Ventastega: A tetrapod perched on a different branch

Of the few courses of value I have enrolled in while at Rutgers, one of my most favorite was the paleontology class taught by William Gallagher from the NJ State Museum (which, coincidentally, has just re-opened!). Much of the course dealt with invertebrates, the lectures being more oriented towards geologists than paleontologists, but there were still a few juicy lectures towards the end that involved vertebrate diversity and evolution. During these lectures he briefly mentioned the Permian temnospondyl Eryops, and he noted that it was probably an aquatic ambush predator, or "crocodiling before there were crocodiles." Looking like a gigantic killer salamander, Eryops may have even consumed its prey like modern crocodylians, tossing food towards the back of the throat to be swallowed rather than politely chewing it (this is called inertial feeding). For whatever reason Dr. Gallagher's phrase stuck with me, and I couldn't help but think of it when I saw the new fossils of the much younger "fishapod" Ventastega published this past week in Nature.

Unlike it's celebrity relative Tiktaalik, Ventastega is not something brand new. Parts of it were discovered in 1994 but not enough to know where along the transition from water to land it fell, whether it was more of a "fishapod" (now exemplified by Tiktaalik) or to the developed tetrapods (like Acanthostega). New discoveries in 2001 provided a better picture of Ventastega, primarily parts of the top of the skull, hips, and shoulder girdle, and it is this new collection of material that forms the basis of the Nature paper by Ahlberg, et al.

(It should also be noted that at the end of the paper the authors note that some of the material mentioned in the study was recovered as early as 1970. If my assumption is correct, this again illustrates that there are still treasures to be found in museum drawers; we just might not know their significance until other pieces to the puzzle are found.)

The skull of Ventastega makes up most of the new material and is certainly important, allowing comparison with the skulls of other related creatures that have been discovered in recent years. (As with whales, feathered dinosaurs, and other groups, the last 30 years have seen an revolution in the study of early tetrapods, especially as more complete skeletons have come out of the ground.) Looking at the reconstruction of the skull from the side, the premaxilla is turned slightly upwards, making Ventastega look like it is sneering at whatever might be in front of it. The eyes are higher up on the skull, coming up to a bit of a short angle almost like what is seen in living caimans. The jaws are studded with minutely curved, sharp teeth, the middle of the jaw holding several larger coronoid fangs and the teeth towards the back being smaller and much closer together. What Ventastega might have been eating in its tidal habitat, I don't know, but it has the profile of ambush predators that often lurk at the water's edge.

From the top the skull of Ventastega looks like it has been somewhat squeezed compared to the skulls of Tiktaalik, Acanthostega, and Ichthyostega, again giving it something of an alligator-like appearance. (I am not proposing that Ventastega lived and ate just like living alligators do. I am just noting the superficial resemblance shared by different groups of aquatic predators.) With Tiktaalik and Acanthostega in particular, the skull is more bowed and comes to a relatively sharper point, almost like an arrow, while the skull of Ventastega has a smaller angle along the sides and is blunter, like a spade. Indeed, while there are certainly similarities found in Ventastega that link it to both creatures like Tiktaalik and Acanthostega, it looks like a different sort of tetrapod, a representative of diversity and not straight-line evolution.

Other material was discovered, of course, primarily from the shoulder-region and from the hips. Although the actual limbs of Ventastega have yet to be discovered the characteristics of the pectoral and pelvic girdles led the researchers to hypothesize that Ventastega had developed limbs with digits like Acanthostega. Confirming this, however, will require more fossils. Likewise an isolated fragment of bone very similar to the bones found in the tail of Acanthostega may have belonged to Ventastega, and if this is true then Ventastega probably had a tail fin much like what is known from the more complete Acanthostega.

Now that more of the skeleton has been uncovered the question of where Ventastega fits into the evolution of tetrapods comes into play. While it is one of the youngest early tetrapods presently known (about 365 million years old) it consistently comes in below Acanthostega and Ichthyostega in phylogenetic trees, preserving older traits and perhaps representing a difference in evolutionary rates. It seems to bear a mosaic of features, the skull having more in common with Tiktaalik but possibly having limbs more like the tetrapod Acanthostega, the mix of features potentially putting it on a different branch of tetrapod evolution.

Ventastega is not just another link in a chain from lobe-finned fish to amphibious tetrapods; it confirms that evolution is a branching process even if we mostly concern ourselves with how more direct lines of descent may have been formed. As the authors themselves state, Ventastega "... demonstrates the presence of considerable morphological diversification among the earliest tetrapods." Although creationists may see fit to deride the fossil as of little importance given its perch on a side branch, it is important to note that the discovery of more Ventastega material does confirm evolutionary predictions. Not only is Ventastega a different kind of tetrapod but it also fits in with what tetrapods during the close of the Devonian would look like, Ventastega fitting snugly into (rather than breaking down) the mass of evidence accumulated so far.

Ahlberg, P.E., Clack, J.A., LukáeviÃÂs, E., Blom, H., ZupiÃâ Ã¡, I. (2008). Ventastega curonica and the origin of tetrapod morphology. Nature, 453(7199), 1199-1204. DOI: 10.1038/nature06991