Pleistocene burrows in the Mar del Plata area
(Argentina) and their probable builders
SERGIO F. VIZCAINo, MARCELo zAnłTE' M. sUSANA BARGO,
andALEJANDRO DONDAS
Yizcaino,S.F.,Zdrate, M., Bargo, M.S., & Dondas, A. 2001. Pleistocene burrows in the
Mar del Plata area (Argentina) and their probable builders. - Acta Palaeontologica
Polonica 46, 2, 289-301.
Structures discovered near Mar del Plata are attributed to palaeoburrows built by fossil
animals on the basis of moqphological patterns, transgressive boundaries in relation to
the sedimentary units, and the presence of claw marks on the walls and roofs. They are
discrete features of several metres in length, and with subrounded cross sections. Their
diameters range from 0.80 to 1.80 m, with the width generally exceeding the height.
These structures occur in Pleistocene deposits containing marnmals referable to the
Ensenadan and Lujanian Ages. Several Xenarthra are good candidates as builders of
these burrows. Palaeobunows were attributed before to the large Pleistocene armadillos
Propraopus, Eutatus, and Pampatherium. We consider the possibility that the mylodontid ground sloths were responsible for excavating the burrows. The similar diameters
of the burrows and the sloths are consistent with this observation. Anatomical,
allometńc, and biomechanicalanalysis of sloths limbs indicates that they were well designed to perform such activity. The shape of some claw marks preserved on the sides and
roof of the burrows fits the form of their hand skeleton. Thus, the mylodontid sloths
Scelidotherium and Glossotherium are considered as possible builders for the large late
Cenozoic burrows present in the Pampean region.
Key words: Pleistocene,burrows, Mylodontidae, Xenarthra, South Ameńca.
Sergio F. Vizcaino [[email protected]],
M. SusanaBargo Imsbargo@
museo.fcnym.unlp.edu.arl, Departamento Cientifico Paleontologia de Vertebrados,
Museo de La Plata, Paseo del Bosque s/n, 1900 La Plata, Argentina. CONICET CIC.;
Marcelo Zdrate, CONICET-IANIGI'Ą/CRICYT, cc 330, 5500, Mendoza, Argentina;
Alejandro Dondas, Museo Municipal de Ciencias Naturales 'Lorenzo Scaglia', Plaza
Espafia,7600 Mar del Plata, Argentina.
Introduction
Mammalian burrows of different sizes are commonly mentioned,but not specifically
described,in the literaturereferredto the late Cenozoic depositsof the Buenos Aires
Acta Palaeontol.Pol. 46, 2,289-301.
290
Pleistocene burrows in Argentina: YIZCAINO
et al.
Province (Argentina),especially in the coastal region (Ameghino 1908;Frenguelli
I9fI,1928; Kraglievichl95f).In thelastdecadesomespecificstudieshavebeenperformed.Genise (1989)and Scognamillo (1993)analyseand describein detail small
burrows of approximately13 cm of diameter,found in the coastalcliffs of Mar del
Plata area(BuenosAires Province)and attributedto the fossil rodentActenomys.
Large structureswith diametersof approximately1m were studiedin the proximity
of La Plata,NE of BuenosAires Province(Imbellone& Teruggi1988;Imbelloneet al.
1990),and Rio Grande do Sul in southernBrazll (Bergqvist& Maciel I994).In Mar
del Plata, Quintana (1992)reporteda burrow of the samediameter,providing the first
morphologicaldescriptionof this kind of tunnels.This was of particularinterestbecauseit was branched,23metreslong, with sometrackson the walls and theroof, and
devoid of sedimentaryfilling. The constructionof all theselarge burrows was attributed to giant armadillos (Dasypodidaeand Pampatheriidae).
In recentyears severallargerburrows were discoveredin the areaof Mar del Plata.
Zdrateet al. (1998)examinedtheselargestructuresfocusingon their stratigraphicand
sedimentologicalimplications. In this paper, we analyzefaunistic, morphological,
biomechanical,and ichnological evidence to propose their probable builders in a
palaeoecologicalcontext.The termburrow or palaeobuffowis usedfollowing Bergqvist & Maciel (1994),i.e.,includingbothpalaeocaves(hollows)and crotovines(filled
with sediments).
Stratigraphy,
morphology, and fillings of the burrows
Although theburrowsin lateCenozoic sedimentsnearMar del Platahavebeenknown
for more than a century,they were not described nor interpreteduntil very recently.
Zdrate et al. (1998)recordedand described42 structuresin the Mar del Plata arca
(BuenosAires Province) (Fig. 1). The time rangeof theseburrowsextendsfrom the
Pliocene to the late Pleistocene,and possibly until as recently as late glacial maximum-late glacial times. Based on the shallowestempty burrows found in the area,
Zźrateet ąI. (1998)estimateda probabledepthof 5_6 m from the sufface.Thus, the
structuresare stratigraphicallytransgressive,which constrainsthe chronological resolution of their time of formation,the prevailing palaeoenvironmentalconditions when
formed, and estimationsof their relative abundanceat any stratigraphiclevel.
The stratigraphicrelationships,sizes, and closed boundariesare the main criteria
for field recognitionof thesestructures(Z6rateet ąI. 1998).once thecontextualframework has beenestablishedbasedon thesecriteria,the sedimentaryfillings and the associatedfaunal remainsarefurtherapplicablecriteria to aid in determiningthe origin.
The structures,formed in cohesivematerialcomposedof very fine sandstoneor
siltstones(loess-like sediments),are located either in floodplain or in interfluvial
palaeoenvironments.The tunnelsare filled with thinly stratifiedfine sandsand silts alternatingwith laminatedclay layers.
The morphology of the structuresis characterisedby discrete boundaries and
subcircularto slightly elliptical cross-sections.Some of them show flat floors and
archedroofs. Longitudinal sectionsare also frequent,but not usually reported,possibly becausethey have been misinterpretedas fluvial features.Their maximum width
ACTA PALAEONTOLOGTCA POLONTCA (46) (2)
A
291
sourH AMERTcA
z
tr
zUJ
o
t.
Fig. 1. A. Map of South America showing the location of Buenos Aires Province (in black) and the approximate areasof natural caves with ground sloths remains (open circles). B. Map of Buenos Aires Province
showing the areaswhere large Pleistoceneburrows (open circles) were found.
varies between0.80 and 1.80 m, and usually the width slightly exceedsthe height
(Fig. D. Tunnelsrecentlydiscoveredin Mar del Plata and devoid of sedimentaryfilling reachup to 1.80m in width and40 m in length.The major size variationmight reflect differentanimal diggers;size variationsbetween15-30 cm are possibly caused
by measurementof width along a plane oblique to the longitudinal axis of the structure
(due to difficulty in interpretationof the true axis); thosebetween5 and 10 cm ure attributableto measurementelror causedbv boundariesthat are difficult to distinsuish
from the surroundingsediments.
Associated fauna
A large numberof mammaliantaxa have been recordedfrom the late Cenozoic exposures along the Mar del Plata-Miramar sea-cliffs.There is an extensiveaccountof
them in a recentcompilationeditedby Alberdi et al. (1995).Many taxa can be discarded as the builders of these burrows as they do not show adaptationsto dig
(proboscideans,
and some
artiodactyls,perissodactyls,litopterns,mostnotoungulates,
large carnivores).Othersare too small (rodents,notoungulatessuch as hegetotheres,
and othercarnivores).
On the otherhand,many representativesof the superorderXenarthrafulfill both requirements:they have morphological adaptationsto dig and they are large enough.
They comprise three main lineages:cingulates,including armadillos (Dasypodidae
f92
Pleistocene burcows in Argentina: YVCA(NO
et at.
and Pampatheriidae)and their relatives the glyptodonts;vermilinguas, the anteaters
(Myrmecophagidae);
and tardigrades,the groundsloths(Mylodontidaeand Megatheriidae),relativesof the highly specialisedliving treesloths(Bradypodidaeand Megalonychidae).
Recent papersconsiderthe possibility of armadillosas the builders of large burrows. Quintana(1992)proposedtwo dasypodids,Eutątusseguini andPropraopus sp.)
or a pampatherid,Pampatherium typum,as probablebuilders of a palaeocavein Mar
del Plata area.This has also been suggestedfor burrows in other areasof southern
South America. Near LaPlata (NE of Buenos Aires Province,Argentina),Imbellone
and Teruggi (1988)and Imbelloneet al. (1990)also tentativelyattributedsome burrows (with diametersof approximately1 m) to Eutatus seguini or Pampatherium
typum.Bergqvist& Maciel (1994)describedlargecrotovines(about1 m width)in Rio
Grande do Sul (southernBrazil) which they attributedto the pampatheresPampatherium andHolmesina,and to the extinctdasypodidPropraopus.Living armadillos
are powerful diggersand scratchersthat usually live in undergroundburrows.Their
skeletonsshow all the adaptationsrequiredfor a digger.The skull is low and narrow,
the limb bones presentstrong processesand scars for muscle attachment,and the
ungualphalangesarewide andrelativelyflat (usuallyoneor two aremore specialised).
Both Eutątus andPropraopus are approximatelythe samesize as the largestliving armadillo Priodontes maximus,so they must have been close to 50 kg in body mass
(Farińa&Ytzcaino 1997),and a maximumtransversebody diameterof about50 cm.
Both show the sameskeletaladaptations,more or less developedto the samedegree,
so they can easily be identifiedas diggers.
Pampatheresare larger than dasypodid armadillos.Pampatheriumtypum must
have reached200 kg and its estimatedmaximum transversediameterwas about80 cm.
It appearsthattheywerenot as good diggersas dasypodids.Edmund (1985)described
limbs of pampatheresas being intermediatein proportionsand specialisationbetween
those of glyptodontsand the living armadillo Dasypus, including graviportal adaptations for the hind limb. The threecentral fingers of the manus are equally developed,
and no especially developedclaws are present.The relative length of the olecranon
processis considerablyless thanin the fossorialarmadillos.A long olecranonprocess
improves the momentarm of triceps,which acts as an ęxtensorof the forearm.Within
mammals,this designis appropriatefor diggers,such as the armadillos.Even in some
unspecialisedcladesof mammals,speciesthatareknown to dig morefrequentlyshow
relativelylongerolecranonprocessesthanthosethatdo not dig (Vizcainoet al. 1999).
The largestburrowshave been attńbutedto other xenarthrans.Frenguelli (1921:
p. 3a! mentionedthe large glyptodonts Glyptodon, Neuryurus, and Doedicurus
(Cingulata, Glyptodontidae) and, even more, emphasisedthat frequently there were
remainsof theseanimalsin thoseburrows.The transversediametersof theseglyptodonts range between90 and 140 cm, and their massesbetween800 and 1800 kg
(Fariflaet ąl. 1998).Kraglievich (1934)consideredthatglyptodontswerenot functionally suited to dig. Although there is no specific study on the matter,Quintana (I99f)
Fig. f . Palaeoburrowsin the Mar del Plata (Playa Santa Isabel) seacliffs. A. Frontal view of an eroded
palaeoburrowwith stratifiedinternal filling at the background;scale bar 0.2m. B. Tunnel sectionrunning
parallel to the sea-cliff face exhibiting open ends; scale bar 1.8 m. C. Protruding internal filling of a
palaeoburrowwith host sedimentsremoved by erosion; scale bar 0.2 m.
ACTA PALAEONTOLOGTCA POLONTCA (46) (2)
,.--
f93
.:l$r'.'
ii!sry!*.::r""r!:';
l-'*. "
,
:-d
tĘ....-:.
294
Pleistocene burrows in Arsentina: YIZCAINO
Fig. 3 . Skeleton of the ground sloth Scell dotherium leptocephalum (fromLydekker
et al.
1894). Scale bar 1 m.
concurs with Kraglievich, arguing that the catapacęis relatively rigid (in comparison
with armadillos) and fused to the pelvic girdle, the dorsal and lumbar vertebraeare
fused forming a tube, and they lack other structuresin the skull and limbs typical of
burrowingmammals.
Mylodonts
as burrowers
Frenguellt(I9f8: pp.77,89; 1955)reporteda burrow filled with volcanic ash thathe
attributedto the groundsloth Scelidotherium(Tardigrada,Mylodontidae) since the animal was foundwithin it. Interestingly,
this assignmenthasnot beenmentionedsince.
Even thoughmany researchersemphasizedthatthelegs of mylodontids(Figs.3, 4)
werewell adaptedfor digging(Owen 1842,1851;Winge I94I; Aramayo 1988;McDonald 1987,1995;Cuenca Anaya 1995;White 1997),they consideredthemmainly
used to dig for food. Moreover, Cuenca Anaya (1995) extensively describedthe
forelimb of Scelidotheriumand assertedthatit was clearly adaptedfor digging,but he
Ęected thepossibilityof fossorialitybasedonly on its largesize.Thebody maSS,maximum transversediameter and length of Scelidotheriumwere approximately 800 kg
(Farifraet al. 1998),100 cm andf70 cm, respectively.The samefigures for Glossotheriumwere 1200kg (Fariflaet ąl. 1998)' If} cm and325 cm.
The morphologyof themanusof somemylodontids(G/ossotheńum,Proscelidodon,,
andScelidotheńum)was extensivelydescribedby Cartelle(1980)'Aramayo(1988)and
Cuenca Anaya (1995),respectively.The carpus, metacarpusand first phalangesare
closely articulatedforming a shovel-like structure,and the most remarkablefeatureis
the greatdevelopmentof the secondand third fingers, specially the ungual phalanges
(Fig. 5,Ą, B). The other fingers are reduced and cany no ungual phalanx in the
scelidotheres.In Glossotherium,they arereducedtoo, but the first finger is less reduced
and retainsan ungualphalanx.The functionalanalysesby Aramayo (1988)and Cuenca
Anaya (1995)indicatethatthe forearmand the manusof the scelidothereswere able to
ACTA PALAEONTOLOGTCA POLONTCA (46) (f)
295
Fig. 4. Skeleton of the ground sloth G/ossotheriumrobustum(from Lydekker 1894).Scale bar 1 m.
peform ample movements of flexion, extension, pronation and supination, which
Aramayo (1998)colrelateswith digginghabits.
Our own anatomical,allometric and biomechanicalstudies on Pleistocenemylodontidsdemonstratethatthe limb bones of Scelidotherium(Fig. 3) and Glossotherium
(Fig. 4), frequentlyrecordedin the Mar del Plata area,were well designedto dig (Bargo
et aL.2000)and are summarisedas follows.In GlossotheriumandScelidotherium,values of strengthindicatorscalculatedon limb bones are equivalentor even higher than
thoseof large moderngalloping mammals,indicating that they are suitedto withstand
strongbendingforces.However,the generalmorphologyand proportionsof the limbs,
particularly of the manus and pes preclude them from such locomotion.Bargo et al.
(2000)proposedthatthe strenuousactivity implied by such tensionscould be digging.
The largeolecranonprocessof ScelidotheriumandGlossotherium(Figs. 3-5) suggeststhat the forearm was betteradaptedfor strengthratherthan speed.Additionally,
the large,relatively flat, wide and straightungual phalangesare also of an appropriate
designfor digging (White 199'7).Application of an equationdevelopedby Fanfraetal.
(submitted)to Scelidotheriumand Glossotheriumindicates that their elbows were
threeto four timeslongerthanrequiredmerelyfor walking. Such valuesstronglysuggest that the limbs were designedto perform other strenuousactivities,such as digging, and include a safetyfactor (Alexander1981).
Finally, theestimatedpositionof thecentreof massin mylodontids,nearlyover the
hind feet,implies thata bipedal posturecould easily have been attained.A bipedal,
thoughnot necessarilyfully erect,postureis an essentialrequirementfor digging,as
the forelimbs must be liberatedfrom their function of supportingthe body weight so
that they may act on the substrate.This applies especially when the tunnel runs horizontally. A good analogue may be the highly specialised armadillo Priodontes
maximus,which walks essentiallyon its hindlimbs while maintainingthe backbone
roughly parallel to the ground,the forelimbs flexed, and the hands,turnedposteriorly,
dragging along the ground (Frechkop 1949).
The generalbody shapesize andtheform of skull aremoretaperedtn Scelidotherium
thanin Glossotheńum.This suggeststhatthe former would be a bettercandidateto dig
296
Pleistocene buruows in Arłentina: VIZCAINO
et al.
Fig. 5. A. Skeletonof theforelimbof Glossotheriumrobustun.B. Rightmanusof Scelldotherium(MLP 3-574).
C. Cast of the claw marks found in the tunneldescribedby Quintana(I99f; MMP 2c). Scale bars 10 cm.
its own tunnels.Those are featureslargely consideredtypical of fossorial mammals
(Shimer 1903).Nevertheless,this interpretationabouta differentialfossorial ability betweenboth forms is far from being definitive and needsfurtheranalvses.
Discussion
There is little doubt that the largestdasypodidarmadillosEutatus and Propraopus
were the buildersof many of the largeburrowsrecordedin severallocalitiesin South
America. Perhapsthe giganticpampathereswould have dug largerburrows,but it is
not so clear thatthey were the buildersof the largeststructuresfound in the Mar del
Plata sea-cliffs.More probablysometypeof mylodontidgroundsloth,suchas Scelidotheriumor Glossotheriwm,could be suspectedas responsible.
Once it is establishedthatcertaingroundslothsarewell suitedfor digging,andthat
the tunnelsare dug in cohesivesediments,otherevidencedeservesto be considered.
First, the size of the largesttunnelsfits betterwith hypothesisthatthe slothswere responsibleratherthanarmadillos.As describedabove,thelargestdasypodidarmadillos
ACTA PALAEONTOLOGTCA POLONTCA (46) (2)
f91
have a maximum width of 50 cm. Consequentlya tunnel of circular sectionof over
100 cm diameterimplies thatthey shouldremovea volume of materialequivalentto
four times their body volume each time they progresstheir own body length during
digging (assumingthatthe volume of the tail is not relevant).To minimise the energy
expenditurewhen constructinga tunnel, the tunnel diameteris expectedto be relatively constantand to approximatethe generalbody diameterof the digger (Vleck
1979;Hickman 1990).The size of the tunnelssuggeststhatthe groundslothswerethe
likely builders.
Anotherpiece of evidenceare sometracksrecordedin somepalaeocaves.Casts of
the tracks reportedby Quintana(1992)are kept in the Museo Municipal de Mar del
Plata 'Lorenzo Scaglia' (MMP 2C). They consist of pairs of parallel groovesof approximately30 cm long and4 cm wide (Fig. 5C). They fit well with themorphologyof
the manus of both Glossotherium and Scelidotherium,which evidently bore large
claws on well-developeddigits II and III (Fig. 5B). The tracks are too large to have
been formed by the largestdasypodidarmadillos,Eutatus and Propraopus.Pampathereswere Largeybut did not bear large, specialisedclaws, and digits II to IV were
subequalin size and shapeand carriedstrongpointedunguals(Edmund 1985).Thus
they would producethreegrooveson the substratumratherthantwo.
Thirdly, Frenguelli(1928) reportedfinding of remainsof a Scelidotheriumskeleton
within a burrowfilled by volcanic ash.This could be takeas strongcircumstantialevidence,but actuallyonly demonstrate
thattheanimalwas there,andnot necessarilythat
it had built theburrow.However,thepositionof thetracksin thepalaeocavewalls and
roofs strengthensthe possibility thatit dug the burrow.
As indicatedabove,thepossibilityof mylodontidsas burrow-buildershasnot been
postulatedsince Frenguelli (1928).Perhapssubsequentresearchersdid not note that
brief reportin his very extensiveandmainly geologicalwork. Possibly also thelack of
appropriateexamplesamongliving mammalsresultedin the assumptionthat an animal of that size could not be burrower.
The largestliving fossorial mammal is the aardvark(Orycteropus,Order Tubulidentata),with a body massup to 100kg (Nowak I99I). However,this value doesnot
necessarilyreflectan upper size limit for fossorialmammals.The AustralianPleistocene giant wombat(Phascolomusgigas, Marsupialia)was even betteradaptedto dig
thanits well-knownburrowerliving relatives(Stirling 1913).Following Rich (1990)it
was the largestfossorial animal thatever lived. It was twice the heightand lengthof
any living wombat (Rich 1990)and, if geometricsimilarity is assumedit must have
had a body massabout200 kg to 300 kg, still much less thanthepampeanmylodonts.
Perhaps,as postulatedby Rayner (1996)with respectto the size of birds and the capability for flight, the upper limit of the body size for a fossorialmammalmay be constrainedby environmentaland ecological ratherthan physical factors.Such factors
may furnish clues for cave-buildingbehaviourin some mylodontids.They are discussedas follows.
Glossotherium andScelidotheriumrrr?!,for example,have dependedon burrows
to escapepredation.During the Pleistocenetherewere two large-sizedcarnivores,the
sabretooth catSmilodon andthe bearArctodus (body masses:350 kg and 300 kg, respectively;Farifia et ąl. 1998)'thatwere possiblepredatorsof thesesloths.The much
298
Pleistocene burrows in Argentina: YIZCAńNO
et al.
largerbody size of the coexistentmylodontidLestodonprobablyrenderedat leastthe
adult individualsimmuneto predation,as occurstoday amongelephants.
Another possibility includes climatic and physiologicalfactors.It is possiblethat
slothsused burrowsto avoid alternativelyexcessivelycold or warm climatic conditions and to conserveenergy and water.During the early Pleistocene(most of the
Ensenadan)the climate was warmerthan today (Tonni & Cione 1995).The prevailing
climate of the Pampeanregion during the last part of the Pleistocene(Lujanian)was
mainly colder and much more arid than today,and a dry steppedevelopedin this area
(Tonni & Cione 1997).In such an environment,the mylodontidsmay have neededa
warmerand humid place to breed,or even to survive,duringthe colder season.
Living xenarthranshave low body temperatures,
low basal ratesof metabolismand
high thermalconductance;theseinfluencethe geographicallimits of their distributions
(McNab 1985).Although thereis no directevidenceof the energeticsof mylodontids,it
is worthnotingthatsomePleistocenemylodontids,suchasMylodon, areknown to have
occupied,at least seasonally,cooler environmentsfurthersouthward,nearthe southern
extremeof continentalPatagonia.Hibernationis not known to occur in living xenarthrans,but thePatagonianarmadillo, Zaedyuspichiy, entersa stateof torporduringwinter,whentheavailability of insects,one of its main food resources,declinesdramatically.
McNab (1985)proposedthatcold temperaturetoleranceof somegroundslothsin North
America (includingGlossotherium),probablystemmedfrom a thick fur coatandcontinuously available food. However, only Mylodon (Moreno & Woodward 1899) and
Nothrotheriops(Lull 1929)are known to have possesseda good thick fur coat.Nothing
is known of the externalappearanceof ScelidotheriumandGlossotherium,but it is conceivablethat,if they did possessa poorerfur coat,they might have compensatedby using caves,at leastduringunfavourableseasons.It mightalsobe suggested
that,as in living wombats(bunowermarsupialswith a very slow metabolicrate),burrowscould help
the groundslothsto reducethe needfor water (Barboza 1995).
It is noteworthythat remains of late Pleistoceneground sloths,including many
mylodontids,were found in naturalcaves and rockshelters.This suggeststhatthe use
of caves,whateverthereason,was a commonbehaviouralpatternin groundsloths.Recordsof groundslothsin naturalcavesand cavernsare speciallyabundantin Brazil,in
the States of Bahia and Minas Gerais (Lund 1842; Winge 1915; Cartelle I99L).
Ground slothswere also recordedin naturalcavesin coolerregionsat higherlatitudes
and/oraltitudesof Patagonia(Moreno & Woodward 1899;Scillato-YanÓ 1976) and
WesternNorth America (Akersten& McDonald I99I; McDonald et al. 1996).For
North American forms,Akersten& McDonald (1991)proposedthattherelativelystable environmentsof caves actedas an effective temperaturebuffer,explaining the frequentoccuffenceof slothremains.Although Scillato-YanÓ(1976)proposeda physiological adaptationfor cold-tolerancein Patagoniansloths,a behaviouraladaptation
such as thatproposedfor the North American onesis also plausible.
In the areaof Mar del Plata,as in mostof theremainingPampeanregion,cavesand
rocksheltersarerestrictedto someplaces of theTandilia and Ventaniamountainranges
of Buenos Aires, and otherrangesflanking westwardthe Pampas.Thus, if the Pampeanmylodontidsneededto dwell in relativelyclosedsheltersin suchextensiveplains
environment,theywould havehad to built thecave sheltersthemselves,andtheywere
well-adaptedfor that.
ACTA PALAEONTOLOGTCA POLONTCA (46) (2)
f99
Acknowledgements
We would like to thank Cecilia Deschamps and Richard Farifia for the critical revision of the manuscript. This paper is a contribution to the projects N336, Facultad de Ciencias Naturales y Museo,
Universidad Nacionalde La Plata and Fundación Antorchas No. 4-13740/I-18 and PICT 06348.
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