Tetrapod evolution without Ichthyostega and Acanthostega

Two recent papers,
(Clack 2009, Long et al. 2018, Figs. 1, 2), included traditional cladograms of tetrapod evolution ranging from taxa with fins to taxa with legs. Both included Ichthyostega and Acanthostega, taxa traditionally considered essential to any discussion of taxa documenting the transition from fins to legs.

Figure 1. Modified from Clack 2009 showing the taxa in the transition from fins to feet.

Figure 1. Modified from Clack 2009 showing the taxa in the transition from fins to feet.

The two studies do not have the same taxon list.
In Clack 2009 (Fig. 1) Panderichthys is a penultimate most basal taxon. In Long et al. 2018  (Fig. 2) Panderichthys is nearly a penultimate most derived taxon.

From Long et al. 2018, their cladogram of taxa in the transition from fins to feet.

Figure 2. From Long et al. 2018, their cladogram of taxa in the transition from fins to feet.

By contrast
the large reptile tree (LRT, 1586 taxa; subset Fig. 3), which employs many more pertinent taxa, nests Ichthyostega and Acanthostega distinctly off the main line leading from jawless Silurian fish to amniotes (= reptiles) and relegates them to the sidelines where they give rise to no other taxa. Apparently these two terminal (= dead end) taxa were evolving secondarily to a more aquatic niche or role. They both have no known descendants in the LRT. The LRT represents a new hypothesis of interrelationships from 2017 requiring confirmation or refutation with a similar taxon list.

Today I’ll summarize the subset topology recovered by the LRT
by graphically listing the included taxa that were transitional between jawless fish in the Silurian and basalmost reptiles in the Early Carboniferous. The list includes many taxa that have been traditionally omitted from prior more focused studies, like Clack 2009 and Long et al. 2018. The LRT minimizes taxon exclusion by testing all 1586 included taxa against one another, minimizing traditional biases and omissions.

Figure 2. Updated subset of the LRT focusing on basal vertebrates (fish). Arrow points to Hybodus. This tree does not agree with previous fish tree topologies.

Figure 3. Updated subset of the LRT focusing on basal vertebrates (fish). Arrow points to Hybodus for no reason during this post.  This tree does not agree with previous fish tree topologies. See figures 1 and 2.

And here they are:
(Figs. 4–6) from Silurian jawless fish like Thelodus to Early Carboniferous Silvanerpeton.

Figure 3. Basal vertebrates in the lineage of reptiles, part 1.

Figure 4. Basal vertebrates in the lineage of reptiles, part 1.

Figure 2. Basal vertebrates in the lineage of reptiles, part 2.

Figure 5. Basal vertebrates in the lineage of reptiles, part 2.

Towards the end,
of figure two fingers and toes first appear in a phylogenetic sense, not a chronological sense. Greererpeton is Early Carboniferous (320 mya) while Ichthyostega and Acanthostega are Latest Devonian (360 mya). To most paleontologists those 40 million years make all the difference permitting omission of Greererpeton and similar taxa To the LRT, Greererpeton is a late survivor from an earlier, perhaps Middle Devonian, radiation.

Figure 3. Basal vertebrates in the lineage of reptiles, part 3.

Figure 6. Basal vertebrates in the lineage of reptiles, part 3.

In this final group,
(Fig. 6) we find Tulerpeton, another taxon from the Latest Devonian (360 mya). It is very nearly a reptile, just two nodes apart from Silvanerpeton, the last common ancestor of all living reptiles. So Silvanerpeton laid amniotic eggs despite its otherwise amphibian-like appearance, and this increases the probability that the more primitive Greererpeton was a late survivor of an earlier Mid Devonian radiation.

Figure 1. From the Beginning - The Story of Human Evolution was published by Little Brown in 1991 and is now available as a FREE online PDF from DavidPetersStudio.com

Figure 7. From the Beginning – The Story of Human Evolution was published by Little Brown in 1991 and is now available as a FREE online PDF from DavidPetersStudio.com

I wish I knew back then
what I know now when I designed, wrote and illustrated “From the Beginning—The Story of Human Evolution” (Wm. Morrow 1991; Fig. 7). But then, it would have been a much bigger book.


References
Clack JA 2009. The fish-tetrapod transition: new fossils and interpretations. Evolution: Education and Outreach 2(2):213–223.
Long JA, Clement AM and Choo B 2018. Early Vertebrate Evolution. New insights into the origin and radiation of the mid-Palaezoic Gondwann stem tetrapods. Earth and Environmental Science Transactions of the Royal Society of Edinburgh, 1–17.

6 thoughts on “Tetrapod evolution without Ichthyostega and Acanthostega

    • Thank you, Werner. The abstract reports, “the majority of trees place Parmastega as a sister group to all other tetrapods.” After analysis Parmastega appears to be congeneric or nearly congeneric with Acanthostega, so that nests it well within the clade Tetrapoda and off the main line leading to Reptilia. I will post on this later.

  1. Dave, perhaps rather than reporting the findings of the LRT as facts that prove traditional paleontology wrong, you should start considering them as hypotheses. For example, your hypothesis (not fact, not even theory) that taxon inclusion is more important than character formulations. Or your hypothesis that any tetrapod matrix that includes only Eusthenopteron as an outgroup is immediately flawed. There is at least one comprehensive analysis (Marjanovic & Laurin’s peerj study) which includes an equally impressive list of amphibian-grade stegocephalians, and yet it continues to recover Acanthostega and Ichthyostega close to the base of four-limbed animals.

    The LRT considers Acanthostega to be in a clade with Ossinodus and Ichthyostega to be a basal embolomere. M & L have Ossinodus and embolomeres, and yet they continue to recover the traditional phylogeny. Based on this post, I assume that you have one of two hypotheses for this discrepancy, which can be summed up with “they didn’t include enough fish” or “the authors are lying pawns of big paleo who make up scores to protect their careers” (or as you said directly in an earlier post, “bowing to academic tradition”). I’m afraid I can’t help you resolve the persistent conspiracy theorist paranoia you possess that led to the second hypothesis.

    As for the first hypothesis, which you consider fact, it can be assessed by considering how the character distribution would change upon inclusion of certain taxa. How would including Squatina, Cheirolepis, Rhizodus, etc. make Ichthyostega more similar to embolomeres? What effect would they actually have? Try creating a variant of the LRT with a taxon sampling identical to that of M & L, and see what differences arise. If the LRT still has Acanthostega sister to Ossinodus and embolomere Ichthyostega, you know that that specific typology is not a result of your expanded taxon list, and that the presence of more fish taxa in the LRT seems to be irrelevant to amphibian-grade phylogeny. This is the easiest way to test your hypothesis, you need to stop assuming that it is a fact a priori.

    • Neil, You don’t see where I state the findings of the LRT as hypotheses needing confirmation or refutation. If not, please re-read.

      Taxon inclusion IS vastly more important than character formulations. For instance, if you want to test where manatees come from, you MUST include elephants and hyraxes. I won’t even mention pterosaurs and Cosesaurus, turtles and Elginia, etc. etc.

      Acanthostega and Ichthyostega are indeed close to the base of tetrapods. Not sure where you find disagreement where there is none. But other taxa are closer to the lineage that leads to reptiles in the LRT. Marjanovic and Laurin did not include enough reptiles to recover a most basal reptile, Silvanerpeton. Other dominoes fall from there.

      No paranoia here. I’ve always been clear about which taxa were omitted. It’s important to do so when that is the problem. Did Marjanovic and Laurin include illustrations to show the bones they identified? No. So, by comparison, that is their shortcoming. Bowing to academic tradition (= doing what your professor/mentor wants you to do) is widespread. Just ask around.

      “Which you consider fact” is a false statement. As stated earlier, I state these findings as hypotheses. You must become a more careful reader. When you do you’ll become a better scientist, Neil.

      Ichthyostega nested with Proterogyrinus before the basal fish entered the LRT. So not sure what your question refers to.

      re: Taxon sampling identical to Marjanovic and Laurin? I’m getting there. But the problem remains, when will they provide illustrations of their data so we can be sure one of us has not misidentified a bone or two.

      You are correct. The presence of more fish taxa in the LRT did not effect the tree topology of amphibians, which were nested first. Back in 2017, 500 taxa fewer ago, when I was adding basal tetrapods, I noted that Ichthyostega and Acanthostega appeared to be secondarily more aquatic. Details here: https://pterosaurheresies.wordpress.com/2017/12/15/ichthyostega-and-acanthostega-secondarily-more-aquatic/

      However, the list of fish taxa preceding basal tetrapods in the LRT is novel, as far as I know.

      re: ‘assuming that it is a fact a priori.’ You may be pointing the finger at yourself, or projecting, or misunderstanding. I present findings recovered by software… and reasons why those findings reflect the gradual accumulation of traits expected in the theory of evolution. Often that involves taxon exclusion. Since every newly included taxon is tested against every previously included taxon, a large study like the LRT is largely self-correcting IF self-reviewed before publication. During such reviews mistakes jump out as unique traits for a clade. Rarely if ever do I use the work ‘fact’ for such recoveries/hypotheses of interrelationships. You set yourself up for criticism when you say I do so. Instead, as you know, I often report on the tens of thousands of corrections I have made during the course of this project. Those I openly admit.

      If you don’t like what I report, or disagree with the findings because they break with academia or tradition, a good thing to do is to run your own analysis and present the differences and similarities on whatever media you choose. Along the same lines, I appreciated your encouragement to untangle earlier work by Nesbitt. Thanks again, Neil.

      • The phrasing in your blog posts certainly sounds like you are making statements of fact rather than hypotheses. “Vancleavea is NOT an Archosauriform”, “It turns out that Ventastega was reverting to becoming more aquatic, not more terrestrial”, “Plesiadapis ,a rodent relative traditionally and wrongly considered a basal primate with rodent-like teeth”, etc. Any unbiased observer can see you avoid subtlety and ambiguity when presenting your hypotheses. The problem is that you present them like facts, regardless of whether you think of them as hypotheses. Even so..

        “Taxon inclusion IS vastly more important than character formulations”. Oh look, a hypothesis stated as a fact. Taxon inclusion is important, no doubt. An analysis which doesn’t consider elephants or hyraxes in sirenian systematics is fundamentally flawed, as there are many characters in those groups which make them worthy candidates to connect to sirenians. That is the most stable evolutionary pathway for sirenian evolution. But evolution is based on changing traits, and taxa lucky enough to be fossilized or live to the present act as “checkpoints” in the speciation of life, allowing us to estimate what characters were present when lineages split. The lineages themselves rely on characters, as do phylogenetic analyses. Characters are how we compare taxa.

        The LRT does compare taxa using characters, but only a small amount of characters. There are many characters which contradict the reconstructed evolutionary pathways you have found, and yet you refuse to properly account for them. You’ve been assuming a priori that all characters not coded in the LRT are useless, convergent, or impossible to properly assess, citing teeth morphology or a certain stubborn ornithologist. But that certain stubborn ornithologist fell by the wayside because his opponents demonstrated that a more robust evolutionary pathway existed, supported by previously ignored characters. I certainly see a parallel here. You may want to check out Grandcolas et al. (https://www.academia.edu/23021689/) for the logical fallacies that result from your beliefs regarding character inclusion and exclusion.

        Finally getting back to Marjanovic & Laurin, I wasn’t talking about their minimal reptile sampling, I was talking about how your hypothesis of an Icthyostega sister to embolomeres (or secondarily aquatic) is contradicted by an analysis which assesses the pathways in that area of the tree of life much better than yours does. We both know that Icthyostega’s placement in the LRT is not taxa-reliant, it was connected to Proterogyrinus regardless of increased taxon sampling. So why do you continue to present a flawed hypothesis using the same matter-of-fact rhetoric you always use when there is a much better alternative? Lastly, Marjanovic and Laurin did not add many fossil illustrations or photographs because those already existed in their sources. When they did present novel observations not visible in other imagery (such as Casineria toes), they provided new photos. Scientific journals place limits (or fees) on how many figures can be published within a single publication, so blame those rather than the authors.

      • re: “Vancleavea is NOT an Archosauriform”, etc. Part of what I do is science, making the discovery. Part of what I do is journalism, getting the word out. Those are headlines announcing results when more taxa are included. If they are in error, please let me know. They remain hypotheses of relationships. It’s standard operating procedure to announce such hypotheses in the form of short concise headlines. After such an announcement, if possible, you or others are invited to prove that wrong. That’s part of the scientific process. That’s what I do when others announce errant discoveries. You should do the same.

        Downloaded your link titled: “Phylogenetics and Ecology: As Many Characters as Possible Should Be Included in the Cladistic Analysis.” This will make a good blog post after reading and digesting it.

        re: ‘Much better than yours does.’ How so? Let’s keep it specific rather than hypothetical. Let’s keep it generic and avoid larger clades. As before: Which three taxa in the LRT are misplaced and where should they go? Keep it to three and we can discuss candidate hypotheses. I look forward to making the corrections, if needed. You know I’ve done it before. You act like I don’t make corrections whenever needed (e.g. “So why do you continue to present a flawed hypothesis…”)

        re: limits or fees for images. That only works for print. The only version of Marjanovic and Laurin I’ve seen has been online with lavish use of color.

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