Bone Surface Modifications, Reasonable Certainty, and Human Antiquity in the Americas: The Case of the Arroyo Del Vizcaíno Site

Abstract

Modifications on bone surfaces are taphonomic features that allow, among other aspects of environmental reconstruction, the assessment of human presence. The agents that cause such marks are diverse and of both biotic and abiotic origin. Among the former, marks made by human tools are of paramount importance for archaeologists and paleontologists to identify. Although it is possible to erroneously assign trampling marks to cut marks, several criteria have been recently developed so as to avoid such risks. These methods are applied here to the 30,000-year-old site of Arroyo del Vizcaíno (Uruguay), where over one thousand megafaunal remains have been collected. Some of them show marks that have been interpreted to be the result of the action of human tools. Using a database built up from previous studies of experimentally made marks as an actualistic model, it was concluded that the marks in the Arroyo del Vizcaíno site are unlikely to have been made by trampling, hence leaving human agency as the most feasible cause. This has important consequences for the debate on the human peopling of the Americas and on the process of extinction of the Pleistocene megafauna.

Full text

The study of surface modifications on bones
is a valuable archaeological approach per-
mitting the identification of the agent that
created them. Among the most common modifi-
cations, root etching, carnivore scratching, rodent
gnawing, trampling and tool-made marks provide
information on the environment. Moreover, when
modifications of biotic agency are present, they
can be considered ichnofossils and can be used
to make inferences about the presence of a par-
ticular taxon. Such modifications are important
for archaeology, in particular when the species is
our own or a close relative.
Among many other examples, this approach
has been useful for some decades in cases such
as that of moas processed by metal tools in his-
torical times in New Zealand (Duff 1956), the
habits of hominids in Olduvai (Potts and Ship-
man 1981), and bone tool use in butchering a
proboscidean in Pleistocene North America
(Shipman et al. 1984). With the availability of
scanning electron microscopy (SEM), it has be-
come easier to distinguish cut marks made by
stone tools from gnaw marks made by nonhuman
predators or scavengers and from marks made
by an excavator’s or preparator’s tools (Potts
BONE SURFACE MODIFICATIONS, REASONABLE CERTAINTY, AND
HUMAN ANTIQUITY IN THE AMERICAS: THE CASE OF THE
ARROYO DEL VIZCAÍNO SITE
Richard A. Fariña
Modifications on bone surfaces are taphonomic features that allow, among other aspects of environmental reconstruction,
the assessment of human presence. The agents that cause such marks are diverse and of both biotic and abiotic origin.
Among the former, marks made by human tools are of paramount importance for archaeologists and paleontologists to
identify. Although it is possible to erroneously assign trampling marks to cut marks, several criteria have been recently
developed so as to avoid such risks. These methods are applied here to the 30,000-year-old site of Arroyo del Vizcaíno
(Uruguay), where over one thousand megafaunal remains have been collected. Some of them show marks that have been
interpreted to be the result of the action of human tools. Using a database built up from previous studies of experimentally
made marks as an actualistic model, it was concluded that the marks in the Arroyo del Vizcaíno site are unlikely to have
been made by trampling, hence leaving human agency as the most feasible cause. This has important consequences for the
debate on the human peopling of the Americas and on the process of extinction of the Pleistocene megafauna.
Las modificaciones en la superficie de los huesos son características tafonómicas que permiten, entre otros aspectos de la
reconstrucción del ambiente en que vivía el organismo, la evaluación de la presencia humana. Los agentes que causan esas
marcas son diversos y de origen tanto biótico como abiótico. Entre los primeros, es de la mayor importancia para disciplinas
como la arqueología y la paleontología la identificación de aquellas marcas hechas por herramientas humanas. Aunque es
posible asignar erróneamente marcas de pisoteo a marcas de corte, varios criterios se han desarrollado recientemente para
evitar tales riesgos. Esto se aplica aquí al yacimiento del Arroyo del Vizcaíno (Uruguay) de fecha 30.000 años aP, en el que
más de mil restos de megafauna ya han sido colectados. Algunos de ellos muestran marcas que fueron interpretadas como
productos de la acción humana. Usando una base de datos tomada de la bibliografía de marcas hechas experimentalmente
como un modelo actualista, se concluye que es muy improbable que esas marcas sean debidas al pisoteo, dejando así la acción
humana como la causa más probable. Esto plantea importantes consecuencias en el debate del poblamiento de América y en
el proceso de extinción de la megafauna pleistocena.
Richard A. Fariña 䡲Sección Paleontología, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, Uruguay
(fari~a@fcien.edu.uy)
American Antiquity 80(1), 2015, pp. 193–200
Copyright © 2015 by the Society for American Archaeology
DOI: 10.7183/0002-7316.79.4.193
193

194 AMERICAN ANTIQUITY [Vol. 80, No. 1, 2015]
and Shipman 1981; Shipman 1981; Shipman and
Rose 1983).
However, when analyzing evidence for human
presence and interaction with animals, the most
important issue is avoiding the misidentification
of trampling marks (Behrensmeyer et al. 1986).
The phenomenon through which different agents
can yield similar results in open systems is called
equifinality, a concept borrowed from systems
theory (von Bertalanffy 1968) and applied to the
study of cut marks and taphonomy in general
(Lyman 2004). Quantitative approaches, such as
the pioneering work by Long and Walker (1977)
and Walker (1978), aided in minimizing this prob-
lem, although only in the last decade (e.g., Bello
and Soligo 2008; Bello et al. 2009; Domínguez-
Rodrigo et al. 2009) has major progress been
made in distinguishing between the surface mod-
ifications caused by the two main and potentially
most misidentifiable agents, i.e. trampling and
human tools.
Here I build a probabilistic model to be ap-
plied to those archaeological sites that show var-
iedly originated surface modifications on bones
as a way to set limits on equifinality in inter-
preting those modifications. In particular, this
report aims to provide a quantitative probability
of the correct identification of the trampling and
cut marks found in the 30,000 year-old site of
Arroyo del Vizcaíno, Uruguay (Fariña, Tam-
busso, Varela, Czerwonogora, et al. 2014). Given
the old age of this site, if cut marks can be iden-
tified with reasonable certainty, this study might
become a very important contribution to the de-
bate on human colonization of the Americas and,
given the proposed role of our species in that
process, the extinction of the megafauna (Fariña
et al. 2013).
Arroyo del Vizcaíno:
Main Characteristics of the Site
The megafaunal site of Arroyo del Vizcaíno, near
the town of Sauce, Canelones, Uruguay (Figure
1), is fully described in Fariña, Tambusso, Varela,
Czerwonogora, et al. (2014). First found in 1997
during a severe drought, the site is formed by a
streambed in a place where the Vizcaíno stream
becomes deeper, forming a natural pond on a
substrate of Cretaceous silicified sandstone. More
than 1,000 remains of at least 27 specimens of
South American Pleistocene megafauna have
been collected during the four field expeditions
undertaken to date (Fariña, Tambusso, Varela,
Czerwonogora, et al. 2014), mostly belonging to
the giant sloth Lestodon armatus but with about
6 percent of remains of other ground sloths,
glyptodonts, Toxodon platensis, a fossil horse, a
deer, a proboscidean, and the sabertoothed felid
Smilodon populator.
With the exception of a few redeposited ele-
ments in an upper bed, nearly all fossils of the
site were found in situ in a .60–.80 m thick bed
of greenish muddy sandy gravel (facies a) to
brownish muddy sand (facies b), the occurrence
of which may be due to slightly different sedi-
mentary conditions or perhaps differential post-
depositional oxidation events. No predominant
orientation of limb and elongated bones was
found. The taphonomy of the site suggests a bio-
genic origin, according to the representation of
bones with different hydraulic transportability
(Voorhies 1969). Although the existence of two
populations of bones cannot be determined to
date, it should not be ruled out that the bones
from the two facies of the fossiliferous bed could
have had different taphonomic histories. This is
particularly valid for those remains with surface
modifications that were carefully studied in Far-
iña, Tambusso, Varela, Czerwonogora, et al.
(2014), as explained below. The representation
of the anatomical units (percent minimal animal
units [MAU]) resembles those in kill sites asso-
ciated with gourmet consumption (Meltzer 2006).
Moreover, the mortality profile shows that 90
percent were adults, 2.6 percent were (mostly
subadult) juvenile individuals, and 7.4 percent
were old individuals. This profile is similar to
that seen at kill sites but different from those
found in attritionally, catastrophically, or acci-
dentally accumulated assemblages (Stiner 1990).
Nine radiocarbon assays were obtained at three
laboratories (URU, Laboratorio 14C, Comisión
Nacional de Arqueología, Cátedra de Radio-
química, Facultad de Química, Universidad de
la República, Uruguay; Beta Analytic Radiocar-
bon Dating, Miami, Florida, USA; and Oxford
Radiocarbon Accelerator Unit, University of Ox-
ford, UK). Samples were taken from both purified
and non-purified bone collagen, as well as from

wood. Although different procedures were un-
dertaken at different times and in different labo-
ratories, the ages reported are consistently close,
at about 30,000 radiocarbon years before present
(B.P.) (Fariña and Castilla 2005; Fariña, Tam-
busso, Varela, Czerwonogora, et al. 2014).
No carnivore tooth marks were identified.
Nearly 59 percent of the bones collected showed
modifications, with features identifiable as tram-
pling marks (Behrensmeyer et al. 1986). More
than one-third of the bones exhibited trampling
abrasion marks on over 25 percent of their surface
area. On the other hand, the suggested potential
human activity is supported by the surface mod-
ification present in about 40 bones (about five
percent of the identified specimens). A total of
15 of those marks were carefully studied in Far-
iña, Tambusso, Varela, Czerwonogora, et al.
(2014) with light microscopy under magnifica-
tions of 20x, 30x, and 45x. Pictures were taken
at different focal depths, a three-dimensional
model of each mark was built, and the cross-sec-
tional profiles were then digitized and measured.
Little lithic material has been collected to date,
due to the site having been only partially ex-
plored. Nonetheless, a small piece of translucid
silcrete was found that has macroscopical features
compatible with a scraper. Using a SEM at 700-
4300x, a rather dull and coarse area was observed
in that piece, with circular microdepressions that
are darker than the rest of the surface and that
extend along a large portion of one edge, which
is consistent with a second-stage micropolish, as
produced by working on dry hide.
Given the old age of the site, finding those
pieces of evidence was unexpected (but see Dille-
hay and Collins 1988; Guidon and Delibrias
1986). Unfortunately, the site has been only par-
REPORTS 195
Figure 1. Geographic location of the Arroyo del Vizcaíno site.

tially excavated and the landscape cannot be prop-
erly reconstructed with the current geological ev-
idence gathered. Therefore, at present it cannot
be determined whether this location is a kill or
butchering site.
In sum, there is a strong need for further dis-
cussion of the evidence found. In particular, it
seems crucial to make the best effort to correctly
assign the marks to the agent that created them,
as conducted in the recent study by Hockett and
Jenkins (2013) in Paisley Caves, Oregon.
Material and Methods
The database in Domínguez-Rodrigo et al.
(2009:Table 5) was used as the basis for the model
described below. The data consist of the absolute
values and percentages of 14 categorical variables
in their experimental sample of reproduced tram-
pling marks (n= 251), cut marks made with sim-
ple flakes (n= 246), and cut marks created with
retouched flakes (n= 105) in modern bones.
Three variables were chosen here according to
196 AMERICAN ANTIQUITY [Vol. 80, No. 1, 2015]
Figure 2. Microphotographs of bone surface modifications in remains from the Arroyo del Vizcaíno site: (a) cut mark on
the rib CAV 451, showing its straight trajectory; (b) 3D reconstruction of the same mark; (c) same reconstruction, with
a horizontal plane representing the bone surface before its modification (and note that the shoulders are observed to pro-
trude above that plane); (d) V-shaped cross section of the same mark; (e) trampling mark on the surface of the femur
CAV 722, showing its wide, U-shaped cross section (and its curvilinear trajectory is not apparent because only a short
part is shown); (f) 3D reconstruction of the same mark, in which no shoulder effect can be observed.

their likelihood (and easiness) of observation in
fossil bones (Figure 2): groove trajectory (straight,
curvy, or sinuous), groove section shape (V-
shaped or otherwise), and shoulder effect (present
or absent). It should be noted that this categorical
classification is not entirely devoid of some de-
gree of subjectivity, a topic that merits its own
discussion elsewhere. Other variables, such as
length, presence of a barb, and microstriae, are
more difficult to find preserved in prehistoric ma-
terial (Behrensmeyer et al. 1986; Domínguez-
Rodrigo et al. 2009). The variables selected were
among those with the highest loadings in two
first dimensions, i.e., those that allowed for better
differentiation of cut from trampling marks in
the multidimensional analysis (categorical prin-
cipal components analysis) used by Domínguez-
Rodrigo et al. (2009). The results are provided as
a probability of reclassification between 0 and 1.
Higher values indicate a greater probability of a
consistent reclassification.
Bello and Soligo (2008) used another approach
based on quantitative variables measured on ex-
perimentally inflicted marks on modern bones:
the angles between the slopes of the cut mark and
the unaffected bone surface, the angle between
both slopes, the angle of the bisector of the open-
ing angle of the cut mark relative to the unaffected
bone surface, the height of the shoulders formed
on either side of the cut, the radius of a circle
fitted to the floor of the cut-mark profile, and the
radius of a circle fitted to the floor of the cut-
mark profile. Although they focused their ap-
proach on the microscopic features that allow for
identifying cut marks made by flint tools and
metal knives at different hand positions, here I
combined their data with those in Fariña, Tam-
busso, Varela, Czerwonogora, et al. (2014) to in-
clude surface modifications made by trampling.
Here I present results based on all 15 marks
in Fariña, Tambusso, Varela, Czerwonogora, et
al. (2014) identified as the possible product of
human tools. They are found in the following
material, selected among those that had macro-
scopically promising marks and little to no tram-
pling, at least in the same area: tibia (CAV 395—
not CAV 385, as erroneously written in Fariña,
Tambusso, Varela, Czerwonogora, et al. (2014)—
1 mark studied), rib (CAV 451, 3 marks studied),
rib (CAV 452, 3 marks studied), long bone frag-
ment (CAV 453, 2 marks studied), rib (CAV 458,
1 mark studied), radius of a Glyptodon sp. (CAV
459, 1 mark studied), rib (CAV 475, 1 mark stud-
ied), hyoid (CAV 476, 1 mark studied), ulna (CAV
520, 1 mark studied), mandible (CAV 897, 1 mark
studied). Unless otherwise stated, all remains
were assigned to the giant sloth Lestodon arma-
tus. All these specimens showed excellent preser-
vation and were classified as Behrensmeyer’s
(1978) weathering stage 1, with smooth surfaces
and no evidence of cracks, such as those made
by alternating episodes of drying and wetting.
Results
Among the 251 trampling marks in the database
provided by Domínguez-Rodrigo et al.
(2009:Table 5), 75 (29.8 percent) are straight,
while this feature is observed in 94.6 percent (332
out of 351) of the retouched and unretouched cut
marks (Table 1). Therefore, the probability of re-
classifying a cut mark showing a straight trajec-
tory is .76, which results from the normalized ra-
tio of straight cut marks to all straight marks:
94.6/(29.8+94.6). Accordingly, if that straight
mark is due to trampling, it will be correctly re-
classified with a probability of .24.
Only 10 (or 4 percent) trampling marks show
a V-shaped section. Among cut marks, 280 show
this characteristic, increasing the appropriate per-
centage to 79.8, making the probability of cor-
rectly reclassifying a cut mark with a V-shaped
section .952. Therefore, this probability is only
.048 for a V-shaped trampling mark.
Finally, the shoulder effect is present in 15
(5.9 percent) trampling marks and in 159 (45.3
percent) cut marks. Thus, the correct reclassifi-
cation of a shouldered cut mark has a probability
of .885. In turn, a shouldered trampling mark has
a .115 probability of being correctly reclassified.
Using those figures as an actualistic model to clas-
sify marks of unknown origin and assuming that
the three chosen variables are independent of each
other, it follows that a mark that is straight, V-
shaped, and with shoulders has a probability of
1.3 x 10-3 to have been made by trampling.
In Figure 3, two variables considered by Bello
and Soligo (2008), depth in ?m and opening angle
in degrees, are shown for some marks. The ex-
perimental marks (open symbols) are from data
REPORTS 197

in Bello and Soligo (2008) representing marks
inflicted by a handaxe or by a flint flake at dif-
ferent angles. Those represented by solid symbols
are observed in bones from the Arroyo del Viz-
caíno site (Fariña, Tambusso, Varela, Czerwono-
gora, et al. 2014).
Discussion
In Fariña, Tambusso, Varela, Czerwonogora, et
al. (2014), 15 marks with all of the three features
discussed above were studied on bones belonging
to megafaunal species in the Arroyo del Vizcaíno
site. Those elements belong mostly to the giant
ground sloth Lestodon armatus, but also to a
glyptodont, and were assigned to human agency.
According to the model presented here, the prob-
ability that none of them were correctly identified
as human made and, instead, that all of them were
made by trampling is only 6 x 10-44. Given the
unlikelihood of equifinality, the conclusion
reached in Fariña, Tambusso, Varela, Czerwono-
gora, et al. (2014) and further discussed in Fariña,
Tambusso, Varela, Di Giacomo, et al. (2014) as-
signing human agency to these marks can be con-
sidered sound, thus yielding reasonable certainty
about their proposal.
The opening angle yields an unambiguous cri-
terion for distinguishing marks made by cutting
from those made by trampling, as shown in Figure
2. Depth, alternatively, had been considered in-
formative by some authors (e.g., Olsen and Ship-
man 1988), but the trampling marks studied in
Fariña, Tambusso, Varela, Czerwonogora, et al.
(2014) are about as deep as those in Bello and
Soligo (2008). However, the cut mark on a
Lestodon bone from the Arroyo del Vizcaíno site
plotted in Figure 2 is much deeper than trampling
marks observed here. A third readily observable
variable, the height of the shoulder, was not de-
picted here because it was not present in the two
trampling marks studied.
It should be noted that, apart from the variables
chosen, which have been thoroughly demon-
strated to be useful for the analysis in this paper,
most, if not all, of the marks studied show addi-
tional features that have been considered relevant
in the literature (Bello and Soligo 2008;
Domínguez-Rodrigo et al. 2009), such as internal
microstriations, Hertzian cones, asymmetry, and
small floor radius. Moreover, sharp points, a fea-
ture considered relevant by Hockett and Jenkins
(2013), were observed in several of the 15 marks
referred to here, while most of them have roughly
the same size. Finally, a few other bones (but
none of those listed above) show multiple inci-
sions, forming a “Y” or “braided” shape near one
end of the cut.
Although quite a few sites are considered to
have broken the Clovis Barrier (Hockett and Jenk-
ins 2013), the results presented here stress the
unexpected nature of the Arroyo del Vizcaíno
site. The evidence found in this locality suggest-
ing human presence in South America at 30,000
B.P. is not well explained by the current view of
the peopling of the Americas, rightly summarized
by Pitblado (2011) as a model of two migrations
based on both archaeological and genetic evi-
198 AMERICAN ANTIQUITY [Vol. 80, No. 1, 2015]
Table 1. Number of Cases in the Database from Domínguez-Rodrigo et al. (2009).
Straight trampling marks 75 V-shaped trampling marks 10 Shouldered trampling marks 15
Total trampling marks 251 Total trampling marks 251 Total trampling marks 251
Percentage 29.8 Percentage 4 Percentage 5.9
Straight unretouched cut marks 230 V-shaped unretouched cut marks 238 Shouldered unretouched cut marks 81
Total unretouched cut marks 246 Total unretouched cut marks 246 Total unretouched cut marks 246
Percentage 93.5 Percentage 96.7 Percentage 32.9
Straight retouched cut marks 102 V-shaped retouched cut marks 42 Shouldered retouched cut marks 78
Total retouched cut marks 105 Total retouched cut marks 105 Total retouched cut marks 105
Percentage 97.1 Percentage 40 Percentage 74.3
Straight cut marks 332 V-shaped cut marks 280 Shouldered cut marks 159
Total cut marks 351 Total cut marks 351 Total cut marks 351
Percentage 94.6 Percentage 79.8 Percentage 45.3
Probability reclassification of .240 Probability reclassification of .048 Probability reclassification of .115
trampling marks trampling marks trampling marks
Probability reclassification of .760 Probability reclassification of .952 Probability reclassification .885
cut marks cut marks cut marks

dence. The first pulse must have occurred around
16,000 or 15,000 years B.P. by watercraft along
the coast of Beringia and western North and South
America, while the second must have taken place
1,000 years later, with proto-Clovis hunters trav-
elling through the ice-free corridor.
Instead, the Arroyo del Vizcaíno site should
be included among the unanticipated findings,
vividly described by Pitblado (2011:360) as be-
longing to the “only sure bet: that at least a few
pending peopling finds will be of the ’wow— I
never saw that coming’ variety.” As such, Arroyo
del Vizcaíno should be added to the pre-Last
Glacial Maximum (and controversial) sites in the
Americas with evidence of human presence,
which include Toca do Boqueirão da Pedra Fu-
rada (Guidon and Delibrias 1986) in Brazil and
Monte Verde I (Dillehay and Collins 1988) in
Chile. It can be claimed that until datable human
remains are found, the association between the
evidence and the age found by Fariña, Tambusso,
Varela, Czerwonogora, et al. (2014) is the closest
possible, as in some cases it is the same object
(the marked bone) that has been dated and shows
evidence of human-made marks. Because in some
cases the very same object (the marked bone)
that has been dated shows the evidence of human
presence, it can be claimed that the association
between that evidence and the age in the Arroyo
del Vizcaíno site is the closest possible, at least
until datable human remains are found. Moreover,
the approach used here must be developed with
more complete databases and more refined mod-
els, in order to enhance and further reduce the
risks posed by equifinality in the study of marks
on bones.
Acknowledgments. This is a contribution to the project “
Prospección de nuevos sitios fosilíferos y arqueológicos en
el área del Arroyo del Vizcaíno,” funded by CSIC, Universidad
de la República. Roberto Bracco was an important influence
in developing the ideas conveyed here. Ángeles Beri, Patricia
Grigny, Christine Lucas, and Luciano Varela read previous
versions of this manuscript. I received substantial help with
Figures 1 (Eva Fariña) and 2 (Luciano Varela and Sebastián
Tambusso). Three anonymous reviewers made many useful
suggestions.
Data Availability Statement. Data are published in Bello and
Soligo (2008), Domínguez-Rodrigo et al. (2009), and Fariña,
Tambusso, Varela, Czerwonogora, et al. (2014).
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Submitted June 12, 2014; Revised August 18, 2014;
Accepted September 1, 2014.
200 AMERICAN ANTIQUITY [Vol. 80, No. 1, 2015]