Scientists have long wondered how the earliest common ancestors of all four-limbed animals--from reptiles to humans--made the transition from fins to feet. Now, a study of the partial remains of a 375-million-year-old fish found in the Canadian Arctic has revealed skeletal features similar to those of animals with legs. Such findings challenge a widely held theory of evolution that such strong, mobile hind limbs evolved only after animals began living on land, and suggest that the leap from fins to feet may have been much shorter than we think.
In 2004, a team of paleontologists in the Canadian Arctic excavated fossils belonging to a crocodile-like fish called Tiktaalik roseae. The discovery intrigued the scientific community because of the fish’s unusual makeup: Though it had scales and gills, it also had strong limb-like front fins, a flexible neck and flat head with eyes on top, features similar to amphibians or other four-footed vertebrates known as tetrapods. But because they only found fossils from the fish’s front half, including its skull, shoulders and front fins, the paleontologists were forced to guess at what its rear portion looked like.
In the years following their initial discovery, however, the researchers turned to additional blocks of rock recovered in the Arctic but considered low priority. Through painstaking excavations in the Philadelphia laboratory of Drexel University’s Academy of Natural Sciences, the rocks revealed the rear portion of Tiktaalik roseae, including the pelvis and partial pelvic fin material. According to the results of the new study, published this week in the Proceedings of the Natural Academy of Sciences, Tiktaalik had an unusually large pelvic girdle for a fish: The top portion of the pelvis, known as the ilium, was large enough to have contact with the vertebral column, a key component of walking on all fours.
More importantly, the size and shape of the fish’s hip joint suggested the presence of strong, mobile hind limbs. This contradicts the so-called “front-wheel-drive” hypothesis, which holds that such sophisticated appendages evolved only later, after creatures left the oceans and made the transition to land. According to study author and paleontologist Edward Daeschler of the Academy of Natural Sciences, “These are four-wheel-drive animals, not just front-wheel-drive only.” Along with Neil Shubin of the University of Chicago and Farish A. Jenkins Jr. of Harvard University (who died in 2012), Daeschler led the team whose excavations unearthed Tiktaalik.
Because some bones in Tiktaalik’s pelvis (as in its front portion) remained fish-like and primitive, while others were more advanced, the fish seems to represent a type of development known as mosaic evolution. According to this phenomenon, the parts nearest the body get modified first, while extremities such as fins or distinct digits are the last to change. According to Daeschler, “Tiktaalik was a combination of primitive and advanced features. Here, not only were the features distinct, but they suggest an advanced function. They appear to have used the fin in a way that’s more suggestive of the way a limb gets used.”
The first tetrapods, salamander-like creatures still tied to water but able to survive on land, are thought to have fully evolved 10 million years after Tiktaalik, who lived in the Late Devonian period (around 395 to 362 million years ago) along with other transitional species. Several million years before the Late Devonian, plants began growing on dry land, where previously there had been only sand and mud. Such land-based ecosystems would have provided the earliest incentives for animals to start leaving their watery environs. The new findings suggest Tiktaalik may have had an evolutionary advantage in being able to move around and push off in shallow water and mud, most importantly to evade predators.
Jennifer Clack, curator of vertebrate paleontology at the University of Cambridge’s Museum of Zoology in the United Kingdom, stressed the importance of the Tiktaalik findings. “It’s what we’ve all been waiting for,” said Clack, who was not involved in the new study but has been a pioneer in investigating the fish-to-tetrapod transition. “Until this discovery, we weren’t able to see the changes by which the pelvic fins of the fish became much larger and more robust, and gradually turned into the tetrapod hind limb.”