1. CONICET, Av. Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina
2. Universidad Nacional de La Pampa, Av. Uruguay 151, 6300 Santa Rosa, La Pampa, Argentina
3. Museo Paleontológico "Egidio Feruglio", Av. Fontana 140, 9100 Trelew, Chubut, Argentina

Correspondence to: Ricardo N. Melchor^1,2 Correspondence and requests for materials should be addressed to R. N. M. (e-mail: Email: melchor_r@cpsarg.com).

Abstract

The study of fossilized footprints and tracks of dinosaurs and other vertebrates has provided insight into the origin, evolution and extinction of several major groups and their behaviour; it has also been an important complement to their body fossil record^{1, 2, 3, 4}. The known history of birds starts in the Late Jurassic epoch (around 150 Myr ago) with the record of Archaeopteryx⁵, whereas the coelurosaurian ancestors of the birds date back to the Early Jurassic⁶. The hind limbs of Late Triassic epoch theropods lack osteological evidence for an avian reversed hallux and also display other functional differences from birds⁷. Previous references to suggested Late Triassic to Early Jurassic bird-like footprints have been reinterpreted as produced by non-avian dinosaurs having a high angle between digits II and IV^{8, 9} and in all cases their avian affinities have been challenged¹⁰. Here we describe well-preserved and abundant footprints with clearly avian characters from a Late Triassic redbed sequence of Argentina^{11, 12}, at least 55 Myr before the first known skeletal record of birds. These footprints document the activities, in an environment interpreted as small ponds associated with ephemeral rivers, of an unknown group of Late Triassic theropods having some avian characters.