title: Tracing the evolution of avian wing digits.
abstract: It is widely accepted that birds are a subgroup of dinosaurs, but there is an apparent conflict: modern birds have been thought to possess only the middle three fingers (digits II-III-IV) of an idealized five-digit tetrapod hand based on embryological data, but their Mesozoic tetanuran dinosaur ancestors are considered to have the first three digits (I-II-III) based on fossil evidence. How could such an evolutionary quirk arise? Various hypotheses have been proposed to resolve this paradox. Adding to the confusion, some recent developmental studies support a I-II-III designation for avian wing digits whereas some recent paleontological data are consistent with a II-III-IV identification of the Mesozoic tetanuran digits. A comprehensive analysis of both paleontological and developmental data suggests that the evolution of the avian wing digits may have been driven by homeotic transformations of digit identity, which are more likely to have occurred in a partial and piecemeal manner. Additionally, recent genetic studies in mouse models showing plausible mechanisms for central digit loss invite consideration of new alternative possibilities (I-II-IV or I-III-IV) for the homologies of avian wing digits. While much progress has been made, some advances point to the complexity of the problem and a final resolution to this ongoing debate demands additional work from both paleontological and developmental perspectives, which will surely yield new insights on mechanisms of evolutionary adaptation.
authors.affiliation: Key Laboratory of Vertebrate Evolution and Human Origin of Chinese Academy of Sciences, Institute of Vertebrate Paleontology & Paleoanthropology, Chinese Academy of Sciences, Beijing, 100044 China. xu.xing@ivpp.ac.cn
authors.forename: Xing
authors.initials: X
authors.lastname: Xu
authors.forename: Susan
authors.initials: S
authors.lastname: Mackem
mesh_terms: D000818:Animals; D005075:Biological Evolution; D001717:Birds / Q000196:embryology / Q000502:physiology*; D025061:Dinosaurs / Q000502:physiology*; D005121:Extremities; D005580:Fossils; D051379:Mice; D023421:Models, Animal; D010163:Paleontology; D010802:Phylogeny; D014921:Wings, Animal / Q000033:anatomy & histology* / Q000502:physiology*