First walking tetrapod couldn’t walk after all
8 Jun 2012 by Evoluted New Media
An ancient tetrapod thought to be the first ever to walk on land couldn’t actually walk at all suggests new research.
The four-limbed creature probably hauled itself out of the primordial ooze with its two front limbs, and dragged the back limbs behind the body, using them for balance.
“These early tetrapods probably moved in a similar way to living mudskipper fishes in which the front fins, or arms, are used like crutches to haul the body up and forward,” said Dr Stephanie Pierce from the Royal Veterinary College and the University of Cambridge.
Researchers scanned dozens of fossil specimens belonging to the Ichthyostega – a tetrapod that lived around 360 million years ago – digitally separating the bones from the surrounding rock. They constructed a whole skeleton using animation software to create the first ever 3D computer model of the tetrapod’s skeleton.
They carefully manipulated the model to estimate each joint’s range of motion before comparing to similar models of other animals they had developed to verify mobility. The model revealed the creature wouldn’t have been able to move its hip and shoulder joints much, and that its limbs couldn’t rotate along their long axis – essential for locomotion in today’s land animals. This means it couldn’t push its body off the ground and move each of its limbs in turn.
“All this points to the idea that limbs may have evolved before the ability to actually walk,” said Pierce. “It also shows that just because you have limbs, it doesn’t mean you can walk.”
Professor Jennifer Clack said: “Our reconstruction demonstrates that the old idea, often seen in popular books and museum display, of Ichthyostega looking and walking like a large salamander, with four sturdy legs, is incorrect.”
“Remarkably, earlier fishes (called tetrapodomorphs) had the ability to rotate their fins, so it seems that just as vertebrate were experimenting with terrestrial movement, the limbs became confined to mainly back-and-forth and up-and-down motions,” said Professor John Hutchinson. “It wasn’t until tetrapods became more competent on land that they recovered the ability to rotate their limbs around their long axis.”
The findings – published in Nature – suggests that some of the 400 million-year-old footprints discovered in 2010 thought to have been made by similar tetrapods might be attributable to altogether different four-legged animals.
