Nunavut Fossil Study Challenges Long-Held Theory on Why Early Tetrapods Developed Necks

For twenty years, a 380-million-year-old fossil from Ellesmere Island, Nunavut, has stood as a landmark in the story of how animals came out of the water. Tiktaalik roseae was the first known tetrapod, a four-limbed animal, to show a natural separation between its skull and its shoulders. In other words, it had something like a neck. A new study published this month in the journal Nature suggests the usual explanation for that neck may be wrong.

The long-held idea was simple. As animals adapted to land, a neck would have let them move the head more freely relative to the body. Dana Korneisel, a postdoctoral researcher at Carleton University and lead author of the study, describes the pre-existing view as one in which the anatomical neck came with the ability to bend and twist and to move the head independently of the body.

To test it, a team led by researchers from Carleton and the University of Ottawa turned to the lake sturgeon, the closest modern anatomical comparison to Tiktaalik roseae. Researchers from Penn State University and the University of Washington also took part. The team used fossils of Tiktaalik roseae and other prehistoric tetrapods to establish neck measurements, then applied those measurements to the backbone of a modern sturgeon on a universal testing machine at the University of Ottawa. The work benefited early on from a three-dimensional reconstruction of Tiktaalik's neck region made by co-author Tom Stewart of Penn State.

The results did not match the old expectation. The researchers estimated that Tiktaalik roseae could likely bend its neck less than six degrees from side to side. Korneisel says mobility was very limited both side to side and up and down, in roughly the same range as in a modern fish. She notes that little research had previously looked at how far early tetrapods could bend or twist their necks at all.

If the neck did not exist so the head could move more, why did it appear? Korneisel puts the question directly: some evolutionary driver separated the shoulder girdle from the skull, and if the benefit was not greater head movement, something else must explain it. She says the answer most consistent with other researchers' data is that the pectoral girdle and forelimb were being used for new kinds of motion and locomotion.

Hillary Maddin, a Carleton paleontologist and co-author, frames it as a new hypothesis. The neck, she says, may have had more to do with providing independent, robust stabilization of the limbs and shoulder girdle against the ground as the animal began to move on land. In the paper, the researchers conclude that the low neck mobility was most likely suited to more efficient swimming and to supporting the shoulders as the forelimbs moved.

Maddin also ties the finding to our own lineage. Life on Earth at that time had very few vertebrates, the group that includes humans, so she calls this a key step in the evolution of our lineage and of essentially all vertebrates on land.

The team is not finished. Korneisel hopes to run similar tests on other well-preserved early fish, which she says could show how Tiktaalik roseae and other early tetrapods moved through their whole bodies, not just the neck.