Bird tracks preserved in fluvial channel facies of the Río Negro Formation (Neogene), La Pampa Province, Argentina

Abstract

The vertebrate ichnofauna of the Neogene Río Negro Formation is composed of abundant mammal footprints and rare bird footprints. This ichnofauna has been extensively recorded from sea cliff exposures located south of the mouth, and also on the margin of the Negro river downstream of Carmen de Patagones city (figure 1). One of the remarkable aspects of this ichnofauna is the presence of trackways of large mammals, including those named as Megatherichnium oportoi Casamiquela, 1974, which provides evidence for bipedal locomotion in Megatherioidea (Casamiquela, 1974, Leonardi, 1994; Casinos, 1996). The tracks of other large mammals display a lower quality of preservation and have been recognized as Macrauchenichnus rector Casamiquela in Angulo and Casamiquela, 1982 and cf. Mylodontidichnum Aramayo and Manera de Bianco, 1987 (Angulo and Casamiquela, 1982; Aramayo, 1999; Aramayo et al., 2004). The materials used to erect other ichnotaxa of possible large mammals are poorly preserved tracks or undertracks and the status of that ichnotaxa is uncertain. This is the case of Falsatorichnum calceocannabius and Caballichnus impersonalis, which were tentatively assigned to Megatherioidea and Equidae by Casamiquela (in Angulo and Casamiquela, 1982). The remaining mammal trace fossils are Porcellusignum conculcator Casamiquela, in Angulo and Casamiquela, 1982 (purported Hydrochoeridae tracks); large meniscated burrows (compared by Casamiquela with Octodontidae burrows), and a digitigrade trackway assigned to a carnivorous marsupial (Angulo and Casamiquela, 1982; Leonardi, 1994; Aramayo et al., 2004). The record of bird tracks are restricted to one undertrack assigned to a phororhacid from the vicinity of El Cóndor town (Aramayo et al., 2004) and undetermined tracks of small birds found near Ingeniero Jacobacci (Casamiquela, 1969, p. 308; Leonardi, 1994, p. 35), both from Río Negro province. Despite that the outcrops of the Río Negro Formation cover a large area in Buenos Aires, Río Negro and La Pampa provinces, no findings of continental trace fossils have been reported for other localities. The purpose of this communication is to describe, to assign ichnotaxonomically, and to infer the paleoenvironmental significance of small bird tracks from outcrops of the Río Negro Formation at La Adela town, La Pampa province.