Fossils reveal how fish evolved into four-legged land animals

SCIENTISTS FIRST DISCOVERED TIKTAALIK IN 2004 WHILE HUNTING FOR FOSSILS IN THE CANADIAN ARCTIC

WASHINGTON: The fossilised remains of an ancient beast have revealed how prehistoric life hauled itself from the water and took its first unsteady steps along the path that led to fourlegged land animals.

Clues to the seminal moment in the history of life were found in the bones of Tiktaalik, a 375m-year-old freshwater creature that grew upto three metres 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, placed Tiktaalik half way between fish and the earliest four-legged land animals.

In work published on Monday, researchers described fossils of the back half of Tiktaalik for the first time. The report shows that the animal had a large, robust pelvic girdle, a prominent hip joint, and long hind fins. The powerful fins could have propelled the beast in the water, but also helped it walk on riverbeds, or scramble around on mudflats.

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 emphasised 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 had already happened in the 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 Nanuvut, northern Canada. The scientists are yet to find a Tiktaalik hind fin bone, or any remains that might shed light on the origins of toes.