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It has been known for a number of years that there have been tetrapod tracks predating Stegocepgalians by tens of millions of years at the least, and now this article has put forward the claim the tracks were made in a non-marine environment. There also seems to be evidence of digit impressions and a lack of signs of body or tail drag, inferring this was an organism well developed for terrestrial locomotion.

It seems very problematic though that these tracks long predate all of the 'famous' transitional forms, and even Ichthyostega is thought to have dragged itself along with it's front limbs, the back limbs not load-bearing. The earliest vertebrates that appear suited for terrestrial motion don't appear in the fossil record until ~350 mya, but these tracks are 40 myr before that.

What exactly does this tell us about tetrapod evolution? Could all of these transitional species we've found belong to a lineage that went completely extinct while all living tetrapods come from the line that made these tracks? Or even more interestingly, perhaps the amphibian group developed from Stegocephalians while Amniotes come from the other? It seems like the craziest story of convergent evolution ever, two completely different lineages gaining traits for terrestrial locomotion within the same time frame as one another.

I contain an amount of skepticism about these tracks, it seems like too fantastic a claim to me that in a span of 30 mya, we basically went from Entelognathus to vertebrates fully suited to walking on land based on these tracks. We still don't know very much about the tracks or the evolution from aquatic to terrestrial living in general, so there's a lot to still discover. It's all very fascinating how non-linear the whole thing seems to be, and the terapod family tree could be completely different to how we've previously thought of it.

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closed as unclear what you're asking by Bryan Krause, Remi.b, iayork, theforestecologist, David Feb 12 at 16:40

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    $\begingroup$ What is your actual question? There seems to be a lot of text here without a specific question. Obviously this is an active area of research, what do you plan to get on SE that hasn't been addressed in the paper you link? $\endgroup$ – Bryan Krause Feb 11 at 16:57
  • $\begingroup$ First non-marine is not the same as terrestrial, marine means ocean, a freshwater swamp or lake would be non-marine. Also I am not aware of anyone who claims ichythostega's hind legs were not load bearing, do you have a source for this. $\endgroup$ – John Feb 11 at 21:51
  • $\begingroup$ All the known references and information are probably here: en.wikipedia.org/wiki/Elpistostegalia $\endgroup$ – com.prehensible Feb 13 at 13:35