The transition from fishes to tetrapods was one of the most dramatic events in the evolution of vertebrates, but many pivotal fossils are incomplete, resulting in gaps in the data that are used for phylogenetic reconstruction. Here we present new observations from the most complete, acid-prepared Devonian tetrapodomorph fish yet discovered, Gogonasus1,2, which was previously placed just crownward of Kenichthys and rhizodontids3,4, the most primitive taxa on the tetrapod lineage. Unexpectedly, Gogonasus shows a mosaic of plesiomorphic and derived tetrapod-like features. Whereas the braincase and dermal cranial skeleton exhibit generalized morphologies with respect to Eusthenopteron5 or Panderichthys6, taxa that are traditionally considered to be phyletically close to tetrapods7,8, the presence of a deeply invaginated, wide spiracle, advanced internal spiracular architecture and near-horizontal hyomandibula are specialized features that are absent from Eusthenopteron9. Furthermore, the pectoral fin skeleton of Gogonasus shares several features with that of Tiktaalik, the most tetrapod-like fish10. A new phylogenetic analysis places Gogonasus crownward of Eusthenopteron as the sister taxon to the Elpistostegalia. Aspects of the basic tetrapod limb skeleton and middle ear architecture can now be traced further back within the tetrapodomorph radiation.
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We thank P. E. Ahlberg for discussion of the new material and access to new tomographic data on the hyomandibula of Eusthenopteron, and for unpublished information on the pectoral fin of Panderichthys, and M. I. Coates for discussion of the material. J.A.L. and G.C.Y. are supported by the Australian Research Council, T.J.S. acknowledges support under an ARC Research Fellowship, and the X-ray Tomography Facility is supported by an ARC LIEF grant. T.H. is supported by a Monash University Dean’s Postgraduate Scholarship, and E.M.G.F. is supported by an Australian Postgraduate Award. Author Contributions The specimen was prepared and described by J.A.L., G.C.Y. and T.H.; T.J.S. conducted X-ray tomography of the specimen including visualization; E.M.G.F. conducted the phylogenetic analyses and study of character evolution with data matrix input from J.A.L., G.C.Y. and T.H.
The animation shows the right posterolateral corner (quadrant) of the otico-occipital. The dermal bone is depicted in translucent brown, and endocranial bones rendered translucent beige. (See Supplementary Notes for further details.)*Note that this file was only uploaded on 23 October 2006.
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