Before the evolution of legs from fins, the axial skeleton -? including the bones of the head, neck, back and ribs -; it was already undergoing changes that would eventually help our ancestors support their bodies to walk on land. A research team that included a Penn State biologist has completed a new reconstruction of the skeleton of Tiktaalik, the 375-million-year-old fossil fish that is one of the closest vertebrate relatives. The new reconstruction shows that the fish’s ribs are likely connected to its pelvis, an innovation thought to be critical for body support and the eventual evolution of walking.
A paper describing the new reconstruction, which used micro-computed tomography (micro-CT) to scan the fossils and reveal vertebrae and ribs of the fish previously hidden under rock, appeared April 2 in the journal. Proceedings of the National Academy of Sciences.
Tiktaalik was discovered in 2004, but key parts of its skeleton were unknown. These new high-resolution micro-CT scans show us Tiktaalik’s vertebrae and ribs and allow us to do a complete reconstruction of its skeleton, which is crucial to understanding how it moved through the world.”
Tom Stewart, assistant professor of biology in the Eberly College of Science at Penn State and one of the leaders of the research team
Unlike most fish, which have vertebrae and ribs that are the same throughout the length of the body, the axial skeletons of limbed vertebrates show dramatic differences in the vertebrae and ribs from the head region to the tail region. The evolution of this regionalization allowed the performance of specialized functions, one of which was a mechanical connection between the ribs in the sacral area with the pelvis that allowed the body to be supported by the hind limbs.
The pelvic fins of fishes are evolutionarily related to the hind limbs in tetrapods -? four-legged vertebrates, including humans. In fish, the pelvic fins and pelvic girdle bones are relatively small and float freely on the body. For the evolution of walking, the researchers explained, the hind limbs and pelvis became much larger and formed a connection with the spine as a way to support the forces associated with supporting the body.
“Tiktaalik is remarkable because it gives us a glimpse into this important evolutionary transition,” Stewart said. “Throughout its skeleton, we see a combination of features typical of fish and aquatic life, as well as features seen in land-dwelling animals.”
The original description of the Tiktaalik focused on the front of the skeleton. The fossils were meticulously prepared to remove the surrounding rock matrix and expose the skull, shoulder girdle and pectoral fins. The ribs in this area were large and enlarged, suggesting that they may have supported the body in some way, but it was not clear exactly how they would have worked. In 2014, the pelvis of the fish, discovered in the same location as the rest of the skeleton, was also cleared of the matrix and described.
“From previous studies, we knew that the pelvis was large and we had a feeling that the hind fins were also large, but until now we couldn’t tell if or how the pelvis interacted with the axial skeleton,” Stewart said. “This reconstruction shows, for the first time, how it all fits together and gives us clues about how walking might have evolved in the first place.”
The researchers explained that, unlike our hips where our bones fit tightly together, the connection between Tiktaalik’s pelvis and axial skeleton was likely a soft-tissue connection of ligaments.
“Tiktaalik had specialized sides that would connect to the pelvis through a joint,” Stewart said. “It’s really amazing. This creature has so many features—a large pair of hind legs, a large pelvis, and a connection between the pelvis and the axial skeleton—that were key to the origin of walking. And while Tiktaalik probably didn’t walk across land, he was definitely doing something new. This was a fish that could probably prop up and push with its back fin.”
The new skeletal reconstruction also sheds light on specializations for head mobility in Tiktaalik and new details of the fish’s pelvic fin anatomy.
“It’s incredible to see the skeleton of Tiktaalik captured in such vivid detail,” said Neil Shubin, Robert R. Bensley Distinguished Service Professor of Organismal Biology and Anatomy at the University of Chicago and one of the authors of the paper. “This study sets the stage for those investigating how the animal moved and interacted with its environment 375 million years ago.”
In addition to Stewart and Shubin, the research team includes Justin B. Lemberg, Emily J. Hillan, and Isaac Magallanes at the University of Chicago and Edward B. Daeschler at Drexel University’s Academy of Natural Sciences.
Support from the Brinson Foundation, the University of Chicago Department of Biological Sciences, an anonymous donor to the Academy of Natural Sciences at Drexel University, and the US National Science Foundation funded this research. Fieldwork was made possible by the Project Polar Continental Shelf of Natural Resources, Canada. the Department of Heritage and Culture, Nunavut; Nunavut’s Resolute Bay and Grise Fiord settlements. and the Iviq Hunters and Trappers of Grise Fiord.
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Journal Reference:
Stewart, TA, et al. (2024) The axial skeleton of Tiktaalik roseae. PNAS. doi.org/10.1073/pnas.2316106121.
