The first forests and the evolution of limbed tetrapods

[Newt]

Here's a little speculation I've been chewing on. It's probably untestable, but I'd like to hear your thoughts on it.

The first trees show up in the mid-Devonian; by the Late Devonian there are extensive forests worldwide, dominated by the progymnosperm Archaeopteris. 

This is the same time-frame in which the lobe-finned tetrapodomorph fishes develop strange new forms: first the finned "fishapods" or elpistostegalians, including Tiktaalik, Panderichthys, and Elpistostege; then such fully limbed forms as Tulerpeton, Ventastega, Acanthostega, and Ichthyostega. Traditionally it was thought that the evolution of these forms was driven by a need to move about on land - either to escape predation pressure or drying water bodies (as some modern fishes do) or to exploit the newly burgeoning populations of terrestrial invertebrates. However, closer examination of the fossils shows that these Devonian animals were poorly adapted for terrestrial locomotion and were completely or almost completely aquatic.

What, then, was the evolutionary pressure that caused perfectly cromulent fishes to lose their dorsal and anal fins (likely at great cost to their swimming efficiency), and begin converting their paired fins into limbs? 

My speculation is that these animals were exploiting the brand new environment of submersed trees and branches. If you spend much time in the water, you will no doubt have seen how attractive such structures are to aquatic invertebrates and small fishes, in part because they are difficult for "traditional" predatory fishes to navigate. Anguilliform predators (eels, aquatic snakes, etc.) do well in these environments, but the Devonian seems peculiarly lacking in such forms. So if you are a big predatory fish in the Devonian that can clamber through these dense environments, the world is your oyster! And antiarch. And acanthodian. And limulid. The point is, fallen trees created a novel aquatic environment, which drove the novel adaptations of our Devonian ancestors.

[irimali]

Your hypothesis makes a lot of sense.  I've been thinking about this too lately.

I would add to your idea, that after those first tetrapods had evolved, they'd have a second advantage over similar sized predatory fish.  Perhaps they had more opportunities to disperse further inland, by crawling along the bottoms of shallow/fast moving streams (similar to large aquatic salamanders today) or by moving into seasonally flooded marshes and swamp forests and ending up in new lakes and rivers.

Maybe their first steps on land happened in the shade of those Archaeopteris trees when the waters receded, on a carpet of leaf litter and moss, and not the vast barren plain of cracked mud depicted in so many early reconstructions of ichthyostega.     

[Halichoeres]

Do we know if any of these plants had emergent roots?

[irimali]

Do we know if any of these plants had emergent roots?

Not sure.  I did find this:

https://www.dcmurphy.com/devoniantimes/opportunity/flooding.html

and this:

https://scispace.com/pdf/terrestrialization-in-the-late-devonian-a-palaeoecological-2ikc5g2vfq.pdf

which discuss a lot of the things mentioned in this thread.

I know there's some carboniferous plants with prop/stilt roots, but haven't seen anything similar regarding devonian plants as of yet. 

[Newt]

Thanks for the links, @irimali . That Cressler et al. paper is a goldmine of Late Devonian ecology information.

[Libraraptor]

This is interesting stuff to work through! And a plausible idea indeed.

[Halichoeres]

Thanks, @irimali!

A limb could be a good way to stay put if water is moving, or to perch just a bit outside the water, but within striking distance of your prey. Of course I don't know if early tetrapodomorphs' appendages were any better at hanging tight than they were at walking.

[Torvosaurus]

The evolution of tetrapods to fully terrestrial animals was likely to happen at some point. The resources available on land, in terms of plants and terrestrial invertebrates, would provide unique possibilities for evolution. The simple reduction of "toes" from eight to six to five, etc. toes better adapted to a terrestrial lifestyle seems to support this.

By no means am I saying your ideas are incorrect, and in fact they are supported indirectly by the literature that has been provided. Initially the creatures mentioned were primarily aquatic. However, I think this phase where they exploited these areas was an intermediate step in the process that led to full terrestrial life rather than the creatures adapting specifically to exploit those areas directly. Even with fingers, the creatures listed are still quite large and still would have ran into problems with exploiting those areas. I also wonder how strong their limbs were. In other words, were they strong enough to move and break underwater branches enough to make it worth their while? I don't know, just considering the possibilities.

The other issue is the possibility that amniotes may have developed much earlier than is currently believed. Footprints (from the Snowy River formation in Australia) dating to ~355 mya suggest that amniotes evolved much faster than the early to mid-Carboniferous guessed at prior. When put in perspective, Acanthostega lived ~365 million years ago. In ten million years creatures evolved from an anamniotic form to an amniote. That's a pretty short length of time in terms of evolution.

Just some thoughts.

Torvo

[Papi-Anon]

I recall reading somewhere that lobed fins could have initially functioned as ways of propelling through rooted waterways by pushing the fish in bursts to catch prey and to avoided BEING preyed upon. Earlier tetrapodomorphs (as stated) were not really well-designed for full-on climbing onto muddy or foliage-covered land (YOU try climbing up an inclined slip'n'slide) but the proto-limbs could still be used for aquatic 'climbing' in a sense. Fingers could very well have been initially an adaptation for gripping roots and fallen branches for maneuvering underwater, not unlike arboreal tetrapods would one day (at least at a rudimentary level).