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.

Figure 1. Distribution of the solid and open palate architectures in basal tetrapods in the LRT topology.

Figure 1. Distribution of the solid and open palate architectures in basal tetrapods in the LRT topology.

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.

Figure 2. Size distribution among basal tetrapods in the LRT topology

Figure 2. Size distribution among basal tetrapods in the LRT topology

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.

Figure 3. Distribution of single vertebrae among basal tetrapods in the LRT.

Figure 3. Distribution of single vertebrae among basal tetrapods in the LRT. Note: Phlegethonita now nests with the microsaurs. 

Single piece centra
appear in frogs + salamanders and microsaurs. Intercentra appear in all other taxa.

Figure 6. Distribution of palatal fangs among basal tetrapods in the LRT.

Figure 6. Distribution of palatal fangs among basal tetrapods in the LRT.

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

Figure 7. Distribution of the otic notch among basal tetrapods in the LRT.

Figure 7. Distribution of the otic notch among basal tetrapods in the LRT.

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

  1. 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.
  2. Salamanders and frogs that greatly reduce posterior skull bones.
  3. 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.

 

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3 thoughts on “Distribution of ‘key’ traits in basal tetrapods

  1. Good to know you’ll address the comments. :-)

    the large reptile tree introduces a non-traditional topology in which lepospondyls have a single origin

    Well, depends on what you mean by “traditional”. Lepospondyl monophyly has been orthodox since the mid-1990s. For further discussion, see the section “The lepospondyl problem” in the Discussion of my preprint. :-)

    Traditional topologies attempt to put all open palate taxa into a single clade. Here the open palate evolved three times by convergence.

    You distinguish only three states of that character? Check out the character in my preprint whose name begins with “VOM 5” and the three characters whose names begin with “INT VAC”. Then download Data S1 and trace these characters on the trees in Mesquite. You’ll find a few interesting things there, I’m sure.

    BTW, the palate closes gradually in seymouriamorph ontogeny; Kotlassia has a completely closed one; and what is that “Seymouria tadpole” taxon you have? The aquatic stage of Seymouria is wholly unknown; do you mean Discosauriscus, which stayed in the water for 10 years or so before losing its lateral-line grooves and coming out?

    The length of basal tetrapods
    falls below 60 cm in Eucritta

    Keep in mind that Eucritta is exclusively known from juveniles. (That’s also why its palate isn’t closed yet.)

    Intercentra appear in all other taxa.

    …They were already there. They’re present in Panderichthys and Eusthenopteron. Do you just mean “are found” when you say “appear”?

    In your fig. 3 you’ve indicated lack of intercentra in Gerobatrachus. They’re there; they’re easily visible in the published illustrations, and even indicated in the drawing.

    Exceptionally, Seymouria also had palate fangs.

    That’s not exceptional at all, it’s normal. Palate fangs are much more widespread than your fig. 6 shows!!! Of the “traditional temnospondyls”, every single one except for Doleserpeton and Gerobatrachus, and Gerrothorax as you mention, has them. Oh, and Spathicephalus – but why do you count baphetoids as “traditional temnospondyls”??? They haven’t been considered temnospondyls in thirty years!

    (You will find literature claiming that Amphibamus doesn’t have them. The largest individual – the neotype, YPM 794 – does; evidently they appeared late in ontogeny in that taxon.)

    There are even “microsaurs” with, well, in any case fang-sized teeth on the palate.

    Those without an otic notch include

    One specimen of Phlegethontia that loses posterior skull bones

    All of them, not just one.

    Some of these, like Greererpeton, have figure data that lack an otic notch, but photos that have one.

    Then, well, please look again. Greererpeton in particular is neither a temnospondyl (hasn’t been considered one in 20 years), nor does it have any trace of an “otic notch” or “temporal embayment” or “spiracular notch”.

    Salamanders and frogs that greatly reduce posterior skull bones.

    Frogs do have that embayment in the squamosal that houses the eardrum. And if you look at Triadobatrachus, its homology with the embayment found in amphibamids becomes reasonably plausible (I still think it’s not homologous, but that’s for phylogenetic reasons).

    All microsaurs more derived than Microbrachis

    All “microsaurs”, without exception, including Microbrachis. You know why no trace of a notch has ever been reported for Microbrachis? I’ve seen a bunch of specimens (maybe 10–20?) and am pretty sure why.

    Let me know
    if I overlooked or misrepresented any pertinent data.

    I haven’t repeated my comments to earlier posts of yours that are relevant here. To pick just one example, your “Diplovertebron” OTU is a duplicate of Gephyrostegus as misinterpreted by Watson (1926) – the real Diplovertebron is known from one partial skeleton that probably belongs to an embolomere and is quite different from Gephyrostegus. I forgot what Eoserpeton was; something like Trachodon or Procheneosaurus in dinosaur terms, IIRC.

    • I note once again that, instead of doing such a review, you’ve been churning out new posts which depend on the issues I’ve pointed out here.

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