You heard it here first: Ichthyostega and Acanthostega were secondarily aquatic

In this YouTube video from 2018
Dr. Donald Henderson starts his online slide video presentation by repeating the traditional fin-to-finger story (Fig. 1).

Unfortunately
that story was already out-of-date in 2018 due to taxon exclusion in comparison to and competition with the phylogenetic analysis found in the large reptile tree (LRT, 1817+ taxa; subset Fig. 5).

Not surprisingly, Dr. Henderson thought it was “very peculiar”
that Middle Devonian tetrapod trackways preceded the Late Devonian fossils of tetrapods by tens of millions of years. The LRT solves this problem. Acanthostega and Ichthyostega are not transitional taxa, but dead end taxa with polydactyly not found in other tetrapod taxa. Their phylogenetic ancestors filled the gap between the Middle and Late Devonian, but those fossils have not been found yet in those strata, only in later strata as late survivors of those earlier radiations.

In the middle of the presentation
Dr. Henderson presented his alternative view: that Ichthyostega and Acanthostega were secondarily aquatic tetrapods. His YouTube video is dated January 11, 2018. Only a short month earlier the LRT recovered Ichthyostega and Acanthostega as secondarily more aquatic tetrapods, time-stamped here.

Evidently that was an idea whose time had come.
Or else Dr. Henderson read that hypothesis here and embraced it. Either way, Dr. Henderson did not employ phylogenetic analysis, but came to his solution as a notion to reconcile the Middle Devonian tracks to the late Devonian fossils.

Otherwise
Dr. Henderson’s presentation was mundane. Henderson’s customary family tree of vertebrates (Fig. 1) indicates he had no idea how clades of fish are related to one another at a species level (Fig. 2). He never tested traditional hypotheses, but accepted them without reservation.

Figure 1. Slide from Henderson's YouTube video with connections between clades highlighted in frame 2.

Figure 1. Slide from Henderson’s YouTube video with connections between clades highlighted in frame 2.

The fish phylogeny problem was resolved
here in 2019 and continues to evolve with every added taxon.

Figure 4. Shark skull evolution according to the LRT. Compare to figure 1.

Figure 2. Shark skull evolution according to the LRT. Compare to figure 1.

Dr. Henderson also presents a traditional lineup
of tetrapods (Fig. 3) that was improved by the LRT by simply adding overlooked taxa (Fig. 4).

Figure 3. Slide from Henderson YouTube presentation modified in frame 2 to reflect the order of basal tetrapods in the LRT. Missing here is Trypanognathus (Fig. 3) and kin, basal tetrapods in the LRT.

Figure 3. Slide from Henderson YouTube presentation modified in frame 2 to reflect the order of basal tetrapods in the LRT. Missing here is Trypanognathus (Fig. 4) and kin, basal tetrapods in the LRT.

Henderson’s traditional lineup is lacking several taxa,
like Trypanognathus (Fig. 4), that are also long, low and with tiny limbs, like Tiktaalik and Panderichthys, but are traditionally never included in fin-to-finger cladograms, other than here in the LRT.

Figure 6. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs.

Figure 4. Dorsal and ventral views of Panderichthys and several basal tetrapods demonstrating the low, flat skulls and bodies with small limbs and relatively straight ribs.

It’s nice to have a notion, like Dr. Henderson had.
After all, that’s where all scientific inquiry has its genesis. But you can’t beat a good old, wide gamut phylogenetic analysis to make your notion into a testable hypothesis that covers all the other competing hypotheses. Let’s hope that someday PhDs will adopt a taxon list comparable to the LRT and then let the taxa and their taxonomy tell the tale.

Figure 4. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Figure 5. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

Colleagues,
follow up those notions with testable analyses. It’s hard work, but it’s the professional thing to do.


References
https://pterosaurheresies.wordpress.com/2017/12/15/ichthyostega-and-acanthostega-secondarily-more-aquatic/

3 thoughts on “You heard it here first: Ichthyostega and Acanthostega were secondarily aquatic

  1. Donald Henderson surely did not need to read your blog: he suggested Ichthyostega being a secondarily aquatic form in 1999.
    It would be fair to check the literature before claiming authorship of a hypothesis.
    Ref: Henderson, D. M. 1999. Late Devonian amphibians as secondarily aquatic tetrapods. In Hoch, E. & Brantsen, A. K. (eds) Secondary Adaptation to Life in Water. University of Copenhagen, p. 18.

    • Good to know! I had no idea. Turns out Google has no idea, either. Seeking “Late Devonian amphibians as secondarily aquatic tetrapods” on Google produces no results for Henderson 1999. Not sure why. You wrote, “It would be fair to check the literature…” Granted, and always a good idea. Unfortunately checking the literature online yields no results. Wikipedia (Evolution of Tetrapods) likewise does not cite Henderson. Even so, thank you, Andrea for revealing this hidden gem. If you have it, please send it. Evidently Henderson 1999 did not perform a phylogenetic analysis to support his notion. Otherwise his YouTube presentation would have included that cladogram, not the traditional one he presented (see above) that nested those taxa as transitional forms. Send the paper so proper credit can be given for the proper evidence.

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