You can’t tell
by looking at the skeleton in lateral view (Fig. 1), but the roadrunner pelvis (Figs. 1–3) is definitely different in dorsal and ventral view.
On a recent trip to the Sam Noble Museum
(Oklahoma Museum of Natural History, OMNH) in Norman, Oklahoma, I happened to look down at a roadrunner skeleton (genus: Geococcyx, Figs. 1–3) in the kid’s section. That pelvis struck me as quite odd and indeed it is, relative to other birds, other theropods and other dinosaurs. Even the road-running ostrich (genus: Struthio, Fig. 4) does not have such a wide pelvis.
Roadrunners are ground cuckoos,
better at sprinting than flying. The heavily muscled hind limbs of roadrunners are well anchored on this laterally expanded pelvis. Truth be told: I have not, but would like to see a muscle comparison between a roadrunner and ostrich (Fig. 4)… then try to figure out why the roadrunner pelvis is so different.
Geococcyx californum (Lesson 1829, Wagler 1831; up to 60cm long) the extant roadrunner is a small terrestrial cuckoo/trumpeter and a basal neognath with a posteriorly rotated pedal digit 4, unrelated to parrots and toucans with a similar toe. Geococcyx nests with the cuckoo, Coccyzus and both nest with the long-legged trumpeter, Psophia.
(chickens, turkeys, peacocks, curassaws, also have a posterior wide pelvis. These are also active terrestrial birds.
Lesson RP 1828, 1829. Genera des Oiseaux u Nort de l’Amérique, et Synopsis des especes qui vivent aux Etats-Unis; par Charles-Lucien Bonaparte. Féruss. Bull. 2 sect 13:122-125.
Wagler 1831. Einige Mitheilungen über Thiere Mexicos. Oken’s Isis 24:510–535.
Zinoviev A 2007. Apparatus of bipedal locomotion of cuculiforms (Aves, Cuculiformes): Scenario of an adaptive radiation. Zoologichesky Zhurnal 86(10):1–9.