Early 4-Legged Animal Moved Like Inchworm, Study Says

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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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