Early 4-Legged Animal Moved Like Inchworm, Study Says

August 31, 2005

One of the first four-legged creatures that walked on land had an inchwormlike gait, a new study says.

The creature, known as Ichthyostega, lived in a floodplain environment on Greenland during the Devonian period, about 360 to 410 million years ago.

About three feet (one meter) long, Ichthyostega looked like a cross between a fish and a crocodile. Its four limbs allowed it to move, for short distances, on land.

Since Ichthyostega's environment had both wet and dry periods, the animal needed to be able to swim in open water, walk over dry land, and poke about in shallow water, said Per Ahlberg, a paleontologist at Uppsala University in Sweden.

Knowing how Ichthyostega and other early tetrapods—animals with backbones and two pairs of limbs—navigated this environment is important to understanding the earliest stages of four-footed-animal evolution and how they emerged from the sea.

Two Gaits

Reporting in tomorrow's issue of the science journal Nature, Ahlberg and colleagues show that Ichthyostega's backbone would not have allowed the side-to-side movement common to fishes and primitive land animals such as salamanders. Rather, the tetrapod's spine had limited up-and-down movement.

As such, the researchers hypothesize that Ichthyostega probably used two different gaits on land, depending on how fast it needed to move.

"On the one hand it could have 'walked' with the body held rigid and the limbs moving in [an] alternating diagonal sequence," Ahlberg wrote in an e-mail to National Geographic News.

In this gait the strong front limbs likely allowed the creature to hold its body off the ground, while the flipperlike hind limbs and rear end dragged behind, Ahlberg noted.

In the other, inchworm-like gait Ichthyostega likely used the limited up-and-down movement of the backbone in combination with symmetrical limb movement "to achieve a weird gait approximating to a slow and extremely stumpy-legged gallop," Ahlberg said.

Scrunching up the backbone would bring the shoulder and pelvis closer together, while straightening the backbone would push them apart, he explained.

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