The Water’s Edge

Between fin and limb

Tiktaalik roseae, found in Arctic Canada, has a mix of fish and tetrapod features that helped researchers understand the order in which four-limbed bodies were assembled. It stands in for the “fishapod” stage our ancestors passed through.

What the evidence shows

• Tiktaalik had scales, fin rays and a fish-like jaw, alongside a shortened skull roof, a mobile neck and a functional wrist joint.
• The rocks it came from suggest life in shallow, marginal waters, sometimes exposed to air.
• Trackways from Poland, about 18 million years older than the oldest tetrapod body fossils, suggest limbed animals already existed before Tiktaalik. Their interpretation has been debated.

What this means

• If those trackways are right, Tiktaalik was a late-surviving relative showing the body plan, not the first animal to take the step.

Sources

• Daeschler, Shubin & Jenkins 2006. A Devonian tetrapod-like fish and the evolution of the tetrapod body plan. Nature. (opens the publisher’s site)

  What it supports: Tiktaalik’s mix of fish and tetrapod features, including a mobile neck and a functional wrist joint, and a setting of shallow, marginal waters. It does not show how far the animal travelled on land.

• Niedźwiedzki et al. 2010. Tetrapod trackways from the early Middle Devonian period of Poland. Nature. (opens the publisher’s site)

  What it supports: Trackways that suggest limbed animals existed millions of years before Tiktaalik. Their interpretation has been debated.

Summaries here are our own. Some publishers limit access to full papers; learning on this site is never paywalled.