Here’s another tetrapodomorph fish to consternate the creationists. These Devonian/Carboniferous animals just keep popping up to fill in the gaps in the evolutionary history of the tetrapod transition to the land—the last one was Tiktaalik.
This lovely beastie is more fish than frog, as you can tell—it was a marine fish, 384-380 million years old, from Australia, and it was beautifully preserved. Gogonasus is not a new species, but the extraction and analysis of a new specimen has caused its position in the evolutionary tree to be reevaluated.
Here’s a little more detail on the skull:
And here’s the pectoral limb:
That limb shows some interesting similarities to that of Tiktaalik, and a more detailed evaluation of the newly revealed characters has pushed Gogonasus a little higher in the tetrapod family tree. It also suggests that the pattern of limb bones is more ancient than previously thought, and some features regarded as characteristic of the tetrapod transition are rooted farther back, in more fish-like functional states.
One of the success stories of evolutionary theory is that we keep finding these organisms that fit so well into an evolutionary framework, and another is that these discoveries lead to further predictions. Gogonasus is no exception: its discovery in Australia suggests that there ought to be more transitional tetrapodomorphs waiting to be found there, and the researchers have already started looking for them.
Finally, we note that strata of similar age to those producing Gogonasus, Panderichthys and Tiktaalik, or slightly younger, have yielded tetrapod jaws in Australia and China, and also trackways attributed to two different unknown tetrapods on the Gondwana supercontinent. This indicates that the initial radiation of tetrapods from elpistostegalian fishes, with evidence currently confined to the northern hemisphere landmass of Euramerica, was probably an extremely rapid global event. Migration of some Middle?Late Devonian fishes from a Gondwana place of origin to Euramerica has been well documented. With Gogonasus now positioned phyletically close to elpistostegalians, we suggest that the current lack of fossil evidence for elpistostegalian fishes in Gondwana could be due to poor sampling. We are now exploring areas in Australia with undescribed Devonian sarcopterygian fish remains to test this hypothesis.
Any bets on whether they’ll find them?
Long JA, Young GC, Holland T, Senden TJ, EMG Fitzgerald(2006) An exceptional Devonian fish from Australia sheds light on tetrapod origins. Nature <doi:10.1038/nature05243>