The Chañares Formation: a window to a Middle Triassic tetrapod community

The Cha~nares Formation: a window to a Middle Triassictetrapod community

ADRIANA C. MANCUSO, LEANDRO C. GAETANO, JUAN M. LEARDI,

FERNANDO ABDALA AND ANDREA B. ARCUCCI

Mancuso, A.C., Gaetano, L.C., Leardi, J.M., Abdala, F. & Arcucci, A.B. 2014: TheCha~nares Formation: a window to a Middle Triassic tetrapod community. Lethaia,Vol. 47, pp. 244–265.

The Cha~nares Formation is known worldwide for its diverse and well-preserved Ladi-nian non-marine tetrapod assemblage, including a wide variety of archosauriform rep-tiles (proterochampsids, early offshoots of the crocodilian line and dinosaurianprecursors) and synapsids represented by dicynodonts and cynodonts. This tetrapodrecord offers an opportunity to evaluate, within a taphonomic context, the palaeoecol-ogy of this Middle Triassic fauna. The taphonomic analysis of the Cha~naresassemblage, under precise stratigraphical control, indicates that it is a good representa-tion of the original faunal composition allowing us to address the palaeoecologicalinteractions between its components. Mass estimations and morphology-basedpalaeobiological inferences of Cha~nares tetrapods are used to reconstruct the trophicstructure of the community. Cha~nares tetrapod fauna was numerically dominated bymiddle-sized herbivorous and small faunivorous cynodonts, whereas middle-sizedfaunivorous cynodonts and large dicynodonts were less common. In contrast to thetherapsids, which show a low species-richness and high abundance, the archosauri-forms are less abundant, but are the most taxonomically diverse group. The largeparacrocodylomorphs (estimated body masses between 350 and 500 kg) are identifiedas the top predators of the community, and the traversodontid cynodonts and dic-ynodonts (estimated body masses reaching approximately 43 and 360 kg, respectively)are indentified as the base herbivores of the trophic pyramid. We conclude that theworldwide faunal composition in the Ladinian reveals two continental assemblages: aneastern Laurasian assemblage dominated by temnospondyl amphibians; and a westernGondwanan assemblage dominated by therapsids but including a wide diversity ofarchosauriforms. □ Cha~nares Formation, community reconstruction, Ischigualasto-Villa Union Basin, Ladinian faunas, taphonomy.

Adriana C. Mancuso [[email protected]], IANIGLA, CCT-CONICET,Mendoza, Adri�an Ruiz Leal s/n, Parque Gral., San Mart�ın, 5500 Mendoza CC330,Argentina; Leandro C. Gaetano [[email protected]], Juan M. Leardi [[email protected]], Departamento de Ciencias Geol�ogicas, Facultad de Ciencias Exactasy Naturales, IDEAN, CONICET Universidad de Buenos Aires, Ciudad Universitaria Pa-bell�on II, 1428 Buenos Aires, Argentina; Fernando Abdala [[email protected]],Evolutionary Studies Institute, University of the Witwatersrand, Private Bag 3 WITS2050 Johannesburg, South Africa; Andrea B. Arcucci [[email protected]], Areade Zoologia, Universidad Nacional de San Luis, Chacabuco 917 5700 San Luis, Argen-tina; manuscript received on 11/09/2012; manuscript accepted on 09/09/2013.

Terrestrial faunas from the upper Middle Triassicare poorly represented globally. In Gondwana, fau-nas of this age are best documented in South Amer-ica, particularly through discoveries in the SantaMaria Formation in southern Brazil (Dinodontosau-rus Assemblage Zone) and the Cha~nares Formationin western Argentina (Romer & Jensen 1966; Rogerset al. 2001; Langer et al. 2007). Several taxa areshared between these assemblages (Rogers et al.2001; Langer et al. 2007), suggesting that they wereapproximately contemporaneous. In addition, twotaxa from these South American faunas (i.e. the car-nivorous cynodont Chiniquodon and the dicynodontStahleckeria) were recently identified in the upper-most strata of the Omingonde Formation of central

Namibia, indicating a putative Ladinian age for theselevels as well (Abdala & Smith 2009; Abdala et al.2013).

The Cha~nares Formation is part of an entirelynon-marine succession deposited in the Ischigualas-to-Villa Uni�on Basin, which represents one of themost continuous continental Triassic successions inSouth America (Stipanicic & Marsicano 2002). Thediverse and well-preserved Ladinian non-marine tet-rapod assemblage from the Cha~nares Formationincludes a high diversity of archosauriforms (e.g.proterochampsids, pseudosuchians, ornithodirans)and high abundance of synapsids (large dicynodontsand smaller cynodonts). Because of this abundanceand diversity, the fossil remains recovered from this

DOI 10.1111/let.12055 © 2013 The Authors, Lethaia © 2013 The Lethaia Foundation

formation constitute one of the best records of Mid-dle Triassic tetrapods (Romer & Jensen 1966; Rogerset al. 2001). It also provides a key to understand theevolution of terrestrial faunas during the Triassic,including the origin and early diversification ofdinosaurs and the dominance of therapsids (Sereno& Arcucci 1993, 1994; Abdala & Giannini 2000,2002).

Romer (1966) was the first to establish back-ground knowledge about the Cha~nares assemblage.He performed extensive fieldwork in La RiojaProvince and collected abundant vertebrateremains during 1964 and 1965 (Romer 1966). Hethereafter produced a series of publications consist-ing of mostly anatomical studies and a sketch ofthe geology of the Triassic units at the Cha~naresand Gualo localities (Romer 1966, 1967, 1969,1971a,b, 1973; Romer & Jensen 1966; Cox 1968;Jenkins 1970; Romer & Lewis 1973). Beginning in1970, Bonaparte conducted several field trips tothe area, including Talampaya National Park wherehe worked almost continuously for a decade. Heproduced an important collection of fossils fromthe Cha~nares Formation, currently housed at theInstituto Miguel Lillo, Universidad Nacional deTucum�an, Argentina. Bonaparte has also publishedon the geology (Bonaparte 1967; Stipanicic &Bonaparte 1979) and vertebrate palaeontology ofthe Cha~nares Formation (Bonaparte 1975). Palae-ontological contributions on Cha~nares fossils havesteadily continued from the 1980s to the present(Arcucci 1986, 1987, 1990, 1996, 2011; Sereno &Arcucci 1994; Abdala 1996; Arcucci & Marsicano1998; Abdala & Giannini 2000, 2002; Desojo &Arcucci 2008; Lecuona & Desojo 2011; Trotteyn &Desojo 2011; Leardi 2012). In addition, an exhaus-tive taphonomic study (Rogers et al. 2001) charac-terized the vertebrate preservation of the Cha~naresfauna from a particular concretion-hosted assem-blage. Rogers et al. suggested that extensive volca-nism in the area was the main cause of massmortality and created a high potential for carcasspreservation.

The main objective of this contribution is to dis-cuss the palaeoecology of the Cha~nares tetrapodcomponents within a palaeoecological and tapho-nomic framework. For this, we gathered anatomicaland taphonomical information of nearly all the fossilmaterial available from the unit and combined withadditional evidence obtained from fieldworks con-ducted by the authors over the last 10 years. Consid-ering the high preservational potential, the diverseand abundant Cha~nares faunal record allows us tohypothesize about the interaction between the com-ponents of the community and its role in the under-

standing of other Middle Triassic faunas fromGondwana.

Geological setting

Triassic non-marine sediments in Argentina are con-centrated in half-graben rift systems located alongthe western Gondwana margin and associated withthe pre-break-up of Pangaea (Uliana & Biddle1988). The Ischigualasto-Villa Uni�on Basin is one ofthe most extensive of these rift basins, both in itsgeological record and surface extent. It is elongatedin a NW–SE direction and filled with a 2000–6000 m non-marine succession of alluvial, fluvialand lacustrine sediments (Stipanicic 2002). SeveralTriassic localities with abundant faunal and floralremains are known from this basin (Artabe et al.2001; Marsicano et al. 2001). The best-exposed out-crops are located along the border between San Juanand La Rioja provinces (Fig. 1A).

The Triassic deposits of the Ischigualasto-VillaUni�on Basin rest unconformably upon thick conti-nental Palaeozoic deposits (Caselli 1998). The ini-tial infilling of the basin (Fig. 1B) is representedby the red alluvial fan, ephemeral fluvial and playalake deposits of the Talampaya and Tarjados for-mations (L�opez-Gamund�ı et al. 1989; Caselli1998). The latter is unconformably overlain by theAgua de la Pe~na Group, which comprises theCha~nares, Los Rastros, Ischigualasto and Los Col-orados formations (Mancuso 2005a,b). TheCha~nares Formation (Fig. 1B) is characterized bytuffaceous sandstones and siltstones deposited influvial–lacustrine environments, with orthocon-glomerates and paraconglomerates deposited inalluvial fans in the northwest part of the basin(Rogers et al. 2001; Mancuso et al. 2004; Mancuso2005a,b). The unit is transitionally replacedupward by the lacustrine deltaic sandstones andblack shales of the Los Rastros Formation, whichin turn is overlain by the fluvial sandstones, mud-stones and tuffs of the Ischigualasto Formation.Finally, the mudstone and sandstone red beds ofthe Los Colorados Formation, interpreted as amoderate-sinuosity fluvial system deposit (Caselliet al. 2001), represent the end of the Triassic suc-cession. A regional unconformity separates the Tri-assic sequence from the fluvial conglomerates andsandstones of the putative Cretaceous Cerro Raj-ado Formation (L�opez-Gamund�ı et al. 1989; Casel-li et al. 2001).

The Agua de la Pe~na Group is considered tospan the Middle–Late Triassic (Fig. 1B) (Stipanicicet al. 1996; Kokogian et al. 2001), with a LateCarnian–Early Norian age (following the Geologic

LETHAIA 47 (2014) Triassic vertebrate fauna from Argentina 245

Time Scale – see Ogg 2012; see also Irmis et al. 2011)for the lower and upper sections of the IschigualastoFormation (231.4 � 0.3 Ma and 225.9 � 0.9 Ma,respectively; Martinez et al. 2011). A Ladinian age isinferred for the Cha~nares Formation based on its fau-nal composition and its stratigraphical relationshipswith the Ischigualasto Formation (Kokogian et al.2001; Rogers et al. 2001).

Cha~nares Formation palaeoenvironment

The Cha~nares Formation in Talampaya NationalPark (La Rioja Province; Fig. 1) is characterized atits base by a sandstone–siltstone fluvial facies withdistinct lower and upper levels (Table 1 and Fig. 2).The lower levels of this facies are composed of lightolive grey fine-grained sandstones (Sh, Sp) with silic-ified root traces, scattered pebbles and locally abun-dant small brown carbonate concretions (5–25 cmin diameter). This facies covers the palaeotopogra-phy of the silicified top of the Tarjados Formation(Mancuso & Caselli 2012). The upper levels of thefluvial facies include fine-grained sandstones andsiltstones that preserve vertebrate remains.

The overlaying tuffaceous claystone–siltstonefacies (TFm) is laterally persistent, and the beds dis-play ‘popcorn’ weathering typical of bentoniticdeposits (e.g. Terry et al. 1998). Abundant large

brown calcareous concretions, approximately 2 m indiameter, crop out in two discrete levels (Fig. 2).Additionally, some smaller and randomly distributedconcretions are present in this facies. The concretionsare composed of a blueish grey silty sandstone; in thinsection, the matrix is composed of grains of quartz,plagioclase and corroded lithic fragments with cus-pate glass shards that have been altered to calcite andiron oxide (Rogers et al. 2001). Most Cha~nares verte-brate fossils have been recovered from the lower con-cretionary level in the mid section of the TFm facies,whereas the upper concretionary level lacks body fos-sil remains (Romer & Jensen 1966; Rogers et al. 2001;personal observations). This upper concretionarylevel is associated with a white tuff layer, and some ofthese concretions contain vertical burrows (1 cmdiameter; Fig. 2). Some vertebrate remains showing arandom areal distribution were found directlyembedded in the tuffaceous clay–siltstones or in smallconcretions below the mid section concretionarylevel. Lithological and geometrical features (Table 1)suggest that the tuffaceous sandstones were depositedby river channels, whereas the tuffaceous claystonesand siltstones were deposited on alluvial floodplains(Rogers et al. 2001; Mancuso 2005a). The dominanceof bedload sedimentation in the fluvial system isprobably associated with the influx of pyroclastic sed-iment (Rogers et al. 2001; Mancuso 2005a).

Fig. 1. Geological map of the Ischigualasto-Villa Uni�on Basin in La Rioja Province, Argentina. Modified from Mancuso (2005a); Triassiclithostratigraphy and chronostratigraphy in the Ischigualasto-Villa Uni�on Basin. Dates in millions of years are from the Geologic TimeScale (Ogg 2012). *231.4 � 0.3 and 225.9 � 0.9 Ma radioisotopic age of Ischigualasto Formation (Martinez et al. 2011); *the top of theLos Colorados Formation is Middle Norian in age (Santi Malnis et al. 2011).

246 Mancuso et al. LETHAIA 47 (2014)

The succession ends with laterally persistent,light grey and pale olive claystones and siltstones(Fm, Fl; see Table 1 and Fig. 2) that also show‘popcorn’ weathering. Abundant sub-vertical inver-tebrate burrows (10–12 cm long and 0.8 cm wide)and some exhibiting meniscate infillings (Fig. 2)are dispersed randomly throughout the succession

and are assigned to the ichnogenus Taenidium(Rogers et al. 2001). The fine-grained lithologiesand sedimentary structures (along with other fea-tures summarized in Table 1) support the inter-pretation that these mudrocks were deposited in ashallow lake environment (Rogers et al. 2001;Mancuso 2005a).

Table 1. Summary of the facies associations of the Cha~nares Formation.

Facies andinterpretation Lithology Structures Bedding Fossil content

Lateral and verticalrelationships

CH-A Fluvial Moderate, sorted, lightolive grey, fine-grainedsandstone and siltstone,with scattered thinquartzite pebbles.(Sh, Sp)

Horizontal andlow-anglelaminated,smallcarbonateconcretions.

Individual beds aretabular with 0.2–0.5 mof thick and extendlaterally for tens ofmetres, and haveplanar non-erosionalboundaries.

Root traces, horizontalburrows, occasionaltetrapod remains.

Sh and Sp faciescommonlyoverlie TarjadosFormation andis overlain byTFm facies.

Light bluish greytuffaceous very fine-grained sandstone,siltstone, and claystone.(TFm)

Massive tonodular,with largecalcareousconcretions.

Beds are tabular with0.15–0.3 m of thickand laterally persistentfor hundreds ofmetres, and have non-erosional boundaries.

Abundant tetrapodremains inside andoutside concretions

TFm faciescommonlyoverlies Sh andSp facies and isoverlain by Fmand Fl facies(CH-B).

CH-B Shallowlake

Light grey, pale olive,tuffaceous siltstone–claystone. (Fm, Fl)

Massive tonodular, andhorizontallylaminated.

Beds are tabular and0.05–0.1 m in thicknessand persist laterally forhundreds of metres,and have non-erosionalboundaries.

Commonly burrowedwith sub-verticalburrows 10–12 cmin length and0.8 cm in width.

Fm and Fl faciesoverlie TFmfacies (CH-A)and are overlainby Los RastrosFormation.

Fig. 2. Schematic section of the Cha~nares Formation in the Gualo area showing the fossiliferous levels. Pictures show details of the sec-tion and structures: (1) Palaeotopography of the silicified top of the Tarjados Formation; (2) Silicified root traces in the Cha~nares Forma-tion; (3) Abundant invertebrate vertical burrows in the upper concretionary level of the Cha~nares Formation; (4) Sub-verticalinvertebrate burrow exhibiting meniscate infillings; (5) Nodular weathering.


Materials and methods

We have examined all the palaeontological collec-tions housing Cha~nares vertebrate remains: BernardPrice Institute for Palaeontological Research, Uni-versity of the Witwatersrand, Johannesburg, SouthAfrica (BPI), Facultad de Ciencias Exactas y Natu-rales, Universidad de Buenos Aires, Argentina(CPBA-V), Centro Regional de Investigaciones de LaRioja, Argentina (CRILAR), Museo Argentino deCiencias Naturales ‘Bernardino Rivadavia’, Argen-tina (MACN-PV), Museum of Comparative Zool-ogy, Harvard University, Cambridge, USA (MCZ),Natural History Museum, London, United Kingdom(NHMUK), Colecci�on Paleontolog�ıa Lillo, Universi-dad Nacional de Tucum�an, Argentina (PVL) andMuseo de Paleontolog�ıa, Universidad Nacional deLa Rioja, Argentina (UNLR). The abundance of eachtaxon (at the genus level) and diversity of repre-sented lineages were estimated by taking intoaccount the number of specimens and taxa observedin the collections.

The ecological role of the fossil taxa was inferredthrough comparison of their anatomy with that ofextant taxa. Identification of mainly herbivorous orcarnivorous taxa was based on the morphologicalanalysis of teeth. Taxa bearing sectorial (i.e. labiolin-gually compressed) teeth were interpreted as primar-ily carnivorous or insectivorous, whereas edentulousforms or those with labiolingually expanded andbasined teeth are regarded as mainly herbivorous(Romer 1967, 1972a; Crompton 1972; Kemp1982,2005; Abdala & Giannini 2000). It should be noted,however, that the diet of some extant mammals witha specialized carnivorous dentition can include alarge proportion of fruits (e.g. foxes, see Catlin 1988and references there cited).

We used body mass estimations together withmorphological traits to infer possible feeding habits(Table 5). Body mass estimations provide a rationalefor inferring possible prey items for each faunivo-rous taxon. Most of the taxa analysed are knownfrom more than one specimen (Table 3) represent-ing individuals of different body sizes. Thus, there isoverlap among the ranges of body size estimations ofvarious taxa. Nevertheless, based on the estimatedbody mass of the largest specimen (assumed to havebeen an adult; see Table 3), we assigned each taxonto one of four body size categories.

Archosauriform and synapsid body masses wereestimated using equations based on extantcrocodilians (Farlow et al. 2005) and mammals(van Valkenburgh 1990). We are aware that theestimates calculated from these equations might

not be precisely accurate because the extant taxaused to produce the formulas (Fari~na et al. 1998;Farlow et al. 2005; Pol et al. 2012; Taborda et al.2013) could have different body proportions andthus not be very appropriate analogies. Neverthe-less, we regard these formulas as the best proxiesavailable for estimating the body mass of theCha~nares animals within an order of magnitude.To normalize the implicit error in the body massestimates among taxa, when possible, we apply asingle equation for all forms included in eachmajor taxonomic group (i.e. archosauriforms, cy-nodonts and dicynodonts). Hence, we expect themass estimates species within the same taxonomicgroup to be similarly biased.

Because we are dealing with extinct taxa subjectedto the biases and problems inherent in the fossilrecord, it is not possible to be certain about the max-imum and minimum body sizes for any particulartaxon. We adopted a conservative methodology byconsidering that the body mass of the largest speci-men known for each taxon represents the maximumsize that could be reached by the species, althoughthis may result in an underestimation of the maxi-mum size that a particular taxon could reach.

Farlow et al. (2005) base their estimations onmeasurements of the crocodilian femur and skull.We used the equations based on femoral and, in spe-cial cases, skull length for our proxies. Estimatesbased on femoral length were preferred because thefemur was known for most taxa; otherwise, skulllength was employed. The equations using skull (1),or femoral length (2), as proxies are given below:

log (body mass) ¼ 3:48log(skull length

in millimetres) � 6:97ð1Þ

log (body mass) ¼ 3:33log(femoral length


Several body mass estimators defined for cranial,dental and limb measurements, as well as for totalbody length (Anderson et al. 1985; Janis 1990; Scott1990; van Valkenburgh 1990; Fari~na et al. 1998), areavailable for modern mammals (e.g. ungulates, car-nivorans, felids and ursids). The body masses fordicynodonts and cynodonts were estimated usingthe equations for ursids (3) and carnivores (4),respectively (van Valkenburgh 1990):

log (body mass) ¼ 2:02log (skull length

in millimetres)� 2:80ð3Þ

log (body mass) ¼ 3:13log (skull length



The selection of these mammal groups as proxiestakes into account the general body size and propor-tions inferred for these therapsids.

The specimens were measured by us wheneverpossible, and in other cases, measurements wereobtained from the literature (Abdala 1996; Abdala &Giannini 2000, 2002; Domnanovich 2010). Only onespecimen’s measurements (UNLR 057) wereobtained from a published figure (Desojo & Arcucci2009).

Data for the taphonomic and environmentalinferences were obtained from fieldwork and thereview and study of specimens housed in collec-tions (including semi-prepared fossil-bearing con-cretions). Taphonomic analysis of in situ fossilswas conducted according to current methodologies(e.g. Behrensmeyer 1991; Rogers 1994; Eberth et al.2007). The taphonomic attributes were docu-mented in each accumulation, regardless ofwhether the fossil specimens were inside or outsideconcretions. However, these data were used fortesting whether there are some differences betweenthe fossil assemblages located inside or outsideconcretions.

The degree of articulation of skeletons wasassessed as one of four different states: articulated(A) where the bones retain their anatomical posi-tions; partially articulated (PA) where only someof the bones are in their anatomical positions; dis-articulated but associated (DA) where the bonesare all separated but remain in the immediatevicinity; and isolated and dispersed (ID) where thebones are widely separated. Voorhies groups (i.e.VG I, VG II and VG III) are indices of the resis-tance of skeletal elements to movement and dis-persal (Voorhies 1969; Fiorillo et al. 2000). VG Iincludes individual skeletal elements that are mostreadily transported such as vertebrae or phalanges,and VG III being those that are most resistant totransport such as skulls and mandibles (Lyman1994). The frequencies plotted in the ternary dia-gram were estimated from the total number of ele-ments preserved in each accumulation. Theattitude of the bones in each accumulation isrecorded to understand their relationship with theentombed sediment. Thus, we were able to deter-mine whether the skeletal elements in the accumu-lation show some spacial orientation and whetherthe skeletal elements cross cut the bedding planes.This helps to resolve any reorientation by an agentof transport and if they were deposited before ortogether with the entombing sediment. Finally,bone weathering stages were measured sensuBehrensmeyer (1978, 1991).

Palaeocological aspects, diversity andabundance of the Cha~nares tetrapods

Cynodonts. – Despite being represented by onlythree species (Chiniquodon theotonicus, Massetogna-thus pascuali and Probainognathus jenseni), cyno-donts are by far the most common tetrapods foundin the Cha~nares Formation (Fig. 3), comprising73.7% of the tetrapod specimens recovered(Table 2). Fragmentary cynodont bones, mostly ofMassetognathus, were rarely collected by previousfield parties, resulting in an underrepresentation inthe collections of cynodonts as a whole, and of Mas-setognathus specifically. Indeed, one of the hallmarksof the Cha~nares assemblage is the numerical domi-nance of traversodontid cynodonts, with Masseto-gnathus pascuali representing 62.3% of the cynodontsample and 46.0% of all amniote remains recovered(Table 2). Additionally, the number of sectorial-toothed cynodonts is approximately half as abun-dant as Massetognathus in the fauna (Table 2). Thedominance of traversodontid cynodonts in theCha~nares Formation is not an isolated phenomenon.High percentages of these forms are also known inolder and younger Triassic faunas from Gondwana,such as the Anisian assemblages from the Cerro delas Cabras Formation in western Argentina and theLifua Member of the Manda Formation in Tanzania(Crompton 1955; Bonaparte 1969; Abdala et al.

A

B

Fig. 3. A, taxonomic diversity of Cha~nares fauna (species level).B, taxonomic abundance of the different taxa of the Cha~narestetrapod assemblage.


2009) and the lower Carnian faunas from the Santa-cruzodon Assemblage Zone, Santa Mar�ıa Formationin Brazil (Soares et al. 2011). A large number of tra-versodontid cynodonts are also recognized in theLate Carnian Ischigualasto Formation, althoughrhynchosaurs are the most abundant vertebrates inthis unit (Martinez et al. 2011).

Massetognathus pascuali is a medium-sized cyn-odont represented by specimens which documentdifferent ontogenetic stages. It attains the largest sizeof any cynodont in the Cha~nares assemblage (skulllength ranging from 72 to 204 mm; Abdala &

Giannini 2000) with a maximum inferred body sizeof approximately 43.57 kg (Table 3). Massetogna-thus is the only cynodont from the Cha~nares Forma-tion with clear adaptations for herbivory, withbasined, labiolingually expanded upper and lowerpost-canines, ensuring a rudimentary dental occlu-sion (Romer 1967, 1972a,b; Crompton 1972).

Chiniquodon theotonicus is a medium-sizedcarnivorous cynodont with sectorial post-caninesthat lack cingula and have a strongly recurved maincusp and smaller accessory distal cusps (von Huene1936; Romer 1969; Abdala & Giannini 2002; Oliveira

Table 2. Relative abundance of Cha~nares taxa.

No ofspecimens

% of totaltetrapods

Therapsida (82.7%) Anomodontia(9.0%)

Dicynodontia (9.0%) Dinodontosaurus 11 3.3Dicynodont indet 19 5.7

Theriodontia(73.7%)

Cynodontia (73.7%) Massetognathus 154 46Chiniquodon 29 8.7Probainognathus 57 17Cynodont indet 7 2.1

Archosauriforms(17.3%)

Non-archosaurianarchosauriforms(7.8%)

Proterochampsida(7.2%)

Chanaresuchus 12 3.6Tropidosuchus 10 3Gualosuchus 2 0.6

Doswelida (0.6%) Tarjadia 2 0.6Pseudosuchia(2.7%)

Basal Suchia (1.8%) Gracilisuchus 6 1.8Paracrocodylomorpha(0.9%)

Luperosuchus 2 0.6Paracrocodylomorphs indet. 1 0.3

Ornithodira(4.5%)

Non-dinosauriformsdinosauromorphs (1.5%)

Lagerpeton 5 1.5

Non-dinosauriandinosauriforms (3%)

Lewisuchus 1 0.3Marasuchus 5 1.5Pseudolagosuchus 4 1.2

Archosauriformsindet. (2.4%)

Archosauriforms indet. (2.4%) Archosauriformes indet. 8 2.4

Table 3. Body mass estimations for the Cha~nares taxa considering skull or femoral length. Skull length was measured from the anterior-most tip of the snout to the basi-occipital condyles. Asterisk depicts measurements from published figures.

Taxa SpecimenSkull length(mm)

Femorallength (mm)

Estimatedmass (kg)

Archosauriforms Chanaresuchus MCZ 4035 – 128.7 20.18MCZ 4036 – 138.3 25.64MCZ 4038 – 124.88 18.25PVL 4575 – 124.4 18.02

Gualosuchus PVL 4576 – 98.07 8.16Tropidosuchus MCZ 9482 (1) – 62.49 1.82

MCZ 9482 (2) – 50.6 0.9PVL 4601 – 68.2 2.43

Marasuchus PVL 3871 – 62.97 1.87Pseudolagosuchus PVL 4629 115 13.87Lagerpeton MCZ 4121 – 62.6 1.83Gracilisuchus MCZ 4117 – 10.889 1.31Luperosuchus UNLR 04 – 55.45* 379.12

Cynodonts Probainognathus PVL 4673 84 – 2.71Chiniquodon PVL #? 121.4 – 8.58

PVL 44 126.7 – 9.81Massetognathus PVL 4728 182.5 – 30.74

CRILAR-PV914 135 – 11.97UBA 14182 92 – 3.6

Dicynodonts Dinodontosaurus platyceps UNLaR 14 450 – 362.65Dinodontosaurus brevirostris UNLaR 15 310 – 170.82


et al. 2009). Only a few cynodonts (including extantmammals) have post-canines with strongly curvedcusps, among them, pinniped carnivorans (Hillson1990), in which the recumbent cusps prevent theescape of prey from the mouth of the animal. Thissuggests that Chiniquodon fed on animals smallenough to allow the orientation of the post-caninecusps to ensure the posterior direction of food. Thesame interpretation is provided for the Early Triassiccynodont Galesaurus, which also has strongly curvedmain cusps lacking dental occlusion (Abdala et al.2006). Chiniquodon probably caught, cut and swal-lowed whole prey items, just as most living carnivo-rous lizards do. The specimens of Chiniquodontheotonicus recovered from the Cha~nares Formationexhibit skull lengths ranging from approximately67–159 mm (see also Abdala & Giannini 2002) withan inferred body mass of 19.97 kg for the largest size(Table 3). Chiniquodon is the least commoncynodont in the Cha~nares assemblage (11.7% of allcynodonts and 8.7% of all tetrapods; Table 2). Repre-sentation of different ontogenetic stages is poor incomparison with Massetognathus pascuali, and thereis an underrepresentation of very early growth stages.Considering the evidence at hand, it can be stated thatthere is an overlap in the size ranges of ChiniquodonandMassetognathus (Table 3).

Probainognathus jenseni is the smallest cynodontfrom the Cha~nares Formation. The relative abun-dance of this species is about twice that of Chiniqu-odon (23.1% of cynodonts and 17.0% of tetrapods;see Table 2). Although a large sample of the ontoge-netic stages of Probainognathus is lacking, small andrelatively large representatives are known, showing awide size range (Abdala 1996). At present, the largestspecimen known (84 mm in skull length) has a bodysize around 2.7 kg, smaller than any of the speci-mens assigned to adult Massetognathus or Chiniqu-odon (Table 3). However, the smallest specimen ofMassetognathus is 72 mm in skull length and that ofChiniquodon is only 70 mm and therefore smallerthan the largest Probainognathus specimen. Thesmall size and complex, cingulate sectorial post-canines of Probainognathus indicate a more complexfaunivorous diet, probably consisting of arthropodsand/or small vertebrates.

Dicynodonts. – Dicynodont remains represent 9.0%of all individual specimens from the Cha~naresassemblage (Fig. 3), and the only genus identified,Dinodontosaurus (Domnanovich 2010), representsaround 3.3% of the total amniotes in the Cha~naresassemblage (Table 2). This genus has three recog-nized species: Dinodontosaurus platyceps, representedby two specimens, D. brevirostris with eight

specimens, and D. platygnathus, which is knownonly by one incomplete specimen (Domnanovich2010).

Dinodontosaurus is a typical South Americantaxon and characterizes the DinodontosaurusAssemblage Zone of the Santa Mar�ıa Formation insouthern Brazil. In contrast to its poor representa-tion in the Cha~nares assemblage, this dicynodont isby far the most common tetrapod from the contem-poraneous Brazilian fauna (Langer et al. 2007), rep-resenting 61% of specimens recovered (Azevedoet al. 1990).

Dinodontosaurus is mid-sized by dicynodont stan-dards, with D. brevirostris somewhat smaller thanD. platyceps and D. platygnathus. The body sizes forD. brevirostris and D. platyceps were estimated fromthe skull length and ranged between approximately170 kg and 362 kg (Table 3). Although D. platygna-thus body mass could not be estimated due to theincompleteness of the only specimen known,Domnanovich (2010) suggested that this species wasof the same size of D. platyceps. All Triassicdicynodonts (including Dinodontosaurus) lackpost-canine teeth, and the anterior portion of thepre-maxilla and the dentary is interpreted to havebeen covered by a keratinous beak (King 1990). Thismorphology would have been useful in dealing withdifferent kinds of vegetation. Dinodontosaurus alsofeatures a pair of well-developed maxillary tusks. Ananalysis of dicynodont cranial morphology suggeststhat these animals used the anterior part of the pal-ate for crushing, whereas the sharp rims of the pre-maxilla and lower jaw would chop and slice food(King 1990). Dinodontosaurus was stocky and proba-bly not an agile animal, comparable in size to a smallextant rhinoceros (King 1990).

Surkov & Benton (2008) classified Permian andTriassic dicynodonts in three categories, reflectingtheir probable feeding habits. Using cranial measure-ments as proxies for muscular development andcommon head movements, those authors inferredthat Dinodontosaurus fed on low vegetation (i.e. at orbelow the level of the head). Dinodontosaurus andMassetognathus have a significant difference in bodysize, but the evidence above suggests that both taxamade use of the same food source, feeding on groundlevel vegetation or on the lower branches of tallerplants and shrubs. Perhaps the overall larger size inDinodontosaurus allowed it to feed on additionalresources not accessed by adultMassetognathus.

Archosauriforms. – Unlike cynodonts, which showa low species-richness and occur in great abundance,archosauriforms are represented by several clades inthe Cha~nares fauna, all with rather low abundance,


but they are quite diverse (Fig. 3). This lineagerepresents 17.3% of the Cha~nares amniote speci-mens recovered (Table 2). The non-archosaurianarchosauriforms, including proterochampsids anddoswelliids, are the most common members of thisclade (Fig. 3), representing more than half of theknown specimens of archosauriforms (Table 2).Proterochampsids are the most abundant (41.4% ofall archosauriforms; Table 2), and they are the mostspecies-rich, with three different taxa (Chanaresu-chus bonapartei, Gualosuchus reigi and Tropidosuchusromeri). Chanaresuchus and Tropidosuchus (repre-sented by at least eleven and ten specimens respec-tively) are the most abundant proterochampsids andarchosauriforms recorded from the Cha~nares For-mation, whereas Gualosuchus is known only fromtwo specimens. Proterochampsids show one of thelargest size ranges among archosauriforms, as Tropi-dosuchus had an estimated weight of less than 3 kg,whereas the largest Chanaresuchus known weighedapproximately 25.6 kg (Table 3).

Proterochampsids have historically been regardedas semi-aquatic and probably piscivorous (Sill 1967;Romer 1971a,b, 1972a,b) because of their low, longsnout with numerous teeth and ornamented skull,combined with an inferred quadrupedal sprawlingposture. However, a recent review of this group(Arcucci 2011) emphasized differences between pro-terochampsids and the aquatic or semi-aquatic croc-odiles with which they are frequently compared. Thelong and low snouts of proterochampsids do notcontain a complete secondary palate as in crocodiles(although there are some fully aquatic reptiles likephytosaurs that lack a bony secondary palate;Chaterjee 1978; Sereno 1991). The number of mar-ginal teeth is approximately the same as in otherarchosaurs (15 in both the upper and lower jaws),and they are laterally compressed and recurved withno apparent similarity to those of confirmed piscivo-rous animals such as Gavialis and sauropterygians,in which these teeth are increased in number, conicaland lack any type of carinae or keel (Massare 1987;Pierce et al. 2009). Moreover, there is no evidencethat the skull ornamentation present in several pat-terns and degrees in many archosauromorph lin-eages (e.g. aetosaurids, euparkeriids, ‘protosuchians’and rauisuchids) has any relationship with life in anaquatic environment. In addition, the post-cranialskeleton of proterochampsids does not show evi-dence of aquatic adaptations: the limbs do not showsigns of digit reduction or enlargement of the distalelements, and the tail is not dorsoventrally tall, butinstead is wide and thick with very low neural spinesand well-developed transverse processes (Arcucci2011). The limbs are long and slender in relation to

other archosauromorphs, such as the larger proter-osuchids and erythrosuchids, suggesting that theposture was not sprawling, having some degree ofadduction of the limbs, although not fully vertical.Given the absence of clear aquatic adaptations, theproterochampsids from Cha~nares are regarded asmainly terrestrial forms, as are most archosauriforms(Nesbitt et al. 2009).

Tarjadia ruthae was originally regarded as anarchosauriform of uncertain affinities (Arcucci &Marsicano 1998), but recently has been identified asa dosweliid (Desojo et al. 2011). Doswelliids havebeen interpreted either as terrestrial or aquatic (We-ems 1980). The latter view has been the most widelyaccepted, based on the animal’s heavily ornamentedskull, broad osteoderms, long mandible with conicalteeth and dorsally directed orbits (Desojo et al.2011). Little can be said about the body size ofT. ruthae because the two known specimens fromthe Cha~nares Formation are very fragmentary. Eventhough most of the above-mentioned characterscannot be observed in T. ruthae, we considered thistaxon to be a semi-aquatic, like other better-knowndosweliids.

The Cha~nares archosaurian record includes repre-sentatives of both major clades in almost equalabundance (pseudosuchians 2.7% and ornithodirans4.5% of all amniotes in the Cha~nares fauna) andspecies-richness (each group represented by threetaxa: Table 2 and Fig. 3). Pseudosuchians are repre-sented by the basal pseudosuchian Gracilisuchus sti-panicicorum, the paracrocodylomorph Luperosuchusfractus and isolated paracrocodylomorph remainsthat represent arguably the largest archosaur of theformation (CRILAR 417; see Leardi 2012). Thepseudosuchians are clearly divided into two verydifferent size categories (Table 3): Gracilisuchus rep-resents one of the lightest forms (with an estimatedweight of less than 1.5 kg) and, in strong contrast,the large paracrocodylomorphs have estimatedmasses around 350–500 kg (see Leardi 2012). Thelatter are very scarce in the Cha~nares Formation(only 5.2% of the archosauriform specimens), beingrepresented by two Luperosuchus specimens (UNLR04 and 057), only one of which is a large animal(Table 3), and by the large unnamed taxon reportedby Leardi (2012). With the exception of Revuelto-saurus, aetosaurs and shuvosaurids, pseudosuchiansare usually regarded as predatory animals (Nesbitt2011). Additionally, paracrocodylomorphs are con-sidered predators on the basis of their huge, labio-lingually compressed teeth with serrated carinae(Gower 2000) and are usually the largest predatoryforms in their community. Gracilisuchus does nothave any particular adaptations for a specialized


diet, and it is considered faunivorous in concor-dance with most of the members of the Pseudosu-chia, probably preying on arthropods or smallvertebrates.

As mentioned previously, ornithodirans represent25.9% of Cha~nares archosauriforms and compriseexclusively non-dinosaurian dinosauromorphs: thebasal dinosauromorph Lagerpeton chanarensis andthe dinosauriforms Marasuchus lilloensis, Lewisuchusadmixtus and Pseudolagosuchus major (a potentialsynonym of the Lewisuchus -Nesbitt et al. 2010).This clade is homogenous in body mass, being com-prised of small animals whose estimated weights donot exceed 2 kg (Table 3), except in the case of Pseu-dolagosuchus, which is interpreted as a significantlylarger form. We were not able to estimate the massof Lewisuchus, as the material available for this taxon(UNLR 01) does not allow for a quantitative esti-mate of body size. Even though few tooth-bearingjaws of basal dinosauromorphs have been found,they are usually regarded as generalized predatorsprobably feeding on small arthropods or small verte-brates, as they lack any particular adaptations toomnivorous or herbivorous diets (Nesbitt et al.2010; Barret et al. 2011). Lewisuchus has beenassigned to the Silesauridae (Nesbitt et al. 2010; Nes-bitt 2011), a group showing adaptations to omni-vory or herbivory (Nesbitt et al. 2010), but a recentphylogeny recovered the taxon outside silesaurids(Bittencourt et al. 2011). Given its contested phylo-genetic position and putative lack of omnivorous/herbivorous adaptations (ABA, personal observa-tion; Bittencourt et al. 2011), Lewisuchus is hereconsidered as a generalist faunivore.

For diversity and abundance estimates, specimensassigned to Lagosuchus talampayensis are consideredas indeterminate archosauriforms (2.4% of allamniotes in the Cha~nares fauna). The assignment ofLagosuchus to Dinosauriformes is controversial (seeSereno & Arcucci 1993) as the affinities of these taxaare problematic. In many cases, individuals assignedto this genus include specimens that do not evenbelong to Archosauria but represent proterochamps-ids (ABA and JML, personal observation Lagosuchus:UNLR 09, PVL 3871, MCZ 4137, MCZ 9483R, Pseu-dolagosuchus: MACN 18954). To solve this problem,a thorough revision of the material assigned to thesetaxa is needed, but that remains is beyond the scopeof this present study.

Taphonomy

In Talampaya National Park, the Cha~nares Forma-tion outcrops cover approximately 22 km2. Thisunit also crops out in Ischigualasto Provincial Park

and the Cerro Bola area, but to date, without anyfossil vertebrates having been found. While theCha~nares area has long been considered the richestfossil locality of the unit (Romer & Jensen 1966;Rogers et al. 2001), the highest abundance anddiversity of fossils actually come from the R�ıoCha~nares-R�ıo Gualo outcrops in TalamapayaNational Park (Fig. 1). In this area, approximately330 vertebrate specimens were found in the last50 years resulting in an average fossil density of 15specimens/km2. The fossils have a random areal dis-tribution (Romer & Jensen 1966; Rogers et al. 2001;Mancuso 2005a) and are confined to levels in thelowest 15 m of the formation where they occur inlight olive grey and light bluish grey tuffaceous silt-stones (TFm) without any perimineralization, andwithin brown calcareous concretions (Fig. 2). Thefossil-bearing large concretions are restricted to asingle, lower concretionary level (Fig. 2) 15 m upfrom the base of the unit, sensu data of Romer &Jensen (1966), Rogers et al. (2001) and personalobservations. Below this lower concretionary level,the fossils are found outside the concretions or insmall concretions with random stratigraphical distri-bution.

Taphonomic attributes of fossils located both out-side and inside concretions (O-CF and I-CF, respec-tively) are documented here. These data includeboth field- and collection-based observations andare summarized in Figures 4–6.

Some 63.2% of the O-CF and I-CF fossil assem-blages preserve remains of a single individual, 23.4%two or more individuals of a single taxon and 13.2%remains of two or more taxa (Fig. 4A). Thus, theassemblages include different combinations ofarchosauriforms, cynodonts and dicynodonts. Theassemblages with two or more individuals of a singletaxon consist in general of cynodonts. Mixed fossilaccumulations are composed of associations ofarchosauriforms and therapsids, different therapsidclades (cynodont and dicynodont) or differentgroups of archosauriforms. It is worth noting thatlarge archosaurian and dicynodont remains arenever perimineralized and are only found in the low-ermost levels.

Different states of articulation are present in theremains recovered from the Cha~nares fauna(Fig. 4B). There is dominance of articulated andpartially articulated remains inside the concretionsand of isolated and dispersed remains outside them(Fig. 4C). The archosauriforms are mostly articu-lated, partially articulated or disarticulated but asso-ciated. The articulated and partially articulatedarchosauriform remains are principally preserved inconcretions, whereas the remains found outside the


concretions are mainly disarticulated but associated.The majority of dicynodonts are found as isolatedand dispersed bones. The cynodonts are preserved inall four categories; however, there is a dominance ofpartially articulated remains within the concretionsand isolated and dispersed bones outside concre-tions.

The skeletal representation observed for differenttaxa between O-CF and I-CF (Fig. 5) suggests thatthe taphonomic processes acted differently. TheI-CF cynodont remains show a higher representationof skeletal elements that are more abundant in theskeleton. On the other hand, there is an anormalousabundance of mandibles and skulls among the O-CFcynodont remains. In general, dicynodont post-cra-nia are underrepresented in both O-CF and I-CF.The archosauriforms show a similar distribution ofskeletal elements between O-CF and I-CF assem-blages, with a dominance of post-cranial elements.

For the taphonomic analysis, the Voorhiesgroups of the Cha~nares fossils were dividedbetween O-CF and I-CF and grouped according

to the relative abundance of skeletal elementsassignable to each Voorhies group. The frequenciesfor each assemblage were plotted in a ternary dia-gram (Fig. 6A). The frequency distribution of O-CF and I-CF fossil assemblages do not show signif-icant differences, suggesting that both were sub-jected to similar hydrodynamic conditions duringburial. The results (Fig. 6A) show a concentrationof assemblages in VG III representing assemblagesthat suffered the removal of low-mass skeletal ele-ments such as vertebrae, ribs and phalanges.Another relatively large concentration close to VGI includes nearly complete skeletons that representfossils accumulating without transport. The assem-blages concentrated near VG I, excluding elementsof VG II or VG III, represent the accumulationswhere the most readily transported elements werecarried by low-energy currents.

A significant number of fossils studied here werediscovered in concretions not collected by us, andmany lack contextual information, including theprovenance and original position. In these cases,

A

C

B

Fig. 4. Taphonomic attributes. A, percentage of the assemblage with single individuals (63.2%), two or more individuals of a taxon(23.4%) and two or more taxa (13.2%). B, abundance of different stages of fossil inside concretion articulation. C, fossil outside concre-tion proportion of the stages of articulation in the different taxa. archo, archosauriform; cyno, cynodont; dicyno, dicynodont; A, articu-lated; PA, partially articulated; DA, disarticulated but associated; ID, isolated and dispersed; I-CF, inside concretion fossil; O-CF, outsideconcretion fossil.


only the orientation of the bones can be used toanalyse the influence of taphonomic agents, andinferences regarding global trends for the concre-tion-bearing level are not possible. Figure 6B showsthe percentage of long bone long axis orientationsfor the O-CF and I-CF assemblages. I-CFassemblages have a higher proportion of skeletal ele-ments oriented and cross cutting the bedding planethan O-CF assemblages (Fig. 6B). Aligned bones inI-CF assemblages are less common, although thecross-cutting bones in I-CF suggests more interactionbetween the remains and the entombing sediment.

Most skeletal elements lack surface modification(e.g. abrasion, rounding or tooth marks). Thebreakage patterns are transverse fractures attrib-uted to post-mineralization damage. In general,there is no evidence of macroscopic weathering onthe bone surfaces (weathering stages between 0and 1; Fig. 6C). The I-CF bones show equalnumber of weathering stages 0 and 1, whereas the

O-CF assemblages present weathering stage 0 twiceas often as stage 1. Rogers et al. (2001) observed adegree of superficial oxidation and corrosion inbackscatter electron images of selected bones insideconcretions.

Discussion

Taphonomy

The fossil accumulations of the Cha~nares assemblageare considered to be the product of two differenttaphonomic pathways (Rogers et al. 2001; Mancuso2005a): (1) attritional accumulation associated withnatural deaths of individuals by predation, diseaseand old age; and (2) mass mortality of animals asso-ciated volcanic events. Here, we discuss whether allthe evidence gathered so far supports this interpreta-tion and test these findings with our new data.

Fig. 5. Taphonomic attributes. Skeletal representation for different taxa. The greyscale in the skeletons represents the relative abundanceof skeletal elements collected.


The remains found in lowest levels (below thelower concretion level), outside concretions and insome small concretions, are considered by us asproducts of attritional mortality. They have randomstratigraphical and areal distributions and areusually found directly embedded in the olive grey

and light blueish grey tuffaceous siltstones or insmall concretions. Most of these accumulationsinclude a single individual represented by isolatedand dispersed, and disarticulated but associatedbones and, to a lesser extent, partially articulatedbones. Regarding cynodont specimens, the dentary,skull and limbs are overrepresented, whereas dic-ynodonts are mainly represented by skulls. Tapho-nomic sorting removed most of the readilytransported skeletal elements, especially from disar-ticulated cynodont and dicynodont carcasses. Inaddition, the remains of large taxa, including somearchosauriforms and dicynodonts, were found out-side concretions. The Cha~nares fossil record fromthe lowest level represents mainly VG III accumula-tions (Fig. 6A), suggesting that low- and medium-mass skeletal elements (e.g. vertebrae, ribs, girdle,limb bones) were removed by low-energy currentsand entombed basinward. The 3D orientation showsan interaction between the remains and the entomb-ing sediment. Post-mortem modifications of thebones, including the weathering stage, suggest thatthe remains represent accumulation by attritionalmortality and did not suffer trampling or surfaceexposure for very long periods of time. Thus, afterthe death of the animals, the skeletons remainedexposed on the floodplain for some time, duringwhich they were disarticulated and dispersed byscavengers and/or low-energy hydraulic flows thatcaused the sorting recorded in these assemblages.

In the lower concretion level, anomalous concen-trations of articulated tetrapod remains are inter-preted as evidence of a mass mortality event. Thestratigraphical distribution is restricted (Fig. 2),whereas the areal distribution is extensive, at least inTalampaya National Park. The assemblages rangefrom a single individual to several individuals of oneor more taxa, which are usually articulated or par-tially articulated, with some occurrences of disartic-ulated but associated bones. Small- to medium-sizedcynodonts and archosauriforms are best representedin the concretions, and, in general, these groupsshow similar frequencies of skeletal parts. The massmortality individuals recovered in concretions showless evidence of sorting agents, with the assemblagesconcentrated near VG I (predominance of completeskeletons, Fig. 5). The accumulation plotted nearVG I represents untransported remains, whereas theaccumulation near VG I, lacking elements of VG IIIor II, includes remains that were transported bylow-energy currents. Bones in concretions show nopreferred orientation in plan and profile views, sug-gesting a combined depositional event for theremains and the entombing sediment. The massmortality event seems to have preferentially affected

A

B

C

Fig. 6. Taphonomic attributes. A, ternary diagram of frequenciesof the skeletal elements showing the clustering of the assemblageswith regard to Voorhies groups. The main concentration ofassemblages is indicated by grey circles. B, percentage of assem-blages with orientation on the surface and in depth. C, weather-ing stages sensu Behrensmeyer (1978). I-CF, inside concretionfossil; O-CF, outside concretion fossil.


small- to medium-sized cynodont and archosauri-form taxa. The carcasses remained on the floodplainfor a very short time (much less than the carcassesproduced by attritional mortality) and were rapidlyburied by a low-energy flow that reoriented someelements. The mass mortality event not only causedthe death of the animals, but also supplied the sedi-ment that facilitated their rapid entombment, pre-venting scavenging, trampling or exposure toweathering agents.

In summary, two clearly different patterns – attri-tional and mass mortality – are supported by thecurrent taphonomic analysis (Table 4). Bone accu-mulations in the lowest levels (outside concretionsor in small concretions) are explained as havingaccumulated through attritional mortality, and thesepreserve the remains of the largest animals of thefauna. The tetrapod record of the attritional accu-mulation is nearly 3:1:1 (cynodont:dicynodont:archosauriforms). A signature of this pattern is thedisarticulation and dispersion of skeletons producedby scavengers and/or low-energy hydraulic flows.Accumulations of carcasses in concretions aremostly a by-product of mass mortality and resultedin the preservation of a large quantity of completelyor partially articulated skeletons and, on rare occa-sions, some sorting followed by rapid burial. In thiscase, the accumulation is nearly 20:1:7 (cynodont:

dicynodont:archosauriforms). In the mass mortalityevent, there is a clear bias towards preservation ofindividuals representing smaller-sized to mid-sizedtaxa (e.g. Massetognathus). The underrepresentedlarge taxa (dicynodonts and paracrocodylomorphs)in the concretion level is not only the result of thelower abundance in the fauna, but also that the massmortality agent affected differentially the small- tomedium-sized taxa and large taxa. The differencesbetween attritional and mass mortality assemblagesfrom the Cha~nares Formation include not only a dif-ferent mode of death, but also the intensities in theoccurrence of the taphonomic processes such asweathering exposure, disarticulation and sorting.Thus, the taphonomic characteristics and the con-centrated stratigraphical interval, in which theCha~nares fauna is recorded, permit the accurate pal-aeoecological examination of this tetrapod record asa community and allow for the inference of trophicinteractions between the different taxa.

The Cha~nares tetrapod community

The largest forms of the Cha~nares fauna, having bodymasses estimated in hundreds of kilograms, arerepresented by three species of the dicynodontDinodontosaurus (D. platyceps, D. platygnathus andD. brevirostris) and paracrocodylomorphs (Luperosu-chus and CRILAR 417; Table 3). The medium-sizedforms, with body masses estimated in tens of kilo-grams, include Chanaresuchus, Chiniquodon, Gualosu-chus, Massetognathus and Pseudolagosuchus. Finally,the most diverse group includes the smallest taxa,which probably did not exceed 3 kg, represented byGracilisuchus, Marasuchus, Lagerpeton, Lewisuchus,Probainognathus and Tropidosuchus (Table 3).

The Cha~nares assemblage is clearly dominated byforms in the range of tens of kilograms, mainlytherapsids, representing ~60% of the specimens(Fig. 7). The next most common body size is offorms weighing no more than 3 kg, representing

Table 4. Features of the two taphonomic modes.

Attritional modeMass mortalitymode

Killingmechanism

Natural death bypredation, disease or oldage

Catastrophicvolcanic event

Stratigraphicdistribution

Random in lowest 15 mof the unit

Restricted to lowerlarge concretionlevel

Arealdistribution

Random Extensive

Host-rock Olive grey and light bluishgrey tuffaceous siltstonesor sometimes in smallconcretions

Large concretions

Accumulation Single individual Multi-individual,multi-taxa

Taxon size Complete spectrum Small and mediumCompleteness Low completeness High completenessArticulation Isolated and dispersed/

disarticulated butassociated

Articulated/partiallyarticulated

Weathering 0–1 0, and few 1Scavenging Evidenced by dispersion Not evidenceVoorhiesGroups

VIII VI, and few VIIand VIII

Orientation 3D orientation, entombedwith sediment

No preferredorientation

Burial Rapid Very rapidBreakage Transverse fracture Transverse

fracture

Fig. 7. Relative distribution of the interpreted ecological guildsrecognized for vertebrate fossils from the Cha~nares fauna.


approximately 26% of the individuals, and clearlydominated by archosauriforms. The largest forms(over 100 kg) are the least common (~10% of thetetrapods) and are represented mainly by dic-ynodonts.

In a trophic reconstruction of the environment(Table 5), 55% of the specimens are herbivorous and45% are faunivorous. All the herbivores are theraps-ids, and the most abundant by far is Massetognathus,representing 83.7% of all herbivorous specimens.Probainognathus and Chiniquodon are the mostabundant faunivorous forms. Tarjadia represents theonly inferred piscivorous taxon (0.6% of the fauna).

Studies on extant non-scavenging carnivores haveshown that these animals usually prefer prey smallerthan their own body size, except when organized inhunting groups or in amphibious forms such ascrocodylians (Troost et al. 2008). Considering infor-mation on predator behaviour in modern ecosys-

tems (Troost et al. 2008), three inferences can bemade for the terrestrial carnivore feeding behaviour:(1) larger animals would have fed on smaller ones;(2) similar-sized predators would have preferred thesame sized prey; and (3) predatory animals wouldhave fed on herbivores (predation on carnivores bycarnivores is uncommon).

These modern analogues are helpful when recon-structing the food chains of past ecosystems. Thefirst one supports the use of body size to establishthe predator ranking and the preferred prey of eachfaunivorous form. The second inference highlightsthe problem of comparing body mass estimatesobtained through different equations and corre-sponding to animals with dissimilar anatomical pro-portions. When extinct archosauriforms arecompared to therapsids of similar body mass, theskull size of the archosauriforms is relatively smaller(see reconstructions by Cox 1965; Jenkins 1970; Ro-mer 1972a,b; Parrish et al. 1986; Kemp 2005). Thus,the body size of the proposed prey of a terrestrialarchosauriform should be smaller than that of a syn-apsid with similar body mass. Finally, the thirdinference favours a predator–prey relationshipbetween to faunivorous–herbivorous forms.

Due to the difficult task of assigning a scavenginghabit based solely on fossil data and to enable us toreconstruct the trophic network (see below), we con-sider all faunivorous taxa to be active hunters. Tarj-adia is excluded from this discussion as it isconsidered here as a piscivorous form, based on theanatomy of the mandible of other known dosweliids(Doswellia: Dilkes & Sues 2009).

Small faunivores ate invertebrates and tinyvertebrates, whereas medium-sized faunivores likelyfed preferably on juvenile Massetognathus anddicynodonts. Additionally, the slightly largerChanaresuchus and Pseudolagosuchus also preyed onsub-adult individuals of Massetognathus. Luperosu-chus and the unnamed paracrocodylomorph repre-sent the top predators in the reconstructedcommunity. They certainly preyed on all the othermembers of the assemblage, including fully growndicynodonts and Massetognathus (Table 3). How-ever, predation of relatively large herbivores by med-ium-sized carnivorous forms (e.g. Chiniquodon)hunting in groups cannot be ruled out. Consideringthe abundance of the herbivorous cynodontMasseto-gnathus, it is clear that this taxon represents the mainfood resource in the Cha~nares assemblage. Consider-ing its overwhelming role as main prey, there is noevidence of predatory bone modification on any ofthe manyMassetognathus bones recovered. This find-ing is in contrast to what is expected of most carni-vore–carcass interactions.

Table 5. Abundance and ecological guilds of Cha~nares tetrapodfauna.

TaxaNo ofspecimens Side

Ecologicalrole

Therapsida 277AnomodontiaDicynodontia 30 HerbivoresDicynodont indet 19 >100 kgDinodontosaurus 11 >100 kg

TheriodontiaCynodontia 247Cynodont indet 7Massetognathus 154 >10 kg HerbivoresChiniquodon 29 ~10 kg CarnivoresProbainognathus 57 <3 kg Carnivores/

InsectivoresArchosauriforms 58

Archosauriforms indet. 12Non-archosaurianarchosauriforms

26

Proterochampsida 24Chanaresuchus 12 >10 kg CarnivoresTropidosuchus 10 <3 kg Carnivores/

InsectivoresGualosuchus 2 ~10 kg CarnivoresDoswelida 2 PiscivoresTarjadia 2 >100 kg

Pseudosuchia 9Basal suchians 6 CarnivoresGracilisuchus 6 <3 kgParacrocodylomorpha 3 CarnivoresParacrocodylomorpha

indet.1 >100 kg

Luperosuchus 2 >100 kgOrnithodira 15 Carnivores/

InsectivoresNon-dinosauriformesdinosauromorphs

5

Lagerpeton 5 <3 kgNon-dinosauriandinosauriforms

10

Lewisuchus 1 <3 kgMarasuchus 5 <3 kgPseudolagosuchus 4 ~10 kg


Only a few forms were capable of preying on fullygrown Massetognathus; therefore, a high predationpressure on infant, juvenile and sub-adults isexpected, and this, together with a high reproductiverate, may explain the overwhelming abundance ofMassetognathus bones preserved. Predation pressureis also expected to have affected fully grown dic-ynodonts, but the predicted predator–prey relation-ship does not explain the low observed proportionof dicynodont fossils in the Cha~nares area.

Traversodontid cynodonts and dicynodonts arethe only specialized herbivores from the Cha~naresFormation. The interpretation of the dicynodontDinodontosaurus as feeding on low vegetation, belowor level with their heads (Surkov & Benton 2008),indicates that they likely fed on the same foliage asMassetognathus.

Extant specialized medium- and large-sizedherbivorous mammals usually form herds and arenumerically abundant in their habitats (McNaugh-ton 1986; Estes 1991). There is evidence of gregari-ous behaviour in Permian dicynodonts such asDiictodon (King 1990; Ray & Chinsamy 2003). Morecompelling is the evidence of dicynodont herds inthe Los Rastros Formation (Marsicano et al. 2010)and of gregarious behaviour recently proposed forDinodontosaurus from the Brazilian Santa MariaFormation (de Oliveira Bueno et al. 2011; de Olive-ira Bueno 2012). Considering this subsidiary evi-dence, we propose that the Dinodontosaurus fromCha~nares were also gregarious. However, it is diffi-cult to reconcile this kind of behaviour in Cha~narestaking into account the scarce representation of thetaxon in the fauna (3.3% of the total tetrapods). Onthe contrary, Massetognathus record suggests a gre-garious habit, in particular concretions includingseveral articulated specimens. This behaviour hasbeen reported for some cynodonts that are known tohave lived in burrows (Groenewald et al. 2001;Damiani et al. 2003; Tałanda et al. 2011).

The attritional record shows the relative abun-dance of dicynodonts andMassetognathus to be simi-lar, with slight dominance of the latter, whereas in themass mortality event, dicynodonts have much poorerrepresentation than Massetognathus. No large taxa(i.e. dicynodonts and paracrocodylomorphs) have asyet been registered in the mass mortality record, butthey are represented in the attritional levels. In addi-tion, an ichnological record of large taxa was reportedfrom the immediately overlying levels of the Los Ra-stros Formation (Marsicano et al. 2010). The lowabundance of dicynodonts in the mass mortalityassemblage, when compared to the almost even ratioof Dinodontosaurus/Massetognathus remains in theattritional levels, is remarkable. We leave the final

cause of this open, just highlighting two possibilities:(1) the mass mortality affected these taxa in a differ-ent way, with increased severity on small young and/or medium-sized forms; or (2) dicynodonts were notregular inhabitants in this area of the basin.

Trophic chain in the Cha~nares-Los Rastrosassemblage

The reconstructed Cha~nares tetrapod communitycan be enhanced by the inclusion of fossils from theLos Rastros Formation (Fig. 8). This is justifiedbecause the two formations have a transitional rela-tionship with each other within the same deposi-tional system, the difference between them beinglithostratigraphical (Mancuso & Marsicano 2008;Mancuso & Caselli 2012). Fossils from the Los Ra-stros Formation include plants, insects, fishes andtemnospondyl amphibians as well as fossil tracksrepresenting two sizes of dinosauromorphs andherds of dicynodonts (Marsicano et al. 2007, 2010).

The base of the tetrapod trophic chain is repre-sented by traversodontid cynodonts and dic-ynodonts. Small predators, including basal suchias,ornithodirans, proterochampsids and cynodonts,preyed on invertebrate or small tetrapods. Thepiscivorous role is occupied by doswelliids, whichpreyed on fish from the lake and deltas of the LosRastros Formation. The next carnivorous level isdominated by medium-sized forms which includesome proterochampsids, chiniquuodontids anddinosauromorphs. This guild preyed on relativesmall cynodonts and dicynodonts individuals. Themajor predators in the Cha~nares-Los Rastros trophicstructure are paracrocodylomorphs and large dino-sauromorphs. They may represent the only directpredators of fully grown herbivorous cynodonts anddicynodonts.

Comparative analysis of Ladinian tetrapodcommunities

As mentioned before, there are few Ladinian terres-trial fossil assemblages that can be compared withthe Cha~nares fauna. The closest comparison geo-graphically and taxonomically is the Dinodontosau-rus Assemblage Zone from southern Brazil. Threetherapsid taxa, Chiniquodon, Dinodontosaurus andMassetognathus, are common to both faunas (Langeret al. 2007). Regarding archosauriforms, thesefaunas have records of proterochampsids (Dilkes &Arcucci 2012), but the Brazilian fauna shows lowdiversity of these forms. But even there are taxo-nomic coincidences among members of these fau-nas, there are major differences in the abundance of


specimens in different taxa and in their distributionstratigraphical and areally. The massive abundanceof Massetognathus in Cha~nares can be comparedwith a similar predominance of the dicynodont Din-odontosaurus in the Brazilian fauna, representing61% of occurrences (Azevedo et al. 1990; Schultzet al. 2000). Among archosauriforms, the abundanceof proterochampsids in Cha~nares contrasts with thatof large ‘rauisuchians’ in the Brazilian fauna, where amonotaxic accumulation including nine individualswas recently discovered (Franc�a et al. 2011). Insummary, even though taxonomic similarities are

present between the Cha~nares and BrazilianDinodontosaurus Assemblage Zone faunas, a majordifference between them is the predominance oflarge animals in the Brazilian fauna.

Another proposed Ladinian Gondwanan assem-blage is represented by the fossils from the top of theUpper Omingonde Formation of Namibia (Smith &Swart 2002; Abdala & Smith 2009; Abdala et al.2013). This assemblage is similar to Cha~nares in theclear dominance of traversodontid cynodonts,whereas dicynodont, large archosauriforms and car-nivorous cynodonts are underrepresented.

Fig. 8. Trophic structure inferred from the recovered Middle Triassic Cha~nares community. The greyscale represents taxon abundancelevels in number of specimens. The fossil record from the Los Rastros Formation (plants, insects, fishes, temnospondils and therapsids,dinosauromorphs and archosauromorphs tracks), added to complement the data from Cha~nares, is not sensu greyscale. Number code:(1) Dinodontosaurus; Dicynodont indet.; (2) Massetognathus; (3) Probainognathus; (4) Tropidosuchus; (5) Chiniquodon; (6) Chanaresu-chus, Gualosuchus; (7) Pseudolagosuchus and Los Rastros footprints; (8) Luperosuchus, paracrocodylomorphs indet.; (9) Tarjadia; (10)Lagerpeton, Marasuchus; (11) Gracilisuchus.


Continental Ladinian faunas are also known fromwestern Europe, where the Erfurt Formation has aparticularly important record of temnospondylamphibians, as well as osteoderms of plagiosaurids,small archosaurs, ganoid scales, plant material(Schoch 2006, 2011a,b) and an isolated tooth of atraversodontid cynodont (Hopson & Sues 2006). InRussia, the Bukobay Formation, in the southernCis-Urals and in the Cis-Caspian Depression, isdominated by temnospondyl amphibians and alsoincludes plagiosaurid amphibians, erythrosuchid,rauisuchid archosaurs and dicynodonts (Lucas 1998;Shishkin et al. 2000; Benton et al. 2004). On thebasis of faunal composition, two kinds of Ladiniancontinental faunas can be recognized: (1) easternLaurasian faunas, in which temnospondyls are theclearly dominant tetrapods; and (2) western Gon-dwanan faunas, in which therapsids are dominantassociated with fairly diverse archosauriforms.

Conclusion

Taphonomic analysis of the Cha~nares tetrapod fos-sil assemblage highlights two different modes ofaccumulation, namely attritional and mass mortal-

ity (Table 4). The attritional mortality assemblagepreserves the remains of the largest animals of thefauna and is characterized by a disarticulation andskeleton dispersion pattern produced by low-energyflowing water. The mass mortality assemblage, witha large quantity of complete or partially articulatedskeletons, shows some post-mortem sorting fol-lowed by rapid burial. The differences betweenattritional and mass mortality assemblages are thekilling mechanisms and the duration and intensityof the taphonomic processes that affected the car-casses. The attritional accumulation preserves theremains of the largest animals of the fauna and isassociated with the natural death of individuals bypredation, disease and/or old age. After death, thecarcasess were exposed on the floodplain for a shortperiod of time, during which they were disarticulat-ed and dispersed by scavenging and low-energyflowing water that sorted the remains. In contrast,the fossil associations recorded in the lower concre-tionary level show evidence of a catastrophic event.The mass mortality assemblage, with a large quan-tity of complete or partially articulated skeletons,shows some post-mortem sorting, followed by rapidburial that prevent the carcasses from beingscavenged, trampled or exposed to weathering. The

Fig. 9. Artist’s reconstruction of the Cha~nares environment during the Middle Triassic. Note the herds of Massetognathus and Dinodon-tosaurus in the front and back of the scene, respectively, feeding from the same vegetal resources; archosauriform piscivores in the waterstream; and the volcanic activity with the sky darkened by abundant falling ash. Art by Jorge Fernando Herrman.


volcanic activity in the area is identified as the causeof mass mortality and the origin of the sedimentthat facilitated the rapid entombment of part of thefauna in that level.

The gregarious herbivorous taxa at the base of thetrophic chain of the Cha~nares-Los Rastros assem-blage are the traversodontind cynodont Massetog-nathus and the dicynodont Dinodontosaurus. Lowerpredation levels are characterized by basal suchians,ornithodirans, proterochampsids and cynodontsthat preyed on invertebrates or small and/or youngtetrapods. Medium-sized carnivores are representedby chiniquodontid cynodonts, proterochampsidsand dinosauromorphs, which preyed on sub-adultherbivores. As the mayor predators in the commu-nity, paracrocodylomorphs and large dino-sauromorphs were the major predator in thecommunity, representing the only direct predatorsof fully grown herbivorous taxa. Tarjadia is the onlyproposed piscivorous form (Fig. 9).

In Gondwana, faunas of Ladinian age are bestdocumented in South America, particularly in theSanta Maria Formation in southern Brazil (Dinodon-tosaurus Assemblage Zone) and the Cha~nares For-mation in western Argentina (Romer & Jensen 1966;Rogers et al. 2001; Langer et al. 2007). Althoughthere are shared genera between the Triassic faunasin Argentina and Brazil, notable differences in theabundance of therapsids and diversity of archosauri-forms are recognized. The Cha~nares assemblage isdominated by cynodonts and also shows a highdiversity of archosauriforms, whereas the BrazilianDinodontosaurus Assemblage Zone is dominated bydicynodonts with a low diversity of archosauriforms.Another important contrast is that larger forms seemto be better represented in the Brazilian fauna. Theproposed Ladinian Upper Omingonde Formationassemblage of Namibia shows clear dominance oftraversodontid cynodonts and a low representationof other forms, as occurs in the Cha~nares assem-blage. Finally, we can now recognize two kinds ofLadinian terrestrial faunas: an eastern Laurasianfauna with amphibians as the dominant tetrapods;and a western Gondwanan fauna, where therapsidsare dominant along with a fair diversity of archosau-riforms.

Acknowledgements. – We gratefully acknowledge R. Irmis, R.Smith, an anonymous reviewer, the editors, C. Marsicano, M.Carrano and R. Bobe for the critical reading of the manuscriptand their pertinent comments. C. Kemp and L. Backwell helpedwith language and editorial assistance in the last version of themanuscript. In addition, we especially thank C. Marsicano(UBA-CONICET), A. Caselli (UBA), M. Bouguet (Ianigla-CON-ICET), C. Schultz (UFRGS), V. Krapovickas (UBA-CONICET),M. Becerra (CONICET), F. Garberoglio and Tom�as Heredia fortheir support during the fieldwork. New specimens were pre-

pared by M. Iberlucea. For access to specimens in their care, wethank E. Vaccari (UNLR), L. Fiorelli and D. Gorla (CRILAR), J.Powell (PVL), A. Kramarz (MACN) and F.Jenkins, J. Cundiffand C. Schaff (MCZ). For permits, we thank Ver�onica Vargasand Laura Gach�on (Direcci�on de Patrimonio Arqueol�ogico yPaleontol�ogico, Secretar�ıa de Cultura de La Rioja) and Administ-raci�on de Parques Nacionales. We are deeply indebted to the staffof Parque Nacional Talampaya for their constant assistance inthe field. Field research was supported by the PIP CONICET0535/98 (AA), PIP CONICET 5120 (Guillermo Ottone), UBA-CyT 20020100100728 (Claudia Marsicano), the Jurassic Founda-tion, PICT 32236 and PIP CONICET 0209/10 (ACM), Bi-national Cooperation project between South Africa (DST to B.Rubidge) and Argentina (MINCYT to C. Marsicano). Additionalfinancial support was provided by the Consejo Nacional de In-vestigaciones Cient�ıficas y T�ecnicas (CONICET), NationalResearch Foundation of South Africa (FA). This is JML andLCG’s contribution R-70 of the IDEAN (Instituto de EstudiosAndinos Don Pablo Groeber, UBA-CONICET).

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The Chañares Formation: a window to a Middle Triassic tetrapod community