Gallinula disneyi, Boles, 2005

Boles, Walter E., 2005, A New Flightless Gallinule (Aves: Rallidae: Gallinula) from the Oligo-Miocene of Riversleigh, Northwestern Queensland, Australia, Records of the Australian Museum 57 (2), pp. 179-190 : 187

Flightlessness in Gallinula disneyi

The major morphological consequence of flightlessness is a reduction of the pectoral assemblage and forelimb. Concurrent with this is an increase in the size of the pelvic limb. The most obvious morphological change in the legs is an increase in the robustness of the elements. In the wing there are several structures that undergo obvious and characteristic modifications with the loss of flight.

Olson (1975a) and Rich et al. (1985) presented a range of features that characterize the humerus of non-volant birds. These are evident in a comparison between the Riversleigh fossils and the similar-sized humerus of Gallinula ventralis . In the fossils, the incisura capitis is directed more proximodistally, bringing it more in line with the shaft; this is caused by the entire proximal end of the humerus being tilted laterally relative to the rest of the element. By virtue of this tilting, the tuberculum ventrale becomes on the same proximal level as the caput humeri. The caput itself is considerably flattened and elongate. The crista deltopectoralis is thickened, reduced and rotated proximomedially in respect to the shaft. The crista bicipitalis is reduced. The shaft is curved and stout; however, this has not been preserved in the fossils. Other changes are seen in the fossil carpometacarpus. Compared to this element in the volant species, it is reduced, becoming shorter as well as stouter, os metacarpale majus is bowed in anterior view, and os metacarpale minus is more curved. Changes in the coracoid related to flightlessness include a more robust processus acrocoracoideus and a broader and more medially directed processus procoracoideus lacking the ventrally directed twist of the tip.

The proportional reduction in length is not uniform across the wing elements. There is a gradient in this proportion, with increased reduction from the proximal to the distal elements ( Livezey, 1995). This is characteristic of flightless birds, not just rails (see, for example, Gadow, 1902; Livezey, 1989, 1990, 1992; Livezey & Humphrey, 1986; Worthy, 1988). Direct comparison of the fossils with comparable elements of the volant Gallinula ventralis demonstrates that the fossil rails also exhibit this trend ( Fig. 6 ). The proximal end of the fossil humerus is slightly smaller than that of G. ventralis , primarily through the reduction of the caput humeri and cristae deltopectoralis and bicipitalis, while the distal end is larger. The carpometacarpus of the fossil is substantially shorter (78% of length) and more robust. In contrast, all fragments of the fossil’s hindlimb elements are considerably larger than the comparable sections of the bones of G. ventralis . Its larger legs and smaller wings compared to G. ventralis are a good indication that it was unable to fly.

Although Gallinula hodgenorum had greater reduction of the wings and pectoral apparatus, and more pronounced morphological differences from G. ventralis than had G. mortierii ( Olson, 1975b) , it showed trends in the hindlimb that are also evident in the Riversleigh bird. The shafts of both the tibiotarsus and tarsometatarsus are heavier those of G. ventralis . The crista cnemialis lateralis of the tibiotarsus is thicker. The proximal end of the tarsometatarsus is more expanded, as are the trochleae, which are also heavier. It is difficult to compare these usefully with the trochleae of G. disneyi because abrasion to the latter gives an underestimate of their size (see Fig. 6 ).

Figures

Fig. 6. Log-ratio plot of selected osteological measurements (see Table 62) of Gallinula disneyi and living and recently extinct species of native-hens Tribonyx. The standard species is Gallinula ventralis. Abbreviations are co, coracoid; h, humerus; ca, carpometacarpus; f, femur; tb, tibiotarsus; tm, tarsometatarsus; dlc, depth of condylus lateralis; dmc, depth of condylus medialis; dt3, depth of trochlea metatarsi III; dt4, depth of trochlea metatarsi IV; dw, distal width; paf, processus acrocoracoideus to facies articularis humeralis; pd, proximal depth; pw, proximal width; tl, total length.