Even before fins evolved into legs, our ancient ancestors’ skeletons were changing in ways that would help them support their bodies on land. A team of scientists, including a biologist from Penn State University, have created a new reconstruction of Tiktaalik, a 375 million year old fossil fish considered one of our closest relatives that had limbs.

The new reconstruction shows that Tiktaalik’s ribs were likely attached to its pelvis. This is an important innovation because it helped support the body and allowed for the eventual evolution of walking.

The researchers used high-resolution micro-computed tomography (micro-CT) scans to examine the fossil and revealed vertebrae and ribs that were previously hidden inside the rock.

Different aspects of the reconstruction of Tiktaalik roseae
Different aspects of the reconstruction of Tiktaalik roseae. Credit: Thomas A. Stewart et al. / Penn State

Tiktaalik was discovered in 2004, but key parts of its skeleton were unknown until now, said Tom Stewart from Penn State. These new micro-CT scans provide a complete picture of Tiktaalik’s skeleton, which is vital for understanding how it moved.

Unlike most fish that have uniform backbones, vertebrates with limbs have differencies in vertebrae and ribs down the body. This specialization allowed functions like mechanical connections between sacral ribs and the pelvis to support the body weight on hind limbs.

In fish, pelvic fins and hip bones are small and float freely, but for walking to evolve, hind limbs and the pelvis became much larger and attached to the backbone to handle forces from supporting the body.

Reconstructed position and orientation of the pelvic girdle of Tiktaalik roseae
Reconstructed position and orientation of the pelvic girdle of Tiktaalik roseae. Credit: Thomas A. Stewart et al. / Penn State

Tiktaalik is notable because it reveals this important transitional phase with a skeleton that combines fish-like and land animal features.

While early studies focused on Tiktaalik’s front end, the new reconstruction shows how its pelvis interacted with its backbone for the first time. The connection was likely made of soft tissue ligaments rather than our rigid pelvic bones.

Tiktaalik has so many features key to the origin of walking, said Stewart. While it did not walk on land, Tiktaalik was doing something new by possibly bracing and pushing with its hind fins.


Sources

Penn State | Thomas A. Stewart, Justin B. Lemberg, Emily J. Hillan, Isaac Magallanes, Edward B. Daeschler, Neil H. Shubin. The axial skeleton of Tiktaalik roseae. Proceedings of the National Academy of Sciences, 2024; 121 (15) DOI: 10.1073/pnas.2316106121


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