Ichthyostega, Acanthostega, and Tetrapod Origins

PhysOrg.com has a story on new research on Ichthyostega and Acanthostega. The new research was prompted by the discovery of new fossils:

How Cool Is That!

In my recent post on Ichthyostega I mentioned I didn’t have access to Nature. Low and behold when I checked my email On Friday I had all the articles on Ichthyostega – sent by none other than Per Ahlberg the lead author of the paper! Thanks again!

The article is even more interesting than than descriptions of it in the press. Before I go into it a little background information is necessary (Run away! afarensis is going to talk about bones again!).
Below is a picture of human vertebrae. There are three different types pictures: (7)cervical, (12)thoracic and (5)lumbar. Additionally, humans have sacral and caudal (or coccygeal – the remnants of the tail) vertebrae. This differentiation – based on form and function – is called regionalization

Human vertebrae share several common features. First, a round heavy body. Second, a round opening (the vertebral foramen). Third, several processes (two – usually – transverse and one spinous). The tranverse processes and spinous processes (along with the pedicle) form the vertebral arch. Additionally, the thoracic vertebrae contain facets for articulation with the ribs. The human vertebral column forms an S-shaped curve.

If you look closely at the middle picture above you will note that the spinous processes change orientation based on where they are on the vertebral column (I will get back to this later). So what does this have to do with a Devonian fossil?

In order to reconstruct Ichthyostega Ahlberg et al used 9 different fossils to create a composite vertebral column (necessary because none of the fossils had a complete vertebral column). This was possible because the fossils contained overlapping segments. It should be pointed out here that the osteology of these early vertebrates is somewhat different from that of humans. The Ichthyostega vertebrae has several pieces as you can see in the drawing below.

Several interesting features of the new reconstruction stand out. First, was the identification of regionalization in Ichthyostega vertebrae. As I understand it (having a background in anthropology rather than say comparitive anatomy or paleontology) this is not seen in fish and can be construed as an adaptation to life on land. This is where things get interesting. Ichthyostega vertebral arches change in orientation based on where they are in the vertebral column and in the lumbar area there is some overlap. Additional changes in the reconstruction involve the ribs. Previous reconstructions showe Ichthyostega with no neck. Which is not the case in the new reconstruction. Another change involves the ribs – which overlap. Taken all together the osteology of Ichthyostega indicates some interesting things about locomotion. The zygapophysis, pictured above, stiffen the spine and are adaptations to life on land. The morphology of the ribs and vertebrate restricts lateral flexion – but the lumbar vertebrate allow for some vertical flexion. I should also point out that Ichthyostega has ears specialized for underwater hearing. Taken all together the morphology shows one, ultimately unsuccesful, attempt to solve the problems of life on land.

Ichthyostega and the Origins of Land Vertebrates

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:

Palaeos

Tree of Life

Also, I am cross-posting this at Transitions

ADDED LATER: Welcome Pharyngulans! After checking out my site I recommend following the Palaeos link for some interesting additional info – and controversy – surrounding Ichthyosyega.