Morphology, evolution, and function of the avian hallux

The hallux (pedal digit I) of birds is one example of a differentially elaborated serial structure, a common theme in biology. In contrast to their theropod dinosaur ancestors, the avian hallux was specialized for novel behaviors, grasping and perching. The transition from an unreversed to a reversed or opposed hallux, which took place during the early evolution of birds, involved both morphological and functional changes. Hallucal orientations of extant birds fall along a continuum from anteromedially directed to fully reversed, resulting from varying torsion of the shaft of metatarsal I. Identification of torsion, which is the primary determinant of hallucal orientation in extant birds, provides a means of directly estimating hallucal orientation in extinct theropods. Hallucal morphology in non-avian theropods is conservative with an anteromedially oriented and unreversed hallux. Hallucal evolution in Mesozoic birds is more complex than the previously hypothesized unreversed/reversed dichotomy. The hallux of the earliest known bird, Archaeopteryx, is not reversed but is anteromedially directed. Members of the clades Confuciusornithidae and Enantiornithes exhibit the first evidence of hallucal torsion and have medially directed halluces. Due to the incomplete nature of the fossil record, the most basal clade of birds with a “modern” foot remains unknown. Functional implications of hallucal reversion for terrestrial locomotion are compared in two species of birds with differing foot designs. Kinematic, kinetic, and electromyographic data were recorded for European starlings ( Sturnus vulgaris) and common quail (Coturnix coturnix). The prominent reversed hallux of starlings may function for deceleration during early stance phase, indicating that hallucal is not a hindrance to terrestrial locomotion as previously suggested. Muscle activity patterns of the m. flexor digitorum longus and m. flexor hallucis longus were similar both within and between species, suggesting that ancestral motor patterns for these muscles are likely retained in extant birds.