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Terrestrial-style feeding in Acanthostega

Although the very early tetrapod Acanthostega possesses many adaptations for an aquatic lifestyle, recent work by Molly Markey and Charles Marshall of Harvard University suggests it had a more terrestrial-style feeding. This is a pretty cool piece of work as suture morphologies on the skull roof of a modern fish Polypterus was correlated with suture functions during feeding. Stain gauge measurements in the skull of Polypterus show tension in the anterior and compression in the posterior parts of the skull. The cross-sectional morphology of these sutures seem to be correlated well with the strain patterns. (image left taken from here)

The authors then went on to quantify suture morphology in fossil forms, a sarcopterygian Eusthenopteron, an early tetrapod Acanthostega, and a fully terrestrial Phonerpeton. The cross-sections revealed that while Eusthenopteron showed similar suture morphology to Polypterus, Acanthostega and Phonerpeton did not. Extrapolating the relationship between suture function and suture morphology would suggest that Eusthenopteron had a similar strain pattern to Polypterus, indicating a similar feeding pattern, in this case, suction feeding. The suture morphology of Phonerpeton on the other hand was not consistent with the 'tension anteriorly, compression posteriorly' but compression both anteriorly and posteriorly. Since Phonerpeton is fully terrestrial, this form of strain pattern is presumably associated with a feeding style where biting prey is the main source of load.

So what's it like in Acanthostega?

Interestingly enough, Acanthostega is reported to have suture morphology reflecting compression at both the anterior and posterior margins of the skull. This suggests that the skull of Acanthostega did not experience the same kind of strains associated with suction feeding as seen in Eusthenopteron or Polypterus. Rather, it is more consistent with the patterns observed for Phonerpeton.

Conclusions: Acanthostega probably employed a biting-type feeding.

So my two cents are as follows:

This is a nicely done piece of work. It's nice and simple and compelling, mostly because of strain gauge measurements in Polypterus - man, I love it when people actually back things up with extant animals.

However, there are a couple of things I'd noticed.

First of all, I think it would be more compelling if strain gauge measurements were also taken from a fully terrestrial extant animal. That way you know for a fact that certain strain patterns are associated with terrestrial-style feeding. The paper only assumes that the strain patterns observed in Phonerpeton would be associated with a terrestrial-style feeding, an assumption that is understandable but should be taken with care as Phonerpeton is another fossil taxon. But I'm just guessing that peeling off the skin of an iguana or some poor amphibian to stick strain gauges directly onto the skulls is not going to be easy. But acquring cross-sections of modern tetrapod skulls should be possible at the least.

Secondly, it seems to me that the locations where cross-sections were taken are not entirely consistent across the species used. Only the interfrontal (IF) and interparietal (IP) sutures are figured for the two fish species while additional locations, the nasofrontal (NF), frontoparietal (FP), and interpostparietal (IPP) are considered for Acanthostega and Phonerpeton. I wonder what the cross-sections of these locations would show in Polypterus and Eusthenopteron.

Overall, I liked this paper for many reasons. I think this is a pretty cool study, as far as functional morphology goes.


John D. Orcutt said…
One of the things that puzzled me about this study was the choice of extant animals to include. I spoke to one of the authors after an earlier presentation of this study, who made a convincing case for not having included data from lungfish, frogs, or salamanders (all of which have fairly different skull morphologies than early tetrapods). I can also understand why Polypterus is a reasonable analog as far as fish go. What was never satisfactorily explained to me was why they didn't run similar experiments on caecilians, which I understand have skulls that are similar to those of early tetrapods. As amphibians, caecilians are certainly much closer to Acanthostega and company than is Polypterus, a basal actinopterygian. Take this criticism with a grain of salt, though, as my knowledge of biomechanics is extremely limited, and early tetrapod evolution isn't exactly a specialty of mine either...
Well, as far as I'm concerned, the sutures they looked at in the fish were the interfrontal and interparietal, both of which are present in most modern tetrapods. Although they did explain that the frontals and parietals in fish and tetrapods are not strictly homologous they are functionally analogous as they are in the same region of the skull. Then I would wonder why they didn't include modern tetrapods that have these analogous sutures as well...
John D. Orcutt said…
Good point; I guess I'll have to find some other study to be critical of...

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