Triassic bird-like footprints from Gondwana

MARSICANO, CLAUDIA; SMITH, ROGER; SIDOR, CHRISTIAN

Cordoba

Congreso; XIX Congreso Argentino de Paleontología y Bioestratigrafía; 2006

Footprints attributed to avian trackmakers are known from several Cretaceous and Cenozoic localities, mainly in the northern hemisphere (e.g. Lockley et al., 1992; Lockley, 1998; Farlow et al., 2000). A number of characters have been used to assume the avian affinities of their trackmakers, including: the large interdigital angle between digits II and IV, the symmetrical “heel” impression where digits II and IV converged, and the often clear posteromedially to posteriorly directed impression of the hallux, among others (Lockley et al., 1992). Distinctly older levels have produced a number of small bird-like footprints, some of them, remarkably similar to those produced by avian trackmakers (e.g. Lockley et al., 1992; Farlow et al., 2000). Among these records are those originally described by Ellenberger in 1970 (Ellenberger 1970, 1972) from fine-grained sandstones of the Late Triassic Lower Elliot Formation at Maphutseng in southern Lesotho, South Africa.  These are the oldest bird-like tracks to be described from Gondwana and among the earliest for Pangea. A recent re-evaluation of the stratigraphic position, sedimentological context and the ichnocoenosis of Ellenberger’s classic footprint localities in Lesotho allowed us to relocate and redescribe the original material. The Maphutseng footprints are relatively small, symmetrical and are preserved as natural moulds randomly distributed on a smooth sandstone palaeosurface.in the marginal “wings” of a classic ribbon-shaped channel sandstone. The tracks are associated with small sinuous grooves and “beaded” trails probably made by insect larvae underwater. Some 50m along strike the same surface displays large sand-filled desiccation cracks indicating a period of sub aerial exposure. The presence of a wide interdigital angle between digits II and IV combined with a clear halucal impression posteriorly directed in some of the prints make them difficult to be differentiated from footprints undoubtely attributed to avian trackmakers from younger levels (e.g. Lockley et al., 1992; Lockley, 1998; Farlow et al., 2000). Comparable footprints have been recently described from Lower Jurassic levels of Argentina (Melchor et al., 1999; Vizán, 2005). Consider the Maphutseng footprints to be produced by avian trackmakers would imply a significant earlier origin of birds and preceeding nodes that is incompatible with the known celulosaurian body fossil record (see Sereno, 1999). Therefore, these bird-like footprints suggest the presence of a distinctly theropod group with a bird-like pedal configuration already in the Late Triassic-Early Jurassic in western Gondwana. Moreover, this record supports previous assumptions about an extensive phyletic diversification of dinosaurs was taking place in the Late Triassic (e.g. Sereno, 1999), a condition that the body-fossil record has failed to document to date.