The Croc-Lizard Split (Ezcurra et al. 2014)

A new PLOSOne paper
by Ezcurra et al. 2014) provides “new data on early saurian evolution including osteohistology, and present a new morphological phylogenetic dataset. We support previous studies that find that no valid Permian record for Lepidosauromorpha, and we also reject some of the previous referrals of Permian specimens to Archosauromorpha.”

Here’s their tree (Fig.1, color added to show lepido/archo division according to the large reptile tree):

Figure 1. Tree recovered by Ezcurra et al. Colored areas are based on the large reptile tree, the new Lepidosauromorpha (pink) and the new Archosauromorpha (baby blue).

Figure 1. Tree recovered by Ezcurra et al. Colored areas are based on the large reptile tree, the new Lepidosauromorpha (pink) and the new Archosauromorpha (baby blue). This tree minimized the strange bedfellow problem, but still has issues.

The Ezcurra et al. tree is completely resolved. They dropped the rhynchosaur, Hyperodapedon, from the inclusion set, “because its advanced morphology is not congruent with that observed in basal members of the group  (Mesosuchus browni, Howesia browni, Noteosuchus colletti) and the absence of species linking these basal forms and Hyperodapedon could cause artifacts in the optimization of characters within Archosauromorpha.”

Let’s test the Ezcurra tree.

Figure 2. The large reptile tree pruned to match (as closely as possible) the taxon list from Ezcurra et al. Note the two branches found by the large reptile tree are here intertwined, as in the Ezcurra tree. This is due to taxon exclusion.

Figure 2. The large reptile tree pruned to match (as closely as possible) the taxon list from Ezcurra et al. Note the two branches found by the large reptile tree are here intertwined, as in the Ezcurra tree. This is due to taxon exclusion.

When the large reptile tree is reduced to the Ezcurra et al. taxon list (Fig. 2), a similar but not the same tree is produced. I had to make Tseajaia the outgroup, but that’s okay. It is just as primitive as Captorhinus, but certainly not the most primitive reptile.

Also of interest is a similar intertwining of Lepidosaurmorphs and Archosauromorphs in complete resolution. So these taxa do not reveal the diphyletic nature of the Reptilia. This is probably so because basal reptiles and their outgroups are not included.

Here (Fig. 2) many dozens of reptiles are excluded, roughly 90% from the large reptile tree. To their credit no suprageneric taxa were employed.

Why is this so?
The large reptile tree has a Homoplasy Index (HI) over 90%. That means massive convergence. That’s just the reality of the situation. That’s why you need a computer to figure it out.

Two cases in point: 
Caseids, typically and traditionally nest with varanopids, ophiacodontids and other pelycosaurs. But when given the opportunity, as in the large reptile tree, they nest with millerettids, including Eunotosaurus, bolosaurids and Acleistorhinus, all of which also have a lateral temporal fenestra.

Pterosaurs, typically and traditionally nest with archosaurs. But when given the opportunity, as in the large reptile tree, they nest with fenestrasaurs, within the Lepidosauria and outside the Squamata.

The case has to be made
that 366 taxa trumps 36 taxa, offering 10x more opportunities to nest where parsimony is maximized and a priori exclusions are minimized. The large reptile tree may not be the last word on reptile systematics, but it greatly increases the confidence level that the tree echoes actual evolutionary events.

Adding taxa adds insight.

In the Ezcurra tree
1. Diapsids arose from anapsids, like Paleothyris and Captorhinus, evidently without transitional taxa.
2. Coelurosauravus is derived from taxa with a diapsid skull morphology
3. Lepidosaurs lost the lower temporal bar
4. Trilophosaurus and Mesosuchus are derived from protorosaurs
5. Prolacerta is not closely related to Protorosaurus
6. Mesosuchus is not related to rhynchochephalians

Major convergence found by the large reptile tree:
1. Tanystropheids and protorosaurs
2. Youngina and rhynchocephalia
3. Caseasauria and basal synapsida

The large reptile tree does not shy away from nesting any taxa, including the rhynchosaur, Hyperodapedon.

Permian Lepidosaurormorphs?
The large reptile tree found at least two Permian lepidosauromorphs: the oldest known lepidosauriform is Lanthanolania of the Middle Permian, which means its phylogenetic predecessor, Paliguana, must have been older still. Coelurosauravus is also a lepidosauriform. The oldest lepidosaurs I have tested are Lacertulus and the Ascendonanus, both from the Late Permian.

Finally, 
The large reptile tree found the croc-lizard split to happen shortly after the origin of the amniote egg, long before all of the taxa listed by Ezcurra et al. appeared.

References
Ezcurra MD, Scheyer TM and Butler RJ 2014. The Origin and Early Evolution of Sauria: Reassessing the Permian Saurian Fossil Record and the Timing of the Crocodile-Lizard Divergence. PLoS ONE 9(2): e89165. doi:10.1371/journal.pone.0089165

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