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a very early tetrapod indeed

Blogging on Peer-Reviewed ResearchI've been fascinated by the story of early tetrapod evolution (where 'tetrapod' = an animal with 4 legs) for years, since reading Carl Zimmer's wonderful book At the water's edge (1998). Our understanding of when & where tetrapods evolved has been steadily extended by a series of fossil finds, most recently the 'fishapod' Tiktaalik. This was definitely a fish, but one where the fleshy base of its fin contained not only the familiar 1-2-many pattern of bones that we see in our own arms & legs but also a flexible wrist.

And now... there's Ventastega.

Ventastega's a valuable find because it sits somewhere in the gap between the fish Tiktaalik and the definitely tetrapodal Acanthostega Ichthyostega. I say 'somewhere' because it's obviously transitional, in the sense that it shows a mosaic of features (some more fish-like, some more like Acanthostega & other early tetrapods, but Ahlberg & his colleagues note that Ventastega doesn't stand exactly between the two. It lived at roughly the same time as Acanthostega (about 350 million years ago) & may represent a lineage that became extinct. This is in itself an exciting idea as it suggests that early tetrapods were actually a pretty diverse bunch of organisms, with several different lineages in existence at the same time.

So what did this animal look like? The following images are from PZ Myer's Pharyngula article on Ventastega (in turn, from the original Nature paper by Ahlbert et al. cited below).

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Ahlberg & his colleagues think that in life the animal was probably about 1m long. The top image shows the bones they've found, superimposed on an outline of Acanthostega. They've done this because the skeletal similarities suggest that the two animals were fairly similar in form. (Look at that tail - it's shape tells you that this is an animal that still spends a fair bit of time in the water.) The next row of images shows its skull, together with a reconstruction (the final row of pictures). This has a lot of bones in common with the earlier Tiktaalik (likewise the fang-like teeth), but its overall shape is more similar to that of Acanthostega.

In addition to the well-preserved skull, there are also bones from the pectoral (shoulder) girdle and part of the pelvic girdle, and a bone which the team feels is likely to be from the animal's tail. The pectoral girdle is similar to that of other early tetrapods & quite different from that of Tiktaalik. On this basis, the research team conclude that the animal had limbs with digits. (I remember being quite excited, years & years ago now, when I first read - in Stephen Jay Gould's book Eight Little Piggies - that early tetrapods had more than the 5 fingers typical of modern tetrapods: nine, in the case of Acanthostega.)

So there you are: a lovely fossil that enhances our knowledge of a very early period in the evolution of terrestrial vertebrates (including ourselves) :-)

P.E. Ahlberg, J.A. Clack, E. Luksevics, H. Blom & I. Zupins (2008) Ventastega curonica and the origin of tetrapod morphology. Nature 453: 1199-1204  

S.J. Gould (1993) Eight little piggies. Norton.

C. Zimmer (1998) At the water's edge: fish with fingers, whales with legs. Touchstone.

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