NOTA PALEONTOLÓGICA

Large-diameter burrows in the Upper Triassic Ischigualasto Formation, Northwestern Argentina

Carina E. Colombi¹-2, Carolina Jofré², and Brian S. Currie³

¹CONICET. ccolombi@unsj.edu.ar
²Instituto y Museo de Ciencias Naturales, Universidad Nacional de San Juan, España 400 (Norte), 5400 San Juan, Argentina.
³Department of Geology, 114 Shideler Hall, Miami University, Oxford, OH 45056, 513-529-7578. curriebs@muohio.edu

Introduction

During the early Mesozoic, global climate was strongly influenced by the tectonic assembly of the Pangea supercontinent. Due to the paleolatitudinal setting and distribution of the Pangean landmass, the low to mid latitudes of the supercontinent are thought to have experienced a highly seasonal climate regime (Parrish, 1993). Many researchers agree that the extreme climate during the Permian and Triassic may have contributed to the explosive appearance of large burrows as a mechanism to protect the burrower from adverse atmospheric conditions (e.g., Groenewald et al., 2001; Hasiotis et al., 2004). Triassic-aged ichnofossils with diameters greater than 10 cm have been reported from Antarctica, Africa, Asia, and North America (e.g. Groenewald et al., 2001; Miller et al., 2001; Hasiotis et al., 2004). Many of these trace fossils have been interpreted as vertebrate burrows, more specifically therapsids (e.g. Groenewald et al., 2001; Miller et al., 2001; Hasiotis et al., 2004). These types of large fossil burrows, however, have not been previously described from South America.
The purpose of this note is to describe and interpret large-diameter ichnofossils identified in the nonmarine Triassic rocks of the Ischigualasto-Villa Unión Basin, northwestern Argentina. These ichnofossils are the first of their kind described in South America and have important implications concerning the paleobiogeographical distribution of large nonmarine burrowing organisms, as well as the factors controlling the evolutionary development of the burrowing behavior during the early Mesozoic.

Geological and paleoclimatological setting

The Ischigualasto-Villa Unión Basin is one of a series of the continental-rift basins developed along the southwestern margin of Pangea during the early Mesozoic (Milana and Alcober, 1994) (figures 1 and 2). The fossil burrows of this study were identified from the Upper Triassic Ischigualasto Formation in the southern part of the basin (~30.1°S, 67.9°W). The Late Triassic (Carnian) age of the Ischigualasto Formation is based on vertebrate fossils and radiometric ages of altered ash beds (Rogers et al., 1993; Martínez, 1994; Alcober, 1996; Shipman, 2004). Global plate reconstructions indicate that the Ischigualasto-Villa Union Basin was situated in southwestern Pangea at ~40°S latitude at this time (Golonka, 2007). Similarly, sedimentological, paleopedological, and taphonomical evidence indicates that the deposition of the Ischigualasto Formation occurred under a seasonal climate regime (Stipanicic and Bonaparte, 1972; Rogers et al., 1993; Currie et al., 2001; Shipman, 2004; Tabor et al., 2006; Colombi, 2007; Colombi and Parrish, in press).

Figure 1. Geologic map of the Ischigualasto-Villa Unión Basin, showing the location of study area / mapa geológico de la cuenca de Ischigualasto-Villa Unión, mostrando la localización geográfica del área de estudio.

Figure 2. Stratigraphic section of the Upper Triassic Ischigualasto Formation. Black arrows indicate the location of the burrows / columna estratigráfica de la Formación Ischigualasto (Triásico Superior). La localización de las cuevas está indicada por las flechas negras.

Large-diameter burrows

The large (>10 cm) diameter burrows studied are contained within floodplain deposits of the Ischigualasto Formation. The characterization of these ichnofossils is based on the ichnotaxobases, which comprise architectural and surficial morphology of the burrow casts; complexity and tortuosity indexes and type of fill (Hasiotis et al., 2004; Hasiotis et al., 2007). Burrow architectural morphology includes general dimensions, cross section geometry, spatial orientation, type of branching and burrow-element interconnectedness. Surficial morphology refers to both large and diminutive structures on the surfaces of burrow walls. Descriptions and interpretations regarding the origin of these burrows are listed below.

Description. The individual burrow systems, consisting of horizontal to subhorizontal tunnels and short vertical shafts, cover areas of ca. 2 m2, (figure 3.1).

Figure 3. Large-diameter burrow casts of the Upper Triassic Ischigualasto Formation / moldes de las cuevas de gran diámetro encontradas en los afloramientos de la Formación Ischigualasto (Triásico Superior). 1, General view of a large diameter burrow / vista general de una de las cuevas estudiadas. 2, Typical undulating geometry of the tunnel casts/ típica geometría ondulada de los moldes de los túneles. 3, Transverse cross section of the burrow casts, see the shallow U-shape of the burrow floor / corte transversal de los moldes de los túneles, obsérvese la forma de U aplanada del piso de los mismos. 4, Intermediate and terminal enlargements, interpreted as chambers. Note the millimeter bioturbation protuberance marks / ensanchamientos intermedios y finales de los túneles, interpretados como cámaras. Obsérvese las protuberancias milimétricas producidas por bioturbación. 5, Vertical shafts of the burrow complex / entrada vertical en el complejo de cuevas. 6, Central pit of the burrow shaft, produced by differential cementation of the fill material. Penknife used for scale = 7 cm / agujero central en la entrada de las chimeneas producido por cementación diferencial del relleno de la cueva. Bolígrafo usado como escala =7 cm.

Tunnels are straight to slightly winding and reach a length of 1 m (figure 3.2). Their diameter is uniform, 10 cm in average, with roughly elliptical cross sectional geometry. In some sectors of the tunnels, the floor bears a longitudinal medial groove that forms a shallow U-shape when viewed in transverse cross section (figure 3.3). The tunnels contain intermediate and terminal enlargements, interpreted as chambers, which are 25 cm in average diameter (figure 3.4). The greater diameter of the chamber is attained by the gradual increase of the tunnel diameter. Vertical shafts in the burrow complex are less than 20 cm long, although their original length may have been truncated by subsequent erosion (figure 3.5). Shafts, which likely represent entrances to the burrow complex, are commonly located at tunnel intersections or at the beginning of tunnels (figure 3.1 and 3.6). Approximately half of the observed shaft molds contain a central pit produced by differential cementation of the fill material (figure 3.7).
The branching angle of tunnel segments is ca. 90° (figure 3a), forming a T-shaped branching. The surficial morphology along the sides and tops of the burrows presents poorly defined longitudinal ridges, 2-3 mm wide, which could represent scratch marks (Groenewald et al., 2001). All surfaces exhibit a granular texture created by bioturbation (figure 3.8). The tortuosity index of the burrows (T) is close to one, indicating the simple geometry of the branching. The complexity index (C) is hard to define because of the burrow cast preservation. The fill of the studied ichnofosssils consist of brown, medium-grained, carbonate cemented sandstone.

Interpretation. The large-diameter burrows preserved in the Ischigualasto Formation could be assigned to different builders, namely crayfish-decapods, lungfish, or tetrapods (amphibians or therapsid reptiles). However, a detailed comparison of the morphology of these burrows to those of the possible builders shows that the studied structures resemble burrows in which therapsid reptiles have been found (Permo-Triassic Karoo Basin burrows) (Groenwald et al., 2001), or to those interpreted as produced by tetrapods because of the morphological comparison with fossil and extant tetrapod burrows, (Triassic Antarctic and North American burrows) (Miller et al., 2001; Hasiotis et al., 2004). The main morphological features supporting this interpretation are: 1) the large and uniform diameter-size (ca. 10 cm), different from that of crayfish burrows, whose maximum diameter is ca. 8 cm and which are not usually uniform; 2) the elliptical transverse section, unlike the burrows made by crayfish and lungfish that are usually circular in section; 3) the low angle orientation, common for tetrapods, but not common for either lungfish or crayfish, except in places close to the water table; 4) the relatively simple architecture characterized by a mosaic pattern with few openings (very short vertical shafts) that lead to long, nearly-horizontal tunnels and terminal or inter-tunnel-chambers, which differ from the usually more complicate architecture of the crayfish burrows or the bottle-like morphology of the lungfish ones; and finally, 5) the subtle wall ornamentation, characterized by longitudinal ridges, and the lack of the typical millimeter and centimeterscale features common on the walls of decapod burrows (i.e., scrape traces, mud liners and log liners, knobby-hummocky surfaces, pleopod striae and body impressions).
In addition, no lungfish and crayfish fossils are present in Ischigualasto Formation; in contrast, this formation comprises an important paleofauna of small therapsids, whose size agrees with the diameter of the burrows founded. This paleofauna includes the small therapsid tetrapods Ecteninion lunensis (Martínez et al., 1996), cf. Probaignognathus sp. (Bonaparte and Cromptom, 1995), and Probelesodon sanjuanensis (Martínez and Forster, 1996).

Discussion

The large-diameter burrows in the Upper Triassic Ischigualasto Formation are the first of their kind to be described from the Mesozoic of South America. Previous workers investigating Triassic large-diameter continental ichnofossils have suggested that the distribution of burrowing vertebrates may have been greater at high latitudes, and that burrowing behavior may have evolved in vertebrates as a response to latitudinally induced seasonal variations in temperature (Miller et al., 2001). This statement was based primarily on assumptions that modern burrowers are more prevalent at higher latitudes (Reichman and Smith, 1987) and the observation that, despite extensive study of Mesozoic stratigraphic intervals deposited at relatively low paleolatitudes, large-diameter vertebrate burrows had not been described in the literature. However, more recent workers have documented a diverse Triassic vertebrate ichnofossil assemblage from relatively low paleolatitudes (5°- 15°N) (Hasiotis et al., 2004). Our work on the large-diameter burrows in the Ischigualasto Formation provides additional information on the distribution of large vertebrate trace fossils for middle paleolatitudes of Triassic Pangea (ca. 40°S). These new data suggest that there was no pronounced paleolatitudinal variation in the distribution of burrowing organisms.
Early Mesozoic organisms may have used burrowing in order to counter the extreme seasonal climate variation that has been interpreted for that time period (Parrish, 1993). Given the above, the vertebrate burrows from the Triassic of Pangea appear to be an advanced behavior utilized for protection against adverse paleoclimatic conditions associated with extreme seasonality. Although initially developed to combat seasonal temperature fluctuations, and water stress associated with seasonally dry climate regimes at low to mid latitudes, burrowing behavior also allowed organisms to live at high latitudes by circumventing seasonal temperature fluctuations and perhaps serving as a refuge during hibernation (Groenewald et al., 2001; Hasiotis et al., 2004).

References

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Recibido: 19 de diciembre de 2007.
Aceptado: 12 de septiembre de 2008.