Tinamids are small cursorial birds with limited flight ability. They are phylogenetically nested within a clade composed mostly of large, flightless birds (ratites). Their ability to fly and the evolution of flightlessness in the clade are currently not well understood, and this is largely due to a paucity of literature on the osteological and muscular anatomy of these birds.
Two Eudromia elegans (Palaeognathae: Tinamidae) were dissected and four skeletons were examined. The skeletons of Eudromia are characterized by a thin sternum, short and stout humeri, and developed lower limbs. Eudromia maintains some ancestral characters in the postcranial skeleton, such as elongated lateral trabeculae of the sternum, an absent rostral external spine of the sternum, and lack of fusion between the distal ilium and ischium. The lost of the hallux suggests cursorial adaptation occured not only in Strutioformes (Struthio has only two digits) but also even in tinamids. Accordingly, the muscles inserting on the hallux are lost or shifted to other digits. However, the body plan of Eudromia is quite similar to other modern volant birds, like Gallus.
Our specimens showed differences in musclular morphology from previous descriptions of tinamou anatomy, particularly, the absence of M. iliofemoralis internus and the femoral head of M. tibialis cranialis. We also compared Eudromia with other ratites, such as Struthio, Rhea, Apteryx, and Dromaius. In the shoulder girdle, Eudromia has a large M. pectoralis thoracica and M. supracoracoideus, which are used for the downstroke and upstroke, respectively, but these muscles do not have such antagonistic actions in other ratites due to differing origins and insertions. The morphological changes in the pelvic girdle are minor compared with those in shoulder and forelimb regions. Ratite have enlarged Mm. femorotibiales, and M. iliotorochantericus medius and M. ischiofemoralis are diminished in size, except in Apteryx. Eudromia has a thin M. caudofemoralis pars pelvica, while other ratites have lost M. caudofemoralis pars caudalis (in Dromaius and Rhea) or it is considerably diminished (in Struthio). The differences between Eudromia and other ratites we find are attributed to the retention of flight ability in Eudromia.