Osteology and myology of the wing of the Emu (Dromaius novaehollandiae), and its bearing on the evolution of vestigial structures

Abstract

Emus have reduced their wing skeleton to only a single functional digit, but the myological changes associated with this reduction have never been properly described. Moreover, the intraspecific variability associated with these changes has not previously been examined, dissections having been restricted in the past to only one or two individuals. In this paper, the myology and osteology of the Emu wing is described for a sample of five female birds. The Emu showed a marked reduction in the number of muscles in the wing, even compared with other ratites. Many wing muscles showed diversity in structure, origin and insertion sites, number of heads, as well as presence-absence variation. This variability dramatically exceeds that found in flying birds. Evolutionary theory predicts that relaxed selection on vestigial organs should allow more variation to persist in the population, and corresponds to what is observed here. A large amount of fluctuating asymmetry was also detected, indicating reduced canalization of the wing during development.

Limb patterning genes and heterochronic development of the emu wing bud

“…The bony elements of the emu wing are all greatly reduced relative to body size. The humerus, radius, ulna and metacarpals are all reduced, while the typical avian three-digit arrangement is replaced by a single claw-bearing digit (corresponding to embryonic digit III) [5]. However, during emu embryonic development, mesenchymal condensations corresponding to digits II, III, IV and V develop, transiently expressing the chondrogenesis marker, Sox9 [9].…”

“…Within the avian lineage, the most striking morphological variation of the forelimb is associated with the loss of flight among ratites. Modern ratites (emus, ostriches, rheas, cassowary and kiwi) have lost the ability to fly and have structurally altered or vestigial wing elements [5]. Ratites were once considered a monophyletic group, distributed across the world through vicariance following the breakup of Gondwanaland.…”

Twenty-first century advances in knowledge of the biology of moa (Aves: Dinornithiformes): a new morphological analysis and moa diagnoses revised

“…Os scapulocoracoideum, rugose area on cranial facies pars coracoideum for origin of the tendon of m. supracoracoideus: prominent, rounded, and abutting the cavitas glenoidalis (0); pronounced crest (1); low to obsolete, offset sternally of the cavitas glenoidalis (2). Note: see Maxwell & Larsson (2007). Galloanseres are non comparable as have separate coracoid and scapula; moas are non comparable because their coracoids lack a glenoid facet.…”

Ossification sequence of the avian order anseriformes, with comparison to other precocial birds

“…The amount of ossification sequence variability detected is not significantly greater than in more traditional morphological data sets (Berger, 1956;Mabee et al, 2000;Maisano, 2002;Maxwell and Larsson, 2007;Maxwell, 2008). Consistent intraspecific variation in some taxa suggests that ecological factors may also play a role (Sheil and Greenbaum, 2005), but these remain poorly defined.…”