Recent analysis of a fossil discovered 20 years ago reveals new details that may explain how vertebrates evolved to walk on land nearly 400 million years ago.
Tiktaalik, a fish dating back 375 million years, was discovered in Canada and represents a transitional form between fish and early tetrapods. The ancestors of all vertebrate land animals, as well as those that returned to aquatic life (like whales), can trace their origins back to pioneering fish similar to Tiktaalik.
On April 2nd, the latest analysis of Tiktaalik's skeletal structure was published in the Proceedings of the National Academy of Sciences.
Reconstruction of Tiktaalik. Image source: PNAS
Tom Stewart, the lead author of the paper and assistant professor at Pennsylvania State University, stated, "Tiktaalik was discovered in 2004, but crucial parts of its skeleton remained unclear. These new high-resolution micro-CT scans reveal the vertebrae and ribs of Tiktaalik, allowing us to reconstruct its skeleton comprehensively, which is crucial for understanding how it moved around the world."
The reconstruction shows that Tiktaalik's ribs likely attached to its pelvis. This is believed to have aided fish in supporting their bodies on land, a crucial step in the eventual evolution towards walking.
In most fish, the vertebrae and ribs are uniform along the length of the spine. However, in four-limbed vertebrates, the size of the ribs varies significantly. This results in specialized functions in different parts of the trunk, including mechanical connections between ribs, pelvis, and hind limbs supporting the body.
The pelvic fins in fish are evolutionarily related to the hind limbs of four-legged animals. Researchers found that walking evolved with the assistance of a larger pelvis, which is part of the spine and helps support the body against the new forces of walking on land.
Stewart explained, "Tiktaalik is fascinating because it offers us a glimpse into this major evolutionary transition. Throughout its entire skeleton, we see a mix of typical features of fish and aquatic organisms, as well as characteristics of terrestrial animals."
Initial descriptions of Tiktaalik focused on the front part of its skeleton.
Stewart added, "From previous studies, we knew the pelvis was large, and we felt the hind fins were large too, but until now, it wasn't clear whether or how the pelvis interacted with the axial skeleton. This reconstruction shows for the first time how they are integrated and provides us with clues about how walking initially evolved."
For further information, refer to the related paper: https://doi.org/10.1073/pnas.231610612.
