1. Academy of Natural Sciences of Philadelphia, 1900 Benjamin Franklin Parkway, Philadelphia, Pennsylvania 19103, USA
2. Harvard University, Department of Organismic and Evolutionary Biology and Museum of Comparative Zoology, 26 Oxford Street, Cambridge, Massachusetts 02138, USA
3. University of Chicago, Department of Organismal Biology and Anatomy, 1027 E. 57th Street, Chicago, Illinois 60637, USA
4. Field Museum of Natural History, Chicago, Illinois 60605, USA

Correspondence to: Jason P. Downs¹Neil H. Shubin^3,4 Correspondence and requests for materials should be addressed to J.P.D. (Email: downs@ansp.org) or N.H.S. (Email: nshubin@uchicago.edu).

Abstract

Among the morphological changes that occurred during the 'fish-to-tetrapod' transition was a marked reorganization of the cranial endoskeleton. Details of this transition, including the sequence of character acquisition, have not been evident from the fossil record. Here we describe the braincase, palatoquadrate and branchial skeleton of Tiktaalik roseae, the Late Devonian sarcopterygian fish most closely related to tetrapods. Although retaining a primitive configuration in many respects, the cranial endoskeleton of T. roseae shares derived features with tetrapods such as a large basal articulation and a flat, horizontally oriented entopterygoid. Other features in T. roseae, like the short, straight hyomandibula, show morphology intermediate between the condition observed in more primitive fish and that observed in tetrapods. The combination of characters in T. roseae helps to resolve the relative timing of modifications in the cranial endoskeleton. The sequence of modifications suggests changes in head mobility and intracranial kinesis that have ramifications for the origin of vertebrate terrestriality.

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