The roadrunner (Geococcyx) has a funky, wide pelvis

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.

Figure 2. Geococcyx the roadrunner skeleton. Pelvis in several views.

Figure 1. Geococcyx the roadrunner skeleton. Pelvis in several views.

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.

Figure 1. Roadrunner (genus: Geococcyx) in dorsal view from the Sam Noble Museum in Norman OK USA.

Figure 2. Roadrunner (genus: Geococcyx) in dorsal view from the Sam Noble Museum in Norman OK USA. Image flipped left to right.

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.

Figure 2. Closeup of figure 1. with sacrum yellow and ilium green. This is a strange pelvis for a theropod or bird.

Figure 3. Closeup of figure 1. with sacrum yellow and ilium green. This is a strange pelvis for a theropod or bird.

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.

Figure 1. Acetabulum of Struthio.

Figure 4. Acetabulum of Struthio, the ostrich, more typical of birds, theropods and dinosaurs in general.

Galliformes
(chickens, turkeys, peacocks, curassaws, also have a posterior wide pelvis. These are also active terrestrial birds.

References
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.

wiki/Geococcyx

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