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.