Biostratigraphic and biogeographic context for tetrapod origins during the Devonian: Australian evidence

Abstract

The geological context and evidence of age for three Australian occurrences attributed to Devonian tetrapods (trackways from Genoa River and the Grampians in Victoria, and an isolated jaw from Jemalong near Forbes in New South Wales) are discussed and updated. Associated fish remains indicate a Late Silurian or earliest Devonian age for the Grampians occurrence, and a Late Devonian (probably Frasnian) age for the Jemalong and Genoa River occurrences, although a Famennian age cannot be entirely excluded. The very sparse fossil record of Devonian tetrapods from the largest landmass of the time (Gondwana) is noted, as is the possibly earlier (?Frasnian) age for a tetrapod jaw recently reported from China. Consideration of new discoveries of sarcopterygian (lobefinned) fishes in Australia-Antarctica (East Gondwana), and the time-space distribution of some Devonian fishes, in particular phyllolepid and other placoderms associated with tetrapods in both Southern and Northern Hemispheres, but with a different age range, provide grounds for suggesting an alternative to the hypothesis that tetrapods evolved during a narrow time interval (Middle-Late Devonian) in the northern continent of Laurussia. The evidence for Frasnian-Famennian faunal interchange between Gondwana and Laurussia involving various Devonian vertebrate groups is summarised, and the possible utility of some placoderm fish groups as indicator species for tetrapod origins is discussed.