Evolution and Homology of the Astragalus in Early Amniotes: New Fossils, New Perspectives F. Robin O’Keefe, 1 * Christian A. Sidor, 1 Hans C.E. Larsson, 2 Abdoudaye Maga, 3 and Oumarou Ide 3 1 Department of Anatomy, New York College of Osteopathic Medicine, Old Westbury, New York 11568 2 Redpath Museum, McGill University, Montreal, Quebec H3A 2K6, Canada 3 Institut de Recherches en Sciences Humaines, Niamey, Niger Republic ABSTRACT The reorganization of the ankle in basal amniotes has long been considered a key innovation al- lowing the evolution of more terrestrial and cursorial be- havior. Understanding how this key innovation arose is a complex problem that largely concerns the homologizing of the amniote astragalus with the various ossifications in the anamniote tarsus. Over the last century, several hy- potheses have been advanced homologizing the amniote astragalus with the many ossifications in the ankle of amphibian-grade tetrapods. There is an emerging consen- sus that the amniote astragalus is a complex structure emerging via the co-ossification of several originally sep- arate elements, but the identities of these elements re- main unclear. Here we present new fossil evidence bear- ing on this contentious question. A poorly ossified, juvenile astragalus of the large captorhinid Moradisaurus grandis shows clear evidence of four ossification centers, rather than of three centers or one center as posited in previous models of astragalus homology. Comparative material of the captorhinid Captorhinikos chozaensis is also inter- pretable as demonstrating four ossification centers. A new, four-center model for the homology of the amniote astragalus is advanced, and is discussed in the context of the phylogeny of the Captorhinidae in an attempt to iden- tify the developmental transitions responsible for the ob- served pattern of ossification within this clade. Lastly, the broader implications for amniote phylogeny are discussed, concluding that the neomorphic pattern of astragalus os- sification seen in all extant reptiles (including turtles) arose within the clade Diapsida. J. Morphol. 267:415– 425, 2006. © 2006 Wiley-Liss, Inc. KEY WORDS: amniote; ankle; astragalus; captorhinid; homology The tarsus in amniotes differs radically from that of anamniote tetrapods. Relative to amphibian- grade taxa, amniotes have fewer bony elements in the ankle, and the elements play an increased, more active role in locomotion by stiffening and stabiliz- ing the pes (Sumida, 1997). This repatterning of the tarsus has long been considered a key innovation in the transition between amphibian-grade and reptilian-grade tetrapods (Romer, 1956; Gauthier et al., 1988), allowing increased terrestriality and loco- motor efficiency. Unfortunately, this transition is not well understood; the homologies of the various tarsal elements are unclear, rendering the study of specific joint articulations difficult (Sumida, 1997, p. 387). New data and new insights on the developmen- tal and evolutionary repatterning of the amniote ankle are therefore of prime importance, for they bear on one of the key transitions in tetrapod his- tory: the attainment of full terrestriality after the long transition from fin to limb (Clack, 2002). Perhaps the most important novelty within the amniote tarsus is the astragalus, a large, complex bone comprising the primary area of articulation for the distal tibia. The definitive astragalus first ap- pears at the origin of the clade Amniota (Fig. 1A); amphibian-grade tetrapod taxa below this node, such as seymouriamorphs and anthracosaurs (re- gardless of the current debate over their relation- ships to each other and to Amniota; Clack, 2002), generally maintain the pattern of tarsal bones char- acteristic of most anamniote tetrapods, while synap- sids and reptiles possess a true astragalus (relation- ships from Coates, 1996; Sumida, 1997). This obvious correlation between amniote origins and the appear- ance of the astragalus has lead to a long discussion of the homologies of this bone (Gegenbauer, 1864; Zittel, 1932; Romer, 1956; Rieppel, 1993; Kissel et al., 2002; Berman and Henrici, 2003). However, a lack of definitive evidence has precluded a satisfac- tory resolution. Here we present new fossil evidence that may help to resolve this debate. Contract grant sponsor: National Geographic Society; Contract grant number: 7258-02 (to C.A.S.); Contract grant sponsor: New York College of Osteopathic Medicine (research funds to C.A.S. and F.R.O.); Contract grant sponsor: National Sciences and Engineering Research Council, Canada Research Chairs; Contract grant sponsor: Fonds Que ´be ´cois de la Recherche sur la Nature et les Technologies (to H.C.E.L.). *Correspondence to: F.R. O’Keefe, Assistant Professor, Dept. of Anatomy, New York College of Osteopathic Medicine, NYCOM II Rm. 326, Old Westbury, NY 11568-8000. E-mail: frokeefe@nyit.edu Published online 18 January 2006 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/jmor.10413 JOURNAL OF MORPHOLOGY 267:415– 425 (2006) © 2006 WILEY-LISS, INC.