Ichthyostega n'était pas un bon marcheur

Cela n'est pas surprenant. En effet, pourquoi l'un des premiers vertébrés à quitter son habitat aquatique pour s'aventurer sur la terre ferme serait-il automatiquement un expert de la marche? Il est toutefois intéressant d'étudier la façon de se déplacer de ce très lointain ancêtre:


One of Earth's earliest four-footed land animals couldn't walk, a new 3-D model suggests. Instead, the dog-size Ichthyostega likely flopped on land, using only two of its four stubby legs for locomotion.

(...) One of our most distant ancestors, Ichthyostega is also one of the earliest tetrapods known to have crept onto land. Until recently, researchers thought the creature squiggled across the mud on all fours like a salamander. But the first 3-D digital reconstructions of Ichthyostega's skeleton suggest its forearms couldn't twist and turn enough to enable a four-legged gait.

The model also suggests that the creature's hind legs barely touched the ground. At best, they may have propped up Ichthyostega's rump as the animal flopped around like a modern-day mudskipper fish.

(...) Further analysis hinted that Ichthyostega had very limited forearm motion and a stiff spine. The model also suggested that the hind limbs couldn't contribute to a forward-propelling, four-legged gait.

Instead, Pierce and her colleagues concluded, Ichthyostega likely "rowed" its forelimbs front-to-back, much as a mudskipper moves its stubby front fins to slide around in the muck.

Per Ahlberg, a paleontologist at Uppsala University who calls Ichthyostega "a close friend," said Pierce and her colleagues' work is the first and most thorough of its kind for animals transitioning from water to land.

(...) One item Ahlberg took issue with in the study, however, is the notion that Ichthyostega didn't do much with its hindquarters. "There would have been a lot of muscle attachments there, and the pelvis is very large," especially compared with a fish pelvis, he said. That pelvis "had to be doing something significant or it wouldn't be there—the evolutionary cost is too large."

Pierce's team contends, however, that the hind limbs and pelvis were used more in swimming and paddling—Ichthyostega's primary means of motion, the team believes. Uppsala's Ahlberg added, "I bet Ichthyostega's stiff spine made it look bizarre when it was swimming. Sort of like a windup fish toy that you put in a bathtub."

Next, Pierce and her colleagues intend to perform nuanced calculations on the spine and reconstruct a full, lifelike model of movement for the ancient creature.

"The land was an open ecological niche, free to be exploited, and these early tetrapods knew what to do. It was theirs for the taking," Pierce said. "We want to see how they navigated this new environment."