Ichthyostega and the Origins of Land Vertebrates

I have a temp assignment for the next couple of days, so in the meantime I thought I would repost this from my archives. This is the first of two I wrote on the subject. After I wrote the first, as you will see, Per Ahlberg, was kind enough to send me the article so I wrote the second based on the info in the article….

One of the problems with being a science blogger but not a scientist is that stories come along that you can’t really cover in the detail you would like. The Chimp genome is a good example. I don’t have access to Nature (other than the free stuff they have put up)short of wandering into the library of one of the local colleges here in St. Louis – something I don’t always have time to do. Decoding the chimpanzee genome is an incredibly important piece of science that we will all be hearing more about (I may do something on it later).
In the meantime, several interesting pieces of science news are just crying out to be covered.
Ichthyostega Hindlimb
Ichthyostega Skull
The above are fossils of Ichthyostga – one of the first land dwelling vertebrates. They evolved in the Devonian (some 410-360 mya)and are found in Greenland. A recent study – published in the most recent issue of Nature – indicates some interesting facts about Ichthyostega (from National Geographic News):

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.

This has interesting implications for how Ichthyostega moved:

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.

Since most paleontologists assume that land vertebrates evolved from an organism that could flex their vertebral column from side to side, this means Ichthyostega probably wasn’t a direct ancestor of later vertebrates:
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. Below is a fossil Acanthostega.
For more info you can go to:
Tree of Life

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