1. Department of Organismal Biology and Anatomy, The University of Chicago, Chicago, Illinois 60637, USA
2. Academy of Natural Sciences of Philadelphia, 19th and Benjamin Franklin Parkway, Philadelphia, Pennsylvania 19103, USA
3. Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, Harvard University, Cambridge, Massachusetts 02138, USA

Correspondence to: Neil H. Shubin¹ Correspondence and requests for materials should be addressed to N.H.S. (Email: nshubin@uchicago.edu).

Abstract

Wrists, ankles and digits distinguish tetrapod limbs from fins, but direct evidence on the origin of these features has been unavailable. Here we describe the pectoral appendage of a member of the sister group of tetrapods, Tiktaalik roseae, which is morphologically and functionally transitional between a fin and a limb. The expanded array of distal endochondral bones and synovial joints in the fin of Tiktaalik is similar to the distal limb pattern of basal tetrapods. The fin of Tiktaalik was capable of a range of postures, including a limb-like substrate-supported stance in which the shoulder and elbow were flexed and the distal skeleton extended. The origin of limbs probably involved the elaboration and proliferation of features already present in the fins of fish such as Tiktaalik.

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