José L. Sanz, Luis M. Chiappe, Bernardino P. Pérez-Moreno, José J. Moratalla, Francisco Hernández-Carrasquilla, Angela D. Buscalioni, Francisco Ortega, Francisco J. Poyato-Ariza, Diego Rasskin-Gutman, Xavier Martínez-Delclòs

A feathered skeleton of a Lower Cretaceous enantiornithine bird from Spain indicates that the modified diapsid skull of modern birds did not evolve until late in their evolution: Basal birds retained an essentially primitive diapsid design. The fossil provides data clarifying long-standing debates on the cranial morphology of the basalmost bird, Archaeopteryx. It also reemphasizes the notion that the early morphological transformations of birds were focused on the flight apparatus. This fossil was a nestling and suggests that early postnatal developments in the Cretaceous enantiornithine birds and those in their extant counterparts are comparable.

J. L. Sanz, B. P. Pérez-Moreno, J. J. Moratalla, A. D. Buscalioni, F. Ortega, F. J. Poyato-Ariza, Unidad de Paleontología, Departamento de Biología, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain.
L. M. Chiappe, Department of Ornithology, American Museum of Natural History, New York, NY 10024, USA.
F. Hernández-Carrasquilla, Unidad de Arqueozoología, Departamento de Biología, Facultad de Ciencias, Universidad Autónoma de Madrid, 28049 Madrid, Spain.
D. Rasskin-Gutman, Department of Paleobiology, MRC 121, National Museum of Natural History, Smithsonian Institution, Washington, DC 20560, USA.
X. Martínez-Delclòs, Departament de Geologia dinàmica, Geofísica i Paleontologia, Facultat de Geologia, Universitat de Barcelona, 0871 Barcelona, Spain.