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.
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.
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.
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.
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.
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.
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.