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The origin of the internal nostril of tetrapods

Abstract

The choana, a unique ‘internal nostril’ opening from the nasal sac into the roof of the mouth, is a key part of the tetrapod (land vertebrate) respiratory system. It was the first component of the tetrapod body plan to evolve, well before the origin of limbs, and is therefore crucial to our understanding of the beginning of the fish–tetrapod transition. However, there is no consensus on the origin of the choana despite decades of heated debate1,2,3,4,5,6,7,8,9; some have claimed that it represents a palatally displaced external nostril4,6, but others have argued that this is implausible because it implies breaking and rejoining the maxillary–premaxillary dental arcade and the maxillary branch of nerve V2,6. The fossil record has not resolved the dispute, because the choana is fully developed in known tetrapod stem-group members8,10,11. Here we present new material of Kenichthys, a 395-million-year-old fossil fish from China12,13,14, that provides direct evidence for the origin of the choana and establishes its homology: it is indeed a displaced posterior external nostril that, during a brief transitional stage illustrated by Kenichthys, separated the maxilla from the premaxilla.

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Figure 1: Nostril positions on the heads of sarcopterygian fishes.
Figure 2: Photographs of Kenichthys campbelli specimens.
Figure 3: Morphology of Kenichthys.

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Acknowledgements

We thank M. M. Chang for advice and discussions, M. Yang for artwork, and X. Lu for specimen preparation. This work was supported by the Special Funds for Major State Basic Research Projects of China and the Chinese Foundation of Natural Sciences. P.E.A. thanks the Royal Society and Chinese Academy of Sciences for supporting his visit to Beijing in 2002 through their exchange programme.

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Correspondence to Min Zhu.

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Supplementary information

Supplementary Methods

Summary of the phylogenetic analysis. The character list. The data matrix. The consensus tree of 18 most parsimonious trees. The analysis based on a new expanded character suite confirms that Kenichthys is the most basal member of the tetrapod stem group. (DOC 709 kb)

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Zhu, M., Ahlberg, P. The origin of the internal nostril of tetrapods. Nature 432, 94–97 (2004). https://doi.org/10.1038/nature02843

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