AVIAN TRAITS IN THE ILIUM OF UNENLAGIA COMAHUENSIS (MANIRAPTORA, AVIALAE)

FERNANDO E. NOVAS

Feathered dragons: Studies on the transition from dinosaurs to birds

Indiana University Press

Año: 2004; p. 150 - 166

The pelvic girdle of Unenlagia comahuensis (Río Neuquén Formation, Turonian, NW Patagonia) seems to be intermediate in many anatomical respects, between that of dromaeosaurid maniraptorans and that of early birds (e.g., Rahonavis ostromi, Archaeopteryx lithographica, Confuciusornis sanctus). This paper remarks on some details of the Unenlagia comahuensis ilium that concern with the acquisition of avian characteristics. In the ilia of Saurornitholestes langstoni MOR 660, Deinonychus antirrhopus AMNH 3015, MCZ 4371, and Velociraptor mongoliensis IGM 100/985, a precursor of the avian processus supratrochantericus is expressed as a faint transverse enlargement of the posterodorsal iliac margin. Unenlagia comahuensis, instead, is more derived towards the avian condition because such process is more prominent and triangular-shaped. Also, it is associated with a ridge that runs posterodorsally the lateral surface of ilium above the acetabulum. The preacetabular blade of Unenlagia comahuensis resembles that of early birds in being cranially extended and rounded, in contrast with less derived coelurosaurs in which the cranial margin of ilium is straight (e.g., oviraptorosaurs, ornithomimids) or notched (e.g., Tyrannosaurus rex, most dromaeosaurids). Contrary to recent claims supporting that dromaeosaurids are more derived than Unenlagia comahuensis in the loss of a medial antiliac shelf for M. cuppedicus, it must be said that such crest is present and prominent in Deinonychus antirrhopus AMNH 3015, Saurornitholestes langstoni MOR 660, Velociraptor mongoliensis IGM 100/985, and Rahonavis ostromi UA 5686. The morphological features enumerated above strength the hypothesis depicting Unenlagia comahuensis as closer to birds than dromaeosaurids.