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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The illustrations are the work of K. Monoyios. Specimen preparation was performed by C. F. Mullison. This research was made possible by permits from the Nunavut Ministry of Culture, Languages, Elders and Youth, the Grise Fiord Hamlet and HTA, with logistical support from the Polar Continental Shelf Project and collections support from the Canadian Museum of Nature. M. Brazeau improved interpretation of P. rhombolepis and offered input. Assistance in the field (1999–2006) was provided by W. Amaral, B. Atagootak, J. Conrad, M. Davis, S. Gatesy, A. Gillis, B. Kilbourne, S. Madsen, K. Middleton, J. Miller, K. Monoyios, C. Schaff, M. Shapiro, R. Shearman and C. Sullivan. This research was supported by an anonymous donor, the Academy of Natural Sciences of Philadelphia, the Putnam Expeditionary Fund (Harvard University), the University of Chicago, the National Science Foundation, and the National Geographic Society Committee for Research and Exploration.
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Downs, J., Daeschler, E., Jenkins, F. et al. The cranial endoskeleton of Tiktaalik roseae. Nature 455, 925–929 (2008). https://doi.org/10.1038/nature07189
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