Re: [dinosaur] Fish-to-tetrapod forelimb function evolution (free pdf)

["Hutchinson, John" <JHutchinson@rvc.ac.uk>]

> We infer that the earliest steps in tetrapod forelimb evolution were related 
> to limb-substrate interactions, whereas specializations for weight support 
> appeared later. Together, these results suggest that competing selective 
> pressures for aquatic and terrestrial environments produced a unique, 
> ancestral "early tetrapod" forelimb locomotor mode unlike that of any extant 
> animal.

Isn't it much easier to conclude that these animals walked underwater, e.g. 
through dense vegetation, and not on land at all? That walking on land is an 
exaptation of walking, not a synonym of it?

That's not a new idea I just came up with either. It goes back to the 1980s at 
least.

>>With Clack as the co-author of the new paper and major constructor of "the 
>>Clack theory" of exaptation of walking via underwater walking and similar 
>>behaviours, yes, this is consistent with those ideas too. It does not 
>>challenge the Clack theory, but clarifies important details. Whether it's 
>>"easier" or not depends on how one distinguishes the evidence for/against 
>>underwater walking vs. any ability at all to move on land. Limb-substrate 
>>interactions would be involved in both. Weight support is still (but less) 
>>relevant underwater yet of course becomes more and more important as animals 
>>spend more time on land and support more weight with limbs vs. rest of body. 
>>Earlier Devonian tracks complicate the matter of when some degree of 
>>terrestriality evolved but don't falsify that there was underwater walking as 
>>an exaptation, either. And precisely who made them remains open to 
>>interpretation. Our earlier work showed, and so far no one has falsified it, 
>>that Ichthyostega (and here Acanthostega too, to some degree) didn't move 
>>like a "typical" (extant, crown) tetrapod, e.g. with lateral sequence walking 
>>gaits and no body/tail dragging. But we inferred that there's no reason it 
>>couldn't have moved (clumsily; crutching; with some body support likely) at 
>>all on land. How often it or other stem tetrapods did that (were they 100% 
>>aquatic or not?) is trickier but if you take fossil footprints at face value 
>>there's merit in the idea that they didn't entirely stick to water. Certainly 
>>there's a huge need for more discovery of earlier-Devonian fossils. The new 
>>study shows that we can't think of early/stem tetrapods more generally as 
>>having forelimbs (including muscles but also joints) that functioned like 
>>those of extant tetrapods. So instead of the simple sequence (lungfish-like 
>>sauropterygian-tetrapodomorph)-(salamander-like tetrapod) there is an 
>>intermediate stage(s) in musculoskeletal function with non-trivial locomotor 
>>differences from both extant homologues/analogues.

By the way, Tetrapoda is defined as the crown-group in Phylonyms. The 
tetrapodomorphs investigated in this paper are pretty far away from the crown.

>>We're aware of trends in phylogenetic taxonomy but also alternative practices 
>>in the field. Hopefully people can still understand the messages of the paper 
>>either way.

-John H