ONLY SEVEN months ago the report of the discovery of Tiktaalik, a tetrapod-like fish, was greeted with elation by the science-minded. Although this animal was clearly a fish, it had front fins that were more like primitive forelimbs (a fossil of the posterior half has not yet been found). Examination of these limbs showed they were capable of propping the fish up. Additionally, Tiktaalik has many other features unusual for a fish showing it was becoming adapted to living in extremely shallow waters with possible forays onto dry land.

This week we greet another unusual fish. John Long and coworkers report in Nature‘s advance online publication the discovery of Gogonasus. While members of this genus have been discovered previously, their state of preservation was poor and this fish’s unusual features were not appreciated.

The first feature is a large, wide spiracle. The spiracle began in fish as a small opening, a gap in the gill covering to allow water to pass in. Large spiracles are present in modern cartilaginous fish such as rays, and in these animals they are used to pull water past the gills without the fish inhaling sand as it lies on the ocean floor. In tetrapodomorphs the spiracle initially is thought to have been used in this way, and possibly also in some for air breathing. The process of forming spiracles involved a slow decrease in the size of various bones making up the skull. In later evolutionary history the spiracle evolved into the middle ear. Instead of a spiracle, tetrapod have an otic notch that was probably covered by a tympanum. Ultimately the reduced skull bones became our modern mallus, stapes, and incus.

To take one of my typical side-tracks, some anti-evolutionists seem to think that the evolutionary process requires traits to appear suddently intact and functioning for their modern purposes. In actuality traits often appear incidental to other modifications, are adapted to serve one original purpose, and then later are modified to serve entirely different purposes. This process is illustrated by the evolution of the tetrapod ear from the fish’s spiracle.

One fish previously considered typical of early tetrapodomorphs is Eusthenopteron. This lobe-finned fish has some early tetrapod features, but its spiracle is still small, as in other fish. The excellent preservation of this fossil of Gogonasus shows that Gogonasus had a large spiracle intermediate between that of Eusthenopteron and Panderichthys, whose well-developed spiracle has been quite useful in tracing the evolution of the spiracle and otic notch. Some unusual features of Gogonasus‘ spiracle absent in later tetrapodomporphs indicate that spiracular breathing may have evolved independently several times, although more research is required to clarify this.

Gogonasus also has bony front fins that to these untrained eyes very closely resemble those of Tiktaalik, although less robust and with a simplified bony structure. However, this fish lacks the modifications of the pectoral girdle that make Tiktaalik so unusual. Additionally, while Tiktaalik‘s forelimb’s bones showed articulation that would allow flexing of the “wrist”, Gogonasus‘ apparently did not have this mobility.

This fossil was discovered in strata about 384-380 million years old, while Tiktaalik is about 375 million years old. The discovery of Gogonasus has enabled us to more clearly map out the appearance and evolution of tetrapod traits in the tetrapodomorph phylogenetic tree.

Finally, Tiktaalik was discovered in Canada, while Gogonasus was discovered in Australia. During Devonian times Canada was contained in the continent of Euramerica, while Australia was part of Gondwana. Most of the data on tetrapod evolution so far are based on fossils found on Euramerica. The authors note that tetrapod jaws and trackways found on land that was part of Gondwana and now the presence of this tetrapodomorph on the shores of Gondwana suggest that there may indeed be early tetrapod evidence in Gondwanan strata, and they are currently searching for these fossils in Australia. Since Gondwana was quite close to Euramerica at this time and some Devonian fish are known to have migrated from Gondwana to Euramerica, it’s possible some fossils as impressive as Tiktaalik await paleontologists there.

“An exceptional Devonian fish from Australia sheds light on tetrapod origins.” Long, J.; Young, G.; Holland, T.; Senden, T.; Fitzgerald, E. Nature. 18 October 2006, advance online publication.

“Tetrapod-like middle ear architecture in a Devonian fish.” Brazeau, M.; Ahlberg, P. Nature. 2006, 439, 318-231.

“The pectoral fin of Tiktaalik roseae and the origin of the tetrapod limb.” Shubin, N.; Daeschler, E.; Jenkins, F. Nature. 2006, 440, 764-771.