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Geobios 41 (2008) 133–143

Original article

Vertebrate taphonomy in loess-palaeosol deposits: A case studyfrom the late Miocene of central Argentina

Taphonomie de vertébrés des dépôts de lœss-paléosols :un exemple dans le Miocène supérieur d’Argentine centrale

Claudia I. Montalvo a,*, Ricardo N. Melchor a,b, Graciela Visconti a, Esperanza Cerdeño b,c

a Facultad de Ciencias Exactas y Naturales, Universidad Nacional de La Pampa, Uruguay 151, 6300 Santa Rosa, La Pampa, Argentinab CONICET, Universidad Nacional de La Pampa, 6300 Santa Rosa, La Pampa, Argentina

c Departamento de Geología y Paleontología, IANIGLA-CONICET, Avda. Ruiz Leal s/n. Casilla de Correo 330, 5500 Mendoza, Argentina

Received 4 November 2005; accepted 4 September 2006

Available online 3 December 2007

Abstract

This paper deals with the taphonomic analysis of the late Miocene bone assemblage from the Cerro Azul Formation at Telén (La PampaProvince, Argentina). The faunal assemblage was assigned to the Huayquerian mammal age (late Miocene). The fossiliferous section shows ahomogeneous lithology, and is interpreted as a loess deposit with two similar and slightly developed palaeosols, classified as calcic vertisols. Thestudied sample comprises 5598 remains anatomically and taxonomically determined. They were collected from an area of about 48,000 m2,appearing randomly distributed through the section and with low density. Most remains are small- to very small-sized, disarticulated, and veryfragmented. Different taphonomical histories are inferred for microvertebrates and macromammals. The microvertebrate assemblage is interpretedas the result of predator activities. After a brief period of pre-burial exposure, remains were dispersed from the original depositional area. On theother hand, a natural and gradual death process is envisaged for macromammals, followed by a long period of exposure to weathering and dispersalby physical agents. Remains of both groups, once buried, suffered the diagenetic processes of the host rock. Consequently, the fossil assemblagefrom Telén would represent a condensed assemblage corresponding to two distinct time spans, i.e., the accumulation of microvertebrates took placein a short time interval whereas that of macromammals occurred over a longer period, coincident with the development of both soils.# 2007 Elsevier Masson SAS. All rights reserved.

Résumé

Ce travail présente l’analyse taphonomique de l’association faunique de la Formation Cerro Azul de Telén (Miocène supérieur de la Province deLa Pampa, Argentine). La faune est caractéristique du Huayquerien (Miocène supérieur). Le profil sédimentologique des niveaux fossilifèresmontre une lithologie homogène interprétée comme des lœss, avec deux paléosols peu développés. L’échantillon étudié comprend 5598 restesanatomiques et taxonomiquement déterminés. La plupart des restes sont de petite ou très petite taille, désarticulés, très fragmentés et cassés. Deuxdifférentes étapes taphonomiques ont été établies pour les microvertébrés et les macromammifères. L’assemblage des microvertébrés est interprétécomme le résultat de l’action d’un prédateur. Après une période brève d’exposition avant l’enfouissement, les restes ont été dispersés à partir del’aire de déposition originelle. Par ailleurs, un processus de mort naturelle et graduelle a été établi pour les macromammifères, suivi d’une longuepériode d’exposition aux agents météorologiques et de dispersion. Une fois enterrés, les restes des deux groupes auraient subi des processusdiagénétiques dans la roche mère. En conséquence, l’assemblage fossile de Telén représenterait un assemblage condensé correspondant à deuxpériodes différentes. L’assemblage des microvertébrés a eu lieu pendant un intervalle de temps court, alors que celui des macromammifères auraitduré un lapse de temps plus long, coïncidant avec le développement des deux sols.# 2007 Elsevier Masson SAS. All rights reserved.

Keywords: Taphonomy; Vertebrates; Mammals; Cerro Azul Formation; Late Miocene; La Pampa; Argentina

Mots clés : Taphonomie ; Vertébrés ; Mammifères ; Cerro Azul Formation ; Miocène supérieur ; La Pampa ; Argentine

* Corresponding author.E-mail address: cmontalvo@exactas.unlpam.edu.ar (C.I. Montalvo).

0016-6995/$ – see front matter # 2007 Elsevier Masson SAS. All rights reserved.doi:10.1016/j.geobios.2006.09.004

Fig. 1. a: Geographic location of Telén, La Pampa, Argentina; b: Stratigraphicsection of the Cerro Azul Formation.Fig. 1. a : Localisation géographique de Telén, Province de La Pampa,Argentine ; b : Section stratigraphique de la Formation Cerro Azul.

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1. Introduction

Taphonomic studies of Cenozoic continental vertebrateassemblages from Argentina are scarce (Bown and Larriestra,1990; Tauber, 1997a, 1997b; Cladera et al., 2004). Thiscontribution presents the results of a taphonomic study on asection of the Cerro Azul Formation exposed at Telén, north ofLa Pampa Province, central Argentina. This section has yieldeda very rich and diversified continental vertebrate fauna,consisting mainly of mammals. In several outcrops of theCerro Azul Formation, the fossiliferous levels correspond topalaeosols (Montalvo, 2002b, 2004). The analysis of verte-brates associated with palaeosols provides interesting biostrati-graphic information, since they commonly have beeninterpreted as formed during a brief period of time (Bownand Kraus, 1981).

The fossil assemblage from Telén is associated with twodistinct palaeosols, but it is treated as a whole, since it wasrecovered from all parts of a lithologically homogeneoussection, and taphonomic attributes evidence homogeneityduring the pre- and post-burial processes. The main purposeof this paper is to interpret the taphonomic history of thisassemblage. The analysis involves the study of taphonomicvariables within a well-defined sedimentological context.Preliminary taphonomic and sedimentological data from Telénwere advanced in different contributions (Melchor et al., 2000;Montalvo, 2000, 2002a), and some vertebrate taxa from this sitehave been already described (Goin and Pardiñas, 1996;Montalvo et al., 1998; Goin et al., 2000; Esteban et al.,2001; Cerdeño and Montalvo, 2001; Albino et al., 2006).

2. Geological setting

The sedimentological analysis of the Cerro Azul Formationat Telén included field logging and thin section micromorphol-ogy. Micromorphological descriptions follow the terminologyproposed by Bullock et al. (1985). Dry rock samples werecompared with the rock colour chart of Goddar et al. (1948).

The Cerro Azul Formation crops out in most of La PampaProvince, except in the southern region (Linares et al., 1980). Itis a continental, nearly flat-lying unit composed of massive,pale red to pinkish siltstones and fine-grained sandstones withinterbedded, poorly developed palaeosols (e.g., Linares et al.,1980; Goin et al., 2000). This lithostratigraphic unit isconsidered the distal portion of a clastic wedge related tothe late Cenozoic Andean fold and thrust belt that is welldeveloped in the neighbouring Mendoza Province. Themaximum outcropping thickness is about 40 m, although theexposures are generally less than 5 m thick. The Cerro AzulFormation has yielded numerous vertebrate remains, speciallymammals, which are referred to the Huayquerian land mammalage, which is of late Miocene age (Pascual and Bondesio, 1982;Montalvo and Casadío, 1988; Goin et al., 2000; Verzi et al.,1991, 1994, 1995, 1999, 2003, 2004; Montalvo et al., 1995,1996, 1998; Esteban et al., 2001; Cerdeño and Montalvo, 2001,2002). The base of the Cerro Azul Formation is covered and itstop is overlain by Pliocene and younger (mostly aeolian)

sediments or is composed of a discontinuous calcrete crust thatforms a resistant layer responsible for the preservation fromerosion of the unit. In the subsurface, the unit reaches �370 mthick overlying older Cenozoic sediments or basement rocks(De Elorriaga and Tullio, 1998).

The studied section of the Cerro Azul Formation near Teléntown (northern La Pampa Province, Fig. 1a) crops out in thewest margin of a dry pond located less than 1 km north of thetown (3681501300S–6583004100W). A preliminary account on thepalaeoenvironmental setting and taphonomy of this section hasbeen reported by Montalvo et al. (1998) and Melchor et al.(2000).

The present relief of this region combines modern aeoliansand dunes and residual mesas composed of Quaternaryvolcaniclastic sands that overlie the Cerro Azul Formation,which crops out at the edge of these mesas. The local surfacedrainage is concentrated in an elongated depression that ismostly dry (Cano, 1980). The analysed site is situated at themargin and escarpment of this depression and reaches an areaof �48,000 m2. The slope of the depression margin is 38 to 58.

3. Lithofacies and palaeoenvironment

The Telén section of the Cerro Azul Formation is 6 m thick(Fig. 1b) and displays two slightly developed palaeosols(Melchor et al., 2000). The lithology of the section ismonotonous, and mostly composed of carbonate-cementedgreyish orange pink (5 YR 7/2) siltstones and fine-grainedsandstones. The distinction of horizons in the soil profiles is

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poor and the stacked palaeosols are 1.7 m (lower) and 1.9 m(upper) thick. Both palaeosols are separated by a 2 to 7 cmthick, discontinuous bed of laminated, greyish-pink mudstonewith mudcracks. Beneath the lower palaeosol lies a 0.5 m thickinterval with poor pedogenic modification, which is a siltstonewith abundant glass shards and no carbonate cementation. Eachpalaeosol exhibits a 1.2 to 1.4 m thick interval with greatercarbonate cementation that is considered as a calcic horizon.The upper soil profile shows a second uppermost horizon,0.30 m thick, more clayey than the rest of de soil, with fineprismatic peds and Celliforma isp. (fossil bee cell). Mostconspicuous features of both palaeosols include pervasive(although not uniform) carbonate (micritic calcite) cementa-tion, pedogenic slickensides and clastic dykes. Carbonatecementation is not homogeneous in the section. Calciumcarbonate content is minimum (�1%) in the lower part of eachpalaeosol, increasing upward (�10%). At the field, carbonatecement appears as calcareous concretions with dominantly sub-vertical arrangement (up to 15 cm long and 5 cm diameter) andscarce nodules (1 to 2 cm in diameter). Pedogenic slickensidesconstitute curved, striated, clayey surfaces with a circularroughly concentric arrangement in plan view and a conical 3Dpattern. These structures are 0.2 to 0.7 m in diameter and 1 mdeep. Clastic dykes are 1 to 7 cm thick, sub-vertical, and filledby two to four pairs of symmetrical muddy laminae showingsubtle lithology and colour contrasts. The classic dykes show nopreferred orientation.

Under the microscope, both palaeosols display similarfeatures. The main pedofeatures identified under the micro-scope include root traces with clay-rich complete infilling,bounded by clay laminated coatings. In addition, voids, grains,and aggregates with pendant hypocoatings, calcareous nodulesand septaria were also identified. Other textural pedofeaturesare limpid clay coatings in most samples, and dusty claycoatings in two of them. Amorphous features are Fe–Mn orthicimpregnative nodules, sub-rounded and dispersive, showinghigh rugosity, generally with sharp boundaries. Sometimes,there are pellets (1 to 2 mm in diameter) with high rugosity,slightly darker than the matrix. Pellets contain mineral grainsand pendant coatings.

Thick, monotonous, massive continental successions ofsiltstone showing good sorting are typical of loess deposits,whose aeolian origin is well established (e.g., Johnson, 1989;Leeder, 1999). The described palaeosols display manydiagnostic features of vertisols, including thick homogeneousprofile, pedogenic slickensides, and deeply penetrating clasticdykes (e.g., Blodgett, 1985a, 1985b; Retallack, 1990; Parrish,1998). Vertisols are excellent indicators of warm climate withseasonal rainfall, and can develop in hundreds of years, mainlywhen the parent material contains a high amount of smectiticclays (Blodgett, 1985a, 1985b; Retallack, 1990; Parrish, 1998).Vertisols are characterised by seasonal shrinking and swelling,which is responsible for the homogeneous nature of the soilprofile (e.g., Buol et al., 1990), and are mainly found in flatlandscapes, located near gentle slopes (Retallack, 1990).Strongly seasonal, semi-arid climates will generate calcicvertisols (Parrish, 1998), which are characterised by a calcic

horizon, as those described herein. Carbonates in soilsaccumulate in areas with annual precipitation lower than1000 mm (Quade and Roe, 1999), although the annualdistribution of precipitation is also an important factor(Retallack, 2000). Fe–Mn oxides commonly accumulate insoils with seasonal waterlogging (Fanning and Fanning, 1989),although these palaeosols do not exhibit hydromorphicfeatures. The interval between both palaeosols, with lowcarbonate content and mudcracked mudstones, is considered asa deposit of small puddles, and probably suggests punctuatedepisodes of increased precipitation. The combined presence oflimpid and dusty clay coatings reflects the translocation of claysin a soil with sparse vegetation cover (Kemp, 1999).

4. Material and methods

Fossil material is stored at the palaeontological collection ofthe Facultad de Ciencias Exactas y Naturales, UniversidadNacional de La Pampa (GHUNLPam). The Telén site(600 m � 80 m) was visited several times and every exposedremain was collected. Square digging and sediment sievingwere not performed, since exploratory analyses using thismethodology produced a few remains.

The taphonomic analysis follows the methodology proposedby Alcalá Martínez (1994) and Fernández López (2000), andwas applied observing the attributes present on each element.The measured taphonomic attributes are:

� a

natomical and taxonomical determination; � m ineralization degree; � w eathering degree:� unweathered bone,� bone surface shows flaking associated with cracks,� deeper cracking and extensive flaking. � p resence/absence of concretions (cemented sediment

strongly attached to the fossil);

� p resence/absence of corroded surfaces and/or dissolved

fragments;

� b reakage degree; � d ispersal degree; � s keletal elements distribution pattern: original or modified,

grouped, uniform or randomly;

� ta phonomic removal degree (accumulated, resedimentated or

reelaborated).

Different indexes were used to quantify the specimens inorder to estimate the abundance of each taxon within theassemblage. The number of identified elements per taxon(NISP; Badgley, 1986) was calculated. Badgley (1986) statedthat this number is affected by the differential fragmentation ofspecimens and the results are distorted if isolated remains arecounted equal to the associated ones. In our case, the presenceof associated bones is very rare and, therefore, that problemdoes not modify the interpretation. However, the number offragmentary remains is very high and so it is the estimated NISPvalue. Consequently, it is possible that some skeletal elementswere counted more than once. Another calculated index is the

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minimal number of skeletal elements per taxon (MNE;Badgley, 1986). This index focuses on fragmentation degree,and the remains considered as coming from the same skeletalelement are grouped in order to count each element just once. Athird index is the minimal number of individuals (MNI;Badgley, 1986), calculated from the most abundant skeletalpart. The MNI reflects the minimal number of individualsnecessary to provide the whole identified skeletal elements inthe sample (Alcalá Martínez, 1994). Most remains from Telénbelong to mammals, and teeth are the most abundant elements.Therefore, the most frequent tooth was chosen to obtain theMNI for each taxon. Dasipodids and glyptodonts (XenarthraCingulata), as well as tortoises (Reptilia, Chelonia), arerepresented by few isolated scutes, and the MNI value wasconsidered 1 for these taxa. In the case of birds and lizards, MNIwas calculated after the femora and dentary respectively.Representativity of each skeletal element was evaluated withreference to the MNI of the assemblage: [MNEi/(Ei �MNI)] � 100, where MNEi is the particular MNE andEi is the expected number of each element in an individual.

Body mass of each taxon was estimated after comparisonwith extant and fossil species of similar size, mainly using thelate Miocene faunal assemblage from the Cerro Azul Formationat Salinas Grandes de Hidalgo, La Pampa (Vizcaíno and Fariña,1999). Materials were organised into two groups: microverte-brates (less than 1 kg of estimated body mass) and larger taxa(more than 1 kg).

5. Palaeoenvironmental and palaeoecological inferencesfrom the faunal association

The recovered fauna (Table 1), notoungulates and rodentsspecially, is indicative of steppes or herbaceous plains.Particularly, the hegetotheriid Paedotherium minor (Notoun-gulata) was a typical dweller of these environments in dry tohumid warm climates (Bond et al., 1995), very abundant in thisregion during the late Miocene (Zetti, 1972; Cerdeño and Bond,1998; Montalvo, 2004). Among rodents, Caviidae, Abrocomi-dae, Chinchillidae, and some Echimyidae are typical of openenvironments in an arid to semiarid climate (Vucetich, 1995;Vucetich and Verzi, 1995; Verzi et al., 1994; Montalvo et al.,1998). On the other hand, Vetelia and Macroeuphractus(Dasypodidae, Euphractini) suggest relatively warm climaticconditions (Scillato-Yané, 1982; Urrutia, 2004).

6. Taphonomic analysis

The collection at Telén includes all materials from surfaceprospecting. The low gradient of the modern margin of thedepression favoured the erosive action by physical processesthat loosened the fossils from the host material. For this reason,bone remains are frequently found in small concentrationsproduced by running water during rains. During a single visit tothe site, 1955 specimens were recovered, 94.53% of them loose,and just 5.47% within the host material. Both the loose and insitu remains appeared throughout the exposed surface of the twopalaeosol levels (Fig. 1b), randomly distributed, disarticulated

and dispersed. The in situ material was found scattered in thehost siltstone as well as in calcite concretions.

6.1. Sample size

No complete skulls or skeletons were found in the studiedassemblage. The whole recovered sample comprises 11,647pieces, including 5598 anatomically and taxonomically iden-tified remains, and 6049 undetermined fragments, mostlysplinters.

Taxonomically determined remains include 1277 isolatedpostcranial elements attributed to undeterminable Mammalia,23 to undeterminable Aves, 2894 scutes of xenarthrancarapaces, and five tortoise scutes. Besides, there are 1396cranial elements (including isolated teeth) assigned to differentmammal taxa and three to Teiidae reptiles.

6.2. Number of taxa, body mass, MNI, and age spectrum

The faunal assemblage is composed of at least 47 vertebratetaxa, including 44 mammal taxa (Table 1). Out of the totalnumber, 29 are microvertebrate taxa (remains of mammals, birdsand reptiles less than 1 kg of estimated body mass) with a MNI of246, whereas 18 taxa are medium to large-sized mammals with aMNI of 27. Most frequent taxa are micromammals (81.3% ofindividuals): the notoungulate Paedotherium minor (Hegetother-iidae; 47.78%) and rodents from families Caviidae (Caviinaeindet., Dolichotinae indet. and Palaeocavia sp.), Chinchillidae(Lagostomus sp.), and Abrocomidae (Protabrocoma sp.).Among cranial elements with teeth, nine specimens of micro-mammals and three of macromammals with emerging teeth werecollected (Table 1). In addition, from the whole sample of cranialand postcranial mammal specimens, 2.78% presents littleconformed bone and not fused long bone epiphyses, bothfeatures indicating their juvenile condition. About 75% ofjuvenile individuals correspond to Paedotherium minor.

6.3. Anatomical representation, MNE, and relativeabundance

The bone assemblage from Telén has several obviousanatomical biases. Skeletal elements were grouped after theiranatomical position as: (1) tortoise and xenarthran carapacescutes; (2) postcranial elements; and (3) cranial elements. Fig. 2shows the abundance of scutes, which implies, on one hand, ahigh degree of disarticulation and breakage of these skeletalelements and, on the other hand, a high preservation of flattenedspecimens. Moreover, it can also be observed that cranialremains (including teeth) occur in number completelydisproportionate to the postcranial elements.

Xenarthran scutes (NISP = 2894) have a high degree ofdisarticulation; just 3.46% correspond to two or a fewarticulated scutes. Considering that a carapace can becomposed of about 800 scutes and that 13 xenarthran taxaare present in this assemblage, a high loss of these skeletalelements is implied. Within the whole sample of isolated scutes,only 26.84% are complete. In general, their breakage surface is

Table 1Vertebrate taxa in the faunal assemblage from Telén. MNI, minimal number of individuals; 1, microvertebrates (less than 1 kg of estimated body mass); 2,macromammalsTableau 1Vertébrés présents dans l’association faunique de Telén. MNI, numéro minimum d’individus ; 1, microvertébrés (moins de 1 kg de masse corporelle estimée) ; 2,macromammifères

The number of specimens with emerging teeth for each taxon is indicated in bracketsLe numéro de spécimens avec des dents émergents de chaque taxon est indiqué entre parenthèses

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smooth, indicating that the specimen was already mineralizedwhen broken.

The postcranial elements in the sample correspond to Mam-malia indet. (NISP = 1277) and Aves indet. (NISP = 23). The

most complete of them are either the smallest bones or those withthe highest density (Fig. 3); 76.98% belong to small individuals.

Cranial elements mostly correspond to different mammaltaxa (NISP = 1396) and only three elements to Teiidae reptiles.

Fig. 2. Percentage of skeletal elements following their position.Fig. 2. Pourcentage d’éléments squelettiques d’après sa position.

Fig. 4. Cranial elements distribution grouped following the estimated bodymass.Fig. 4. Distribution des éléments crâniens groupés selon la masse corporelleestimée.

Table 2Comparison of MNE and NISP valuesTableau 2Comparaison entre les valeurs de MNE et NISP obtenues

MNE NISP

Xenarthra 13 2894Tortoise 1 5Other taxa 2138 2699

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They also show preservation biases: specimens attributed tosmall individuals (90.78%) and cranial portions of higherresistance (Fig. 4).

In the context of the whole outcropping beds, thefossiliferous density is very low: 11,647 remains/48,000 m2 = 0.25 fossils/m2. Even though the possibility ofcounting more than once the same skeletal element is low, justfor the low density and the high degree of dispersion beforeburial, the total MNE was calculated for the entire assemblagenot for each taxon. Considering that one carapace per taxon wascalculated for armoured xenarthrans and one carapace for alltortoise remains, the MNE of reptiles, birds and mammals is2152. If the NISP of scutes is not considered, due to their highdegree of disarticulation and fragmentation, the obtained valueof the total MNE is very close to the total NISP of theassemblage (Table 2).

Excepting mandibles, the relative abundance of the skeletalelements is very low in the sample (Fig. 5). Isolated teeth aremuch more frequent than vertebrae (ratio = 3.56), but theyshould appear in a similar number if the association had acomplete representation of the skeletal elements. That indexwas proposed by Behrensmeyer and Dechant Boaz (1980) inorder to check whether a sample was hydrodynamically biasedbefore burial.

Fig. 3. Postcranial skeletal elements distribution.Fig. 3. Distribution des éléments squelettiques post-crâniens.

6.4. Articulation and breakage degrees

As said before, most remains were found isolated. From thespecimens found in the host material, 0.25% was foundpartially articulated indicating that they were buried intact.

Fig. 5. Relative abundance of skeletal elements.Fig. 5. Abondance relative des éléments squelettiques.

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They correspond to Cingulata carapace scutes, autopodialbones, and small mammal vertebrae. Very few skeletalelements were found complete; they are small and relativelyrobust elements as autopodial bones of the microvertebrategroup, mainly metapodials. Skeletal elements more susceptibleto breakage, such as vertebrae, skulls, mandibles, and limbbones from this group were commonly broken. Some of them(2.75%) have spiral fractures common in fresh bones, so theywere probably broken before burial. Generally, the fracture areain limb bones is close to the epiphysis, and 7.85% of them havea spiral fracture in this region. Also bones without epiphyses orisolated epiphyses are commonly found, indicating theirjuvenile condition. On the other hand, post burial fracturesproduced on mineralised bones (cranial and postcranialelements), perpendicular to the major axis of the bone, arefrequent (59.13%) within the sample. Among microvertebrates,seven skull fragments preserve different portions, but lack theoccipital area. Half-palates and hemimandibles with teeth arethe most common cranial pieces. Among palate remains,10.03% show freshly separated unfused sutures. All hemi-mandibles lack the articular zone; when part of the ascendingramus is preserved, the posterior border shows spiral fractures(5.08%).

The breakage degree of macromammal bones is higher thanon microvertebrate bones. All specimens from this group arevery fragmentary. Only the smallest elements were foundcomplete. Cranial elements are represented by few fragments ofmandible, palate, and isolated teeth. Postcranial elements arevery scarce except for the scutes. Noteworthy, manyundetermined remains, which could be assigned to this group,are so fragmented and deteriorated that it is impossible todistinguish any bone structure that allow their taxonomical oranatomical determination. This characteristic can be attributedto a long exposure to atmospheric agents.

6.5. Weathering degree

A relationship between the weathering degree and the size ofspecimens is observed. Those corresponding to medium andlarge-sized mammals are mostly fully weathered. Loss of

compact bone, surface cracking, and/or longitudinal fracturesare evident, and specimens have been considered as belongingto categories 2 and 3. Most materials of that group are, however,too fragmented to be evaluated for this character.

On the other hand, cranial and postcranial remains ofmicrovertebrates are better preserved. They have little or nobone surface deterioration attributable to weathering. In thesecases, the loss of bone material can be generally due toprocesses other than weathering, such as post-burial corrosions.

6.6. Abrasion degree

The high post burial fragmentation degree, evidenced byperpendicular smooth fractures, makes it difficult to evaluatethe abrasion effect on the remains. This attribute was bestobserved on the Glyptodontidae scutes, 1.87% of them havingrounded edges, polished surfaces and worn relieves. This kindof scutes is almost devoid of compact bone, and is quickly wornwhen transported by any physical agent. These marks appear onmineralised scutes with spongy bone cavities filled withcemented and abraded sediment, which would indicate that thephysical wear happened before exhumation of the fossilremains.

6.7. Mineralization

Every specimen in the assemblage is permineralised and98.62% present their cavities filled with cemented sediment.Among remains found in situ, those included in calcareousconcretions also have cemented sediment. Analysis of thinsections of mandibles of Paedotherium minor suggestspreservation of the original histological structure.

6.8. Impregnation and crust development

In the sample, 17.51% of the specimens have thickcalcareous cementation that sometimes includes completelythe specimen. This feature is common for microvertebrateremains, and can be directly related to late diagenetic processesthat affected palaeosol levels. Wright and Tucker (1991)

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indicated that this kind of accumulation occurs in climatic areaswith seasonal humidity deficiency, which facilitated theprecipitation of calcium carbonate.

Carbonate rhizoliths were observed on 0.45% of specimens.They are small, empty calcite cylinders of regular diameteradhered to the specimens. Their origin is related to sedimentaccumulation and cementation around roots (e.g., Klappa,1980). These structures provide information about soilcolonisation by roots and in this case, they also indicate thatthe level where rhizoliths were formed would be close to theexposed surface.

Finally, 1.32% of the remains present a thin micrite coating(less than 1 mm) that may be stained with Fe–Mn oxides. Thistype of crust mainly affects microvertebrate remains, and ispresent on specimens with different degree of impregnation,which only in a few cases also affects the thick calcareouscementation. As it happens in the Caleufú assemblage(Montalvo, 2004), such calcareous crust would have beenformed soon after burial, protecting the specimens from otherpedogenic processes.

All specimens have some degree of Fe–Mn oxideimpregnation. In some cases (16.20%), it is slight and fossilsare light coloured (pale orange, 10 YR 8/2 and white, N9);about a third of the remains (32.74%) display moderateimpregnation with dendritic, medium grey stains (N5); andmore than a half of the remains (51.06%) exhibit a strongimpregnation and dark grey colour (N3). The sediment(siltstone or calcareous concretions) associated with the mostimpregnated remains also presents a fine opaque superficialfilm of the same colour.

A relationship between size and colour (degree ofimpregnation with Fe–Mn oxides) of the specimens was noted.Large remains exhibit slight impregnation, and small-sizedvertebrate remains commonly display a strong impregnation,sometimes complete. The presence of the later characterindicates humid environments (Díez et al., 1999).

6.9. Corrosion

From the whole sample, 1005 specimens present some typeof corrosion that affects the bony and/or dental surfaces. In27.06% of cases, corrosion was attributed to root activitydeveloped in the interface skeletal element-sediment. Twodifferent types of biogenic structures attributed to rootdevelopment have been identified (Mikulás, 1999): sphenoich-nia and corrosichnia. According to their characteristics, thesemarks could have been produced at different times of thetaphonomical history (Montalvo, 2002a). Sphenoichniaappears on remains with different Fe–Mn oxide impregnationdegree, but it produces no colour change in the affected area;this trace fossil would have occurred in the early stages of theburial process. In contrast, corrosichnia produces a greaterdegradation of the bone surface and a colour modification of thewhole affected area, and was interpreted to have occurredduring late burial stages, even after impregnations (Montalvo,2002a). In 2.69% of cases, corrosion can be assigned to lichensthat used the specimens as substratum. Bioerosive modification

by lichen activity on the bony surface appears as degraded areaswith parallel bands at the bottom of the feature (FernándezJalvo, pers. com.). These corrosions appear together withcolour changes of the affected area. Lichen corrosion showsthat remains were reworked. The other corroded remains(70.25%) have different sized areas with loss of superficial boneand colour change, and were attributed to soil microorganism’sactivity and acids. Most of these cases are due to processesoccurred once the specimen was already mineralised andimpregnated with Fe–Mn oxides.

6.10. Other surface marks

Among cranial and postcranial elements, 0.85% presentstooth punctures and/or gnaw marks, like those produced bymammalian carnivores (Andrews and Evans, 1983) and 2.18%has other type of marks represented by two or three parallelscratches. There are also specimens (0.26%) with pressuremarks, evidenced by parallel fractures on broken compactbones.

7. Discussion

The taphonomic analysis of the Telén fossil assemblagesuggests that all specimens suffered similar conditions afterburial, and that microvertebrate and macromammal remainsexperienced different taphonomic processes before burial.

Several of the observed characters that are linked toprocesses that suffered the host sediments (e.g., differentimpregnation degree; fine calcareous crust development;specimens included in calcareous concretions; many corrosionfeatures; and development of rhizoliths) suggest that skeletalelements were gradually incorporated to the sediment duringthe palaeosol development in an open environment, beingsubject to soil-forming processes. Palaeosols have beeninterpreted as vertisols, whose quick development suggeststhat the faunal remains were deposited in a brief period of time.Furthermore, the uniform lithology of the whole sectionindicates that the superficial process operating duringsedimentation was similar. The development of both palaeosolswas probably linked to increased stability of the landscape, butunder the same environmental setting. Bown and Kraus (1981)proposed that vertebrate concentrations in palaeosols from theearly Eocene of Wyoming (USA) were the result of gradualmortality. However, several taphonomic attributes observed inthe Telén assemblage, already present on the bones when theywere buried, indicate a different taphonomical process.

The low density of fossils in Telén, scattered and randomlydistributed throughout the outcrops, could be explained by adisplacement from the original depositional area. Absence ofpreferential orientation of the in situ remains suggests aeoliantransport; although it could also be due to an aqueous flow. Thisremobilisation could explain, at least for microvertebrates, thedominance of small to very small remains with a highpercentage of undetermined fragments.

The preservation of the most resistant bones and manyundetermined fragments indicates a high degree of breakage

C.I. Montalvo et al. / Geobios 41 (2008) 133–143 141

and fragmentation previous to burial, both factors supportingalso the existence of transport, although biostratinomic agents(weathering, trampling, and predation) could also haveinfluenced.

The abrasion modifications on the Glyptodontidae scutes, aswell as on their sediment filling, would indicate theirdisplacement once mineralised. Bioerosive modification bylichen activity affected mineralised and impregnated speci-mens, suggesting that these were re-exposed and newly buried.Both characteristics also support the existence of transport fromthe original depositional area.

Once established the post-burial characteristics of theassemblage, the question is to elucidate which was the causeof death of each individual, and the process suffered by thespecimens to be transported to the burial area where palaeosolswere developing. The possible cause of death, includingnatural, catastrophic, and predation death (Andrews, 1990), wasanalysed for microvertebrates and macromammals.

With respect to microvertebrates, a natural or catastrophicdeath process would produce remains that will be incorporatedto the host material (Andrews, 1990). However, considering theopen environment of the region during the late Miocene(Pascual and Bondesio, 1982; Melchor et al., 2000; Montalvo,2000), some characters can not be explained by that origin,specially the lack of articulated elements, the homogeneity ofweathering and breakage degrees, and the bias in anatomicalrepresentation, since all skeletal elements should be betterrepresented. The index of isolated teeth/vertebrae (Behrens-meyer and Dechant Boaz, 1980) in the Telén assemblage (3.56)indicates some influence of sorting during transport.

The high taxonomic diversity of microvertebrates, and thehigh percentage of juvenile individuals could indicate a preyselection, referred to individual age and body mass. However,there is no evidence for digestion on any skeletal element.

The anatomical representation in the assemblage does not fitwell with any known association of modern predators(Andrews, 1990), and this difference could be the result ofpost-burial processes. However, the relative abundance of theskeletal elements are similar to those found in the late MioceneCaleufú assemblage (Cerro Azul Formation, La Pampa,Argentina), where a selective use of anatomical parts by acarnivore predator was inferred based on the presence of toothand gnaw marks (Montalvo, 2004). In Telén, the latter attributeswere only observed in 30 specimens.

On the other hand, the fracture and breakage patternobserved in skulls, mandibles, and long bones of microverte-brates (see description) may have been produced before burialby predation and/or trampling (Andrews, 1990). However, thehigh frequency of smooth edge fractures, considered as relatedto exhumation of the remains, hampers the evaluation of theorigin of other kind of fractures. The fractured fragmentsindicate that only the most resistant areas were preserved, andthere is no important loss of fresh teeth. Concerning postcraniallong bones, the observed spiral fractures near the epiphysis canoccur during the prey death, but they can also be attributed totrampling activity. However, other features typical of tramplingcharacteristics have not been observed (e.g., high percentage of

isolated teeth, scarcity of mandibles and maxillaries, fragmen-tation of preserved mandibles, and shallow striation due tosediment friction; Fernández Jalvo, 1996).

8. Conclusions

Sedimentological analysis of the section of the Cerro AzulFormation from Telén suggests that the main depositional agentwas transport of fine-grained siliclastic sediments by windunder a semi-arid, probably warm, and seasonal climate.Palaeoclimatic constraints come mainly from the presence ofpalaeosols with vertic features and a calcic horizon, and fromthe faunal assemblage. Vertisols may develop rapidly, whichmight imply that the faunal remains were deposited in a briefperiod of time, even though they are randomly distributedthroughout the section. These inferences are in agreement withthe palaeoecological constrains from the faunal assemblage.

It is suggested that the microvertebrate assemblage wouldhave been produced by predation, although posterior processeswould have overprinted some characteristics typical of thisprocess. Predator activity producing little modification on preybones would explain the lack of digestive features, and the bitemarks would have been produced by scavengers (Andrews,1990). These remains would have been incorporated rapidly tothe soil in an area close to the modern outcrop, since there arefew evidences for pre-burial weathering. Accepting thepredator activity, even the predator could have a specialbehaviour favouring the burial, such as the use of burrows orshelters or the trampling of the area. As mentioned before,corrosion by lichen activity indicates the remains wereunearthed during one or more exhumation episodes; and othercharacters, such as high dispersion and low bone density in theprospected area, could indicate that remains were displacedfrom the original depositional area. Considering the prevailingaeolian origin of the sediments and the dearth of evidence offluvial transport, a short distance aeolian transport for theremains is likely. The latter would have been a slow process thatgradually buried the bone remains.

Macromammal remains would have suffered a differentprocess. The detailed taphonomical characteristics of thisgroup, such as high loss of skeletal elements, mainly happenedprevious to burial. High fragmentation and weathering degrees,and abrasion evidence related to exhumation episodes support anatural and gradual death in an open environment where theremains would have been exposed to weathering and physicalagents. In this context, some skeletal elements became buried,thus increasing their potential of preservation.

In sum, the performed analysis allowed the differentiation oftwo groups within the faunal assemblage, with differenttaphonomical histories. Microvertebrate assemblage is inter-preted as the result of predator activities. After a short period ofexposure before burial, they were probably dispersed and sortedby action of wind from the original depositional area.Macromammals would have suffered a natural and gradualdeath, followed by a long period of exposure to weathering andphysical agents. Both groups of remains, once buried, sufferedpedogenic and diagenetic processes.

C.I. Montalvo et al. / Geobios 41 (2008) 133–143142

The fossil assemblage from Telén would represent acondensed assemblage corresponding to two distinct timespans, i.e., the accumulation of microvertebrates took place in ashort interval whereas that of macromammals occurred over alonger interval, coincident with the development of both soils.

Acknowledgements

This work has been partially funded by projects of theFacultad de Ciencias Exactas y Naturales de la UniversidadNacional de La Pampa (C. M. and G. V.) and the CONICET(E. C.). We thank L. Alcalá and an anonymous reviewer fortheir useful comments that improved the manuscript.

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