(Schoch and Voigt 2019 report on a new dvinosaur,
Trypanognathus (Fig. 1) preserving both cranial and post-cranial data down to a few tiny fingers and one preserved toe phalanx + four metatarsals). In the large reptile tree (LRT, 1441 taxa, subset Fig. 2).
According to Schoch and Voigt 2019:
“The skull closely resembles that of the early Permian dvinosaurian genus Trimerorhachis in outline and suture topology, but the occiput and the palate differ substantially. Derived states are the penetration of vomerine tusks through the splenial and symphyseal
tusks through the premaxilla. Trypanognathus shares with dvinosauroids the lack of a squamosal embayment, an elongated basipterygoid process, a foreshortened palatine ramus exclusively reaching the ectopterygoid, the absence of pterygoid denticles, and enlarged palatal tusks. The body is elongate with well-ossified, but small limbs, the presacral count is circa 28, and the pleurocentra are large and reached ventrally almost as far as the intercentrum.”
Schoch and Voight nested dvinosaurs as derived taxa,
more derived than the basal lepospondyls, Neldasaurus, and Trimerorhachis, just the opposite of the LRT, which nests dvinosaurs as the most basal tetrapods with fingers and toes, instead of fins. Note the flat long body, flat wide head and small limbs. These are primitive traits shared with Panderichthys and Tiktaalik two taxa without fingers and toes.
It is traditional to consider Dvinosauria
as derived taxa within the clade Temnospondyli. Here in the LRT, after testing a wide gamut of basal tetrapods, the Dvinosauria is a basal clade/grade leading to all higher limbed tetrapods, including the clade Temnospondyli and Lepospondyli, and ultimately Reptilomorpha. The retention of robust gill bars along with primitive hands and feet are signs of their primitive status.
Schoch RR and Voigt S 2019. A dvinosaurian temnospondyl from the Carboniferous-Permian boundary of Germany sheds light on dvinosaurian phylogeny and distribution. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2019.1577874.
Does that mean Acanthostega is no longer an Stegocephalian but a real member of Tetrapoda?
In the LRT Acanthostega is a derived tetrapod taxon without descendants. It comes from the first clade to develop relatively large hind limbs, but that clade did not lead to reptiles. Apparently Acanthostega was enjoying a reversal, heading back to a more aquatic niche. This hypothesis was arrived at by adding taxa.