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Early 4-Legged Animal Moved Like Inchworm, Study Says

John Roach
for National Geographic News
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.

Combining a forelimb power stroke with the backbone scrunch, and a hindlimb stroke with the backbone extension, "could push the animal along with reasonable efficiency," Ahlberg said.

Robert Carroll, a paleontologist at McGill University in Montreal, Canada, said Ahlberg and his colleagues' reconstruction and analysis of the Ichthyostega skeleton is "unquestionably" superior to previously published interpretations.

Regionalized Backbone

The team's reconstruction differs from all previously published reconstructions of the animal.

Unlike in other reconstructions, the vertebrae that make up the backbone in Ahlberg's rendering are regionalized: They have different shapes in different parts of the column. Therefore, different parts of the backbone flexed in different ways, Ahlberg speculates.

The shapes of the vertebrae would have prevented Ichthyostega from sideways movement. The vertebrae generally resemble those of mammals, suggesting that this part of the backbone could flex vertically to some extent, Ahlberg said.

While regionalization of the backbone is fairly common in living land vertebrates, it's not seen in the lobe-finned fishes from which Ichthyostega is thought to have evolved. Lobe-finned fishes have thick, fleshy fins, as opposed to the delicate fins of most fish. Only two types of lobe-finned fishes survive today, coelacanths and lungfishes.

"Ichthyostega is actually the first example of such a regionalized vertebral column in the vertebrate fossil record," Ahlberg said.

The finding is unexpected, because most paleontologists are confident that land vertebrates are descended from creatures that could flex their vertebral column from side to side, he added.

In other words, Ichthyostega's body design was a failure. Few, if any, fossils representing descendants of this lineage are known after about 360 million years ago, Carroll noted in a commentary on this research in Nature. The creatures, it seemed, simply died out.

"Remember, the origin of land vertebrates from fish took 15 million years," Carroll said in a telephone interview. That's a long time, he added, for lobe-finned fish to have evolved various designs—with varying degrees of success.

Ahlberg said that another Devonian tetrapod from Greenland, Acanthostega, which is more primitive and less terrestrial looking than Ichthyostega, appears closer to the "main line" of tetrapod evolution.

"You can think about Ichthyostega as a sort of analog of Neanderthal man: close to our line of ancestry, and to some extent informative about ancestral conditions, but really a specialized dead-end side branch rather than an ancestor," Ahlberg said.

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