Diplovertebron and amphibian finger loss patterns

Diplovertebron punctatum (Fritsch 1879, Waton 1926; Moscovian, Westphalian, Late Carboniferous, 300 mya, Fig. 1) was considered an anthracosaur or reptile-like amphibian. That is confirmed by the large reptile tree (LRT, subset Fig. 2), where it nests with  Utegenia transitional between basal seymouriamorpha, like Kotlassia, and basal amphibians, like Balanerpeton (Fig. 3), yet close to the origin of stem reptiles, like Silvanerpeton. Based on the nesting of Tulerpeton in the LRT, Diplovertebron had origins in the Late Devonian.

In Diplovertebron,
the vertebral structure is primitive. The notochord persisted in adults. The ribs were long and slender as in basal taxa, not shortened as in lepospondyl amphibians. Five manual digits were preserved with a 2-3-3-3-4 formula, a formula similar to amphibians, not like reptiles (2-3-4-5-5). The ilium is bifurcate with a long posterior process. The pubis did not ossify, as in several basal tetrapods including Crassigyrinus and derived Amphibia. Small scutes covered the entire torso ventrally, as in basalmost tetrapods and basal reptiles.

The presence of five manual digits
in Diplovertebron and Balanerpeton (Figs. 4, 5) sheds light on their retention in Acheloma + Cacops. There is a direct phylogenetic path between them (Fig. 2). Note that all other related clades lose a finger or more. Basal and stem reptiles also retain five fingers.

Note the narrow frontals,
on Diplovertebron distinct from the wide frontals in Utegenia and Kotlassia, but more similar to those in Balanerpeton (Fig. 4), another basal amphibian, and Silvanerpeton, a stem reptile. Yet none have the hourglass shape found in Diplovertebron.

As reported
earlier, finger five was lost in amphibians,while finger one was lost in temonospondyls. Now, based on the longest metacarpal in Caerorhachis and Amphibamus (second from medial), apparently manual digit one was lost in that clade also, distinct from the separate frog and microsaur clades. In summary, loss from five digits down to four was several times convergent in basal tetrapods.

Finally, we have to talk about
Tulerpeton (Fig. 6). The evidence shows that the sixth manual digit is either a new structure – OR – all post-Devonian taxa lose the sixth digit by convergence, since they all had five fingers. Finger 6 has distinct phalangeal proportions, so it is NOT an exposed finger coincident rom the other otherwise unexposed hand in the fossil matrix.

References
Fritsch A 1879. Fauna der Gaskohle und der Kalksteine der Permformation “B¨ ohmens. Band 1, Heft 1. Selbstverlag, Prague: 1–92.
Kuznetzov VV and Ivakhnenko MF 1981. Discosauriscids from the Upper Paleozoic in Southern Kazakhstan. Paleontological Journal 1981:101-108.
Watson DMS 1926. VI. Croonian lecture. The evolution and origin of the Amphibia. Proceedings of the Zoological Society, London 214:189–257.

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