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, 278 (1716), 2333-7

The Bizarre Wing of the Jamaican Flightless Ibis Xenicibis Xympithecus: A Unique Vertebrate Adaptation

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The Bizarre Wing of the Jamaican Flightless Ibis Xenicibis Xympithecus: A Unique Vertebrate Adaptation

Nicholas R Longrich et al. Proc Biol Sci.

Abstract

Birds have frequently evolved to exploit insular environments by becoming adapted to a terrestrial lifestyle and losing the ability to fly, usually via reducing the wings and pectoral girdle. The enigmatic flightless ibis Xenicibis xympithecus (Threskiornithidae) from the Quaternary of Jamaica provides a rare example of flight loss in ibises. We report on previously undescribed fossils of Xenicibis, and show that the wing differed radically from that of all other birds, flightless or volant. The metacarpus is elongate, grotesquely inflated and has extremely thick walls; phalanges are short and block-like; the radius is distally expanded; and the humerus is elongate. The furcula, coracoid and sternum are all well developed. We propose that the elongate forelimb and massive hand functioned in combat as a jointed club or flail. This hypothesis is supported by the morphology of the carpometacarpus, by features permitting rapid extension of the wing and by the presence of fractures in wing bones. Although other birds use the wings as weapons, none resemble Xenicibis, which represents a unique and extraordinary morphological solution to this functional problem. Xenicibis strikingly illustrates how similar selective pressures, acting on a similar starting point, can result in novel outcomes.

Figures

Figure 1.
Figure 1.
Skeletal reconstruction of Xenicibis xympithecus based on USNM 460349 and fossils from the Red Hills Fissure cave deposits, Saint Andrews Parish, Jamaica.
Figure 2.
Figure 2.
Wing skeleton of X. xympithecus compared with Eudocimus albus and Apteribis glenos. (a) Left carpometacarpus (USNM 543067) in cranial view, (b) dorsal view, (c) ventral view and (d) caudal view. (e) Major metacarpal (USNM 543074) sectioned at midlength (note the greatly thickened anterior wall). (f) Radiograph of carpometacarpus of Eudocimus compared with (g) Xenicibis (USNM 543067). (h) Composite left wing of Xenicibis (USNM 543067-543073) compared with (i) Eudocimus (USNM 266467) and (j) Apteribis.
Figure 3.
Figure 3.
Selected skeletal elements of X. xympithecus from the Red Hills Fissure cave deposits, representing multiple individuals. Skull elements: (a) rostrum (USNM 543075) and mandible (USNM 543076) in lateral view and (b) dorsal view; (c) cranial cap (USNM 543077) in left lateral view and (d) dorsal view. Pectoral girdle: (e) coracoid (USNM 543078), (f) clavicle (USNM 543079) and (g) two fragments of sternal carina (USNM 543081 and 54080). Hindlimb: (h) tibiotarsus (USNM 543084), (i) femur (USNM 543083) and (j) tarsometatarsus (USNM 543085). Pelvis (USNM 543082): (k) dorsal view and (l) lateral view.
Figure 4.
Figure 4.
Fossils of Xenicibis showing evidence of trauma. (a) Fractured left humerus (USNM 543086 and 543087) showing fracture callus (fc): (i) photograph; (ii) radiograph. (b) Carpometacarpus of Xenicibis (USNM 460349) with rugose fracture callus on anterior, dorsal and ventral surfaces of the major metacarpal: (i) photograph; (ii) radiograph showing development of callus inside the medullary cavity.

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