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Home , Buenos Aires Province, Doedicurus, Eutatus, Glossotherium, Holmesina, La Plata

Pleistocene burrows in the area () 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. 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 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 and Ages. Several are good candidates as builders of these burrows. Palaeobunows were attributed before to the large Pleistocene Propraopus, , and Pampatherium. We consider the possibility that the mylo- dontid ground 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 de- signed 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 . Thus, the mylodontid sloths and are considered as possible builders for the large late burrows present in the Pampean region.

Key words: Pleistocene,burrows, , Xenarthra, South Ameńca.

Sergio F. Vizcaino [[email protected]],M. SusanaBargo Imsbargo@ museo.fcnym.unlp.edu.arl, Departamento Cientifico Paleontologia de Vertebrados, Museo de , Paseo del Bosque s/n, 1900 La Plata, Argentina. CONICET CIC.; Marcelo Zdrate, CONICET-IANIGI'Ą/CRICYT, cc 330, 5500, Mendoza, Argentina; 'Lorenzo Alejandro Dondas, Museo Municipal de Ciencias Naturales Scaglia', Plaza Espafia,7600 Mar del Plata, Argentina.

Introduction

Mammalian burrows of different sizes are commonly mentioned,but not specifically described,in the literaturereferred to the late Cenozoic depositsof the

Acta Palaeontol.Pol. 46, 2,289-301. 290 Pleistocene burrows in Argentina: YIZCAINO et al.

Province (Argentina),especially in the coastalregion (Ameghino 1908;Frenguelli I9fI,1928; Kraglievichl95f).In thelast decade some specific studies have been per- formed.Genise (1989)and Scognamillo (1993)analyse and describein detail small burrows of approximately13 cm of diameter,found in the coastalcliffs of Mar del Plata area(Buenos Aires )and attributedto the fossil rodentActenomys. Large structureswith diametersof approximately1m were studiedin the proximity of La Plata,NE of BuenosAires Province(Imbellone & Teruggi1988; Imbellone et al. 1990),and in southernBrazll (Bergqvist& Maciel I994).In Mar del Plata, Quintana (1992)reported a burrow of the samediameter, providing the first morphologicaldescription of this kind of tunnels.This was of particularinterest be- causeit was branched,23metres long, with sometracks on the walls andthe roof, and devoid of sedimentaryfilling. The constructionof all theselarge burrows was attrib- uted to giant armadillos (Dasypodidaeand ). In recentyears severallarger burrows were discoveredin the areaof Mar del Plata. Zdrateet al. (1998)examined these large structures focusing on 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 Bergq- vist & Maciel (1994),i.e., including both palaeocaves (hollows) and crotovines (filled with sediments).

Stratigraphy, morphology, and fillings of the burrows

Although theburrows in lateCenozoic sedimentsnear Mar del Platahave been known for more than a century,they were not described nor interpreteduntil very recently. Zdrate et al. (1998)recorded and described42 structuresin the Mar del Plata arca (BuenosAires Province) (Fig. 1). The time rangeof theseburrows extends from the to the ,and possibly until as recently as late glacial maxi- mum-late glacial times. Based on the shallowestempty burrows found in the area, Zźrateet ąI. (1998)estimated a probabledepth of 5_6 m from the sufface.Thus, the structuresare stratigraphicallytransgressive, which constrainsthe chronological reso- lution 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 (Z6rate et ąI. 1998).once thecontextualframe- work has beenestablished based on thesecriteria, the sedimentaryfillings and the as- sociatedfaunal remainsare further applicable criteria to aid in determiningthe origin. The structures,formed in cohesivematerial composed of very fine sandstoneor siltstones(loess-like sediments),are located either in floodplain or in interfluvial palaeoenvironments.The tunnelsare filled with thinly stratifiedfine sandsand silts al- ternatingwith 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,possi- bly becausethey have beenmisinterpreted as fluvial features.Their maximum width ACTA PALAEONTOLOGTCA POLONTCA (46) (2) 291

A sourH AMERTcA

z tr z UJ o t.

Fig. 1. A. Map of showing the location of (in black) and the approxi- 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. Tunnelsrecently discovered in Mar del Plata and devoid of sedimentaryfill- ing reachup to 1.80m in width and40 m in length.The major size variationmight re- flect differentanimal diggers;size variationsbetween 15-30 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); thosebetween 5 and 10 cm ure at- tributableto measurementelror causedbv boundariesthat are difficult to distinsuish from the surroundingsediments.

Associated fauna

A large number of mammalian taxa have been recordedfrom the late Cenozoic expo- sures along the Mar del Plata-Miramarsea-cliffs. There is an extensiveaccount of them in a recentcompilation edited by Alberdi et al. (1995).Many taxa can be dis- carded as the builders of these burrows as they do not show adaptationsto dig (proboscideans,artiodactyls, perissodactyls, litopterns, most notoungulates, and some large carnivores).Others are too small (,notoungulates such as hegetotheres, and othercarnivores). On the otherhand, many representativesof the superorderXenarthra fulfill both re- quirements:they have morphological adaptationsto dig and they are large enough. They comprise three main lineages:cingulates, including armadillos ( f92 Pleistocene burcows in Argentina: YVCA(NO et at. and Pampatheriidae)and their relatives the ;vermilinguas, the ();and tardigrades,the groundsloths (Mylodontidae and Megathe- riidae),relatives of the highly specialisedliving treesloths (Bradypodidae and Mega- lonychidae). Recent papersconsider the possibility of armadillosas the builders of large bur- rows. Quintana(1992) proposed two dasypodids,Eutątus seguini andPropraopus sp.) or a pampatherid,Pampatherium typum,as probable builders of a palaeocavein Mar del Plata area.This has also been suggestedfor burrows in other areasof southern SouthAmerica. Near LaPlata (NE of Buenos Aires Province,Argentina), Imbellone and Teruggi (1988)and Imbelloneet al. (1990)also tentativelyattributed some bur- rows (with diametersof approximately1 m) to Eutatus seguini or Pampatherium typum.Bergqvist & Maciel (1994)described large crotovines (about 1 m width)in Rio Grande do Sul (southernBrazil) which they attributedto the pampatheresPampa- therium andHolmesina, and to the extinctdasypodid Propraopus. Living armadillos are powerful diggersand scratchersthat usually live in undergroundburrows. Their skeletonsshow all the adaptationsrequired for a digger.The is low and narrow, the limb bones presentstrong processesand scars for muscle attachment,and the ungualphalanges are wide andrelatively flat (usuallyone or two aremore specialised). Both Eutątus andPropraopus are approximatelythe samesize as the largestliving ar- madillo Priodontes maximus,so they must have been close to 50 kg in body mass (Farińa&Ytzcaino 1997),and a maximumtransverse body diameterof about50 cm. Both show the sameskeletal adaptations, more or less developedto the samedegree, so they can easily be identifiedas diggers. Pampatheresare larger than dasypodid armadillos.Pampatherium typum must have reached200 kg and its estimatedmaximum transversediameter was about80 cm. It appearsthat they were not as good diggersas dasypodids.Edmund (1985)described limbs of pampatheresas being intermediatein proportionsand specialisationbetween those of glyptodontsand the living Dasypus, including graviportal adapta- tions for the hind limb. The three central 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 moment arm of ,which acts as an ęxtensorof the forearm.Within ,this designis appropriatefor diggers,such as the armadillos.Even in some unspecialisedclades of mammals, that are known to dig morefrequently show relativelylonger olecranon processes than those that do not dig (Vizcainoet al. 1999). The largestburrows have been attńbutedto other xenarthrans.Frenguelli (1921: p. 3a! mentionedthe large glyptodonts , Neuryurus, and (, Glyptodontidae) and, even more, emphasisedthat frequently there were remainsof theseanimals in thoseburrows. The transversediameters of theseglypto- donts range between90 and 140 cm, and their massesbetween 800 and 1800 kg (Fariflaet ąl. 1998).Kraglievich (1934)considered that glyptodonts were not function- ally 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

ii!sry!*.::r""r!:'; ,.-- .:l$r'.' l-'*. " :-d , tĘ....-:. 294 Pleistocene burrows in Arsentina: YIZCAINO et al.

Fig. 3 . Skeleton of the ground Scell dotherium leptocephalum (fromLydekker 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)reported a burrow filled with volcanic ash thathe attributedto the Scelidotherium(Tardigrada, Mylodontidae) since the an- imal was foundwithin it. Interestingly,this assignmenthas not been mentioned since. Even thoughmany researchers emphasizedthatthe legs of mylodontids(Figs. 3, 4) werewell adaptedfor digging(Owen 1842,1851;Winge I94I; Aramayo 1988;Mc- Donald 1987,1995;Cuenca Anaya 1995;White 1997),they consideredthem mainly used to dig for food. Moreover, Cuenca Anaya (1995) extensively describedthe forelimb of Scelidotheriumand assertedthat it was clearly adaptedfor digging,but he Ęected thepossibility of fossorialitybased only on its largesize.The body maSS,max- imum transversediameter and length of Scelidotheriumwere approximately 800 kg (Farifraet al. 1998),100 cm andf70 cm, respectively.The samefigures for Glosso- theriumwere 1200kg (Fariflaet ąl. 1998)' If} cm and325 cm. The morphologyof the manusof somemylodontids (G/ossotheńum, Proscelidodon,, andScelidotheńum) was extensivelydescribed by Cartelle(1980)' Aramayo (1988) and Cuenca Anaya (1995), respectively.The carpus, metacarpusand first phalangesare closely articulatedforming a shovel-like structure,and the most remarkablefeature is the greatdevelopment of the second and 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 arereduced too, but the first finger is less reduced and retainsan ungualphalanx. The functionalanalyses by Aramayo (1988)and Cuenca Anaya (1995)indicate that the 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)colrelates with digginghabits. Our own anatomical,allometric and biomechanical studies on Pleistocenemylo- dontids demonstratethat the limb bones of Scelidotherium(Fig. 3) and Glossotherium (Fig. 4), frequentlyrecorded in the Mar del Plata area,were well designedto dig (Bargo et aL.2000)and are summarisedas follows.In Glossotheriumand Scelidotherium,val- ues of strengthindicators calculated on limb bones are equivalentor even higher than those of large modern galloping mammals,indicating that they are suitedto withstand strongbending forces. However, the generalmorphology and proportionsof the limbs, particularly of the manus and pes preclude them from such locomotion. Bargo et al. (2000)proposed that the strenuousactivity implied by such tensionscould be digging. The largeolecranon process of Scelidotheriumand Glossotherium (Figs. 3-5) sug- geststhat the forearm was betteradapted for strengthrather than speed.Additionally, the large,relatively flat, wide and straightungual phalangesare also of an appropriate designfor digging(White 199'7).Application of an equationdeveloped by Fanfraetal. (submitted)to Scelidotheriumand Glossotheriumindicates that their elbows were threeto four timeslonger than required merely for walking. Such valuesstrongly sug- gest that the limbs were designedto perform other strenuousactivities, such as dig- ging, and include a safetyfactor (Alexander1981). Finally, theestimated position of thecentre of massin mylodontids,nearly over the hind feet,implies thata bipedal posturecould easily have been attained.A bipedal, thoughnot necessarilyfully erect,posture is an essentialrequirement for 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 hori- zontally. 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,turned posteriorly, dragging along the ground (Frechkop 1949). The generalbody shapesize andthe form of skull aremore tapered tn Scelidotherium thanin Glossotheńum.This suggeststhat the former would be a bettercandidate to dig 296 Pleistocene buruows in Arłentina: VIZCAINO et al.

Fig. 5. A. Skeletonof theforelimb of Glossotheriumrobustun.B. Rightmanusof Scelldotherium(MLP 3-574). C. Cast of the claw marks found in the tunneldescribed by Quintana(I99f; MMP 2c). Scale bars 10 cm. its own tunnels.Those are featureslargely consideredtypical of fossorial mammals (Shimer 1903).Nevertheless, this interpretationabout a differentialfossorial ability be- tweenboth forms is far from being definitive and needsfurther analvses.

Discussion

There is little doubt that the largestdasypodid armadillos Eutatus and Propraopus were the buildersof many of the largeburrows recorded in severallocalities in South America. Perhapsthe giganticpampatheres would have dug largerburrows, but it is not so clear thatthey were the buildersof the largeststructures found in the Mar del Platasea-cliffs. More probablysome of mylodontidground sloth, such as Scelido- theriumor Glossotheriwm,could be suspectedas responsible. Once it is establishedthat certain ground sloths are well suitedfor digging,and that the tunnelsare dug in cohesivesediments, other evidence deserves to be considered. First, the size of the largesttunnels fits betterwith hypothesisthat the slothswere re- sponsiblerather than armadillos. As describedabove, the largest dasypodid armadillos ACTA PALAEONTOLOGTCA POLONTCA (46) (2) f91 have a maximum width of 50 cm. Consequentlya tunnel of circular sectionof over 100 cm diameterimplies thatthey shouldremove a volume of materialequivalent to four times their body volume each time they progresstheir own body length during digging (assumingthat the volume of the tail is not relevant).To minimise the energy expenditurewhen constructing a tunnel, the tunnel diameter is expectedto be rela- tively constantand to approximatethe generalbody diameterof the digger (Vleck 1979;Hickman 1990).The size of the tunnelssuggests that the groundsloths were the likely builders. Anotherpiece of evidenceare sometracks recorded in somepalaeocaves. Casts of the tracksreported by Quintana(1992) are kept in the Museo Municipal de Mar del Plata 'Lorenzo Scaglia' (MMP 2C). They consist of pairs of parallel groovesof ap- proximately30 cm long and4 cm wide (Fig. 5C). They fit well with themorphology of 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 largestdasypodid armadillos, Eutatus andPropraopus. Pampa- thereswere Largeybut did not large,specialised claws, and digits II to IV were subequalin size and shapeand carriedstrong pointed unguals (Edmund 1985).Thus they would producethree grooves on the substratumrather than two. Thirdly, Frenguelli(1928) reportedfinding of remainsof a Scelidotheriumskeleton within a burrowfilled by volcanic ash.This could be takeas strongcircumstantial evi- dence,but actuallyonly demonstratethat the was there,and not necessarilythat it hadbuilt theburrow. However, the position of thetracks in thepalaeocave walls and roofs strengthensthe possibility thatit dug the burrow. As indicatedabove, the possibility of mylodontidsas burrow-buildershas not been postulatedsince Frenguelli (1928).Perhaps subsequent researchers did not note that brief reportin his very extensiveand mainly geologicalwork. Possibly also thelack of appropriateexamples among living mammalsresulted in the assumptionthat an ani- mal of that size could not be burrower. The largestliving fossorial is the aardvark(Orycteropus, Tubuli- dentata),with a body massup to 100kg (Nowak I99I). However,this value doesnot necessarilyreflect an uppersize limit for fossorialmammals. The AustralianPleisto- cenegiant wombat(Phascolomus gigas, Marsupialia)was evenbetter adapted to dig thanits well-knownburrower living 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 massabout 200 kg to 300 kg, still much less thanthe pampean mylodonts. Perhaps,as postulatedby Rayner (1996)with respectto the size of birds and the capa- bility for flight, the upperlimit of the body size for a fossorialmammal may be con- strainedby environmentaland ecological ratherthan physical factors.Such factors may furnish clues for cave-buildingbehaviour in some mylodontids.They are dis- cussedas follows. Glossotherium andScelidotherium rrr?!, for example,have dependedon burrows to escapepredation. During the Pleistocenethere were two large-sizedcarnivores, the sabretooth catSmilodon andthe bearArctodus (bodymasses: 350 kg and 300 kg, re- spectively;Farifia et ąl. 1998)'that were possiblepredators of thesesloths. The much 298 Pleistocene burrows in Argentina: YIZCAńNO et al. largerbody size of the coexistentmylodontid probably rendered at leastthe adult individualsimmune to ,as occurstoday amongelephants. Another possibility includesclimatic and physiologicalfactors. It is possiblethat slothsused burrowsto avoid alternativelyexcessively cold or warm climatic condi- tions 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 steppedeveloped in this area (Tonni & Cione 1997).In such an environment,the mylodontidsmay have neededa warmerand humid place to breed,or evento survive,during the colder season. Living xenarthranshave low body temperatures,low ratesof metabolismand high thermalconductance; these influence the geographicallimits of their distributions (McNab 1985).Although thereis no direct evidenceof the energeticsof mylodontids,it is worth notingthat some Pleistocene mylodontids, such asMylodon, areknown to have occupied,at least seasonally,cooler environmentsfurther southward, near the southern extremeof continentalPatagonia. Hibernation is not known to occur in living xenar- thrans,but the Patagonianarmadillo , Zaedyuspichiy, entersa stateof torporduring win- ter,when the availability of ,one of its main food resources,declines dramatically. McNab (1985)proposed that cold temperaturetolerance of someground sloths in North America (includingGlossotherium),probably stemmed from a thick fur coat andcontin- uously available food. However, only (Moreno & Woodward 1899) and (Lull 1929)are known to have possesseda good thick fur coat.Nothing is known of the externalappearance of Scelidotheriumand Glossotherium, but it is con- ceivablethat, if they did possessa poorerfur coat,they might have compensatedby us- ing caves,at leastduring unfavourable seasons. It mightalso be suggestedthat, as in liv- ing wombats(bunower with a very slow metabolicrate), burrows could help the groundsloths to reducethe needfor water (Barboza 1995). It is noteworthythat remains of late Pleistoceneground sloths,including many mylodontids,were found in naturalcaves and rockshelters.This suggeststhat the use of caves,whatever the reason, was a commonbehavioural pattern in groundsloths. Re- cordsof groundsloths in naturalcaves and caverns are specially abundant in ,in the States of Bahia and Minas Gerais (Lund 1842; Winge 1915; Cartelle I99L). Ground slothswere also recordedin naturalcaves in coolerregions at higherlatitudes and/oraltitudes of (Moreno & Woodward 1899;Scillato-YanÓ 1976) and WesternNorth America (Akersten& McDonald I99I; McDonald et al. 1996).For North American forms,Akersten & McDonald (1991)proposed that the relatively sta- ble environmentsof caves actedas an effective temperaturebuffer, explaining the fre- quentoccuffence of slothremains. Although Scillato-YanÓ(1976) proposed a physio- logical adaptationfor cold-tolerancein Patagoniansloths, a behaviouraladaptation such as thatproposed for the North American onesis also plausible. In the areaof Mar del Plata,as in mostof theremaining Pampean region, caves and rocksheltersare restricted to someplaces of the Tandilia and Ventaniamountain ranges of Buenos Aires, and otherranges flanking westwardthe .Thus, if the Pam- peanmylodontids needed to dwell in relativelyclosed shelters in suchextensive plains environment,they would havehad to built thecave sheltersthemselves, and they were 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 manu- script. 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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