Did early tetrapods walk on land?

“Fishapods” like Tiktaalik and Ichthyostega are among the iconic transitional fossils that mark part of our own evolutionary journey. Understanding how these intriguing animals lived is key to understanding why they ended up out of water, and how evolution took them there. Of course, one of the most important changes that happened to our fishy forebears is gaining the ability to walk. While “walking” may be less of a challenge to fish than we thought, just how good “classical” proto-tetrapods like Ichthyostega were at it still isn’t entirely clear. (Below: model of Ichthyostega emphasising its aquatic capabilities; photo by Dr Günther Bechly, Wikimedia Commons.)

The recent discovery of tetrapod-like trackways in Mid-Devonian rocks in Poland (Niedźwiedzki et al., 2010) added to the pile of evidence on the “pretty good” side. The footprints appeared to come from feet that looked like those of Ichthyostega, and the trackways betrayed an animal that walked essentially like a modern tetrapod, lifting its body clear off the ground. (The fact that these trackways were left in a different environment and 18 million years earlier than any previously known tetrapod is just the icing.)

As is usually the case in science, the paper wasn’t allowed to be simply right. Using a computerised Ichthyostega skeleton and comparisons to modern tetrapods, Pierce et al. (2012) argue that whatever made the Polish tracks and others like them, it couldn’t have been much like Ichthyostega.

The problem is that Ichthyostega had pretty stiff hip and shoulder joints. The joint surfaces are kind of elongated (and, in the case of the shoulder, also twisted), allowing the bones to hinge in various directions but not to rotate around their long axes. The unrotating hip and rigid knee of the hindlimb made it impossible for the animal to put its soles on the ground. The hindlimb looks very paddle-like with its broad, flattened bones – now it appears that paddling is all it was good for. (Below: a decade of Ichthyostega from Dennis Murphy’s wonderful Devonian Times. See his caption for the sources of the reconstructions.)

By all appearances, Ichthyostega couldn’t rotate its arms and legs enough to walk properly, and couldn’t have left even one of the presumed Polish hindlimb prints without dislocating several joints. The most it could have done is haul itself along like a seal or a mudskipper, which, considering its powerful arms and mobile elbows,might have been exactly what it did. Pierce et al. also argue that other known proto-tetrapods were unlikely to be the trackmakers; while they didn’t examine their limb joints in detail, the skeletons share many of the same features that limit the mobility of Ichthyostega‘s limbs.

That raises plenty of questions. If it wasn’t like any of the known early tetrapods, what sort of creature made those footprints? Were there other ancient tetrapod groups with more limber joints that didn’t leave body fossils because of where they lived? Did the known proto-tetrapods go back to a more aquatic existence and more rigid limbs? (Pierce et al. say that Ichthyostega‘s joints were even stiffer than some of its fishier cousins’!) Which kind begat the lineage that gave rise to living tetrapods? What does all of this say about the significance of walking-like movements we observe in living lobe-finned fish?

***

References:

Niedźwiedzki G et al. (2010) Tetrapod trackways from the early Middle Devonian period of Poland. Nature 463:43-48

Pierce SE et al. (2012) Three-dimensional limb joint mobility in the early tetrapod Ichthyostega. Nature advance online publication, 23 May 2012. doi: 10.1038/nature11124

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