Did modern tetrapods descend from Tiktaalik or Ichthyostega? While I understand that in terms of all tetrapods, the lineages of Tiktaalik and Ichthyostega both would be included, but I want to know if their lineages have continued to the modern day. Since they also have 10 million years between them, it's possible that both of their lineages have not continued to the modern day. It's also possible that neither of them have continued to the modern day and it was a later fish species altogether whose lineage has continued to this day in the form of currently living tetrapods even though Tiktaalik and Ichthyostega were the first vertebrates on land. I'm especially interested in knowing which fish species have humans descended from. I'd be grateful if anyone is able to shed any sorta light on this.
3 Answers
Your question cannot be answered.
One of the effects of how incomplete the fossil record is, is that it is impossible to say if a species was a member of a direct line or a side branch. we just do not have the resolution in the vast majority of the fossil record to tell. The answer could be both, neither or one or the other and we would not be able to tell. All we can say is it is likely they were closely related to or part of our ancestral line. But the working assumption is always that you have a related sister species and not the ancestor unless you can show extremely good evidence otherwise, (which is so rare you can assume it is impossible in extinct vertebrates).
Tiktaalik and Ichthyostega only give us a rough idea of what our ancestors would have been like. It is impossible to say with any certainty if either was a direct ancestor or merely a sister species to them. This is part of the reason cladograms are constructed the way they are, so that no fossil is ever put on the direct line.
Note: Tiktaalik probably did not live on land, more likely it was a shallow water air breather.
Also whatever species was the ancestor of tetrapods, all living tetrapods would be its descendants. One aspect of population genetics is for a specific individual in an ancestral population, past a certain point in a linage either every descendant is your descendant or you have no descendants.
Technically they descended both from the tiktaalik and Ichthyostega because Ichthyostega was later, but in the same family and evolutionary line as tiktaalik.
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$\begingroup$ Of all the tetrapods to have ever lived, some would be descendants of Tiktaalik and some of Ichthyostega, yes. This makes sense. However, I'm especially interested in conclusive works that posit both the lineages having continued to this day. I am not sure if there's any work of that sort, I certainly couldn't find any on my own level, hence, posted this question here. I'm also interested in knowing which of the two happen to be ancestral to human beings. I'll edit the question to reflect these. $\endgroup$ Oct 30, 2020 at 3:20
You can study the reeources under "stem tetrapod tree" and tetrapod limbs. you can see the relationship in the list of the second image.
The trees suggest that tiktaalik and ichtiostega are not direct ancestors, they are close ones, so modern tetrapods descended from their common ancestors, not from their species.
vertebra:
lower jaw:
front limbs:
