Microsaurs: the “little lizards” that weren’t little lizards.

This post was updated February 8, 2017 with more taxa and new scores for several previously placed taxa.

Figure 1. Microsaurs and other basal non-amniote tetrapods from the large reptile tree.

Figure 1. Microsaurs and other basal non-amniote tetrapods from the large reptile tree.

Earlier we looked at the Microsauria and earlier we looked at some weird members of that clade. Today we’ll look at all the microsaurs included in the large reptile tree (Fig. 1) to scale and in phylogenetic order (Fig. 2). This list represents just a few of their number.

The variety is amazing
with certain members closely approaching the basal reptile morphology, including the loss of the otic notch (concavity at the posterior skull).  I added microsaurs to test whether they would indeed nest apart from the amniotes.

They do (Fig. 1).

What starts them off?
A sister to Utegenia is basal to the Microsauria (Fig. 1) as it is also basal to living amphibians and their prehistoric kin, represented here by Doleserpeton. So living amphibians are part of the outgroup to the Microsauria. Utegenia is also the outgroup to the pre-amniotes, like Silvernerpeton and Gephyrostegus, that ultimately gave rise to reptiles like Cephalerpeton, which was once considered a microsaur.

Figure 2. The Microsauria to scale and in phylogenetic order. Members shown are those included in the large reptile tree (Fig. 1). The variety is amazing, any approaching the basal reptile morphology. Amphibian interrelations are still contested and this limited inclusion set and tree topology is subject to change with the addition of more pertinent taxa. Some of these are definitely related to one another. Others I'm not so sure about. More taxa would help, but then, it's a reptile family tree, not a microsaur family tree.

Figure 2. The Microsauria to scale and in phylogenetic order. Members shown are those included in the large reptile tree (Fig. 1). The variety is amazing, any approaching the basal reptile morphology. Amphibian interrelations are still contested and this limited inclusion set and tree topology is subject to change with the addition of more pertinent taxa. Some of these are definitely related to one another. Others I’m not so sure about. More taxa would help, but then, it’s a reptile family tree, not a microsaur family tree.

What others say:
Wikipedia describes the Microsauria as “the most diverse and species-rich group of lepospondyls.” Recently, Microsauria has been considered paraphyletic, as several other non-microsaur lepospondyl groups such as Lysorophia seem to be nested in it. Microsauria is now commonly used as a collective term for the grade of lepospondyls that were originally classified as members of Microsauria. The microsaurs all had short tails and small legs (note: not so in Fig. 1) , but were otherwise quite varied in form. The group included lizard-like animals that were relatively well-adapted to living on dry land, burrowing forms, and others that, like the modern axolotl, retained their gills into adult life, and so presumably never left the water. They are possible ancestors of the newts and salamanders, if that group did not arise from thetemnospondyls along with the frogs and toads.”

Differences in cladograms
The Wiki cladogram (modified from Anderson 2001), does not provide a generic outgroup, but uses the suprageneric “Lepospondyli”. Utaherpeton nests at the base there and not far from the base here (Figs. 1,2).

The Wiki cladogram (modified from Anderson 2001), nests Tuditanus far from the “horn heads”, like Diplocaulus, where Eoserpeton was left out, but they are all close kin in the large reptile tree.

The Wiki cladogram (modified from Anderson 2001), nests Batropetes and Rhynchonkos close to Eocaecilia, but they are far apart in the large reptile tree (Fig. 1).

The Wiki cladogram (modified from Anderson 2001), nests Pantylus as a derived taxon, but the large reptile tree nests it close to the origin of the Microsauria. Anthracodromeus is not even listed in the Wiki cladogram because (Carroll and Baird 1972) considered it a basal reptile.

These difference might be accounted for by the fact that all three trees use a different outgroup taxon. I also place less confidence in the order of microsaurs in the large reptile tree, like long-legged, squat Batropetes, because it seems to be a strange bedfellow nesting with elongated kin. More taxa are needed. Unfortunately, microsaurs are bit players because here because the reptiles are the focus of this study.

The more recent Ruta and Coates (2007) tree
nests Microsaurs in three clades with no listed outgroup.

In all three cladograms
Microbrachis nested close to the base along with Odonterapeton (an elongate, small limbed, large-eyed genus) and the similar Hyloplesion. I haven’t tested these, but once again, the elongated small-limb taxa usually do not give rise to short long-limbed taxa.

References
Anderson JS 2001. The phylogenetic trunk: Maximal inclusion of taxa with missing data in an analysis of the Lepospondyli (Vertebrata, Tetrapoda). Systematic Biology 50 (2): 170–193.
Carroll RL and Baird D 1972. Carboniferous Stem-Reptiles of the Family Romeriidae. Bulletin of the Museum of Comparative Zoology143 (5): 321–363.
Carroll RL and Gaskill P 1978. The order Microsauria. American Philosophical Society, Philadelphia, 211 pp.
Ruta M and Coates MI 2007. Dates, nodes, and character conflict: addressing the lissamphibian origin problem. Journal of Systematic Palaeontology 5: 69–122.

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