Distribution of ‘key’ traits in basal tetrapods

Before the advent of phylogenetic analysis,
paleontologists attempted to define clades with a short list of synapomorphies. In this way they were getting close to the dangers of pulling a Larry Martin. Many taxa, like pterosaurs and Vancleavea were (and are) considered enigmas because they seemed to appear suddenly in the fossil record with a short suite of traits that did not appear in other reptiles. That was only true back then because paleontologists were only considering short lists of traits.

After the advent of phylogenetic analysis
considering long lists of traits, the rule of maximum parsimony allowed clades to include members that do not have a short list of key traits. For instance some reptiles, like snakes, do not have limbs, but that’s okay based on the rule of maximum parsimony as demonstrated in the large reptile tree (LRT, 977 taxa, subsets shown in Figs. 1-5).

Before the advent of phylogenetic analysis
Carroll (1988) divided basal tetrapods into labyrinthodonts and lepospondyls and presented short lists of key traits.

Labyrinthodonts

1. evolved directly from rhipidistian fish
2. labyrinthine infolding of the dentine
3. palate fangs and replacement pits
4. vertebral centra composed of more than one element
5. otic notch
6. large in size

Lepospondyls

1. a heterogeneous assemblage of groups with perhaps several origins from among various labyrinthodonts
2. simple (non-labyrinthine) teeth
3. no palate fangs
4. vertebral centra composed of one element
5. no otic notch
6. small in size

By contrast,
the large reptile tree introduces a non-traditional topology in which lepospondyls have a single origin. Below (Figs. 1-5) the distribution of several traits are presented graphically.

Open palate distribution
Basal tetrapods have a solid palate (Fig. 1) in which the pterygoid is broad and leaves no space around the medial cultriform process. Other taxa have narrow pterygoids and large open spaces surrounding the cultriform process. Still others are midway between the two extremes. Traditional topologies attempt to put all open palate taxa into a single clade. Here the open palate evolved three times by convergence.

The length of basal tetrapods
falls below 60 cm in Eucritta and more derived taxa. It also falls below 60 cm in Ostelepis, at the origin of Tetrapoda and Paratetrapoda. Phlegethontia has a small skull, but is otherwise like an eel, and so does not fall below the 60 cm threshold.

Single piece centra
appear in frogs + salamanders, microsaurs and Phlegethontia, by convergence. Intercentra appear in all other taxa.

Palate fangs
appear in all basal paratetrapods and tetrapods except Phlegethontia, Spathicephalus and Gerrothorax. Exceptionally, Seymouria also had palate fangs.

The otic notch
is widespread among basal tetrapods. Those without an otic notch include

1. One specimen of Phlegethontia that loses posterior skull bones
2. Six flat-skulled temnospondyls in which the tabular contacts the squamosal. Some of these, like Greererpeton, have figure data that lack an otic notch, but photos that have one.
3. Salamanders and frogs that greatly reduce posterior skull bones.
4. All microsaurs more derived than Microbrachis

Let me know
if I overlooked or misrepresented any pertinent data. This weekend I should be able to look at and respond to the many dozen comments that have accumulated over the last few weeks.