The fossilized remains of an ancient beast have given scientists ideas to explain how prehistoric life hauled itself from the water and took its first unsteady steps along the path that they say led to four-legged land animals.
Clues to the explanation were found in the bones of Tiktaalik, a 375 million-year-old freshwater creature that grew to 3m long and had aquatic features mixed with others more suited to life on land.
Scientists first discovered Tiktaalik in 2004 while hunting fossils on Ellesmere Island in the Canadian Arctic. Its extraordinary blend of gills, scales, fins and lungs, combined with a movable neck, sturdy ribcage and crocodile-like head, put Tiktaalik half way between fish and the earliest four-legged land animals in the evolutionary scale.
In work published on Monday, researchers described fossils of the back half of Tiktaalik for the first time. The report showed that the animal had a large, robust pelvic girdle, a prominent hip joint and long hind fins. The fins could have propelled the beast in the water, but also helped it walk on riverbeds, or scramble around on mudflats, the report said.
Neil Shubin, professor of anatomy at the University of Chicago and the first author on the paper, said the most surprising find was the size of the pelvis.
“To give you a sense of how giant it is, the pelvis of this animal is the same size as the shoulder, so it’s very clear from understanding these bones that the hind appendage was already being emphasized in the transition to creatures with limbs,” he said.
Shubin had expected the hind fins and pelvis to be small in animals like Tiktaalik, with rear limbs becoming stronger and more prominent only as animals adapted to life on land. He described the transition as moving from “front-wheel drive” in fish to “four-wheel drive” in four-legged land animals, or tetrapods.
“It turns out that the size of the hind appendage was already large in fish and that a good chunk of the transition has already happened in fish before the origin of tetrapods,” he said.
The latest findings are based on the fossils of five Tiktaalik specimens recovered from Ellesmere Island in Nunavut, northern Canada. The scientists have yet to find a Tiktaalik hind fin bone, or any remains that might help them explain the origins of toes.
“The hind fin of Tiktaalik is tantalizingly incomplete,” Shubin told the Guardian.
Details of the fossils are reported in Proceedings of the National Academy of Sciences.
Jennifer Clack, professor and curator of vertebrate palaeontology at Cambridge University Museum of Zoology, said the Tiktaalik fossils answer a long-standing question about life’s transition from water to land.
“There has been a big gap in our understanding of how it happened, because the fossils did not provide any good evidence. This new material is just the sort of thing we hoped to find,” she said.
“The development of a large pelvis had to happen somewhere in the transition, and given its other tetrapod-like features, it’s no great surprise, but very satisfying, that the beginnings of it can be seen in Tiktaalik,” she added.