1. Museum Victoria, P O Box 666, Melbourne, Australia 3001
2. Department of Earth and Marine Sciences, The Australian National University, Canberra, Australia 0200
3. School of Geosciences, Monash University, Clayton, Victoria, Australia 3800
4. Department of Applied Mathematics, Research School of Physical Sciences and Engineering, The Australian National University, Canberra, Australia 0200

Correspondence to: John A. Long^1,2,3 Correspondence and requests for materials should be addressed to J.A.L. (Email: jlong@museum.vic.gov.au).

Abstract

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, Gogonasus^{1, 2}, which was previously placed just crownward of Kenichthys and rhizodontids^{3, 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 tetrapods^{7, 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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