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Among themorphological 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.
The vertebrate transition to land was accompanied by changes in a variety of functional systems including feeding, respiration, support and locomotion. The relative timing of morphological changes associated with these shifts is revealed exclusively through the study of fossil taxa. In recent years, our understanding of vertebrate terrestrialization has been greatly expanded by the description of intermediate forms such as Panderichthys rhombolepis from the late Givetian to early Frasnian of Latvia1,2, Elpistostege watsoni from the early Frasnian of Quebec3,4, and most recently Tiktaalik roseae from the early-middle Frasnian of Nunavut5,6. These and other taxa have revealed that some of the innovations traditionally associated with terrestriality first appeared in aquatic elpistostegalians. Examples of such features include body-supporting forelimbs and pectoral girdle; a neck supporting a head capable of independent motion; and raised, dorsally positioned eyes on a flattened skull. Recent phylogenetic analyses5,7,8,9 distribute these features along the tetrapod stem and, as a result, the origin of tetrapods in the Late Devonian (375-363 million years (Myr) ago) is now recognized as a stepwise process of character acquisition, rather than an abrupt shift. Despite this progress, a morphological gap has persisted between the cranial endoskeleton of tetrapodomorph fish (finned tetrapodomorphs) and that of tetrapods (limbed tetrapodomorphs). As a consequence of this apparent morphological disparity, numerous synapomorphies have been used to distinguish the tetrapod cranial...