First Four-Legged Animals Inched Along

A reconstruction of the skeleton of the first four-legged land animal suggests that it didn't move too nimbly on land - it either shuffled along or crept like an inchworm.

360 million years ago, Ichthyostega (ick-thee-oh-stay-gah) crawled out of the water onto land. Although it was an amphibian, many of its skeletal features were fish-like. But it also had sturdy shoulders and hips, capable of supporting the body's weight on land.

The specialized shoulders and hips also allowed it to move its limbs out of water, making them useful for getting around on land. But Ichthyostega got around unlike any animal seen today.

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"Although the gross anatomy of Ichthyostega follows the familiar land vertebrate pattern - head, backbone, tail, limb girdles - the shapes of the different elements combined in a way that doesn't match anything living today," lead researcher Per Ahlberg of Uppsala University in Sweden told LiveScience.

Ichthyostega's complete skeleton was first described by Erik Jarvik in 1955, but many aspects of its anatomy remained unknown, making it difficult to determine how it moved on land. Ahlberg and his colleagues made a few changes in this construction, mainly in the ribcage, neck, and shoulder regions.

Then they played around with this reconstruction to figure out how the animal might have moved.

"We arrived at the overall functional interpretation by drawing together our functional interpretations of the different parts of the skeleton - forelimb, hind limb, backbone, and so forth - and trying to figure out how they could all make sense in one animal," Ahlberg said.

Through this process, Ahlberg and his colleagues suggest two ways the animal may have gotten around on land.

"On the one hand, it could have 'walked' with the body held rigid and the limbs moving in alternating diagonal sequence - front left and hind right, front right and hind left," Ahlberg told LiveScience. "The forelimbs were robust with bent elbows and could probably lift the front part of the body off the ground, but the hind limbs were more flipper-like so the pelvic region probably dragged on the ground."

Or the animal may have moved more like an inchworm by pulling its hips and back legs up towards its shoulders and then extending its back to move its front legs forward. This series of movements, described by Ahlberg as "a slow and extremely stumpy-legged gallop," would have allowed Ichthyostega to push itself along with reasonable efficiency.

Although Ichthyostega could move around on land, it probably spent plenty of time in the water. In water, its broad tail and flipper-like limbs would have allowed it to swim around.

This finding is detailed in the Sept. 1 issue of the journal Nature.

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