New findings on a 375-million-year-old fish fossil shed light on the evolution of the axial skeleton, potentially shedding light on how our ancestors transitioned from an aquatic to a terrestrial lifestyle.
Even before legs evolved into fins, the axial skeleton, which includes the head, neck, back, and rib bones, had undergone changes that would have helped our ancestors support their bodies for walking on land. A research team, including a biologist from the University of Pennsylvania, has completed a new reconstruction of the skeleton of Tiktaalik, a 375-million-year-old fossil fish, one of the closest relatives of vertebrates.
The new reconstruction shows that the fish’s ribs were most likely attached to its pelvis; This is thought to be crucial for supporting the body and the later evolution of walking.
An article describing the new reconstruction, which used micro-computed tomography (micro-CT) to scan the fossil and reveal the fish’s vertebrae and ribs previously hidden beneath the rock, was published April 2 in the journal. Proceedings of the National Academy of Sciences.
“Tiktaalik was discovered in 2004, but important parts of its skeleton were unknown,” said Tom Stewart, a professor of biology at the Eberly College of Science at the University of Pennsylvania and one of the leaders of the research team. “These new high-resolution micro-CT scans show us Tiktaalik’s vertebrae and ribs and allow us to make a complete reconstruction of his skeleton, which is vital to understanding how he moved around the world.”
Understanding Vertebrate Evolution
Unlike most fish, where the spine and ribs are identical throughout the body length, the axial skeletons of vertebrate limbs show sharp differences in vertebrae and ribs from the head region to the tail region. The evolution of this type of regionalization made it possible to perform specialized functions; one of them was the mechanical connection between the ribs in the sacrum and the pelvis, which made it possible to support the body with the hind legs.
The ventral fins of fish are evolutionarily related to the hind legs of tetrapods, four-legged vertebrates including humans. In fish, the ventral fins and bones of the pelvic girdle are relatively small and float freely within the body. Researchers explain that during the evolution of walking, the hind legs and pelvis became much larger and formed a connection with the spine as a way to distribute the forces involved in supporting the body.
“Tiktaalik is great because it gives us an insight into this important evolutionary transition,” Stewart said. “Throughout its skeleton, we see a combination of features unique to fish and aquatic life, as well as features unique to land-dwelling animals.”
Tiktaalik: A bridge between aquatic and land life
The first description of Tiktaalik focused on the front part of the skeleton. The fossils were carefully prepared to remove the surrounding rock matrix and reveal the skull, shoulder girdle and pectoral fins. The ribs in this area were large and wide; This suggested that they might have supported the body in some way, but it was unclear exactly how they worked. In 2014, a fish basin found in the same location as the rest of the skeleton was also cleared of matrix and identified.
“We knew from previous studies that the pelvis was large, and we had a feeling that the hind fins were large as well, but until now we couldn’t understand whether the pelvis interacted with the axial skeleton and, if so, how.” aforementioned. “This reconstruction shows for the first time how everything fits together and gives us clues as to how walking may have first evolved.”
Unlike our hips, where the bones fit tightly together, the connection between Tiktaalik’s pelvis and the axial skeleton was most likely a soft tissue connection composed of ligaments, the researchers explained.
“Tiktaalik had special ribs that could be attached to the pelvis by ligaments,” Stewart said. “It’s truly incredible. This creature has many features that were key to the emergence of walking: a large pair of hind legs, a large pelvis, and the connection between the pelvis and the axial skeleton. And while Tiktaalik probably wasn’t walking on land, he was certainly doing something new. He was probably doing something new.” “It was a fish that could support itself and move with its rear fin.”
The new skeletal reconstruction also sheds light on the specialization of head movements in Tiktaalik and new details about the anatomy of the fish’s pelvic fins.
“It is incredible to see the Tiktaalik skeleton in such vivid detail,” said Neil Shubin, the Robert R. Bensley Professor of Biology and Anatomy at the University of Chicago and one of the paper’s authors. “This study paves the way for subsequent research that will examine how the animal moved and interacted with its environment 375 million years ago.”
Source: Port Altele
