An exceptional Devonian fish from Australia sheds light on tetrapod origins

TL;DR: New observations from the most complete, acid-prepared Devonian tetrapodomorph fish yet discovered, Gogonasus are presented, which was previously placed just crownward of Kenichthys and rhizodontids, the most primitive taxa on the tetrapOD lineage.

Abstract: The evolutionary transition from water to land exerts a continuing fascination, heightened by recent discoveries of transitional fossils in Canada and the reinterpretation as tetrapods (or near-tetrapods) of fossils once classified as fishes. But signs of land life are detectable even further back. A spectacularly preserved 380-million-year old fossil of the fish Gogonasus from the Devonian of Australia is fish-like in many respects, yet features of its ear and limbs are unexpectedly advanced. 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.