3D model reveals how ancient creature got around
24 May 2012, by Tamera Jones
An ancient four-limbed creature that's thought to be the first ever to walk on land couldn't actually walk at all, researchers have discovered.
Instead, they think the animal, called a tetrapod, simply hauled itself out of the primordial ooze with its two front limbs, using its back limbs merely 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,' explains Dr Stephanie Pierce from the University of Cambridge and The Royal Veterinary College, lead author of the study.
The shift from swimming to walking was a pivotal step in the evolution of land-dwelling animals like us. Rather than fins, creatures destined for the land had to develop limbs, which are essential for getting about.
Early tetrapods ticked this box. So scientists assumed that because they had four limbs – instead of fins – they could walk, 'a bit like a salamander.' The idea was that they'd do this by coordinating each limb in a sequence just like today's four-legged animals.
'It also shows that just because you have limbs, it doesn't mean you can walk.'
Dr Stephanie Pierce, University of Cambridge and The Royal Veterinary College
But since the 90s, researchers have gathered a wealth of evidence to suggest that, despite having limbs, early tetrapods lived primarily in water. This means they might not have simply walked along the ground, but instead used some other form of locomotion. Some scientists have even suggested they might have moved a bit like seals do.
'So, the question is, what did they do with their limbs?' says Pierce.
To find out, she and colleagues from the University of Cambridge and the Royal Veterinary College created the first ever 3D computer model of a tetrapod's skeleton to work out how its limbs might have moved.
They scanned dozens of fossil specimens of a tetrapod that lived around 360 million years ago called Ichthyostega. They digitally separated the bones from the rock surrounding the fossils. Then they put each bone back together, ' like a jigsaw puzzle,' using animation software, before carefully manipulating the model to estimate each joint's range of motion.
To make sure their computer model was reliable, they built similar models of seals, salamanders, platypuses, crocodiles and otters and used the model to test their joint movements. They describe their findings in Nature.
3D image of a tetrapod. © Julia Molnar
They found that the creature wouldn't have been able to move its hip and shoulder joints very much at all. Not just that, but its limbs couldn't rotate along their long axis, a movement that's essential for locomotion in today's land animals.
This means it almost certainly couldn't push its body off the ground and move its limbs in turn.
'All this points to the idea that limbs may have evolved before the ability to actually walk,' says Pierce.
'It also shows that just because you have limbs, it doesn't mean you can walk,' she adds.
The findings suggest that 400 million-year-old footprints discovered in Poland two years ago – thought to have been made by similar tetrapods – may have been made by an altogether different animal.
The research team says the next step is to combine their models of limb movement with similar models of the rest of the skeleton, as well as the muscles to make a more accurate judgement about these animals' gait.
They also want to analyse how another tetrapod, called Acanthostega would have got about.
'We're hoping to get a deeper understanding of how these creatures moved. What was their athletic performance like? Did they move slowly? And we want to know when the ability to walk on all four legs first evolved,' Pierce says.
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