New avian tracks from the neogene of northwest argentina

Abstract

The Argentinean trace fossil record of vertebrate footprints is abundant and profusely studied; however, the one referred to Aves is particularly scarce and less known. Although there are some studies on avian fossil footprints, there are few descriptions and taxonomical assignations. Here we describe a new ichnotaxon that is ascribed to bird activity. Footprints were found at the Río Iruya section (62 Km NW from San Ramón de la Nueva Orán city, Sierras Subandinas Australes),from lower levels of the Guandacay Formation (≈ Río Guanaco Formation; Mio-Pliocene). They are preserved as convex hyporeliefs in brownish-grey pebble to boulder conglomerates overlying reddish - reddish brown mudstones and very fine-grained sandstones. The new ichnotaxon is defined based on a trackway consisting of three well-preserved tridactyle footprints. This track presents a stride of 96 cm long, a track width of 13.6 cm, a step angle of 178° and a femoro-acetabular height of 65.2 cm, approximately. Footmarks present a mean length of 16.3 cm, a mean width of 10.9 cm and the following combination of characters: rhomboidal overall outline with a width/length ratio of 0.67 and the minor axis posteriorly displaced; thick digits (D) broader in their middle sector; DIII longer than DII and DIV, respectively; continuity between DIII and the rounded metabasipodial distal end; DII - DIV internal angle of 62°; DII and DIII with a tapered outline and a welldefined knuckle and mark of claws at the ungueal phalanges; ungueal impression of DIII externally curved and DIV with an oval outline noticeably separated from the meta-basipodial distal end. The presence of a single knuckle by digit, the orientation of the DIII ungueal impression and the absence of a DI (hallux) suggest that these footprints could have been generated by a Rheidae, differing from those referable to other similar sized birds (e.g. Phorusrhacidae). The presence of probable rheid footprints was mentioned for the stratigraphic units outcropping at Quebrada del Yeso (Mio-Pliocene) in La Rioja Province and at the paleo-ichnological site of Pehuen-Co (late Pleistocene) in Buenos Aires Province.