Procolophonid phylogeny – problems and solutions

Cisneros 2008 reviewed the “procolophonids” as traditionally assumed (Fig. 1) and created a  focused phylogenetic family tree with Nyctiphruretus and Owenettids at the base and Procolophon, Hypsognathus and Sclerosaurus as derived taxa (Fig.1). Unfortunately, this topology was not replicated in the large reptile tree. Moreover, the Cisnernos tree appears to be upside down.*

Figure 1. The Procolophonid family tree according to Cisneros 2008.

Figure 1. The traditional procolophonid family tree according to Cisneros 2008. Highlighted taxa are found scattered in the large reptile tree (figure 2). Nyctiphruretus is the outgroup. Owenettidae is the secondary outgroup. The large reptile tree found Orobates to be the outgroup for Procolophon and kin (Fig. 2). Nowhere here are any diadectids, the sister group to Procolophon and kin in the large reptile tree. 

Figure 2. A segment of the large reptile tree with a gamut more or less equal to the Cisneros tree demonstrating the large number of excluded intervening taxa discovered by increasing the inclusion set.

Figure 2. A segment of the large reptile tree with a gamut more or less equal to the Cisneros 2008 tree demonstrating the large number of excluded intervening taxa discovered by increasing the inclusion set. Highlighted taxa are found in the Cisneros 2008 tree (figure 1).

The large reptile tree
found a different topology (Fig. 2) with Hypsognathus and Procolophon at the base derived from a sister to Orobates. Sclerosaurus nested with pareiasaurs. Nyctiphruretus and owenettids nested as derived taxa basal to lepidosaurs (outgroup to Paliguana). Clades were widely separated from one another by numerous taxa.

Background
Cisneros 2008 reviewed the prior literature on the subject, noting with regularity that owenettids were previously not associated with procolphonids and otherwise often are found to each be monophyletic taxa. That all changed when Reisz and Laurin (1991) considered Owenetta to be a procolophonid. DeBraga and Rieppel (1997) found an Owenettidae-Procolophonidae clade. DeBraga (2003) placed Sclerosaurus within the Procolophonidae.

The problem
with Cisneros 2008 and the other prior papers listed above appears to be a reliance on small traditional inclusion sets without basing them on a verified and valid larger gamut study, like the large reptile tree (Fig. 2). Such a large study establishes broader relationships. Then more focused studies, like Cisneros 2008, can proceed with greater confidence and fewer taxon inclusion/exclusion errors.

This is the reason to have a large gamut study of the reptile family tree — to avoid such assumptions based on tradition, not testing. Larger trees also establish outgroups better.

Real procolophonids
Are restricted to Hypsognathus and Procolophon (from the large reptile tree), plus the procolophonid cousins listed in figure 1. Sclerosaurus, nyctiphruretids and owenettids belong to small non-procolophonid clades on the new Lepidosauromorpha branch leading to lepidosaurs, further to the right in the tree, far from the true procolophonids.

References
Cisneros JC 2008. Phylogenetic relationships of procolophonid parareptiles with remarks on their geological record. Journal of Systematic Palaeontology 6(3): 345-366.
deBraga M 2003. The postcranial skeleton, phylogenetic position, and probable life style of the Early Triassic reptile Procolophon trigonicepsCanadian Journal of Earth Sciences 40: 527–556.
DeBraga M and Rieppel O 1997. Reptile phylogeny and the interrelationships of turtles. Zoological Journal of the Linnean Society 120: 281–354.
Reisz RR and Laurin M 1991. Owenetta and the origin of turtles. Nature 349: 324–326.

* We’ve seen such trees before here, with phylogenetic relationships perfectly ordered but upside-down.

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