Anthracosaurus: beware the chimaera!

Anthracosaurus russelli (Huxley 1863, Panchen 1977, Clack 1987; Westphalian, Late Carboniferous, 310 mya, skull length 40cm; Figs. 1, 2) was originally considered a labyrinthodont. The wide, yet pointed, triangular skull and tall orbits recall traits found in labyrinthodonts, like Sclerocephalus, and in the basal tetrapod, Tiktaalik. Here, in the large reptile tree (LRT, 967 taxa),  Anthracosaurus nests with Neopteroplax (Fig. 3) as a derived embolomere, the clade that likely gave rise to Seymouriamorpha, Lepospondyli and Reptilia

At least one orbit
in Anthracosaurus has an inverted teardrop shape. The marginal and palatal fangs are quite large. Although flattened in dorsal view, comparisons suggest the jaw margin was convex, as in Neopteroplax.

Based on its size and nesting,
Anthracosaurus developed a labyrinthodont-like skull by convergence because Proterogyrinus is basal in the Embolomeri. Those giant marginal and palatal fangs indicate a predatory niche.

Clack 1987
illustrated a lateral and dorsal view of Anthracosaurus (Fig. 2) based on a chimaera of specimens. Unfortunately, plugging that data into the LRT produced loss of resolution over several nodes. Using the older single skull in dorsal view had no such problems.

We looked at the problems chimaera taxa produce
earlier here, and in six blogs that preceded that one.

The clade Anthracosauria has had problems
From Wikipedia: “Gauthier, Kluge and Rowe (1988) defined Anthracosauria as ‘Amniota plus all other tetrapods that are more closely related to amniotes than they are to amphibians” (Amphibia in turn was defined by these authors as a clade including Lissamphibia and those tetrapods that are more closely related to lissamphibians than they are to amniotes).”

In this definition non-amniote Anthracosauria does not include Anthracosaurus, but only Silvanerpeton and Gephyrostegus in the LRT because more basal taxa are also basal to amphibians.

“Similarly, Michel Laurin (1996) uses the term in a cladistic sense to refer to only the most advanced reptile-like amphibians. Thus his definition include the (Diadectomorpha and Solenodonsauridae) and the amniotes.”

In the LRT Diadectomorpha and Solenodonsauridae are amniotes.

“As Ruta, Coates and Quicke (2003) pointed out, this definition is problematic, because, depending on the exact phylogenetic position of Lissamphibia within Tetrapoda, using it might lead to the situation where some taxa traditionally classified as anthracosaurs, including even the genus Anthracosaurus itself, wouldn’t belong to Anthracosauria.

Indeed! And that happened in the LRT.

Laurin (2001) created a different phylogenetic definition of Anthracosauria, defining it as “the largest clade that includes Anthracosaurus russelli but not Ascaphus truei”.

In the LRT Ascaphus, the tailed frog, is derived from the large clade, the embolomeri, that includes Anthracosaurus. However the small clade that includes just Anthracosaurus and Neopteroplax does not include the tailed frog.

“However, Michael Benton (2000, 2004) makes the anthracosaurs a paraphyletic order within the superorder Reptiliomorpha, along with the orders Seymouriamorpha and Diadectomorpha, thus making the Anthracosaurians the “lower” reptile-like amphibians. In his definition, the group encompass the Embolomeri, Chroniosuchia and possibly the family Gephyrostegidae.”

In the LRT the Embolomeri are basal to Eucritta and the Seymouriamorpha, which are basal to the Reptilia (= Amniota) and Lepospondyli (including Amphibia). The Chroniosuchia and Gephyrostegus are both amphibian-like reptiles in the LRT.

The clade Reptilomorpha suffers the same definition problems.
As Wikipedia reported, “As the exact phylogenetic position of Lissamphibia within Tetrapoda remains uncertain, it also remains controversial which fossil tetrapods are more closely related to amniotes than to lissamphibians, and thus, which ones of them were reptiliomorphs in any meaning of the word.”

Wouldn’t it be great if someone could put together
a large gamut phylogenetic analysis that could settle all those controversial issues?

References
Clack JA 1987. Two new speciemens of Anthracosaurus (Amphibia: Anthracosauria) from the Northumberland coal measures. Palaeontology 30(1):15-26.
Huxley TH 1863. Description of Anthracosaurus russelli, a new labyrinthodont from the Lanarkshire coalfield. Quartery Journal of the Geological Society 19:56-58.
Panchen AL 1975. A new genus and species of anthracosaur amphibian from the Lower Carboniferous of Scotland and the status of Pholidogaster pisciformis Huxley. Philosophical Transactions of the Royal Society of London, B. 269: 581-640.
Panchen AL 1977. On Anthracosaurus russelli Huxley (Amphibia: Labyrinthodontia) and the family Anthracosauridae. Philosophical Transactions of the Royal Society B. 279 (968): 447–512.