometric dating of the human bones
indicated an age of 6600 370 conv. yr BP.
This work aims to describe the megafauna that survived in this
area until a recent period of the Holocene. It constitutes, to our
knowledge, the most recent occurrence of many elements of the
megafauna of South America. In layers below the megafauna, the
remains of a hearth containing scarce flint flakes, burnt pieces of
bones and fragments of ceramics represent the oldest artefacts in
this sector of the Bolivian Chaco. Similar to other evidence collected
in South America (Gruhn and Bryan, 1984; Markgraf, 1985; Guérin
et al., 1993; Núñez et al., 1995, 2001; Faure et al., 1999; Ficcarelli
et al., 2003; Grill et al., 2007), these findings indicate (i) the coex-
istence of humans and megafauna and (ii) raise the possibility that
humans could have commonly utilised large mammals as part of
their diet. Moreover, the sedimentological and stratigraphic char-
acteristics of the study site allow us to reconstruct the paleo-
environment of this region and the significant changes that
occurred during the Holocene after the arrival of humans.
2. Regional setting
The Chaco is a flat area of ca. 840,000 km
2
that extends between
Bolivia, Paraguay and northern Argentina (Fig.1). The area is located
to the east of the Andean Cordillera, bordered by western verging
fronts of the Sub-Andean fold thrust belt (Baby et al., 1992; Horton
and DeCelles, 2001). In this Sub-Andean zone, thrust fronts affect
the Miocene Chaco Group as well as Early Pliocene deformed and
uplifted littoral and continental sandstones, conglomerates and
marls (MacFadden and Wolff,1981; Uba et al., 2006 and ref. therein).
The piedmont of Cordillera in the western Chaco is dominated by
megafans (Iriondo, 1993; Horton and DeCelles, 2001; May, 2006;
May et al., 2008a,b), associated with the transport and deposition
of mostly medium to fine sediments along the major rivers.
The climate is characterised by a permanent interchange of
warm tropical and cold austral air masses (Nogués-Paegle and Mo,
1997; Cerveny, 1998; Piovano et al., 2006; Garreaud et al., 2009). In
the summer, the area is dominated by northerly winds due to the
interaction of the South American Summer Monsoon and South
American Low Level Jet, which extend the moister and humidity
typical of the tropical region of Bolivia and Brazil to the south.
Summer rains are usually intense and are associated with cold
fronts together with southerly winds, as well as with convective
movements originating from humid tropical air masses, but with
more localised effects. The main rains events, varying from 500 to
700 mm/yr, begin in October and end in March. The mean annual
temperatures vary from 10
e20
to 22
e32
. However, in summer,
temperatures may rise up to 40
C.
At present, the vegetation of the Chaco is characterised by
xerofilous forests, tracts of savannah and swamps. Only ten species
of xerofilous trees are currently present (Cabrera and Willink , 1973;
Navarro and Fuentes, 1999), and the more characteristic among
these trees is Chorisia speciosa, which exhibits a large biconvex
trunk with evident spines. The present day vegetation underwent
important changes in the last threeefour centuries due to
anthropic causes (Adámoli et al.,1990). In fact, after the “Conquista”
and particularly during the XVIII and XIX centuries, these regions
exhibited more open vegetation with few trees. Agriculture and
grazing activities dominated, as documented in the archives of the
Centro di Documentación Ecclesial of the S. Francisco Convent in
Tarija. In recent decades, large parts of the region have been
abandoned due to migration towards the rapidly growing towns,
especially Santa Cruz de la Sierra, leading to the rapid re-growth of
forest vegetation.
3. Methods
Detailed geomorphological mapping of the study region was
conducted, first using photogeology and later through field surveys.
We selected aerial photos from the 1973 because they were
captured only few years after the first investigations of the area
made by Hoffstetter. We investigated the stratigraphic, sedimen-
tological and pedological characteristics of the area. The sedimen-
tological analysis and subsequent facies interpretation was
conducted following the methodology proposed by Miall (1985,
1996 and ref. therein). The soil profiles were described following
the recommendation of the Soil Survey Staff (2010). We also con-
ducted preliminary palaeontological excavations of outcropping
megafauna remains that are now preserved in the Museum of
Tarija. The bones were found in a layer approximately 10 cm thick
layer between successions (Sections) 6 and 7. Detailed identifica-
tion at the species level, when possible, was performed using the
rich collection of the Tarija Museum. Radiocarbon dates were
obtained at the Beta Analytic Lab in Florida (Table 3).
4. Results
In the study area (Fig. 1), which is located in Chuquisaca prov-
ince between the Pilcomayo and Parapeti rivers, hills generally
M. Coltorti et al. / Journal of South American Earth Sciences 33 (2012) 56e67 57
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below 700 m in elevation are modelled over Miocene and Early
Pliocene rocks (Uba et al., 2006). Along the slopes, a series of gullies
generated badlands that are currently experiencing the re-growth
of vegetation cover and stabilisation. At the base of the hills, to
the south and north of Quebrada Ñuagapua, a series of coalescent
flat alluvial fans are present. The bedrock locally crops out in the
plain and forms low hills with a flat surface dipping gently east-
ward. This suggests the previous existence of pediments that con-
nected the reliefs to the plain before aggradation of the alluvial fans
and later down-cutting took place. On the alluvial fan surface,
a series of abandoned channels has been recognised in aerial
photos due to their contrasting colour compared to the surrounding
surface created by the dark rich organic sediments with which they
are filled. They exhibit a sinuous pattern that can be followed for
hundreds of metres to a few kilometres. Many of the channels are
marked by isolated depressions that may host small lakes during
the rainy season (i.e. Pato lake, Fig. 1B). Eastward, the alluvial fans
show a gradual transition to an alluvial plain dominated by silt and
Table 1
List of fossil mammal coming from Ñuagapua Formation: symbol Xindicates the mammal family present; the number of specimen, of each taxon is also indicated. Underlined
genera and species are extinct. The question mark along the row of Ñuapua 2 mean that the number of specimens are undefined. The question mark associated with the taxa
means that the attribution is the most probable.
N. Taxa (Mammalia) This paper Hoffstetter (1968) MacFadden and Wolff
(1981)
Number of fragments
Ñuagapua unit 1 Ñuapua 1 Ñuapua 2 Ñuapua 1 Ñuapua 2
1 Chiroptera, Molossidae, Eumops perotis >10
2 Chiroptera indet. ?
3 Primates, Hominidae X
4 Xenarthra, Glyptodontidae XXX
5 Xenarthra, Glyptodontidae, Panochtus cf. tuberculatus >10
6 Xenarthra, Glyptodontidae, Glyptodon cf. reticulatus >10
7 Xenarthra, Glyptodontidae, Sclerocalyptus cf. ornatus 1
8 Xenarthra, Glyptodontidae, Chlamidotherium <10
9 Xenarthra, Dasypodidae XX X
10 Xenarthra, Dasypodidae, Pampatherium cf. humboldti >10
11 Xenarthra, Dasypodidae, Euphractus sexcinctus <10
12 Xenarthra, Dasypodidae, Chaetophractus villosus >10
13 Xenarthra, Dasypodidae, Zaedyus pichiy >10
14 Xenarthra, Dasypodidae, Dasypus novemcinctus <10
15 Xenarthra, Dasypodidae, Tolypeutes matacus <10
16 Xenarthra, Dasypodidae, Propraopus 2
17 Xenarthra, Megatheriidae XX XX
18 Xenarthra, Megatheriidae, Megatherium cf. americanum 2<10
19 Xenarthra, Mylodontidae XXXX
20 Xenarthra, Mylodontidae, Mylodon darwini >10
21 Xenarthra, Mylodontidae, Scelidodon 1
22 Xenarthra, Megalonychidae, Nothrotherium?1
23 Rodentia, Octodontidae X
24 Rodentia, Hydrochoeridae, Hydrochoerus cf. hydrochoeris 21
25 Rodentia, Caviidae, Galea cf. musteloides <10
26 Rodentia, Chinchillidae, Lagostomus maximus >10
27 Rodentia, Chinchillidae, indet. <10
28 Rodentia, Capromyidae XXX
29 Rodentia, Capromyidae, Myocastor coipus >10
30 Rodentia, Ctenomyidae XX
31 Rodentia, Ctenomyidae, Ctenomys >10
32 Rodentia, Cricetidae XX
33 Rodentia, Cricetidae, Holochilus cf. brasiliensis >10
34 Rodentia, Cricetidae, Zygodontomys cf. lasiurus <10
35 Lagomorpha, Leporidae X
36 Carnivora, Canidae XX X
37 Carnivora, Canidae, Protocyon cf. troglodytes 2
38 Carnivora, Canidae, Dusicyon cf. griseus >10
39 Carnivora, Ursidae, Arctodus cf. pamparum >10
40 Carnivora, Felidae, Panthera onca cf. palustris 2
41 Carnivora, Felidae, Smilodon 2
42 Litopterna, Macraucheniidae XX
43 Litopterna, Macraucheniidae, Macrauchenia cf. patachonica <10
44 Notoungulata, Toxodontidae XX X
45 Notoungulata, Toxodontidae, Toxodon ensenadensis 2>10 ?
46 Proboscidea, Gomphotheriidae, Stegomastodon?>10 ?
47 Proboscidea, Gomphotheriidae, Cuvieronius?2
48 Perissodactyla, Equide XX X
49 Perissodactyla, Equide, Equus cf. curvidens >10
50 Perissodactyla, Equide, Hippidion?1
51 Perissodactyla, Equide, Equus?3
52 Artiodactyla, Tayassuidae X
53 Artiodactyla, Camelidae XX X
54 Artiodactyla, Camelidae, Palaeolama >10 ?
55 Artiodactyla, Camelidae, Vicugna?1
56 Artiodactyla, Cervidae, unknown genus <10
57 Artiodactyla, Cervidae, Morenelaphus <10
M. Coltorti et al. / Journal of South American Earth Sciences 33 (2012) 56e6758
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fine sand deposition. In the plain, lakes are more common and clay
deposition dominates. The plain contains traces of paleo-meanders
that are seasonally occupied by small lakes and swamps.
The Quebrada Ñuagapua (lat. 20
53
0
S, long. 63
01
0
W; Fig. 1)
alluvial fan is currently dominated by down-cutting processes
responsible for incisions up to approximately 15 m into the alluvial
deposits of the fan. Comparison between the aerial photos from
1973 (ca. 1:57,000 scale, Fig. 1C) and Google Earth images from
2005 (Fig. 1B), allowed a multitemporal analysis to be performed, in
addition to the preparation of a geomorphologic sketch map
(Fig. 1D). In 1973, a single deeper channel affected the apical part of
the fan. Five to six hundred metres from the apex, the main channel
ended and generated two major sandy bar lobes. The southern lobe
continued along the present day thalweg for over 2.5 km. It
included a reduced section before crossing the road and then
enlarged again for 2 km. The presence of the road clearly consti-
tuted a source area for accelerated sedimentation down-valley. The
other lobe diverged from the present day course between Sections
5 and 6. It extended to the northeast for approximately 1.5 km, but
it ended before joining a secondary stream coming from the hills to
the north of Ñuagapua. A series of secondary smaller lobes were
recognisable between the two main lobes due to the lighter colour
of the vegetation. They reveal that the whole area between these
lobes was sporadically affected by flooding during major stormy
events. Slope wash and gully erosion were active locally on slopes
in the hills. Although the vegetation was quite thick in some areas,
it was open in vast sectors and exhibited a typical spotty texture.
The southern lobe followed an EW oriented rectilinear tract before
reaching the road. This is the orientation of a series of square areas
that crossed the plain and are still visible in more recent aerial
photos. They represent the limit between properties and stream
deposition, and later incision, was probably activated along the
road running along the border.
In the last 40 years, the stream has lost most of its solid load, and
the fluvial bars are no longer easily recognisable and stabilised by
vegetation. A progressive down-cutting of the fan surface occurred
because, corresponding with Section 4, the stream is over 9 m deep.
Down-cutting of this strength in a very short period of time is not
surprising because it has been recorded elsewhere since 1960 and
is commonly attributed to changes in land use (Goudie, 2006;
Surian and Rinaldi, 2003). In Ñuagapua this was most likely due
to the abandonment of pasture and the simultaneous increase in
forest cover. The old channels are now covered with grassy and
shrub vegetation and still host small lakes during the rainy season
(Fig. 1B).
4.1. Stratigraphic and sedimentological characteristics
Stream incision allowed almost continuous observation of the
stratigraphy and sedimentology of the apical and medium portions
of the alluvial fan, which contain the palaeontological remains. We
investigated a sector that is almost 2.5 km long from where the
stream exits from the hilly slope to the road that connects Rancho
Borda Alta with Estancia Campo Azul (from A to B in Fig.1B). The fan
Fig. 1. A: Location of the study area (a and b); B) 2005 aerial photo taken from Google Earth of the Ñuagapua area with the part of the stream (from point A to B) that contains the log
reported in Fig. 2; point A is located at the head and point B at the toe of the alluvial fan that also corresponds with the intersection of the Quebrada with thelocal road, the area is
vegetated and the stream is affected by incision; C) 1973 aerial photo of the area; D, Geomorphological sketch of the investigated area in 1973: 1) gullies; 2) fluvial escarpments; 3)
abandoned channels; 4) road; 5) boundary of land properties: 6) Quebrada Ñuagapua; 7) alluvial fan surface; 8) depositional areas during 1970; 9) bedrock; 10) human settlements.
M. Coltorti et al. / Journal of South American Earth Sciences 33 (2012) 56e67 59
Author's personal copy
apex is located at 602 m (lat. 20
53
0
09
00
S; long. 63
02
0
58
00
W) and
the road crosses the stream at approximately 505 m (B in Fig. 1B).
The longitudinal profile of the stream shows two main steps that
originate in correspondence with the more resistant K horizons of
buried soils. We report a total of 9 sections (Fig. 2) that show the
principal changes in texture and facies.
We confirmed the subdivision of the sequence into three main
units, as described by previous authors, although in the apical zone
(Sections 9A and 9B; lat. 20
53
0
09
00
S; long. 63
02
0
58
00
W), another
unit (unit 0) is present that does not crop out down-valley and does
not contain palaeontological remains. To avoid confusion, we
described the various units following the indications of previous
authors. Sediments belonging to units 1 and 3 show great lateral
facies and texture variability moving away from the apical to the
middle zone of the fan.
Unit 0 crops out in Sections 9A and 9B, below an unconformity
(Fig. 2a; Section 9). This unit consists of a series of fine gravelly
layers filling very flat channels that rarely exceed a few metres in
width and a few tens of centimetres in depth (Gt; Miall, 1985,1996).
In Section 9A, gravels and sands lie unconformably on a small strath
terrace modelled over bedrock. The gravels are mostly round quartz
recycled from the Chaco Fm; the maximum size of the gravels is
20 cm but they rapidly decrease in size to few centimetres moving
down-valley. The strath terrace and overlying gravels are cut by
a channel (“X”in Section 9, Fig. 2a) filled with trough cross-bedded
and horizontally layered coarse sands (St and Sh). These sands are
again cut by a deep channel (Y) that develops from the top of
Section 9A. Unit 1 was deposited inside this incision. It is composed
of sandy sediments (St and Sh) similar to those of the previous unit
up to a depth of 4 m. In the upper part of Section 9B, there is buried
soil (Bkb; unit 2) that allows easy separation of the various units in
the rest of the Quebrada. In this section, this soil is truncated by
a very flat channel and is buried under sandy sediments (unit 3)
that constitute the present day terraced alluvial fan. The top
depositional surface is formed from almost fresh sediments that are
only locally weathered by a weak organic (A1) horizon.
In Section 8 (lat. 20
53
0
11
00
S; long. 63
02
0
58
00
W), which is still
located in the apical zone of the fan, the basal portion of unit 1
consists of a gravelly layer up to 30e40 cm thick that correlates
with those observed in Section 9B. The gravels lay on bedrock and
contain fragments of rolled pieces of pottery (Fig. 3a). They are
buried under massive sands, with widespread bioturbation. In the
upper part, unit 1 is bounded by the buried soil with a Bkb horizon
of unit 2. Carbonates accumulate through a thick network of small
veins (pseudomycelium).
Slightly down-valley, in Section 7 (Fig. 1; lat. 20
53
0
11
00
S; long.
63
02
0
36
00
W), unit 1 is represented by massive sands containing
fragments of charcoal, burned bones and a few flint artefacts. The
charcoals have been dated to 6870 50 yr BP (7.790e7.610 cal yr BP;
Beta 194692). Inside the sandy layer in Section 6 (lat. 20
53
0
25
00
S;
long. 63
02
0
18
00
W), almost in lateral continuity with the previously
dated layers, we discovered a hearth consisting of a hollow
approximately 1 m in diameter and 25 cm deep cut into the alluvial
sediments (Fig. 4b and c). Inside the hearth, there is a smaller
depression approximately 50 cm in size. Its borders preserve the
remains of reddened baked clay, with the red colour resulting from
the high temperatures reached in the hearth, similar to what has
been seen in structures investigated elsewhere (Nadel et al., 2004).
On the eastern side, the reddened clay reaches a thickness of
approximately 15 cm, indicating relatively long use and/or that high
temperatures were reached during its usage. The depression of the
hearth is filled with sandy silty sediments, small gravels, centimetre
thick charcoals and a large piece of burnt reddish clay detached from
the wall. The charcoals presented an age of 5960 80 conv. yr BP
(7000e6650 cal yr BP; Beta 197969). In the same layer, a few metres
from the hearth, many pieces of charcoal, fragments of burnt clay
and a few pieces of pottery were found mixed with sands. The hearth
is buried under small scale trough cross-bedded sands (St) a few
decimetres in thickness that still contain many small pieces of
charcoal, evidently selected by transport. This layer can be followed
up-valley for many tens of metres and locally becomes coarser
ultimately presenting fine gravels (Gt).
A
ab
B
Fig. 2. a and b: Stratigraphic logs at Quebrada Ñuagapua: 1) through cross bedding; 2) horizontal cross bedding; 3) buried soil with Bk or Ak horizon; 4) present-day and weakly
developed soils; 5) trees with buried stumps; 6) fragments of charcoal; 7) carbonate nodules; 8) bones; 9) flint artifacts; 10) fragments of pottery; 11) fragment of wooden plank; 12)
hearth; 13) unit 0; 14) unit 1; 15) unit 2; 16) unit 3.
M. Coltorti et al. / Journal of South American Earth Sciences 33 (2012) 56e6760
Author's personal copy
The pottery fragments found in the different sections havesmall
dimensions, but one of these fragments (Fig. 3a) is marked with
distinctive thumb-prints made before the pottery was placed into
the oven. However, these few remains clearly indicate widespread
occupation of the area by human groups. Higher up, these layers are
sealed by trough cross-bedded sands where evidence of human
visits is missing. This sandy layer contains a large number of bones
of extinct megafauna, including a greatly damaged mandibular
branch of an individual of Equus sp. (Fig. 4c). The fauna is quite well
concentrated in a single horizon, and therefore, it apparently
records a short period. These sands are inter-layered with buried
soils with a weak Akb profile. The soils are up to 30 cm thick, with
a clear fine sub-angular aggregation and small organic cutans
(colour 5 YR 5/3, reddish brown) along the aggregates. Thin
carbonate veins are widespread and are more common along the
aggregates, with common to abundant small nodules and rare
polmonate shells. Some channels up to 4e5 m in width and depth
have been observed at the top of this unit. They are filled with
weathered clays and silts with local sandy intercalations.
Unit 2 is characterised by a buried soil that locally (Sections 1, 2,
3 and 7) reaches 2 m in thickness (Section 1, Fig. 1; Section 2: lat.
20
53
0
45
00
S; long. 63
01
0
22
00
W; Section 3: lat. 20
53
0
44
00
S; long.
63
01
0
48
00
W).
The soil has a complex profile with a Bkb developed on clayey
loam deprived of carbonates. The aggregates are small and pris-
matic; small clay/organic 5 YR 5/3 reddish brown cutans are
present along the aggregates. In Section 4, this horizon is buried
under a series of sandy and silty layers. The arrival of these sedi-
ments interrupted the pedological evolution. The lowermost
coarser sandy layer exhibits a series of load casts. In these sedi-
ments, thin soils with a weak Akb horizon and a slightly developed
poliedric aggregation are present. In Section 1 the Bkb profile of
unit 2 reaches approximately 1.40 m in thickness, and sandy lenses
are missing, but the soil reaches and overlaps with the less devel-
oped Akb horizons intercalated within unit 3.
The upper part of the sequence (unit 3) is characterised by
sands, sometimes showing local erosional contact with the
underlying soil. Their thickness changes along the stream and is
greatest in the middle part (Sections 7, 6 and 5. Section 5: lat.
20
53
0
25
00
S; long. 63
02
0
18
00
W) where it reaches approximately
5 m. In Section 9B, at the fan apex, the thickness is reduced toa few
tens of decimetres. However, roughly 30 m down-valley, in Section
8, it is ca. 1.5 m thick. Furthest east in Sections 3 and 2, the thickness
is again approximately 1.0 m. These sands can be described as very
flat trough cross-bedded (St), horizontal (Sh) and locally low angle
planar, coarse-to-medium, quite well-sorted sands (Sp).
These sediments bury the soil but also the roots of trunks of
large C. speciosa. The trunks can exceed 1.5 m in diameter, and the
roots depart from the top of unit 2. Many of these trees are still
living, although their roots are buried and are therefore not visible,
except along the scarp of the stream (Fig. 4a). We also observed
many burnt trunks either buried, but in a standing position, or lying
horizontally at the base of the sands, together with lenses of
charcoals and ashes that can reach 20e30 cm in thickness. These
are layers interpreted as volcanic ashes by previous authors. In
Section 1, at the base of unit 1, we found the remains of a well-cut
wooden plank (Fig. 4b) radiometrically dated to 140 50 yr BP (cal
290e0 yr BP; Beta 197971) that is nicely supported by the size of
the buried, but still growing, trees.
4.2. Palaeontology
A few fossil remains have been found in Quebrada Ñuagapua
(Table 2). Most of these are well mineralised, but show some
weathering marks. They are now stored in the Museo Naciónal de
Paleontología y Arqueología de Tarija without any inventory
numbers. Previous authors (Hoffstetter, 1968; MacFadden and
Wolff, 1981) have collected many fossil bones (see Table 1)
belonging to both classical megafauna (unit 1) and other more
recent fauna, such as birds, reptiles and small mammals (unit 2). In
Table 1, it can be seen that there is a large difference in the quantity
and diversity of fossil remains, among the findings of previous
authors. At the family level, the MacFadden collection is richer than
that of Hoffstetter. In the former, two small living mammalian
families (Octodontidae and Leporidae) and a living Tayassuidae
species were included, as well as the previously mentioned H.
sapiens remains. We collected the same taxa described by previous
authors, except for a small Camelidae specimen.
Among megafauna, Equus is represented by four elements, but
only a molar and a radio-ulna fragment have been undoubtedly
ascribed to the genus. A calcaneum fragment, lacking its posterior
portion has been attributed to Vicugna sp., mainly on the basis of its
dimensions and the morphology of its articular surfaces. Xenar-
thrans are represented only by a molariform fragment of the giant
ground sloth Megatherium.Megatherium is different from species
typical of Tarija in both dimensions and morphology. We suggest
Fig. 3. a) Flint artefacts (1, 3 and 5) and fragments of pottery (2 and 4). Item 4 is
decorated with digital impression; b) hearth discovered inside unit 3, Section 6; c)
sedimentological details of the hearth; St, trough cross bedded sands and fine gravels.
M. Coltorti et al. / Journal of South American Earth Sciences 33 (2012) 56e67 61
Author's personal copy
that it belongs to the Megatherium americanum species on the basis
of its paleobiogeographic and biochronologic distribution. We also
found a tusk fragment and a glenoid scapular element likely
belonging to a mastodon species, such as Cuvieronius. Two teeth
fragments were ascribed to a Notoungulata genus, possibly Tox-
odon. Other fossil fragments were too damaged for attributions to
be made. Most of these belong to small animals, probably birds and
reptiles. There were some additional noteworthy bone fragments
found, mainly belonging to equidae and birds, that are poorly
mineralised and somewhat burnt.
Only Equus and Vicugna fossil remains were collected around
the hearth. A greatly damaged mandibular branch of Equus sp.
(Fig. 4c) was found above these layers. Previous authors (Faure
et al., 1999; Ficcarelli et al., 2003) have cited an Early Holocene
faunal association with an Equus species. The former authors cited
the presence of Equus neogaeus (Lund, 1840) in the São Raimundo
Nonato archaeological site, in northeastern Brazil. The associated
layers were dated between 8490 120 yr BP and 6890 60 yr BP.
Ficcarelli et al. (2003) cited the presence of Equus (Amerhippus
santaelenae (Spilmann, 1938) along the coastal area of Guayas
province (Ecuador) dated 8680 80 conv. yr BP
(7914e7541 cal yr BC). Ficcarelli et al. (2003) hypothesised that this
region constitutes a South American refuge area that existed just
before the megafaunal extinction. Therefore, our Equus sp. finding
represents, to our knowledge, the last occurrence of these horses,
before the complete extinction of the genus.
Other megafaunal remains have been surveyed and collected
along the thalweg of the “Quebrada”and at the base of its lateral
slopes. On the basis of their conservation status, the type of min-
eralisation and direction of breaking lines, these remains come
from unit 1. Additionally a molariform fragment of Megatherium sp.
was found that was perfectly divided in half along its weaker
portion. Hoffstetter also noted that many fossil remains of unit 2
“.proviennent d’éleménts brisés et remaniés du niveau inférieur”
(come from broken elements reworked from the older layer).
5. Discussion
The evolution of the study area is revealed by the presence of
two deep incisions in the sedimentary sequence in the apex of the
fan (Section 9). The younger of the incision is Holocene in age based
to the fact that unit 1, representing the first sedimentary filling, is
well dated and contains evidence of human frequentation (a
hearth, pottery, burnt bones, flint artefact, etc.). Elements are not
available to date unit 0, but it is older than 6 ka. It could tentatively
be attributed to the Late Pleistocene, when climatic deterioration
Fig. 4. a) Stratigraphy of Section 2 with a clear distinction of the three layers. Note the trunk still growing at the surface that has the roots inside unit 2 and was deeply buried by
unit 3. It constitute a good chronological marker for the beginning of the deposition of the uppermost unit; b) wooden plank find at the base of unit 2 in Section 1 top of unit 2 and
dated 140 50 yr BP; c) mandibular branch of an Equus sp. found within a sandy layer of unit 1 in Section 6.
Table 2
New fossil findings from Ñuagapua Formation (unit 1): brief description and main
measures. The question mark along the description row means that there are some
uncertainties for the type of tooth. The question mark along the taxon reveal the
most probable attribution.
N. Taxon Description Main measures in mm
1Megatherium
americanum
Molariform (III?) Maximum width ¼56
2Megatherium
americanum ?
Pelvis fragment e
3Equus sp. Mandibular branch Molar toothrow length ¼85
4Equus sp. Tibia fragment (young) Maximum length ¼295
5Equus sp. Radio-ulna Estimated maximum length ¼350
6Equus sp. M2? Inferior Occlusal surface ¼26.5 16.5
7Cuvieronius ?? Tusk fragment e
8Cuvieronius ?? Glenoid portion e
9Toxodon sp. Two fragmentary
molars
e
10 Provicugna sp. Distal portion of
calcaneum
Maximum height ¼21.5;
maximum width ¼23.5
M. Coltorti et al. / Journal of South American Earth Sciences 33 (2012) 56e6762
Author's personal copy
induced slope disequilibrium, producing large amounts of sedi-
ments and causing the growth of an alluvial fan, the remain of
which have been largely cancelled. In alluvial fans slightly to the
north, a similar incision is found at the base of the Holocene
sequence. It follows a period of soil evolution and lacustrine
deposition, with the latter being dated between 12 and
10.5 cal yr BP, resting on LGM sediments (May et al., 2008b). The
deep incision that, at least in this area, almost cancelled the oldest
unit, suggests widespread down-cutting and erosion during the
early Holocene.
Unit 1, which represents the earlier depositional phase associ-
ated with the rapid growth of a sand-dominated alluvial fan, is
characterised by sandy bedforms (SB) due to bars filling the chan-
nels and spreading over the fan surface during overflooding.
Overbank deposits (FF) are interbedded with soils that developed
between flooding events. Thin gravelly lenses constitute sporadic
coarser deposits of channel fill (GB). All of these architectural
elements are characteristic of the High Energy Sandy Bed Braided
pattern of sandy alluvial fans (Model 3 or 11 of Miall, 1985, 1996).
This pattern is commonly found in semi-arid, climates or in envi-
ronments with reduced vegetational cover due to human-induced
deforestation because the slopes are naked in both cases, and
seasonal contrasts favour sheet flooding processes. The buried soil
inter-layered with sandy and silty layers could also indicate
a climate with natural or human-induced seasonal contrasts that
was dry enough to generate veins and nodules of carbonates. In any
case, the development of a Bw or a Bt argillic horizon was prevented
by the frequent deposition of fresh alluvial sediments. Megafaunal
remains were found in a narrow layer above a hearth containing
a baked reddish clay surface, scarce flint artefacts and fragments of
pottery. The fauna were transported, but this sector of the fan is
close to the bedrock, and no other sediments have been found
upstream that could suggest an origin from older layers. The
absence of scattered remains across the unit, especially in the basal
gravelly layers that are more suitable for concentrating large bones,
suggests the occurrence of a single and almost contemporaneous
episode of reworking. The fact that it affected such a large variety of
only slightly transported fauna could be explained by the distur-
bance of a large butchery site located slightly up-valley. Unfortu-
nately the dated material are charcoals from the hearth and there
are no direct dates of the Pleistocene mammal bones as recom-
mended by Barnosky and Lindsey (2010) but in many mammal
bones from sites in Ecuador the collagene was not present.
Unit 1 was dated to the period between ca. 7000 and ca.
5000 yr BP, indicating that degradation of the forest cover began
during the Early Holocene. A similar unit was recognised in the
nearby alluvial fan (May et al., 2008a,b) although the deposition of
the older unit began locally approximately at 9.5 cal yr BP. These
authors found large amounts of charcoal in the coeval sediments
and associated them with forest fires induced by the onset of more
arid climatic conditions. The reduction of forest cover in other parts
of the Bolivian lowlands in areas close to Ñuagapua (Santa Cruz de
la Sierra), as well as in the Amazonian basin between 7000 and
5000 yr BP and again between 3400 and 1400 yr BP (Servant et al.,
1981; Seltzer, 1992), appears to most likely have been associated
with more intense human occupation. In more remote areas, the
forest continued to grow and maintained slope stability as revealed
by the absence and non-contemporaneous ages of deposition.
It is difficult to ascertain weather this reduction of vegetation
was associated with severe climatic worsening towards drier
conditions. Nobody denies the occurrence of climate fluctuations
during the Holocene, as these have been revealed by many proxi
data (Markgraf et al., 2000; Abbott et al., 2003; Piovano et al., 2006),
but it is difficult to establish the importance of these variations at
low elevations and early human impacts on natural vegetation
(Goudie, 2006; Chepstow-Lusty et al., 2009; Williams et al., 2011),
as well as their climatic feedbacks, and these are factors that are
generally rarely considered. The study area exhibits quite high
precipitation and is far from the Arid Diagonal (Piovano et al.,
2006), which is the belt that separate zones with precipitation
levels below 250 mm/yr to the south. Therefore, it is very difficult to
admit that Holocene climatic changes were able to disrupt the
vegetation cover at these latitudes and to generate barren slopes. In
fact, cover of shrubs and grasses prevents slope degradation and
contemporaneous aggradation as demonstrated during the Late
Pleistocene in Italy (Coltorti and Dramis, 1995). If Polylepis forest is
the potential vegetation type of the higher regions of the Altiplano
well above 4000 m (Fjeldså and Kessler, 1996; Gosling et al., 2009;
Chepstow-Lusty et al., 2009 and ref. therein) where precipitation is
much lower than at Ñuagapua, it appears to be clear that without
human disturbance, thick vegetation would also have covered the
area during the drier periods of the Holocene.
In Ñuagapua, the clear evidence of anthropic activity that has
been found suggests that humans may have strongly interacted
with the dynamics of the natural environment, which was recently
also noted to have occurred in Brazil during the same time interval
(Araujo et al., 2005). In the Bolivian Chaco (and probably in the rest
of Rio de La Plata Basin), humans lived together with many members
of the megafauna, among which: Megatherium,Doedicurus (Borrero
et al., 1998); Glossotherium (Prous and Fogaça, 1999); Eutatus
(Vizcaíno et al., 2003); Catonyx (Neves et al., 2004); Lama extinct
species (Nami and Nakamura, 1995), Hippidion (Nami and
Nakamura, 1995; Garcia et al., 2008), Equus (Faure et al., 1999;
Ficcarelli et al., 2003), but also Cuvieronius,Toxodon,Vicugna (this
paper). Unfortunately,the pottery artefacts that have been found are
too fragmented and scarce, and the state of the knowledge too poor,
to allow diagnostic attribution. Similar decorations were used in
different styles and periods throughout the Amazon basin in the
MiddleeLate Holocene (Roosevelt, 1995; Meggers, 1997; Lizzaraga,
2004). Megafauna survived in the area to at least up to 5500 yr BP,
and this constitutes, to our knowledge, their latest occurrence in
South America, and the more recent record from around the world,
with the exception of mammoth remains found in Wrangel Island
offshore of eastern Siberia (Boeskorov, 2006) dated to 3700 yr BP
Table 3
14
C-dated samples in Quebrada Ñuagapua.
Sample Material Analysis Coordinates Laboratory
number
Measured
Radiocarbon Age
13
C/
12
C Conventional
Radiocarbon age
Calibrated Age
(2 Sigma)
C2
Unit 1
Section 6
Organic
sediment
AMS 20
53
0
18
00
S, 63
02
0
36
00
W Beta - 194692 6820 þ/50 BP 21.7 o/oo 6870 þ/50 BP Cal BC 5840 to 5660
Cal BP 7790 to 7610)
Fireplace
Unit 1 Section 7
charred
material
Radiometric
Standard
20
53
0
25
00
S, 63
02
0
18
00
W Beta 197969 5960 þ/80 BP 24.1 o/oo 5980 þ/80 BP Cal BC 5050 to 4700
Cal BP 7000 to 6650
Plank
Unit 2 Section 1
wood Radiometric
standard
20
53
0
21
00
S, 63
02
0
53
00
W Beta 197971 150 þ/50 BP 25.3 o/oo 140 þ/50 BP Cal AD 1660 to 1950
Cal BP 290 to 0
M. Coltorti et al. / Journal of South American Earth Sciences 33 (2012) 56e67 63
Author's personal copy
(Vartanyan et al.,1995). Slightly older Holocene deposits than those
of Ñuagapua have been found along the Ecuadorian coast with quite
similar characteristics (Ficcarelli et al., 2003). Megatherium and
Mastodon remains associated with flint artefacts and dated to
approximately 6060 yr BP have also been described in the Colum-
bian highlands (Correal and Van der Hammen, 2003). The Ñuagapua
fauna association is anomalous because open (equids, glyptodonts,
camelids), semi-open (megatheres, mylodonts, mastodonts,
macrauchenids), forested (pecaries, deers, some typical rodentia,
such as the genus Holochilus), aquatic and semi-aquatic ones (tox-
odonts, hydrochoeridae and capromydae rodents) lived together.
Less demanding species (felids, small canids, armadillos, bats, other
rodents) were also present.
The aquatic and forested species most likely exploited the
alluvial plain, including swamp environments that are still quite
abundant slightly to the east of the study area. These assemblages
occupied the area beginning in the Early Holocene and continued
during the deposition of unit 1 (later than 5900 yr BP). In the latter
period, deforestation opened the top surface of the alluvial fans and
created favourable conditions and refuge areas for open and semi-
open elements (horses, glyptodonts, armadillos, some tardigrads
and camelids). During the Early Holocene, the natural open and
semi-open environments of these species were preserved at higher
latitudes (Tonni, 1990; Borrero et al., 1998; Vizcaíno et al., 2003),
where arid conditions and open vegetation seems to have existed
(Tonni et al., 1999; Pardiñas, 2001). During the Middle Holocene,
forested species continued to exploit the riparian vegetation.
Although the coexistence of hunters and megafauna is documented
in some Late Pleistocene and early Holocene sites (Gruhn and
Bryan, 1984; Markgraf, 1985; Guérin et al., 1993; Nami and
Nakamura, 1995; Núñez et al., 1995, 2001; Massone et al., 1998;
Faure et al., 1999; Prous and Fogaça, 1999; Correal and Van der
Hammen, 2003; Ficcarelli et al., 2003), including Ñuagapua, span-
ning over 5000 years, faunal extinctions occurred in more recent
times. This finding disproves the hypothesis of Firestone et al.
(2007) because megafauna survived well after the Younger Dryas
and the eventual extraterrestrial impact.
Unit 2 is dominated by soil with a rich organic clay horizon that
indicates a period of forest growth. The deeper channels at the base
of the unit indicate a new period of general incision and their slow
filling by sandy blankets and sheets (SB) during major episodic
floods. Solid load reduction and channel incision were therefore,
associated with forest growth. The fauna described in these layers
(Hoffstetter, 1968; MacFadden and Wolff, 1981) confirm the pres-
ence of more humid and forested environments. The human
remains found in unit 2 dated at approximately 6600 conv. yr BP
(MacFadden and Wolff, 1981) indicate rapid accumulation of the
older unit. However, the possibility is not excluded that the remains
came from the top of layer 1 and were incorporated into the profile
by weathering.
At the base of unit 3, we found large stumps (many of which
belong to large trees that are still growing), large fallen and burnt
trunks, charcoal and ash layers. This is clear evidence of an
important abrupt phase of slash and burn deforestation for agri-
cultural and grazing purposes. We had the chance to see the same
situation on the Amazonian side of the Ecuadorian Andes some
years ago. The sands of this unit exhibit similar characteristics to
those of unit 1 and belong to the sandy bedform associations of
Miall’s Model 11 (1996). This is additional evidence of the growth of
a sand-dominated alluvial fan. However, this rapid sedimentation
prevented the evolution of soils that were common in unit 1. The
deposition of unit 3 occurred in the last 200 years, as demonstrated
by the dated wooden plank, the buried stumps and the fact that
sedimentation was still active in 1967. The Bolivian Chaco was
intensively settled during the XIX century, and deforestation
processes were more intense at that time than during the rest of the
Holocene. The deposition of an alluvial fan resulting from the
widespread and intense degradation of natural forests is obviously
a model that can be easily used to explain the deposition of unit.1.
May et al. (2008a,b) found that in other alluvial fans in the region,
a similar depositional phase started almost 900 cal yr BP, which
could be related to the fact that the vegetation of major rivers
became degraded earlier than the vegetation of more remote river
basins. In the Laguna Mar Chiquita (30
54
0
Se62
51
0
W), which is
located on the pampean plains of Argentina, following a period of
desiccation of the lake after 4.7 cal yr BP, increased sedimentation
began after 1.5 cal yr BP (Piovano et al., 2006) but it is worth noting
that the sedimentation rates of the last 1500 yrs have more than
doubled those of the early Holocene.
A later, very rapid stream incision probably began slightly before
1960, when Hoffstetter visited this area for the first time with most
of the incision occurring after this period. It proceeded very quickly,
as Hoffstetter reported a limited thickness of unit 1. This is also
evident in the comparison of aerial photos from 1970 to 2005,
when the surface of the mid-fan was still affected by sandy lobes.
Since 1970, the incision has progressed eastward. Although the last
2e300 years have been a period dominated by important cooling
events on a global scale (Little Ice Age), there is no evidence that the
induced climatic changes were able to modify the vegetation belt
and the associated processes of soil formation in this part of the
Chaco. Additionally, in the last decades, sedimentation has been
interrupted and incision has prevailed due to well-documented
land use modifications.
6. Conclusions
The Holocene was characterised by great climatic variability,
which can mostly be seen through oscillations of lake levels in the
Altiplano and the Argentinean lowlands (Seltzer et al., 1998; Baker
et al., 2001; D’Agostino et al., 2002; Abbott et al., 2003; Piovano
et al., 2006; Williams et al., 2011), in glacial cores (Thompson
et al., 2000) and glacier fluctuations (Glasser et al., 2004).
However it is unlikely that these oscillations were able to eliminate
the vegetation at low elevations such as the study area.
May et al. (2008a,b) attributed most of the sedimentological
changes recorded in a nearby alluvial fan to climatic changes,
although in the Ñuagapua area, which is located well below the tree
line, the observed changes were quite inertial and could have been
mostly driven by human land use changes since the Prehistoric
period. Accumulations of sediments were particularly concentrated
between 6900 and 5900 cal yr BP and after the XVIII century.
As observed at both the Ecuadorian site (Ficcarelli et al., 2003)
and at Ñuagapua, Equus and other savannah-grassland animals
(such as glyptodonts and camelids) lived together with forested
species and animals more suited to humid habitats, such as
mastodonts, deers and megatheres. The opening of forest cover due
to human deforestation created a series of niches at low latitudes
that were rapidly occupied by large herbivores that had survived
during the Early Holocene in the open environment of the Southern
Cone (e.g. Megatherium,Doedicurus and Equus cfr. Nami, 1996;
Doedicurus cfr. Borrero et al., 1998;Mylodon cfr. Long et al., 1998).
During the same period, megafauna (megatheres, other ground
sloths, giant armadillos, glyptodonts, deers, camelids, horses, etc.)
could have been an important part of the everyday diet in paleo-
Indian cultures (Martin, 1967, 1973, 1984; Mosimann and Martin,
1975; Mengoni Goñalons, 1986; Diamond, 1989; Núñez et al.,
1994; Nami, 1996; Alroy, 2001). However, this coexistence lasted
for over 5000 years, which is a time span long enough for the
animals to become aware that humans constituted a natural enemy
(Brook and Bowman, 2003). Recent discoveries in Australia testify
M. Coltorti et al. / Journal of South American Earth Sciences 33 (2012) 56e6764
Author's personal copy
that an even longer period of megafauna survival alongside
humans occurred for over 10,000 years (Trueman et al., 2005). It
appears that a major contribution to the extinction of these species
come from large changes induced by human activities in the
natural environment that affected mammal assemblages that were
already strongly stressed and weakened from the climatic and
environmental changes that occurred at the transition from the
Pleistocene to the Holocene (Ficcarelli et al., 2003; Barnosky et al.,
2004, Barnosky and Lindsey, 2010). The increased human pressure
at the end of the Early Holocene in the Chaco lowlands could also
explain the contemporary “silencio arquelogico”(Grosjean et al.,
2006) in the earlier settled Andean Altiplano, where megafauna
are not recorded after the PleistoceneeHolocene transition. The
extinction of these species should therefore be investigated as the
result of a series of events induced by human disturbances of
fragmented ecological conditions on the continental scale. In this
context, on the local scale, widespread nonspecific or specific
diseases could also have contributed to the extinction of the
megafauna (Lyons et al., 2004; Rothschild and Laub, 2006).
Acknowledgements
This work has been funded by the MIUR (Ministero dell’Is-
truzione, Università e Ricerca) during 2002, titled “Climatic and
environmental changes in sample areas of Bolivia during the Late
Pleistocene and Holocene”. The Siena research unit was coordi-
nated by Mauro Coltorti while the Florence unit was coordinated by
Prof. Lorenzo Rook. We thank Padre Lorenzo Calzavarini of the
Centro Eclesial de Documentacion of the Convent S. Francisco
(Tarija) for the documents regarding the environmental situation of
the Chaco during the last centuries. We also thanks Manuel Arroyo-
Kalin, from Department of Archaeology, University of Cambridge,
for the information regarding the archaeology of the Amazonian
basin and the anonymous referees for their useful suggestions.
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