205 — An exceptional Devonian fish from Australia sheds light on tetrapod origins

Long et al (10.1038/nature05243)

Read on 13 March 2018
#fossil  #paleontology  #Devonian  #fish  #tetrapod  #evolution  #vertebrates  #phylogeny  #x-ray-tomography  #x-ray  #Gogo-formation  #preservation  #tetrapodomorph  #pectoral-limb  #spiracle 

I’ve read a few papers by paleontologist John Long before (#109, #126) and, much like these other papers, this paper made big waves.

The species presented in this research, Gogonasus, took the crown for the most tetrapodomorphic fish discovered to date, helping to close the fossil-record gap between the fully aquatic fishes of the Devonian and the late-Devonian tetrapod-like (and perhaps more famous) Tiktaalik.

One particular, conspicuous feature is the shape and orientation of the spiracles, or respiratory holes in the skull. These wide spiracles — along with a complex internal spiracular architecture — are absent from “earlier” animals like Eusthenopteron. These spiracles are the evolutionary precursors to devoted external respiratory organs, and originally opened directly into the mouth. (They filled with water in these animals — they are not yet for air-breathing respiration this early in the evolutionary progression.)

This specimen, from the Gogo Formation in Australia, was also the only known case of full preservation of a tetrapodomorph pectoral limb, which shed a lot of light on the evolutionary pathway between the aquatic and the amphibious. The fin showed specializations that lean away from finlike bone structure in favor of a stouter, more articulated structure. This new structure in Gogonasus is the most closely related to the structure seen in the limb of Tiktaalik.

Though there is still a great deal to be learned about the transition from water to air, Gogonasus begins to tell the story of our transition from sea to land.