Tetrapod footprints from the Triassic of Patagonia: reappraisal of the evidence

Nadia S. Domnanovich ¹ and Claudia A. Marsicano ¹

¹ Laboratorio de Paleontología de Vertebrados, Departamento de Ciencias Geológicas, Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, Pabellón II, Ciudad Universitaria, 1428 Buenos Aires, Argentina. nadiadom@gl.fcen.uba.ar

Abstract. To date, the only known Triassic tetrapod footprints in Patagonia are those from the Los Menucos Basin, Río Negro province. The footprints occur at several levels in the Vera Formation in finegrained tuffaceous sandstones interbedded with tuffaceous mudstones that contain a rich " Dicroidium type Flora". A revision of the original material described by Casamiquela and study of undescribed specimens from the same locality are presented herein. Previous assumptions about the large number of tetrapod groups represented by the footprint assemblage are not corroborated by the available evidence. However, this new analysis supports former propositions about the relative abundance and high diversity of small therapsids, probably theriodonts. This footprint assemblage suggests a unique Late Triassic tetrapod fauna in northern Patagonia, in which archosaurs (basal archosaurs and dinosaurs) were not the dominant component as in most known Late Triassic Gondwanan tetrapod faunas. This unusual situation might be either the result of a taphonomic bias or the record of an endemic fauna actually dominated by therapsid taxa.

Resumen. Huellas de Tetrápodos del Triásico de Patagonia: reevaluación de la evidencia . Hasta el presente, las únicas huellas de tetrápodos triásicas conocidas para Patagonia son aquellas de la Cuenca de Los Menucos en la provincia de Río Negro. Las huellas se registran en varios niveles de la Formación Vera en areniscas tobáceas de grano fino intercaladas con pelitas tobáceas que contienen una rica "Flora de Dicroidium ". Una revisión del material original descrito por Casamiquela y el estudio de varios ejemplares inéditos de la misma localidad son discutidos en el presente trabajo. Hipótesis previas que proponían un gran número de grupos de tetrápodos representados por la asociación de huellas no es corroborado por la evidencia disponible en el presente estudio. Sin embargo, este nuevo análisis sí respalda propuestas anteriores que sostenían la relativa abundancia y alta diversidad de pequeños terápsidos, probablemente teriodontes, en la asociación. Las huellas sugieren una fauna de tetrápodos triásica poco común en el norte de Patagonia en la cual los arcosaurios (arcosaurios basales y dinosaurios) nos son el componente dominante como ocurre en la mayoría de las asociaciones de tetrápodos del Triásico Tardío de Gondwana. Esta situación inusual podría ser tanto el resultado de un bias tafonómico o el registro de una fauna endémica en efecto dominada por terápsidos.

Key words. Late Triassic; Argentina; Patagonia; Tetrapod footprints; Therapsids.
Palabras clave. Triásico Tardío; Argentina; Patagonia; Huellas de tetrápodos; Terápsidos.

Introduction

In southernmost South America, Triassic tetrapods are only known from two Late Triassic sequences that crop out in Patagonia, Argentina. One corresponds to the El Tranquilo Basin in southern Patagonia (Santa Cruz province), where several prosauropod skeletons and a unique specimen of an ornithischian dinosaur were described from the uppermost levels of its infilling (Casamiquela, 1980; Báez and Marsicano, 2001). The second record occurs in the Los Menucos Basin, located farther north, in the province of Río Negro. In this case, tetrapods are only represented by the ichnites of relatively smallsized animals (Casamiquela, 1964, 1975; Domnanovich, 2003). The tetrapod Patagonian record, thus, contrasts strikingly with the diverse Triassic faunas known from west-central Argentina, such as those from the Ischigualasto-Villa Unión Basin (e.g. Bonaparte, 1973, 1978, 1997; Báez et al ., 1993; Lucas, 1998; Arcucci et al ., 2004), particularly if only the skeletal evidence is considered.
The footprints and trackways from Los Menucos are recorded at several horizons in the volcaniclastic Vera Formation and were originally studied by Casamiquela (1964, 1975). This author described many different ichnotaxa that were referred to several groups of tetrapods (Casamiquela, 1964, 1975). Therefore, he suggested the presence in the ichnocoenosis of amphibians, "mammal-like reptiles", mammals, "thecodontians", lizards, chelonians and procolophonids, among others (Casamiquela, 1964, 1975). Subsequently, Casamiquela´s original ichnotaxa were reanalyzed and mostly attributed to therapsid trackmakers (Leonardi and Henrique de Oliveira, 1990; Leonardi, 1994).
The aim of this paper is to restudy the Los Menucos ichnites, including all the original material described by Casamiquela, plus many undescribed specimens from the same locality, and to evaluate the possible trackmakers in a phylogenetic context. It is noteworthy that at present several studied specimens are in the slabs that pave the sidewalks along the shoreline of Nahuel Huapi Lake in the city of Bariloche, in the province of Río Negro (Domnanovich, 2003). In addition, some uncatalogued slabs are also housed in the Museum of Ingeniero Jacobacci town, in the same province (Domnanovich, 2003).

Institutional abbreviations. MLP: Facultad de Ciencias Naturales y Museo de La Plata, División Paleontología de Vertebrados, Buenos Aires, Argentina; CICRN: Centro de Investigaciones Científicas de Río Negro, Río Negro, Argentina.

Geological setting

The Triassic Los Menucos Basin is located in northern Patagonia, in the central region of the Río Negro province (figure 1). Its infilling consists of 150 m of nonmarine volcaniclastic rocks deposited by ephemeral river systems under explosive volcanic activity and developed over a local low gradient volcanic relief (Llambías, 1999; Kokogian et al ., 2001; Labudía and Bjerg, 2001). The volcanism in the region during the Late Triassic is related to trans-arc extensional phenomena (Franzese and Spalletti, 2001; Macdonald et al ., 2003) that produced the development of volcanic arcs on the active western margin of the continent and an important outpouring of silicic extensional volcanism (Macdonald et al ., 2003).

Figure 1. Location map of the Los Menucos area (modified from Kokogian et al ., 2001) / mapa del área de Los Menucos (modificado de Kokogian et al ., 2001).

The Triassic sequence (Los Menucos Group) is well exposed near the town of Los Menucos, approximately 40° 53' S - 63° 11' W; it unconformably overlies sedimentary and igneous paleozoic rocks and it is covered by Cretaceous sedimentary rocks and Tertiary volcanics (Labudía and Bjerg, 2001). The Triassic succession is lithostratigraphically divided into two units (figure 2), the Vera Formation and the overlying Sierra Colorada Formation (Stipanicic et al ., 1968; Labudía et al ., 1995; Labudía and Bjerg, 2001). The Vera Formation consists of tuffaceous sandstones and mudstones interfingered with volcanic rocks, whereas the Sierra Colorada Formation is exclusively rhyolitic (Stipanicic et al ., 1968; Pesce, 1974; Rosenman, 1979; Labudía and Bjerg, 2001).

Figure 2. Generalized lithostratigraphic section of the Vera Formation, Los Menucos Group, showing the track-bearing levels (modified from Labudía et al ., 1995) / sección litoestratigráfica generalizada de la Formación Vera, Grupo Los Menucos, mostrando los niveles portadores de huellas (modificado de Labudía et al ., 1995).

As mentioned above, the footprints are recorded in the Vera unit from several levels of fine-grained tuffaceous pale yellowish brown and light brown sandstones interbedded with brown grayish mudstones bearing a rich " Dicroidium -type Flora" (Casamiquela, 1964; Stipanicic et al ., 1968; Stipanicic and Methol, 1972; Pesce, 1974; Artabe, 1985, 1986; Labudía et al ., 1995; Labudía and Bjerg, 2001). The sandstones containing the footprints are quarried for slabs; all studied material came from two quarries, the "Cantera Vieja" (= "La Vieja" sensu Leonardi and Henrique de Oliveira, 1990; = "Cantera Tscherig" sensu Leonardi and Henrique de Oliveira, 1990; Leonardi, 1994) and the "Cantera Nueva" (= "La
Cantera Nueva" sensu Leonardi and Henrique de Oliveira, 1990; = "La Nueva" sensu Leonardi and Henrique de Oliveira, 1990; Leonardi, 1994). These quarries are located in the proximity of the Cerro Las Lajas, west of the town of Los Menucos, and they are still under excavation.
Recent radiometric analyses in volcanics from the Sierra Colorada Formation (222 ± 2 Ma, Rapela et al ., 1996) place this sequence in the middle Carnian ( sensu Gradstein et al ., 1995), thus giving a minimum Carnian age to the Vera beds and to the track-bearing levels. This is in agreement with the Late Triassic age previously assigned to the Vera unit based on its stratigraphic relationships, radiometric data and floral content (Stipanicic et al ., 1968; Stipanicic and Methol, 1972; Labudía et al ., 1995).

Description of the footprints and trackways

Morphotype A (" Shimmelia chirotheroides ")

Material. MLP 60-XI-31-1, a slab with a cast of a right pes and a right? manus impression (figure 3.A); MLP 60-XI-31-2, a slab with, apparently, a cast of a left pes impression; three isolated uncatalogued specimens (figure 3.B-C) in the sidewalks of the shoreline of the Nahuel Huapi Lake (Bariloche, Río Negro province).

Figure 3. Morphotype A " Shimmelia chirotheroides ". A, MLP 60-XI-31-1 (holotype), cast of a pes (right) and a manus impression, scale bar = 10 cm; B, natural mould of a manus-pes set and C, natural mould of a right pes impression, both specimens from the sidewalks of Bariloche, scale bar = 5 cm / morfotipo A " Shimmelia chirotheroides ". A, MLP 60-XI-31-1 (holotipo), calco de un pie (derecho) y la impresión de una mano, escala = 10 cm; B, molde de un par mano-pie y C, molde de un pie derecho, ambos especímenes de la avenida costanera de Bariloche, escala = 5 cm.

Locality and horizon. "Cantera Vieja", Cerro Las Lajas, Los Menucos (aprox. 40° 53' S - 63° 11' W), Río Negro province, Argentina; Vera Formation (Los Menucos Group), Late Triassic (Labudía et al ., 2002).
Description. The slab MLP 60-XI-31-1 is a pale yellowish brown fine-grained sandstone which preserved two poorly defined footprints which are probably under prints. They correspond to a pes and a manus and, due to their relative size and position, they do not belong to the same pair. The pes is pentadactyl with digits I-IV anteriorly directed and digit V partially everted. Digit III is longer than II and IV, which are similar in length, and digit I is the shortest of the series; none of them presents claw marks. The manus track is tetradactyl with the digits of similar length and printed relatively spread out from each other.
Discussion. The specimens MLP 60-XI-31-1 (holotype) and MLP 60-XI-31-2 (paratype) were assigned to a new ichnotaxon, " Shimmelia chirotheroides ", by Casamiquela (1964, pl. XV, fig. 2 and pl. XVI, fig. 1) who also suggested that they could belong to a primitive saurischian or derived prosauropod dinosaur. Subsequently, he related them to "pseudosuchian" archosaurs (Casamiquela, 1975). These tracks were also mentioned in the literature by other authors who also related them to "pseudosuchian" archosaurs (Leonardi and Henrique de Oliveira, 1990; Leonardi, 1994). "Chirotheroid" footprints have often been regarded as crurotarsal archosaur prints (Parrish, 1989; Haubold, 1983, 1986; Demathieu and Wright, 1988; Courel and Demathieu, 1995; Haubold and Klein, 2000; Lockley and Meyer, 2000), although there is no evidence preserved in isolated "chirotheroid" tracks that might be used to distinguish among different crurotarsal archosaur taxa (see Sereno, 1991; Parrish, 1993).
As mentioned above, the pes and manus impressions in the holotype specimen do not belong to the same pair. The manus is comparatively large in relation to the pes and, moreover, its position (posteromedially to the pes) is unusual for this type of track in which the manus generally is imprinted in front of the pes (Peabody, 1955; Sarjeant, 1975; Lockley and Hunt, 1995). The isolated track in MLP 60-XI-31-2 and the footprints from Bariloche (see figures 3.B-C) although badly preserved, are considered here to represent the same morphotype.

Morphotype B (" Ingenierichnus sierrai ")

Material. MLP 60-XI-31-3, one slab, which preserves a cast of a trackway of 12 footprints with marks of body and tail drag (figure 4.A); two uncatalogued slabs with a trackway showing marks of body and tail drags (figure 4.B) housed in the Museum of Ingeniero Jacobacci (Río Negro province).

Figure 4. Morphotype B " Ingenierichnus sierrai ". A, MLP 60-XI-31-3 (holotype), cast of a trackway of twelve footprints with marks of body and tail drag; B, cast of a trackway of the same morphotype housed in the collections of the Museum of Ingeniero Jacobacci. Scale bars = 10 cm / morfotipo B " Ingenierichnus sierrai ". A, MLP 60-XI-31-3 (holotipo), calco de una rastrillada de doce huellas con marcas de arrastre de cuerpo y cola; B, calco de una rastrillada asignada al mismo morfotipo de las colecciones del Museo de Ingeniero Jacobacci. Escalas = 10 cm.

Locality and horizon. "Cantera Vieja", Cerro Las Lajas, Los Menucos (aprox. 40° 53' S - 63° 11' W), Río Negro province, Argentina; Vera Formation (Los Menucos Group), Late Triassic (Labudía et al ., 2002).
Description. The slab MLP 60-XI-31-3 is a pale yellowish brown fine-grained sandstone with a smoothly preserved elongated trackway of a quadruped animal crossing its surface. It consists of twelve relatively small footprint impressions with a body drag throughout its length. Each impression shows a circular to oval outline and probably corresponds to the superposition of a manus and pes print. Thus, the more circular impressions might be interpreted as a total superposition and the elongate ones to a partial superposition (see Leonardi, 1987). In the midline of the trackway there is a broad, "sine-wave-shaped dragmark" (see Farlow and Pianka, 2000) as wide as the intermanus distance. Along the trackway, the body drag partially razes the footprints thus suggesting, together with its wideness, that it might correspond to both the belly and tail drag (see Farlow and Pianka, 2000).
The trackway pattern shows a regular progression of the trackmaker, with short steps, and small footprints relatively close to the midline of trackway. Moreover, the moderately low pace angulation (approximately 106°, table 1), and the shape of the body/tail drag indicate an elongated animal with a sprawling gait and having part of its body in continuous contact with the substrate surface during progression (see Boggs, 1987).

Table 1. Measures of Morphotype B " Ingenierichnus sierrai ", MLP 60-XI-31-3. Parameters according to Leonardi (1987) / medidas del Morfotipo B " Ingenierichnus sierrai ", MLP 60-XI-31-3. Parámetros de acuerdo a Leonardi (1987).

Discussion. The specimen MLP 60-XI-31-3 (holotype) was originally included in the new ichnotaxon " Ingenierichnus sierrai " by Casamiquela (1964, pl. XVII, fig. 4) who allied it to several different groups of tetrapods such as amphibians, "lizard-like" reptiles, crocodiles, "thecodonts", and theropod dinosaurs (Casamiquela, 1964, 1975). In the same publication, Casamiquela figured (1964, pl. XIX, fig. 3) a slab with a trackway of " I . sierrai " from the sidewalks of Bariloche which is at present in the collections of the Museum of Ingeniero Jacobacci. Subsequently, Leonardi recognized the holotype specimen as produced by a "lizard-like" reptile (Leonardi, 1994). The trackway probably corresponds to an undertrackway due to the lack of any detail in the footprints beyond their general outline.
The trackway was made by a quadruped animal with a sprawling gait, according to the low pace angulation. The shape of the body drag and the relatively long stride suggest an animal with short limbs and relatively long body, thus not able to maintain the belly away from the substrate. Trackways with a similar pattern have been described for extant salamanders (Peabody, 1959; Brand, 1996), Paleozoic trackways assigned to amphibian trackmakers (Haubold, 1971; Scarboro and Tucker, 1995; Clack, 1997) and extant lizards (Peabody, 1948; Farlow and Pianka, 2000). Nevertheless, the lack of information about the morphology of the manus and pedes in the specimen from Patagonia prevents discrimination between a putative temnospondyl or a lepidosauriform trackmaker. Temnospondyl amphibians are well-known in the Upper Triassic of Argentina although lepidosauriforms are as yet unknown from the Triassic of Gondwana.

Morphotype C (" Calibarichnus ayestarani ")

Material. MLP 60-XI-31-4, one slab with an isolated manus-pes set preserved as a cast (figure 5.A); MLP 66-XI-15-2, one slab with a cast of manus-pes set (figure 5.B); MLP 93-XII-13-2, one slab with a cast of two superimposed footprint impressions (figure 5.C), one uncatalogued specimen with a manus-pes set (figure 5.E) housed in the Museum of Ingeniero Jacobacci (Río Negro province), a manus-pes set (figure 5.D) in the sidewalks along the shoreline of Nahuel Huapi Lake (Bariloche, Río Negro province).

Figure 5. Morphotype C " Calibarichnus ayestarani ". A, MLP 60-XI-31-4 (holotype), a cast of a manus-pes set (manus above, pes below), p1 and p2 pad impressions, scale bar = 1 cm; B, MLP 66-XI-15-2, a cast of a manus-pes set; C, MLP 93-XII-13-2, a cast of two superimposed footprint impressions; D, cast of a manus-pes set from the sidewalks of Bariloche; E, natural mould of a manus-pes set housed in the collections of the Museum of Ingeniero Jacobacci. B-E scale bars = 2 cm / morfotipo C " Calibarichnus ayestarani ". A, MLP 60-XI-31-4 (holotipo), calco de un par mano-pie (mano arriba, pie abajo), p1 y p2 impresiones de almohadillas, escala = 1 cm; B, MLP 66-XI-15-2, calco de un par mano-pie; C, MLP 93-XII-13-2, calco de dos huellas superpuestas; D, calco de un par mano-pie de la avenida costanera de Bariloche; E, molde de un par mano-pie de las colecciones del Museo de Ingeniero Jacobacci. B-E escala = 2 cm.

Locality and horizon. "Cantera Vieja", Cerro Las Lajas, Los Menucos (aprox. 40° 53' S - 63° 11' W), Río Negro province, Argentina; Vera Formation (Los Menucos Group), Late Triassic (Labudía et al ., 2002).
Description. All analyzed material (MLP 60-XI-31-4, MLP 66-XI-15-2, MLP 93-XII-13-2) is preserved on slabs of pale yellowish brown fine-grained sandstones and they are very well impressed. All the tracks correspond to quadrupedal, semi-plantigrade, near-symmetrical, and pentadactyl animals with marked homopody. Each pes and manus impression shows two well defined pads (p1 and p2 in figure 5.A), one located just behind the digit impressions and other, anteroposteriorly elongated, behind the former; this latter pad probably represents a heel impression. The pedes impressions are somewhat longer than wide, whereas the manus prints are slightly wider than long. The manus are imprinted in front of the pedes and pedes digit impressions are anteriorly directed, subequal in size, similar in shape, and with the most external digits (I and V) slightly separated from the others (II, III, IV). All the digits have triangular and broad-based claw marks.
Discussion. The specimen in MLP 60-XI-31-4 is the holotype of " Calibarichnus ayestarani ", Casamiquela, 1964 (pl. XVII, fig. 2) which was attributed to several tetrapod groups such as Chelonia (Casamiquela, 1964) and therapsids (Casamiquela, 1964, 1975; Haubold, 1971; Leonardi, 1994). The tracks in MLP 66-XI-15-2 were formerly assigned to a different ichnotaxon (" Gallegosichnus garridoi ") by Leonardi (1994) which is also present in the Los Menucos assemblage (see below Morphotype F). Nevertheless, the general shape of the prints and the presence of well developed claw marks in MLP 66-XI-15-2 (figure 5.B) are here considered to include it in the Morphotype C group. The undescribed material in MLP 93-XII-13-2 (figure 5.C), in the uncatalogued slab of the collections of the Museum of Ingeniero Jacobacci (figure 5.E) and in the sidewalks of Bariloche (figure 5.D) are here considered indistinguishable from the holotype specimen.
The morphology of the prints (near-symmetrical, marked homopody and pentadactyly) relates the trackmaker to non-mammalian therapsids ( e.g. Haubold, 1971; Olsen and Galton, 1984; Leonardi and Henrique de Oliveira, 1990; Hunt et al ., 1993; Leonardi, 1994; Retallack, 1996; Lockley and Meyer, 2000). Within this clade ( sensu Sidor and Hopson, 1998; Rubidge and Sidor, 2001), the presence of nearly equal sized toes indicates a relatively derived form with an erect gait (Hopson, 1995; Peters, 2000). Moreover, the presence of a well developed heel impression (calcaneal heel sensu Hopson, 1995) and the Late Triassic age of the track-bearing levels suggest that the trackmaker might be allied with the eutheriodontian group (see Jenkins, 1971; Hopson, 1994, 1995).

Morphotype D (" Rogerbaletichnus aguilerai ")

Material. MLP 60-XI-31-5, one slab containing a natural mould of a trackway with four manus-pes sets, two complete and two incomplete impressions with evidence of limb-drags (figure 6) one slab, housed in the Museum of Ingeniero Jacobacci (Río Negro Province), containing a natural mould of a trackway with four manus-pes sets with evidence of limb-drags.

Figure 6. Morphotype D " Rogerbaletichnus aguilerai ". A and A´ , holotype (MLP 60-XI-31-5), a trackway of four manus-pes sets with limb-drags as a natural mould; m, manus and p, pes. Scale bar = 10 cm / morfotipo D " Rogerbaletichnus aguilerai ". A y A ´, holotipo (MLP 60-XI-31-5), una rastrillada de cuatro pares mano-pie como molde con marcas de arrastre de miembros; m, mano y p, pie. Escala = 10 cm.

Locality and horizon. "Cantera Nueva", Cerro Las Lajas, Los Menucos (aprox. 40° 53' S - 63° 11' W), Río Negro province, Argentina; Vera Formation (Los Menucos Group), Late Triassic (Labudía et al ., 2002).
Description. The trackway is on the surface of a light brown very-fined sandstone preserved as a natural mould and faint small symmetrical ripple marks are superimposed on the footprints. The material indicates the presence of a quadrupedal plantigrade animal that drags the limbs during progression. The trackway consists of four manus-pes sets where the pedes prints are larger than those of the manus (table 2) and partially overprint the posterior half of the former. The manus impressions are slightly rotated inwards and are, apparently, tetradactyl with the digit impressions anteriorly directed. The digit impressions are similar in shape and size, with the first inner digit shorter than, and somewhat separated from, the others (II, III and IV). The pedes impressions are anteriorly directed and the digit impressions are subequal in size although they are poorly preserved due to partial superposition over the manus prints. Arcshaped limb drags are present along the trackway, produced by the forelimbs (figure 6). The limb drag marks show striations that run parallel to the margins. These may represent toe drag traces.

Table 2. Measures of Morphotype D " Rogerbaletichnus aguilerai ", MLP 60-XI-31-5, and Morphotype E " Palaciosichnus zettii ", MLP 60-XI-31-6. Parameters according to Leonardi (1987) / medidas del Morfotipo D " Rogerbaletichnus aguilerai ", MLP 60-XI-31-5, y Morfotipo E " Palaciosichnus zettii ", MLP 60-XI-31-6. Parámetros de acuerdo a Leonardi (1987).

The trackway starts with an incomplete left manus print followed by three alternating complete manus-pes sets and ends with an incomplete left pes print. The trackway pattern is broad (low pace angulation, see table 2) and shows short steps suggesting a trackmaker with a relatively short body. Also, the pattern of the limb drags, in an open laterally convex arc, indicates an animal that had its elbows-out, at least for the forelimbs, and laterally flexed the vertebral column during progression (see Smith, 1993).
Discussion. The specimen MLP 60-XI-31-5 (holotype) was originally described by Casamiquela (1964, pl. XVII, fig. 3) as a new ichnotaxon, " Rogerbaletichnus aguilerai ", and which he related to Chelonia (Casamiquela, 1964). In the original publication, Casamiquela assigned an uncatalog specimen as paratype (1964, pl. XIV, figs. 5 and 6) which was part of the same trackway of the holotype (Casamiquela, 1964). This specimen is at present housed in the collections of the Museum of Ingeniero Jacobacci. The trackmaker of " R. aguilerai " was interpreted as a possible cynodont (Casamiquela, 1975) or anomodont therapsid (Leonardi and Henrique de Oliveira, 1990; Leonardi, 1994).
The trackway denotes the progression of a quadrupedal animal with a sprawling gait evidenced by the relative breadth of the trackway and the shape of the limb drag traces. Trackways that show a similar degree of drag to the limbs as that analyzed above are, hitherto now, known for South America. In general, the described trackways frequently preserve only isolated toe dragmarks and they were mentioned for extant amphibians (Peabody, 1948, 1959), paleozoic trackways attributed to amphibians and basal amniotes (Haubold, 1971; Demathieu et al ., 1992; Haubold et al ., 1995), and chelonians (Ellenberger, 1974; Bernier et al ., 1982; Wright and Lockley, 2001). According to the Late Triassic age of the material, temnospondyl amphibians and turtles could be proposed as possible trackmakers. However, the reduction of the manus digits in the specimen to four, if it is not a preservational effect (Peabody, 1959; Brand, 1979, 1996; McKeever and Haubold, 1996), cannot be used as a character to distinguish between these tetrapod groups. Even though temnospondyls have a four digit manus, the same condition has been also described for Triassic turtles such as Proterochersis from west-central Argentina (Rougier et al ., 1995). Another possibility is that the trackmaker may correspond to a Late Triassic non-mammalian therapsid as was previously proposed (Casamiquela, 1964; Leonardi and Henrique de Oliveira, 1990; Leonardi, 1994) although, the lack of details in the trackway does not allow us to confirm the attribution of its trackmaker to this group of tetrapods. In any case, the trackway pattern and the apparent sinuous progression strongly suggests a quite primitive progression of its trackmaker.

Morphotype E (" Palaciosichnus zettii ")

Material. MLP 60-XI-31-6 (figure 7.A), a slab with a natural mould of a trackway of six steps (three manus-pes sets), MLP 60-XI-31-2, a slab with a cast of a trackway of five footprints (two manus-pes sets and one isolated print) all preserved as underprints; several uncatalogued slabs with trackways (figure 7.B) housed in the Museum of Ingeniero Jacobacci (Río Negro province) preserved as a natural mould; six trackways in the slabs of the sidewalks along the shoreline of Nahuel Huapi Lake (Bariloche, Río Negro province).

Figure 7. Morphotype E " Palaciosichnus zettii ". A and A´ , holotype (MLP 60-XI-31-6); A, a natural mould of a trackway of three manuspes sets, scale bar= 10 cm; A´, detail of one of the manus-pes set of the same trackway, scale bar = 1 cm; B, a natural mould of a trackway housed in the collections of the Museum of Ingeniero Jacobacci, scale bar= 10 cm / morfotipo E " Palaciosichnus zettii ". A y A´, holotipo (MLP 60-XI-31-6); A, molde de una rastrillada de tres pares mano-pie, escala = 10 cm; A, detalle de un par mano-pie de la misma rastrillada, escala = 1 cm; B, molde de una rastrillada de las colecciones del Museo de Ingeniero Jacobacci, escala = 10 cm.

Locality and horizon. "Cantera Vieja", Cerro Las Lajas, Los Menucos (aprox. 40° 53' S - 63° 11' W), Río Negro province, Argentina; Vera Formation (Los Menucos Group), Late Triassic (Labudía et al ., 2002).
Description. The specimens are preserved on slabs of pale yellowish brown fine-grained sandstones. The MLP 60-XI-31-6 consists of a well preserved trackway of three manus-pes sets starting with a right manus-pes pair. It corresponds to a quadrupedal, pentadactyl, semi-plantigrade animal. All the tracks are very similar (homopody), with the manus prints a little larger than those of the pedes. In each set the manus is imprinted in front of the pes, and is slightly rotated outwards, whereas the pedes are anteriorly directed. All the footprints have several small rounded palm/sole pad impressions and five very short subparallel toes. In all the prints, digit III is the longest, II and IV are of similar length and digits I and V are the shortest of the series.
The size of the footprints relative to the stride length and the narrowness of the trackway with the slightly laterally placed position of the manus suggest an animal with a parasagittal gait, holding the elbows beneath the body during progression (table 2).
Discussion. The specimen was originally described as the new ichnotaxon " Palaciosichnus zettii " (Casamiquela, 1964, pl. XVIII) and related to different groups of therapsids (Casamiquela, 1964, 1975; Haubold, 1971, Leonardi, 1994). In the original publication, Casamiquela figures several undescribed slabs from the sidewalks of Bariloche (Casamiquela, 1964 pl. XX, XXI and XXII), apparently still in place, which are here considered to represent the same morphotype. Another undescribed specimen is housed in the Museum of Ingeniero Jacobacci (figure 7.B).
The trackway of MLP 60-XI-31-6 shows a quadrupedal animal with homopody and an upright posture that probably relate it to non-mammalian therapsids (e.g. Haubold, 1971; Olsen and Galton, 1984; Hunt et al ., 1993; Retallack, 1996; Lockley and Meyer, 2000). According to recent phylogenetic hypotheses of therapsid relationships (Sidor and Hopson, 1998; Rubidge and Sidor, 2001), the presence of nearly equal-sized toes and the upright posture of the limbs in the trackmaker suggest the presence of a derived therapsid (Jenkins, 1971; Hopson, 1994, 1995; Pough et al ., 1999; Peters, 2000; Blob, 2001). The trackway pattern also suggests that the trackmaker was running during the imprinting of the trackway and the speed was estimated using Alexander's formula (Alexander, 1976; see table 2). The resulting speed is relatively high (4.35 m/s) and the stride length is 2.7 times the estimated trackmaker hip height (relative stride length) thus indicating a trotting/running gait for the trackmaker (Alexander, 1976; Thulborn, 1989, 1990; McKeever, 1994). This result was based on the assumption that h (hip height) was about four times the pes length (mean estimated for erect reptiles sensu Thulborn, 1989, 1990). If calculation of hip height ( h ) is considered less than four times the pedes length (see McKeever, 1994), judging from reconstructions of Triassic cynodonts (e.g. Jenkins, 1971; Benton, 2000), the calculated values indicate an even faster gait for the trackmaker. Therefore, the trackway suggests the presence of quite a small animal that could have developed a derived, fast gait such as galloping. As no synapomorphies are preserved in Late Triassic eutheriodontian footprints that might discriminate between non-mamalian eutheriodonts and mammals as possible trackmakers, mammalian affinities cannot be rejected for the " Palaciosichnus " trackmaker.
The MLP 60-XI-31-2 slab contains one isolated track (see above, discussion of Morphotype A) and a poorly preserved small trackway of five steps. This small trackway is equivalent to that discussed above, and also shows an animal that developed a fast running gait.

Morphotype F (" Gallegosichnus garridoi ")

Material. MLP 60-XI-31-7, a slab containing a trampled surface, including seven footprints as casts (figure 8.A); MLP 60-XI-31-8, a slab which preserves a cast of a trackway of six steps plus two isolated footprints and a tail drag (figure 8.B); MLP 60-XI-31-9, a slab with a cast of part of a trackway of two (left- right) manus-pes sets and an isolated left set; MLP 66-XI-15-1, a slab with a cast of a left manus-pes set and, apparently, an incomplete right set (figure 8.C); MLP 66-XI-15-3, a slab containing a cast of a complete left manus-pes set? (figure 8.D); MLP 93-XII-13-1, a slab which preserves a cast of a probable isolated right pes footprint (figure 8.E); MLP 93-XII-13-3 ( a and b ), a large slab with a natural mould of two trackways, one ( a ) of thirteen steps and the other ( b ) of twelve steps (figure 9); four uncatalogued specimens housed in the Museum of Ingeniero Jacobacci town (Río Negro province) (figures 8.F-G); several slabs of uncatalogued specimens in the sidewalks of the shoreline of the Nahuel Huapi Lake (Bariloche, Río Negro province) (figure 8.H).

Figure 8. Morphotype F " Gallegosichnus garridoi ". A, holotype (MLP 60-XI-31-7), a cast of a trampling surface, including seven footprints, and a detail of one of the footprint (upper left corner) showing the p1 and p2 pad impressions, scale bar = 5 cm; B, paratype (MLP 60-XI-31-8), a cast of a trackway of six steps with a tail drag plus two isolated footprints, scale bar = 5 cm; C, MLP 66-XI-15-1, a cast of a left manus-pes set and, apparently, an incomplete right set; D, MLP 66-XI-15-3, a cast of a complete left manus-pes set?; E, MLP 93-XII-13-1, a cast of a probable isolated right pes footprint, scale bar = 2 cm; F, a cast of a manus-pes set housed in the collections of the Museum of Ingeniero Jacobacci, scale bar = 2 cm; G, a cast of a trackway of the same morphotype housed in the collections of the Museum of Ingeniero Jacobacci; H, a cast of two manus-pes set from the sidewalks of Bariloche, scale bar = 5 cm / morfotipo F " Gallegosichnus garridoi ". A, holotipo (MLP 60-XI-31-7), calco de una superficie de pisoteo, que incluye siete huellas, y detalle de una de las huellas (ángulo superior izquierdo) donde se muestran las impresiones de las almohadillas p1 y p2, escala = 5 cm; B, paratipo (MLP 60-XI-31-8), calco de una rastrillada de seis pasos con arrastre de cola y otras dos huellas, escala = 5 cm; C, MLP 66-XI-15-1, calco de un par mano-pie izquierdo y, aparentemente, un par derecho incompleto; D, MLP 66-XI-15-3, calco de un par ? mano-pie izquierdo; E, MLP 93-XII-13-1, calco de una huella aislada de un probable pie derecho, escala = 2 cm; F, calco de un par mano-pie de las colecciones del Museo de Ingeniero Jacobacci, escala = 2 cm; G, calco de una rastrillada del mismo morfotipo de las colecciones del Museo de Ingeniero Jacobacci; H, calco de dos pares mano-pie de la avenida costanera de Bariloche, escala = 5 cm.

Locality and horizon. "Cantera Vieja", Cerro Las Lajas, Los Menucos (aprox. 40° 53' S - 63° 11' W), Río Negro province, Argentina; Vera Formation (Los Menucos Group), Late Triassic (Labudía et al ., 2002).
Description. All material previously mentioned is preserved on slabs of fine-grained pale yellowish brown sandstones and, in general, they are very well impressed. They correspond to quadrupedal, plantigrade animals with two well defined pads in both the manus and pedes prints (p1 and p2 in figure 8.A). One pad is located just behind the digits and another anteroposteriorly elongated (probably a heel impression) posterior to the former. The manus and pedes tracks are pentadactyl, and quite similar in size (homopody), although the pedes tracks are slightly longer than wide whereas the manus tracks are somewhat wider than long. The manus tracks are imprinted in front of the pedes tracks and are rather rotated inwards; the digits are anteriorly directed, similar in shape, with digit IV slightly longer than the others and the most lateral digits (I and V respectively) slightly separated from the others (II, III, IV). The pedes impressions are anteriorly directed with the digit impressions subequal in size with digit III somewhat longer than the others and digits I and V shorter than all remaining digits. In the manus prints the digit imprints are more spread out than in the pedes prints. None of the studied footprints shows claw marks.
The trackway pattern visible in slabs MLP 60-XI-31-8 (figure 8.B) and a and b in MLP 93-XII-13-3 (figure 9), indicate a regular progression of the trackmakers, with short steps (measurements summarized in table 3), and the manus and pedes prints rather close to the midline of trackway (relatively narrow trackway). Moreover, the pace angulation (between 104° and 124°, table 3) indicates an animal with a sprawling to semi-erect gait. In the specimen MLP 60-XI-31-8, a narrow tail dragmark is preserved along the midline of the trackway (figure 8.B).

Figure 9. Morphotype F " Gallegosichnus garridoi ". A and A´ , MLP 93-XII-13-3; A, a natural mould of three trackways ( a, b and c ), two of them correspond to the Morphotype F ( a and b ). Scale bar = 10 cm / morfotipo F " Gallegosichnus garridoi ". A y A´, MLP 93-XII-13-3; A, molde de tres rastrilladas ( a, b y c ), dos de ellas corresponden al Morfotipo F ( a y b ). Escala = 10 cm.

Table 3. Measures of Morphotype F " Gallegosichnus garridoi ", MLP 93-XII-13-3 and MLP 60-XI-31-8. Parameters according to Leonardi (1987) / medidas del Morfotipo F " Gallegosichnus garridoi ", MLP 93-XII-13-3 y MLP 60-XI-31-8. Parámetros de acuerdo a Leonardi (1987).

Discussion. The ichnites in MLP 60-XI-31-7 (holotype), MLP 60-XI-31-8 (paratype) and in MLP 60-XI-31-9 (paratype) were originally included in " Gallegosichnus garridoi " (Casamiquela, 1964, pl. XV, fig. 1 and pl. XVI, fig. 2) and related to several different groups of synapsids (Casamiquela, 1964; Haubold, 1971; Casamiquela, 1975; Leonardi, 1994). The undescribed slab MLP 93-XII-13-3 preserves three trackways, two of them ( a and b ) are here recognized as the same type as the material described above. The third trackway ( c ) was originally also assigned by Leonardi to " G . garridoi " (Leonardi and Henrique de Oliveira, 1990; Leonardi, 1994), although the manus prints are rotated outwards in contrast to the trackways a and b where the manus prints are rotated inwards. This condition suggests a different position of the fore limbs during progression in the trackmaker of trackway c , with the manus rotated outwards and thus, possibly, the elbows held underneath the body (Jenkins, 1971). The undescribed specimens from Bariloche and Jacobacci were previously partially figured by Casamiquela (Casamiquela, 1964 pl. XIX, figs. 1 and 2) and Leonardi (Leonardi, 1994 pl. XVI, fig. 1).
All the specimens described above denote an animal with a quadrupedal progression, homopody and pentadactyly; this morphology relates them to nonmammalian therapsids (e.g. Haubold, 1971; Olsen and Galton, 1984; Hunt et al ., 1993; Retallack, 1996; Lockley and Meyer, 2000). The presence of a heel pad (calcaneal heel sensu Hopson, 1995) in the prints suggests a relatively derived non-mammalian therapsid trackmaker (Jenkins, 1971; Hopson, 1995), probably a eutheriodont ( sensu Sidor and Hopson, 1998; Rubidge and Sidor, 2001). The inward rotation of the manus suggests an elbows-out gait (Smith, 1993), and the shape of the digits might indicate a basal position of the trackmaker within that clade (see Jenkins, 1971; Hopson, 1994, 1995; Sidor and Hopson, 1998; Peters, 2000). In the three known trackways (MLP 60-XI-31-8 and, a and b in MLP 93-XII-13-3), the speed of the trackmakers was estimated (Alexander, 1976; see above discussion of Morphotype E). In the three cases, the animals were walking, although the relative speeds (table 3) are slightly different. The speed of the trackmaker in MLP 60-XI-31-8 shows that progression was comparatively much slower than in trackways a and b . This trackway is the only one with a tail dragmark preserved and the relatively low pace angulation and the higher manus-pes distance are also consistent with slower progression by its trackmaker (see Peabody, 1959).

Morphotype G (" Stipanicichnus bonetti ")

Material. CICRN 1-X-72-3, one slab containing a natural mould of a trackway of eleven footprints with five complete manus-pes sets and one isolated pes print (figure 10).

Figure 10. Morphotype G " Stipanicichnus bonetti ". A and A´, holotype (CICRN 1-X-72-3), a natural mould of a trackway of eleven footprints (from Casamiquela, 1975); m, manus and p, pes. Scale bar = 10 cm / morfotipo G " Stipanicichnus bonetti ". A y A´, holotipo (CICRN 1-X-72-3), molde de una rastrillada de once huellas (tomado de Casamiquela, 1975); m, mano y p, pie. Escala = 10 cm.

Locality and horizon. "Cantera Vieja", Cerro Las Lajas, Los Menucos (aprox. 40° 53' S - 63° 11' W), Río Negro province, Argent ina; Vera Formation (Los Menucos Group), Late Triassic (Labudía et al ., 2002).
Description. The trackway is well impressed on a slab of a fine-grained sandstone and it corresponds to a quadrupedal, plantigrade, pentadactyl animal. All the footprints are similar in size (homopody). As mentioned above, the trackway consists of four manus-pes sets and one isolated print. In each set, the manus is imprinted medially and slightly anterior to the corresponding pes print. The manus tracks are somewhat rotated inwards whereas the pedes tracks are more anteriorly directed and the interpedes distance is greater than that of the manus tracks spacing. The sole pads are posteriorly elongated which probably denotes the presence of a heel impression. In general, all the digits are subequal, similar in shape and have pointed claw-marks medially directed. The manus digits are more spread out than those of the pedes, in particular digits I and V are more separated than the inner ones. The trackway starts with a left manus-pes set followed by four alternating manus-pes sets and ends with an incomplete right pes print. It shows regular progression by the trackmaker with very short steps and footprints particularly widely separated from the midline of the trackway (broad trackway). Moreover, the very low pace angulation (table 4) indicates an animal with a sprawling gait ( sensu Leonardi, 1987).

Table 4. Measures of Morphotype G " Stipanicichnus bonetti ", CICRN 1-X-72-3. Parameters according to Leonardi (1987) / medidas del Morfotipo G " Stipanicichnus bonetti ", CICRN 1-X-72-3. Parámetros de acuerdo a Leonardi (1987).

Discussion. The specimen in CICRN 1-X-72-3 is the holotype material of Stipanicichnus bonetti Casamiquela, 1975 (pl. 6), which was related to Cynodontia (Casamiquela, 1975). Subsequently, the same specimen was allied to "Therapsida or Mammalia" (Leonardi, 1994).
The morphology of the prints (e.g. homopody) relates the trackmaker to non-mammalian therapsids (e.g. Haubold, 1971; Olsen and Galton, 1984; Leonardi and Henrique de Oliveira, 1990; Hunt et al ., 1993; Leonardi, 1994; Retallack, 1996; Lockley and Meyer, 2000). Moreover, within that group the presence of a probable heel impressions (calcaneal heel sensu Hopson, 1995) might be correlated with relatively derived therapsids (Jenkins, 1971; Hopson, 1995). Nevertheless, the marked broadness of the trackway suggests a sprawling posture for both the forelimbs and hindlimbs. Moreover, the pattern observed in the trackway is very peculiar due to the sideways orientation of the footprints. This arrangement of the prints has been related to animals that walked, probably uphill, across a steep surface (McKeever, 1994). The lack of information about the environmental context where the trackway CICRN 1-X-72-3 was imprinted does not allow us to corroborate this hypothesis.

Discussion

With the exception of Patagonia, Triassic ichnites attributed to non-mammalian therapsids are generally scarce in Argentina. They were mentioned from the Ischigualasto-Villa Unión Basin (Marsicano et al ., 2004) and the Cuyana Basin (Bonaparte, 1966; Romer, 1966; Leonardi, 1994; Marsicano and Barredo, 2004; Marsicano et al ., 2004) in west-central Argentina. In other Gondwanan areas, footprints attributed to this group of tetrapods are also scanty and were described from Late Triassic and Early Jurassic levels of Southern Africa (Ellenberger, 1970, 1972, 1974; Lockley et al ., 2004), the Lower Triassic of Antarctica (Macdonald et al ., 1991) and the Lower Triassic of Australia (Retallack, 1996).
The Middle Triassic Cerro de Las Cabras (Cuyana Basin) and Los Rastros formations (Ischigualasto-Villa Unión Basin) have mainly yielded small isolated tracks (Bonaparte, 1966; Romer, 1966; Leonardi, 1994; Marsicano et al ., 2004) that were recently reviewed and tentatively assigned to non-mammalian therapsids (Marsicano et al ., 2004). No clear evidence of a calcaneal heel impression is present in the material from central-western Argentina, in contrast to the condition observed in almost all of the footprints from Los Menucos. Only in the Morphotype E (" Palaciosichnus zettii ") is the impression of a calcaneal heel absent, although this apparent semiplantigrade condition might be the result of unusually fast progression developed by the trackmaker during the imprint of the trackway.
In the Upper Triassic Portezuelo Formation (Cuyana Basin) two types of tracks attributed to nonmammalian therapsid trackmakers were studied and, in both cases, they are represented by isolated footprints and trackways (Marsicano and Barredo, 2004). One type (Type Q1 in Marsicano and Barredo, 2004) was related to cynodont trackmakers; the described trackway displays an animal developing a rather fast running gait and the prints lack the heel impression present in the specimens from Patagonia. The other type (Type Q2 in Marsicano and Barredo, 2004) corresponds to large rounded footprints that were attributed to members of the dicynodont anomodont clade, probably kannemeyerids (Marsicano and Barredo, 2004). This type of print is, until now, not present in the assemblage from Los Menucos.
In Gondwana, relatively well preserved footprints of non-mammalian therapsids (Eucynodontia) were described from the lower part of the Early Jurassic Clarens Formation in the Karoo Basin (Ellemberger, 1970, 1972; Olsen and Galton, 1984). All these South African footprints mainly consist of isolated material and were reassigned to the Jurassic Patagonian ichnotaxon " Ameghinichnus " by Olsen and Galton (1984). These prints are fairly similar to those from Los Menucos (see Ellemberger, 1970, 1972), particularly to the specimens of morphotypes C (" Calibarichnus ayestarani ") and F (" Gallegosichnus garridoi "). In the Lower Triassic of the Sydney Basin (Australia) a relatively wide trackway of short steps was attributed to an anomodont therapsid trackmaker (Retallack, 1996), although the footprints show quite long subequal and relatively spread-out digits that differ strikingly from the Los Menucos material.
In Laurasia, similar footprints to those of Morphotype F (" Gallegosichnus garridoi ") were described from the Lower Triassic of Germany and England (" Dicynodontipus geinitzi " in Haubold, 1971, pp. 41). They closely resemble those from Los Menucos, particularly in the morphology and orientation of the manus-pes sets: the inward rotation of the manus, the anteriorly directed pedes and the presence of conspicuous heel impressions.
"Chirotheroid" footprints are extensively known in the Triassic footprint assemblages of Pangea. They were described from several basins in southern Africa (e.g., Haubold, 1984; Olsen and Galton, 1984), North America (e.g., Lockley and Hunt, 1995), Europe (e.g., Courel and Demathieu, 1995; Haubold and Klein, 2000; Lockley and Meyer, 2000), and South America (Leonardi, 1994). In Argentina, they are also known from the Middle Triassic Cerro de las Cabras (Rusconi, 1952; Peabody, 1955; Bonaparte, 1966; Marsicano et al ., 2004) and Los Rastros formations (von Huene, 1931; Marsicano et al ., 2004), and the Upper Triassic Los Colorados (Bonaparte, 1997; Arcucci et al ., 2000, 2004) and Portezuelo formations (Marsicano and Barredo, 2004); all these footprints resemble, in general, the ones described herein (Morphotype A) although, as noted above, a chirotherid affinity for Morphotype A is by no means certain.
Trackways comparable to those originally described as " Ingenierichnus " (Morphotype B) by Casamiquela are unknown from Triassic sequences of Argentina. Moreover, the broad continuous "sinewave-shaped dragmark" produced during the progression of the animal was not previously recorded from lower Mesozoic sequences of Gondwana. A similar situation occurs with the material originally assigned to " Rogerbaletichnus " (Morphotype D). Similar tracks with conspicuous and persistent limb dragmarks were neither described for Lower Mesozoic sequences of Argentina nor for other Gondwanan areas.

Faunal significance of Los Menucos assemblage

Nearly all of our knowledge about the Triassic tetrapod faunas in southernmost South America comes from two Late Triassic successions in Patagonia: the El Tranquilo Group in Santa Cruz province and the Los Menucos Group. Whereas the El Tranquilo has yielded exclusively dinosaur remains (prosauropods and ornithischians), the situation represented by the Los Menucos ichnites is completely different.
Until now, all well known Late Triassic Gondwanan tetrapod faunas have been archosaur-dominated (crurotarsal archosaurs and dinosaurs), such as those recorded in the Ischigualasto and Los Colorados formations in central-western Argentina ( e.g. Bonaparte, 1971,1997; Arcucci et al ., 2004), the upper part of the Santa María Supersequence in southern Brazil (Schultz at al ., 2000; Zerfass et al ., 2003), and the Elliot-Clarens succession in southern South Africa ( e.g. , Kitching and Raath, 1984; Olsen and Galton, 1984; Lucas, 1998; Lucas and Hancox, 2001).
The present analysis of Los Menucos tracks, which relied on a relatively large number of specimens, supports the existence of a terrestrial tetrapod fauna characterized by rather small animals. The dominant component of the fauna was largely the non-mammalian therapsids, probably eutheriodonts, both in terms of taxonomic diversity and relative abundance. Nevertheless, the presence of mammals in the assemblage cannot be ruled out. Therefore, the Los Menucos assemblage was unique, relative to other coeval Gondwanan tetrapod faunas, in which archosaurs (basal archosaurs and dinosaurs) are not the dominant component. This unusual situation might be either taphonomically biased toward small, predominantly terrestrial tetrapods or the record of an endemic fauna actually dominated by therapsids. The likelihood of a preservational bias as an explanation is difficult to assert because no taphonomic studies of the collected material are available.
The second interpretation is that the assemblage really represents an endemic fauna. The deposition of the Los Menucos succession was controlled by volcanic processes and deposited by ephemeral river systems under explosive volcanic activity, conditions that probably continuously changed the configuration of the landscape (see Lockley, 1990). This scenario might contributed to the development of an endemic fauna which reflects the instability of the landscape in which the animals lived.

Acknowledgements

The authors acknowledge the contribution of A.M. Báez (Universidad de Buenos Aires) for critical reading of the manuscript and pertinent comments. We thank M. Reguero (Museo de La Plata, Buenos Aires) for access to specimens under his care. Funding for this work was provided by the Universidad de Buenos Aires (UBACyT Grant TX01). Additional financial support was provided by the Consejo Nacional de Investigaciones Científicas y Técnicas (CONICET), PIP 5120.

References

Alexander, R.McN. 1976. Estimates of speeds of dinosaurs. Nature 261: 129-130.         [ Links ]
Arcucci, A.B., Marsicano, C.A., and Caselli, A.T. 2000. Tetrapod footprints from Los Colorados Formation, Upper Triassic of Argentina. Journal of Vertebrate Paleontology 20: 27A.         [ Links ]
Arcucci, A.B., Marsicano, C.A., and Caselli, A.T. 2004. Tetrapod association and paleoenvironment of Los Colorados Formation (Argentina): a significant sample from western Gondwana at the end of the Triassic. Geobios 37: 557-568.         [ Links ]
Artabe, A.E. 1985. Estudio sistemático de la tafoflora triásica de Los Menucos, Provincia de Río Negro, Argentina. Parte I. Sphenophyta, Filicophyta, Pteridospermophyta. Ameghiniana 22: 3-22.         [ Links ]
Artabe, A.E., 1986. Estudio sistemático de la tafoflora triásica de Los Menucos, Provincia de Río Negro, Argentina. Parte II. Cycadophyta, Ginkophyta y Coniferophyta. Ameghiniana 22 (1985): 159-180.         [ Links ]
Báez, A.M. and Marsicano, C.A. 2001. A heterodontosaurid ornithischian dinosaur from the Upper Triassic of Patagonia. Ameghiniana 38: 271-279.         [ Links ]
Báez, A.M., Marsicano, C.A., and Cione, A.L. 1993. Vertebrados Mesozoicos. In: V.A. Ramos (ed.), Geología y Recursos Naturales de Mendoza, 12° Congreso Geológico Argentino y 2° Congreso de Explotación de Hidrocarburos (Mendoza), Relatorio II, 13: 341-348.         [ Links ]
Benton, M.J. 2000. Vertebrate Palaeontology . Blackwell Science Ltd., 452 pp.         [ Links ]
Bernier, P., Barale, G., Bourseau, J-P., Buffetaut, E., Demathieu, G., Gaillard, C., and Gall, J-C. 1982. Trace nouvelle de locomotion de chélonien et figures d'émersion associées dans les calcaires lithographiques de Cerin (Kimméridgien Supérieur, Ain, France). Geobios 15: 447-467.         [ Links ]
Blob, R.W. 2001. Evolution of hindlimb posture in nonmammalian therapsids: biomechanical tests of paleontological hypothesis. Paleobiology 27: 14-38.         [ Links ]
Boggs, S. 1987. Principles of Sedimentology and Stratigraphy . Merrill Publishing Company, Columbus, 784 pp.         [ Links ]
Bonaparte, J.F. 1966. Cronología de algunas formaciones triásicas argentinas basada en restos de tetrápodos. Revista de la Asociación Geológica Argentina 21: 20-38.         [ Links ]
Bonaparte, J.F. 1971. Los tetrápodos del sector superior de la Formación Los Colorados, La Rioja, Argentina. Opera Lilloana 22, 183 pp.         [ Links ]
Bonaparte, J.F. 1973. Edades/reptil para el Triásico de Argentina y Brasil. 5° Congreso Geológico Argentino (Carlos Paz) , Actas 3: 93-129.         [ Links ]
Bonaparte, J.F. 1978. El Mesozoico de América del Sur y sus tetrápodos. Opera Lilloana 26, 596 pp.         [ Links ]
Bonaparte, J.F. 1997. El Triásico de San Juan-La Rioja, Argentina y sus dinosaurios . Museo Argentino de Ciencias Naturales Bernardino Rivadavia, Buenos Aires, 190 pp.         [ Links ]
Brand, L.R. 1979. Field and laboratory studies on the Coconino sandstone (Permian) vertebrate footprints and their paleocological implications. Palaeogeography, Palaeoclimatology, Palaeoecology 28: 126-139.         [ Links ]
Brand, L.R. 1996. Variations in salamander trackways resulting from substrate diferences. Journal of Paleontology 70: 1004-1010.         [ Links ]
Casamiquela, R.M.1964. Estudios icnológicos. Problemas y métodos de la icnología con aplicación al estudio de pisadas mesozoicas (Reptilia, Mammalia) de la Patagonia. Talleres Gráficos Colegio Industrial Pío IX, Buenos Aires, 229 pp.         [ Links ]
Casamiquela, R.M. 1975. Nuevo material y reinterpretación de las icnitas mesozoicas (Neotriásicas) de Los Menucos, Provincia de Río Negro (Patagonia). 1° Congreso Argentino de Paleontología y Bioestratigrafía (Tucumán, 1974), Actas 1: 555-580.         [ Links ]
Casamiquela, R.M. 1980. La presencia del género Plateosaurus (Prosaurópoda) en el Triásico Superior de la Formación El Tranquilo, Patagonia. 2° Congreso Argentino de Paleontología y Bioestratigrafía y 1° Congreso Latinoamericano de Paleontología (Buenos Aires), Actas 1: 143-147.         [ Links ]
Clack, J.A. 1997. Devonian tetrapod trackways and trackmakers; a review of the fossils and footprints. Palaeogeography, Palaeoclimatology, Palaeoecology 130: 227-250.         [ Links ]
Courel, L. and Demathieu, G. 1995. Tentative stratigraphic correlation using ichnological data from continental sandstone series and marine faunas in the middle Triassic of Europe. Albertiana 15: 83-91.         [ Links ]
Demathieu, G. and Wright, R. 1988. A new approach to the discrimination of chiroteroid ichnospecies by means of multivariate statitics. Triassic eastern border of the french Massif Central. Geobios 21: 729-739.         [ Links ]
Demathieu, G., Gand, G., and Toutin-Morin, N. 1992. La palichnofaune des Bassins Permiens Proven?aux. Geobios 25: 19-54.         [ Links ]
Domnanovich, N.S. 2003. [ Análisis paleobiológico de una asociación de huellas de tetrápodos del Triásico Superior de Patagonia (Los Menucos, Provincia de Río Negro). Lic. Thesis, Universidad de Buenos Aires, Facultad de Ciencias Exactas y Naturales, 134 pp. Unpublished.].         [ Links ]
Ellenberger, P. 1970. Les niveaux paleontologiques de premier apparition des Mammiferes Primordiaux en Afrique du Sud et leur Ichnologie: Establissement de zones stratigraphiques detaillees dans le Stormberg du Lesotho, (Afique du Sud) (Trias Superieur a Jurassique). In: S.H. Haughton (ed.), I.U.G.S., 2° Symposium on Gondwana Stratigraphy and Palaeontology. Pretoria, Council for Scientific and Industrial Research, pp. 343-370.         [ Links ]
Ellenberger, P. 1972. Contribution à la classification des pistes de vertèbres du Trias: les types du Stomberg d'Áfrique du Sud (1° partie). Palaeovertebrata , Mémoire Extraordinaire Montpellier, pp.1-152.         [ Links ]
Ellenberger, P. 1974. Contribution à la classification des pistes de vertèbres du Trias: les types du Stomberg d'Áfrique du Sud (2° partie): le Stomberg Superieur-I. Le biome de la zone B/1 ou niveau de Moyeni: ses biocenoses). Palaeovertebrata , Mémoire Extraordinaire Montpellier, pp.1-202.         [ Links ]
Farlow, J.O. and Pianka, E.R. 2000. Body form and trackway pattern in Australian Desert Monitors (Squamata: Varanidae): Comparing zoological and ichnological diversity. Palaios 15: 235-247.         [ Links ]
Franzese, J.R. and Spalletti, L.A. 2001. Late Triassic-early Jurassic continental extension in southwestern Gondwana: tectonic segmentation and pre-break-up rifting. Journal of South American Earth Science 14: 257-270.         [ Links ]
Gradstein, F.M., Agterberg, F.P., Ogg, J.G., Hardenbol, J., Van Veen, P., Thierry, J. and Huang Zehui 1995. A Triassic, Jurassic and Cretaceous time scale. In: W.A. Berggren, D.V. Kent, M.-P. Aubrey and J. Hardenbol (eds.), Geochronology, Time Scales and Global Stratigraphic Correlation: SEPM. Society for Sedimentary Geology, Special Publication 54: 95-126 pp.         [ Links ]
Haubold, H. 1971. Ichnia Amphibiorum et Reptiliorum fossilium. In: O. Kuhn (ed.), Handbuch der Paläoherpetologie , part. 18, Gustav Fischer Verlag, Stuttgart, Germany and Portland, USA, 121 pp.         [ Links ]
Haubold, H. 1983. Archosaur evidence in the Buntsandstein (Lower Triassic). Acta Palaeontologica Polonica 28: 123-132.         [ Links ]
Haubold, H. 1984. Saurierfährten . Wittenberg Lutherstadt, Germany: Neue BrehmBucherei, A. Ziemsen Verlag, 231 pp.         [ Links ]
Haubold, H. 1986. Archosaur footprints at the terrestrial Triassic-Jurassic transition. In: K. Padian (ed.), The beginning of the age of dinosaurs , Cambridge University Press, Cambridge, pp. 189-202.         [ Links ]
Haubold, H. and Klein, H. 2000. Die dinosauroiden Fährten Parachirotherium-Atreipus-Grallator aus dem unteren Mittelkeuper (Obere Trias: Ladin, Karm, ?Nor) in Franken. Hallesches Jahrbuch für Geowissenschaften 22: 59-85.         [ Links ]
Haubold, H., Hunt, A.P., Lucas, S.G., and Lockley, M.G. 1995. Wolfcampian (Early Permian) vertebrate tracks from Arizona and New Mexico. In: S.G. Lucas and A.B. Heckert (eds.), Early Permian footprints and facies , New Mexico Museum of Natural History and Science 6: 135-165.         [ Links ]
Hopson, J.A. 1994. Synapsid evolution and the radiation of the non-eutherian mammals. In: D. Prothero and R. Schoch (eds.), Major features of vertebrate evolution , Short Courses in Paleontology, The Paleontological Society 7: 190-219.         [ Links ]
Hopson, J.A. 1995. Patterns of evolution in the manus and pes of non-mammalian therapsids. Journal of Vertebrate Paleontology 15: 615-639.         [ Links ]
Hunt, A.P., Santucci, V.L., Lockley, M.G., and Olson, T.J. 1993. Dicynodont trackways from the Holbrook Member of The Moenkopi Formation (Middle Triassic: Anisian), Arizona, USA. In: S.G. Lucas and M. Morales (eds.), The nonmarine Triassic , New Mexico Museum of Natural History and Science Bulletin 3: 213-218.         [ Links ]
Jenkins, F.A. 1971. The postcranial skeleton of African cynodonts. Bulletin of the Peabody Museum of Natural History Yale University 36, 154 pp.         [ Links ]
Kitching, J. and Raath, M.1984. Fossils from the Elliot and Clarens formations (Karoo sequence) of the northeastern Cape, Orange Free State and Lesotho, and a suggested biozonation based on tetrapods. Palaeontologia Africana 16: 17-23.         [ Links ]
Kokogian, D.A., Spalletti, L.A., Morel, E.M., Artabe, A.E., Martínez, R.N., Alcober, O.A., Milana, J.P., and Zavattieri, A.M. 2001. Estratigrafía del Triásico argentino. In: A.E. Artabe, E.M. Morel and A.B. Zamuner (eds.), El Sistema Triásico en la Argentina . Fundación Museo La Plata, La Plata, pp. 23-54.         [ Links ]
Labudía, C.H. and Bjerg, E.A. 2001. El Grupo Los Menucos: redefinición estratigráfica del Triásico Superior del Macizo Nordpatagonico. Revista de la Asociación Geologica Argentina 56: 404-407.         [ Links ]
Labudía, C.H., Llambías, E.J., Rapela, C.W., and Artabe, A.E. 1995. El Triásico de Los Menucos: procesos volcánicos y sedimentarios. 2° Reunión del Triásico del Cono Sur (Bahía Blanca), Acta, pp. 17-21.         [ Links ]
Labudía, C.H., Llambias, E.J., Bjerg E.A. and Stipanicic, P.N. 2002. Vera (Formación ...). In: P.N. Stipanicic and C.A. Marsicano (eds.), Léxico Estratigráfico de la Argentina: Triásico , Asociación Geológica Argentina, serie "B" (didáctica y complementaria), 26: 295-296.         [ Links ]
Leonardi, G. (editor) 1987. Glossary and Manual of Tetrapod Footprint Palaeoichnology . Departamento Nacional da Produçao Mineral, Brasilia (Brasil), 117 pp.         [ Links ]
Leonardi, G. 1994. Annotated atlas of South America tetrapod footprints (Devonian-Holocene). Ministerio de Minas e Energia, Companhia de Pesquisa de Recursos Minerais, Brasil, 248 pp.         [ Links ]
Leonardi, G. and Henrique de Oliveira, F. 1990. A revision of The Triassic and Jurassic tetrapod footprints of Argentina and a new approach on the age and meaning of The Botucatu Formation footprints (Brazil). Revista Brasileira de Geociencias 20: 216-229.         [ Links ]
Llambías, E.J. 1999. El magmatismo gondwánico durante el Paleozoico Superior-Triásico. In: R. Caminos (ed.), Geología Argentina , Servicio Geológico Minero Argentino, Instituto de Geología y Recursos Minerales, Buenos Aires, Anales 29: 349-376.         [ Links ]
Lockley, M. 1990. How volcanism affects the biostratigraphic record. In: Lockley, M.G. and Rice, A. (eds.), Volcanism and Fossil Biotas , Geological Society of America, Special Paper 244: 1-12.         [ Links ]
Lockley, M. and Hunt, A.P. 1995. Dinosaur tracks . Columbia University Press., New York, 338 pp.         [ Links ]
Lockley, M. and Meyer, C. 2000. Dinosaur tracks and other fossil footprints of Europe . Columbia University Press, 323 pp.         [ Links ]
Lockley, M.G., Lucas, S.G., Hunt, A.P. and Gaston, R., 2004. Ichnofaunas from the Triassic-Jurassic boundary sequences of the Gateway area, Western Colorado: implications for faunal composition and correlations with other areas. Ichnos 11: 89-102.         [ Links ]
Lucas, S. 1998. Global Triassic tetrapod biostratigraphy and biochronology. Palaeogeography, Paleoclimatology, Palaeoecology 143: 347-384.         [ Links ]
Lucas, S. and Hancox, J. 2001. Tetrapod-based correlation of the nonmarine Upper Triassic of Southern Africa. Albertiana 25: 5-9.         [ Links ]
Macdonald, D.I.M., Isbell, J.L. and Hammer, W.R. 1991. Vertebrate trackways from the Triassic Fremouw Formation, Queen Alexandra Range, Antarctica. Antactic Journal of the U.S. 26: 20-22.         [ Links ]
Macdonald, D., Gomez-Perez, I., Franzese, J., Spalletti, L., Lawver, L., Gahagan, L., Dalziel, I., Thomas, C., Trewin, N., Hole, M. and Paton, D. 2003. Mesozoic break-up of SW Gondwana: implications for regional hydrocarbon potential of the southern South Atlantic. Marine and Petroleum Geology 20: 287-308.         [ Links ]
Marsicano, C.A. and Barredo, S. 2004. A Triassic tetrapod footprint assemblage from southern south America: palaeobiogeographical and evolutionary implications. Palaeogeography, Palaeoclimatology, Palaeoecology 203: 313-335.         [ Links ]
Marsicano, C.A., Arcucci, A.B., Mancuso, A. and Caselli, A.T. 2004. Middle Triassic tetrapod footprints of southern South America. Ameghiniana 41: 171-184.         [ Links ]
McKeever, P. 1994. The behavioral and biostratigraphical significance and the origin of vertebrate trackways from the Permian of Scotland. Palaios 9: 477-487.         [ Links ]
McKeever, P.J. and Haubold, H. 1996. Reclassification of vertebrate trackways from the Permian of Scotland and related forms from Arizona and Germany. Journal of Paleontology 20: 1011-1022.         [ Links ]
Olsen, P.E. and Galton, P.M. 1984. A review of the reptile and amphibian assemblages from The Stormberg of Southern Africa, with special emphasis on the footprints and the age of The Stormberg. Palaeontologia Africana 25: 87-110.         [ Links ]
Parrish, J.M. 1989. Phylogenetic patterns in the manus and pes of early Mesozoic archosauromotph reptiles. In: D.D. Gillette and M.G. Lockley (eds.), Dinosaur tracks and traces , Cambridge University Press, Cambridge, pp. 249-258.         [ Links ]
Parrish, J.M. 1993. Phylogeny of the Erythrosuchidae (Reptilia: Archosauriformes). Journal of Vertebrate Paleontology 12: 198-212.         [ Links ]
Peabody, F.E. 1948. Reptile and amphibian trackways from the Lower Triassic Moenkopi Formation of Arizona and Utah. Bulletin of the Department of Geological Sciences University of California Publications 27: 295-467.         [ Links ]
Peabody, F.E. 1955. Occurrence of Chirotherium in South America. Bulletin of the Geological Society of America 66: 239-240.         [ Links ]
Peabody, F.E. 1959. Trackways of living and fossil salamanders. University of California Publications in Zoology 63: 1-71.         [ Links ]
Pesce, A.H. 1974. [ Geología de la región situada al oeste de Los Menucos, Provincia de Río Negro . Lic. Thesis, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 62 pp. Unpublished.].         [ Links ]
Peters, D. 2000. Description and interpretation of interphalangeal lines in tetrapods. Ichnos 7: 11-41.         [ Links ]
Pough, F.H., Janis, C.M., and Heiser, J.B. 1999. Vertebrate life . Prentice-Hall Inc., 733 pp.         [ Links ]
Rapela, C.W., Pankhurst, R.J., Llambías, E.J., Labudía, C. And Artabe, A. 1996. "Gondwana" magmatism of Patagonia: inner Cordilleran calc-alkaline batholiths and bimodal volcanic provinces. 3° International Symposium on Andean Geodynamics (Saint-Malo, France), pp. 791-794.         [ Links ]
Retallack, G.J. 1996. Early Triassic therapsid footprints from the Sydney Basin, Australia. Alcheringa 20: 301-314.         [ Links ]
Romer, A.S. 1966. Vertebrate Paleontology . The University of Chicago Press, Chicago, 468 pp.         [ Links ]
Rosenman, H.L. 1979. [Geología de la zona comprendida entre La Esperanza, Aguada de Guerra y Los Menucos, Provincia de Río Negro, República Argentina. Ph. D. Thesis, Facultad de Ciencias Exactas y Naturales, Universidad de Buenos Aires, 52 pp., Unpublished.].         [ Links ]
Rougier, G.W., De la Fuente, M.S. and Arcucci, A.B. 1995. Late Triassic Turtles from South America. Science 268: 855-858.         [ Links ]
Rubidge, B.S. and Sidor, C.A. 2001. Evolutionary patterns among Permo-Triassic therapsids. Annual Review of Ecology and Systematics 32: 449-480.         [ Links ]
Rusconi, C. 1952. Rastros de patas de reptiles pérmicos de Mendoza. Revista de Historia y Geografía de Cuyo 3: 2-13.         [ Links ]
Sarjeant, W.A.S. 1975. Fossil Tracks and Impressions of Vertebrates. In: R.W. Frey (eds.), The Study of Trace Fossils , Springer-Verlag, New York, pp. 283-324.         [ Links ]
Scarboro, D.D. and Tucker, M.E. 1995. Amphibian footprints from the mid-Carboniferous of Northumberland, England: Sedimentological contex, preservation and significance. Palaeogeography, Palaeoclimatology, Palaeoecology 113: 335-349.         [ Links ]
Schultz, C., Santos Scherer, C.M., and Barberena, M.C. 2000. Biostratigraphy of Southern Brazilian Middle-Upper Triassic. Revista Brasileira de Geociencias 30: 491-494.         [ Links ]
Sereno, P. 1991. Lesothosaurus , "Fabrosaurids", and the early evolution of Ornithischia. Journal of Vertebrate Paleontology 11: 168-197.         [ Links ]
Sidor, C.A. and Hopson, J. 1998. Ghost linages and "mammalness": assessing the temporal pattern of character acquisition in the Synapsida. Paleobiology 24: 254-273.         [ Links ]
Smith, R.M.H. 1993. Sedimentology and Ichnology of Floodplain Paleosurfaces in the Beaufort Group (Late Permian), Karoo Sequence, South Africa. Palaios 8: 339-357.         [ Links ]
Stipanicic, P.N and Methol, E.J. 1972. Macizo de Somún Curá. In: A.F. Leanza (ed.), Geología Regional Argentina , Academia Nacional de Ciencias (Córdoba), pp. 581-600.         [ Links ]
Stipanicic, P.N., Rodrigo, F., Baulies, O.L. and Martinez, C.G. 1968. Las formaciones presenonianas en el denominado Macizo Nordpatagonico y regiones adyacentes. Revista de la Asociación Geológica Argentina 23: 67-98.         [ Links ]
Thulborn, R.A. 1989. The gaits of dinosaurs. In: D.D. Gillette and M.G. Lockley (eds.), Dinosaur tracks and traces , Cambridge University Press, Cambridge, pp. 39-50.         [ Links ]
Thulborn, R.A. 1990. Dinosaurs Tracks . Chapman and Hall, London, 410 pp.         [ Links ]
von Huene, F. 1931. Die fossilen Färhten im Rhät von Ischigualasto in Nordwest-Argentinien. Palaeobiologica 4: 99-112.         [ Links ]
Wright, J. and Lockley, M. 2001. Dinosaur and turtle tracks from the Laramie/Arapahoe formations (Upper Cretaceous), near Denver, Colorado, USA. Cretaceous Research 22: 365-376.         [ Links ]
Zerfass, H., Lavina, E.L., Schultz, C.L., Garcia, A.J.V., Faccini, U.F. and Chemale Jr., F. 2003. Sequence stratigraphy of continental Triassic strata of Southernmost Brazil: a contribution to Southwestern Gondwana palaeogeography and paleoclimate. Sedimentary Geology 161: 85-105.         [ Links ]

Recibido: 23 de setiembre de 2004.
Aceptado: 5 de mayo de 2005.