[D] Unenlagia comahuensis [sG] [T]
Cretaceous Late [Cenomanian] Turonian Coniacian
Saurischia Theropoda Tetanurae Coelurosauria Maniraptora Unenlagiinae
Rio Neuquen Formation, Sierra del Portezuelo, Neuquen Province, Patagonia, Argentina
Genus - Typespecies
Unenlagia comahuensis (Novas, 1997) > Unenlagia paynemili (Calvo, Porfiri & Kellner, 2004) >> Neuquenraptor argentinus (Novas & Pol, 2005) >> Araucanoraptor argentinus (Novas, 1997)
Based on a partial postcranium
Despite the relatively late appearance of this dinosaur in the fossil record (Upper Cretaceous), several features of Unenlagia are more birdlike than in any other non-avian theropod so far discovered. Unenlagia resembles Archaeopteryx in the morphology of the scapula, pelvis and hindlimb. But several shared primitive features of the pubus, ischium and hindlimb suggest that Unenlagia may represent the sister taxon of the Avialae (=Aves).
The structure of the forelimb suggests that the avian mode of the forelimb folding, and the extensive forelimb elevation necesssary for powered, flapping flight, was already present in cursorial, non-flying theropod dinosaurs. Unenlagia does resemble Archaeopteryx in important details of its forelimb and hip. Foremost among these is the structure of the shoulder joint, which indicates that the animal could hold its arm directly outward, like a bird, as well as fold it against its body.
This posture suggests that it could flap its arm, although Unenlagia was much too heavy to fly with such a short wing. The skeleton of a different and smaller animal will soon be described which is more complete and may contain dromaeosaurid specializations in the foot and tail. It is from strata that are slightly younger than those in which Unenlagia was found.
Were these animals basal birds? No, for they lived at a time when small, highly aerodynamic birds had been in existence for several tens of millions of years. But they probably resembled the true basal birds, which may have been present during middle Jurassic time, 80 million years before Unenlagia was alive.