CLEVELAND, OHIO—Scientists are learning more about how some fish became landlubbers. According to a new study of a "missing link" fossil presented here yesterday at the annual meeting of the Society of Vertebrate Paleontology, these adventurous swimmers were already losing their ability to feed by suction before their fins evolved into limbs. By giving up the skill, which involves deforming the skull to create a vacuum that draws in prey, the fish were more able to raise their heads out of the water--and breathe air.
The missing link, a meter-long predator known as Tiktaalik roseae, was discovered in sandstone in northern Canada in 2004. Like all fishes, it had fins and scales. But in other aspects of its anatomy, it resembled four-limbed animals called tetrapods (Science , 7 April 2006, p. 33). Tiktaalik had a neck, for example, and a relatively flat, elongated skull. It also lacked another classic fish feature, the gill-covering bone called the operculum. Details of the braincase, which in fish consists of a set of bones nestled inside the skull, remained hidden inside the rock, however, and preparators spent years carefully exposing the bones.
Now an analysis by paleontologist Jason Downs of the Academy of Natural Sciences in Philadelphia, Pennsylvania, and colleagues shows how the braincase was becoming more like that of tetrapods. Overall, the braincase was more rigid, Downs says. Fish have very flexible skulls, a loose collection of bones that move around easily and allow them to suction-feed. In Tiktaalik, the joints between various bones in the braincase are more complex, suggesting that they did not allow as much motion.
Another important transition is evident in Tiktaalik's hyomandibular bone, which is essential for gill breathing. In fish, this bone coordinates the motions of the braincase, the palate, and the gill skeleton. But in tetrapods, the bone has lost these connections and shrunk, becoming part of the middle ear. Primitive fish have a boomerang-shaped hyomandibular bone, but Tiktaalik's is shorter and straight--tending toward the tiny dimensions of primitive tetrapods. It is connected to the braincase but not to the gill skeleton, which would have allowed the head more flexibility to move up and down. The braincase is also described by the same authors today in Nature.
The change in the hyomandibular bone suggests that gill respiration was becoming less important for Tiktaalik, says paleontologist Jennifer Clack of the University of Cambridge in the United Kingdom. She notes that all the modifications point in one direction: an animal that it getting better and better at raising its head out of shallow water to breathe air. Rather than a big transition in the braincase, as it had seemed before from other species, Tiktaalik shows how the evolution of the tetrapod braincase is "actually achievable by small, gradual steps," Clack says.