RESEARCH ARTICLE Greater Growth of Proximal Metatarsals in Bird Embryos and the Evolution of Hallux Position in the Grasping Foot JOÃO FRANCISCO BOTELHO 1,2,3,4 , DANIEL SMITH-PAREDES 1 , SERGIO SOTO-ACUÑA 1,5 , DANIEL NÚÑEZ-LEÓN 1 , VERÓNICA PALMA 4 , AND ALEXANDER O. VARGAS 1∗ 1 Departamento de Biología, Laboratorio de Ontogenia y Filogenia, Facultad de Ciencias de la Universidad de Chile, Santiago, RM, Chile 2 Department of Anatomy, Faculty of Medical and Health Sciences, University of Auckland, Auckland, New Zealand 3 Instituto Nacional de Ciência e Tecnologia em Estudos Interdisciplinares e Transdisciplinares em Ecologia e Evolução (IN-TREE), Salvador, BA, Brazil 4 Departamento de Biología, Laboratorio de Células Troncales y Biología del Desarrollo, Facultad de Ciencias de la Universidad de Chile, Santiago, RM, Chile 5 Área de Paleontología, Museo Nacional de Historia Natural, Santiago, RM, Chile In early theropod dinosaurs—the ancestors of birds—the hallux (digit 1) had an elevated posi- tion within the foot and had lost the proximal portion of its metatarsal. It no longer articulated with the ankle, but was attached at about mid-length of metatarsal 2 (mt2). In adult birds, the hallux is articulated closer to the distal end of mt2 at ground level with the other digits. How- ever, on chick embryonic day 7, its position is as in early theropods at half-length of mt2. The adult distal location is acquired during embryonic days 8–10. To assess how the adult phenotype is acquired, we produced fate maps of the metatarsals of day 6 chicken embryos injecting the lipophilic tracer DiI. The fates of these marks indicate a larger expansion of the metatarsals at their proximal end, which creates the illusory effect that d1 moves distally. This larger proximal expansion occurs concomitantly with growth and early differentiation of cartilage. Histological analysis of metatarsals shows that the domains of fattened and prehypertrophic chondrocytes are larger toward the proximal end. The results suggest that the distal position of the hallux in the avian foot evolved as a consequence of an embryological period of expansion of the metatarsus toward the proximal end. It also brings attention to the developmental mechanisms leading to ABSTRACT Confict of interest: None. ∗ Correspondence to: Alexander O. Vargas, Departamento de Biología, Laboratorio de Ontogenia y Filogenia, Facultad de Ciencias de la Universidad de Chile, Santiago, RM 7800003, Chile. E-mail: alexvargas@uchile.cl Received 15 March 2016; Revised 10 August 2016; Accepted 16 August 2016 DOI: 10.1002/jez.b.22697 Published online in Wiley Online Library (wileyonlinelibrary.com). C  2016 WILEY PERIODICALS, INC.