Ichthyostega’s toes – evidence of regeneration?

You might remember
earlier the basal tetrapod Ichthyostega (Fig. 1) shifted its nesting closer to Proterogyrinus (Figs. 2, 3) and Eucritta (Fig. 4) at the base of the Reptilomorpha. One of the reasons for that shift was a reexamination of the pes of Ichthyostega, which has seven digits. Which digits are homologous with the five that are found in many other higher tetrapods?

Metatarsal and phalangeal proportions 
provide clues. If the above digit identities ares used, there is a pretty close match to related taxa. Acanthostega, for instance, has eight pedal digits with metatarsal 3 about twice as long as the more medial metatarsals. Distinct from Ichthyostega, Acanthostega has only one phalanx on digit 1 and only 2 phalanges on digit 2, but in keeping with the ‘one less’ phalangeal formula, digits 3–7 stop at 3 phalanges. In Ichthyostega digits 4 and 5 each add a phalanx, approaching the pattern seen in Proterogyrinus.

Holmes 1984
reconstructed the pes of Proterogyrinus (Fig. 2). If one takes the data from in situ drawings provided by Holmes (Fig. 3), reconstructions of both pedes can be created to check the accuracy of the Holmes reconstruction while removing any freehand bias.

The pes of the related Eucrtta also bears another look.
It is more difficult to reconstruct based on the taphonomic scattering of the elements. If you’ll notice the medial three digits of Eucritta each appear to have one less phalanx, as in Acanthostega.

 Which makes one wonder about Ichthyostega.
The vestigial digit between 2 and 3 in particular gives one pause. We know that salamanders can regrow their extremities. Based on the unusual apparent binding of pedal digits 1 and 2 in Ichthyostega, along with the vestige of a digit between 2 and 3, One may wonder if that unusual morphology is the result of an accident or injury with subsequent imperfect or unfinished regeneration. Another identical Ichthyostega pes would falsify this hypothesis.

References
Ahlberg PE, Clack JA and Blom H 2005. The axial skeleton of the Devonian trtrapod Ichthyostega. Nature 437(1): 137-140.
Clack JA 1998. A new Early Carboniferous tetrapod with a mélange of crown group characters. Nature 394: 66-69.
Clack JA 2007. Eucritta melanolimnetes from the Early Carboniferous of Scotland, a stem tetrapod showing a mosaic of characteristics. Transactions of The Royal Society of Edinburgh 92:75-95.
Holmes R 1984. The Carboniferous Amphibian Proterogyrinus scheelei Romer, and the Early Evolution of Tetrapods. Philosophical Transactions of the Royal Society of London. Series B, Biological Sciences 306: 431-524.
Jarvik E 1952. On the fish-like tail in the ichthyostegid stegocephalians. Meddelelser om Grønland 114: 1-90.
Jarvik E 1996. The Devonian tetrapod Ichthyostega. Fossils and Strata. 40:1-213.
Romer AS 1970. A new anthracosaurian labyrinthodont, Proterogyrinus scheelei, from the Lower Carboniferous. Kirtlandia 10:1-16.
Ruta M, Jeffery JE and Coates MI 2003. A supertree of early tetrapods. Proc. R. Soc. Lond. B (2003) 270, 2507–2516 DOI 10.1098/rspb.2003.2524 online pdf
Säve-Söderbergh G 1932. Preliminary notes on Devonian stegocephalians from East Greenland. Meddelelser øm Grönland 94: 1-211.

wiki/Ichthyostega
wiki/Eucritta
wiki/Proterogyrinus