title: Bird embryos uncover homology and evolution of the dinosaur ankle.
abstract: The anklebone (astragalus) of dinosaurs presents a characteristic upward projection, the 'ascending process' (ASC). The ASC is present in modern birds, but develops a separate ossification centre, and projects from the calcaneum in most species. These differences have been argued to make it non-comparable to dinosaurs. We studied ASC development in six different orders of birds using traditional techniques and spin-disc microscopy for whole-mount immunofluorescence. Unexpectedly, we found the ASC derives from the embryonic intermedium, an ancient element of the tetrapod ankle. In some birds it comes in contact with the astragalus, and, in others, with the calcaneum. The fact that the intermedium fails to fuse early with the tibiale and develops an ossification centre is unlike any other amniotes, yet resembles basal, amphibian-grade tetrapods. The ASC originated in early dinosaurs along changes to upright posture and locomotion, revealing an intriguing combination of functional innovation and reversion in its evolution.
authors.affiliation: Departamento de Biología, Laboratorio de Ontogenia y Filogenia, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Ñuñoa, Santiago 7800003, Chile.
authors.forename: Luis
authors.initials: L
authors.lastname: Ossa-Fuentes
authors.affiliation: Departamento de Biología, Laboratorio de Neurobiología y Biología del Conocer, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Ñuñoa, Santiago 7800003, Chile.
authors.forename: Jorge
authors.initials: J
authors.lastname: Mpodozis
authors.affiliation: Departamento de Biología, Laboratorio de Ontogenia y Filogenia, Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Ñuñoa, Santiago 7800003, Chile.
authors.forename: Alexander O
authors.initials: AO
authors.lastname: Vargas
mesh_terms: D000818:Animals; D000842:Ankle / Q000033:anatomy & histology* / Q000196:embryology; D005075:Biological Evolution; D001717:Birds / Q000033:anatomy & histology / Q000145:classification / Q000196:embryology* / Q000235:genetics*; D025061:Dinosaurs / Q000033:anatomy & histology / Q000235:genetics*; D005580:Fossils; D010802:Phylogeny