1. Subdepartment of Evolutionary Organismal Biology, Department of Physiology and Developmental Biology, Evolutionary Biology Centre, Uppsala University, Norbyv 18A, SE-752 36 Uppsala, Sweden

Correspondence to: Martin D. Brazeau¹ Correspondence and requests for materials should be addressed to M.D.B. (Email: martin.brazeau@ebc.uu.se).

Few fossils show the incipient stages of complex morphological transformations¹. For example, the earliest stages in the remodelling of the spiracular tract and suspensorium (jaw suspension) of osteolepiform fishes^{2, 3, 4} into the middle ear of tetrapods have remained elusive³. The most primitive known tetrapods show a middle ear architecture that is very different from osteolepiforms such as Eusthenopteron ³, with little indication of how this transformation took place. Here we present an analysis of tetrapod middle ear origins that is based on a detailed study of Panderichthys, the immediate sister taxon of tetrapods. We show that the spiracular region is radically transformed from osteolepiforms and represents the earliest stages in the origin of the tetrapod middle ear architecture. The posterior palatoquadrate of Panderichthys is completely tetrapod-like and defines a similarly tetrapod-like spiracular tract. The hyomandibula has lost its distal portion, representing a previously unrecognized advance towards a stapes-like morphology. This spiracular specialization suggests that the middle ear of early tetrapods evolved initially as part of a spiracular breathing apparatus^{5, 6}.

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