MURKY ORIGINS

Until recently, scientists’ grasp of the evolutionary transition between fishes and early tetrapods hinged mainly on several spectacular fossils that seem to bridge these two groups. One is from a fish called Panderichthys rhombolepis from the Baltic region and dates to the Middle to Late Devonian period (around 384 million to 379 million years ago). With its elongated humerus and large radius and ulna (the upper arm bone and forearm bones, respectively, in tetrapods) and its tetrapodlike skull bone pattern, Panderichthys offered the first clues that the group of fishes to which it belongs were the closest fishes to the tetrapods. The group is called the elpistostegalians, after the then poorly known Elpistostege from eastern Canada.

In 2006 Neil Shubin of the University of Chicago and his colleagues announced the discovery of another elpistostegalian fish fossil, 380-million-year-old Tiktaalik roseae from the Canadian Arctic. Tiktaalik was a real game changer in revealing a slew of new data showing that the pectoral fin was highly advanced in these fishes—more so than any other known fossil—with both well-developed arm bones and mobile wrist joints. Its skull also had distinctive features, including a long, flat snout and a specialized braincase—traits shared by tetrapods.

Together, this and the other known elpistostegalian fish fossils suggested that a number of hallmark traits of tetrapods originated in their piscine predecessors, including land-worthy arm bones and joints. But what these fishes did not appear to have were fingers. In the case of Panderichthys, bony elements that many researchers initially thought were rudimentary digit bones were later rejected as such. And the Tiktaalik fossil, for its part, did not preserve the complete tip of the pectoral fin, where one would expect to find digit bones if the animal had them. The available evidence left experts to conclude that fingers were not part of the initial fin-to-limb transition. Instead they appeared to have evolved later, after tetrapods had already staked a claim on terra firma.