PZ Myers. 2005 Sep 01. Ichthyostega. <http://pharyngula.org/index/weblog/ichthyostega/>. Accessed 2008 Aug 29.

Posted on M00o93H7pQ09L8X1t49cHY01Z5j4TT91fGfr on Thursday, September 01, 2005

Ichthyostega

It's not all chimpanzees. Ahlberg et al. have published a detailed description of the anatomy of the Devonian tetrapod, Ichthyostega. It looks to be a very special beastie, even if it was also something of a dead end.

Ichthyostega is special in that it is an early tetrapod that is clearly adapted to walk terrestrially. What this new analysis and reconstructions shows is that the axial skeleton had familiar regional specializations: cervical (neck), thoracic (chest), lumbar (lower back), sacral (pelvic) and caudal vertebrae. It also had massive, flanged and overlapping ribs. The drawing below shows the new reconstruction at the top, below it the older and less detailed version, and at the bottom is a completely different organism, another early tetrapod, Acanthostega.

a, New reconstruction of Ichthyostega, most anterior cervical vertebrae (obscured by skull in lateral view) in grey outline. The reconstruction shows a maximally extended presacral column. Note the revised dentition, orientation of the limb elements and proportional differences (larger limb girdles and shorter tail) compared with b. The forearm is shown close to the beginning of the powerstroke; the manus remains unknown. Neural spines of the cervical and anterior caudal regions are also unknown. Regions are as follows: cervical (1), thoracic (2), lumbar (3) and sacral (4); unlabelled, caudal. Hindlimb based on ref. 7 and new observations. b, Previous reconstruction of Ichthyostega. c, Lateral view reconstruction of Acanthostega based on unpublished illustrations by Coates. Grey shading in a and c shows what are considered to be the sacral neural spines. Scale bars, 100 mm. (click to see larger image)

The skeleton had some limitations. Horizontal flexion, the ability to wiggle like a salamander, wouldn't work. They could bend their backs up and down, though. These observations suggest that it probably had a rather peculiar gait.

If we are correct and the axial morphology did not permit lateral trunk flexion in Ichthyostega, two gaits seem theoretically possible: 'walking' with diagonally synchronized limb movements and rigid elevated trunk; and a bilaterally symmetrical 'shuffling' or 'inchworm' movement, with vertical flexion of the lumbar region as part of the powerstroke for the forelimb and recovery stroke for the hindlimb, and lumbar extension contributing to the converse. Both may have been used at different times. These locomotory hypotheses remain to be tested by further analysis of limbs and joint surfaces.

Acanthostega was a contemporary of Ichthyostega with a different lifestyle.

Acanthostega, in contrast to Ichthyostega, had a laterally flexible trunk and appears better adapted for tail-propelled swimming. The limbs and limb girdles of Ichthyostega are also proportionately much larger than those of Acanthostega. Coupled with differences in the dentition (recurved sectorial teeth in Ichthyostega, conical piercing teeth in Acanthostega) and body size (Ichthyostega is about 30% larger than Acanthostega), plus the fact that the two genera rarely occur together in the same locality within the Greenland sediments, this points to a clear ecological separation between these primitive tetrapods.

In case the raw bones aren't enough to spark your imagination, there was also a nice painting of the fully-fleshed version. Unfortunately, it's based on the older reconstruction.

Length of the animals is about 1 m. These depictions show their capacity for swimming in water using the tail, or walking on land with the trunk supported by muscles attached to specialized neural spines on the vertebrae. (click to see larger image)

Afarensis has more, including photos of the fossils.

Ahlberg PE, Clack JA, Blom H (2005) The axial skeleton of the Devonian tetrapod Ichthyostega. Nature 437:137-140.

1. Thanks for the link. Honesty compels me to point out that these are not pictures of the fossils in the Nature article.
   #: Posted by  on  09/01  at  05:16 PM
2. Interesting new reconstruction. So, if I'm reading this right, Ichthyostega independently evolved a (loosely) mammalian-style flexing of its spine, fresh out of the water?
   #: Posted by HairyMuseum  on  09/01  at  05:32 PM
3. Ichthyostega are very cool. They also have a fascinating ear structure, or as Clack et al describe it, "a structurally and functionally unique modification of the tetrapod otic region, unlike anything seen in subsequent tetrapod evolution" (Clack JA, Ahlberg PE, Finney SM, Dominguez Alonso P, Robinson J, Ketcham RA. A uniquely specialized ear in a very early tetrapod. Nature. 2003 Sep 4;425(6953):65-9.). Probably the spatial arrangements of the structures were selected for by the different sound-transmitting properties of water vs. air at a time when some verts were making the transition from water-dwellers to land-dwellers.
   
   I have a real fondness for the little beasties for coming to the rescue when I needed such an unusual anatomical example for my thesis.
   #: Posted by Raven  on  09/01  at  06:41 PM