Latest fins-to-fingers paper stumbles due to taxon exclusion

Dickson et al. 2020 bring us their views
on the transition from fins to feet at the base of the Tetrapoda.

Unfortunately,
taxon exclusion (Fig. 1), once again, mars this study published in Nature.

Figure 1. Cladogram from Dickson et al. 2020 with an overlay indicating taxa found in the LRT and key LRT taxa with limbs lacking in Dickson et al. 2020.

Figure 1. Cladogram from Dickson et al. 2020 with an overlay indicating taxa found in the LRT and key LRT taxa with limbs lacking in Dickson et al. 2020.

Cherry-picking taxa in Dickson et al. 2020
nested Ichthyostega and Acanthostega as the first taxa to have fingers and toes. These two were highly promoted in earlier works that included co-author, Jennifer Clack, but that should not excuse the exclusion of pertinent taxa. When more taxa are added to a cladogram (Fig. 2), these two famous basal tetrapods, with their large, well-formed limbs, nest not as transitional taxa, but as derived taxa leaving no descendants (subset Fig. 2). Their polydactyl extremities were evolutionary dead-end experiments perhaps reflecting one of the first returns to a more aquatic existence.

Employing a wider gamut of taxa,
in the large reptile tree (LRT, 1766 taxa; subset Fig. 2), pre-tetrapods with flat morphologies and small fins, like Panderichthys (Fig. 3) had only four finger buds. Strongly similar taxa, like the late-surviving basalmost tetrapod, Trypanognathus, likewise had a flat morphology, small limbs and only four fingers. This finger number is typical of basal tetrapods until the advent of finger 5 in five unrelated clades (Fig. 2) leaving no living descendants, except for reptilomorphs, beginning with Tulerpeton, Utegenia and kin, the clade that ultimately evolved reptiles, mammals, primates and humans.

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

Figure 2. Subset of the LRT focusing on basal tetrapods. Colors indicate number of fingers known. Many taxa do not preserve manual digits.

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

Adding taxa 
resolves this problem and many others. Excluding taxa only perpetuates traditional myths and hobbles present research.


References
Dickson BV, Clack JA, Smithson TR et al. 2020. Functional adaptive landscapes predict terrestrial capacity at the origin of limbs. Nature (2020). https://doi.org/10.1038/s41586-020-2974-5

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