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

[David Marjanovic <david.marjanovic@gmx.at>]

Oops, I almost forgot:

Gesendet:ÂSamstag, 23. Januar 2021 um 10:05 Uhr
Von:Â"Hutchinson, John" <JHutchinson@rvc.ac.uk>

> 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.

Well, "any ability at all" may not be a realistic endpoint of that spectrum. In 
wet nights, eels and *Silurus* catfish sometimes wriggle through the grass from 
one body of water to another. Large elongate animals from *Eusthenopteron* to 
anthracosaurs should have had this capability regardless of what their 
extremities were like, even if they weren't any good at it and even if not 
every individual did it in its lifetime. So _that_ should be what we mean by 
"fully/100% aquatic".

The interesting question, instead, is when (and how often, and how) an 
amphibious lifestyle evolved from such a "fully aquatic" one.

> 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.

I'm almost perfectly happy with the idea that *Ichthyostega* was, as I called 
it in the paper you edited, "the Godzilla version of a mudskipper". Despite the 
animal's sheer size, that seems to be the only explanation for its robust build 
and reduced tail combined with internal gills and huge paddle-like feet that 
couldn't be put on the ground, among other things that you've pointed out. 
However, most of this seems unique to *Ichthyostega*, so I think our last 
common ancestor with *Ichthyostega* was "fully aquatic" as described above.

The Devonian footprints and trackways may all have been made in very shallow 
water. That's interesting enough in its own right (i.e. the animals in question 
regularly went into water too shallow to swim in efficiently, and instead 
walked when they were there), and for most of them there's no evidence either 
way, but so far there aren't any that were definitely made on land.

> 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.

Definitely.

> > 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

Sure. It just would have been an opportunity to "promote stability and 
uniformity in nomenclature" by using the most "official" definition that there 
is. Myself, I'm not happy with this one, seeing as paleontologists have almost 
always used Tetrapoda for a larger clade, neontologists have mostly not used 
the name for _specifically_ the crown group, and the definition leaves us 
unsure if *Seymouria* is a tetrapod; but so far I prefer going with the 
definition from Phylonyms and seeing if it works.

I really wouldn't call it "taxonomy", though. It's nomenclature. It doesn't 
dictate which phylogenetic hypotheses you apply it to.