Strange Bedfellows – Nesbitt (2011) – part 1 – Mesosuchus

Sometimes the details escape us. Sometimes we miss the big picture.
Sterling Nesbitt (2011) recovered a family tree of the Archosauriformes that is substantially different from the large reptile tree. How do we account for those differences? Earlier we looked at the characters proposed for certain clades. Today we’ll look at pictures, which should be worth a thousand words, or a few hundred character trait scores.

Here then, for your approval and disapproval are comparisons between closest kin found by the Nesbitt (2011) tree versus those found by the large reptile tree. We’ll start at the base, because at the base we start with a problem.

The base of the Nesbitt (2011) tree

Figure 1. The base of the Nesbitt (2011) tree with featured taxa in yellow.

Nesbitt rooted Mesosuchus (Figs. 1, 2) at the base of his archosaur tree with Prolacerta and Proterosuchus following. Certainly Prolacerta and Proterosuchus were morphologically similar, but, taking a larger view, the large reptile tree found Mesosuchus to be closer to rhynchocephalians (sphenodontians) and lizards on the other branch of the tree. Youngina should have been chosen as the outgroup taxon. Here the two competing nestings are presented together for visual comparison.

The first three taxa in the Nesbitt (2011) tree.

Figure 2. The first three taxa in the Nesbitt (2011) tree. Here Mesosuchus is the strange bedfellow, an odd planteater with a derived dentition. Nesbitt (2011) did not test Mesosuchus against other rhynchocephalians, but the large reptile tree does test and demonstrate this relationship.

And here is the alternative, with Youngina replacing Mesosuchus. Note the more gradual transition from generalized to derived traits, particularly in the skull and pelvis. I’m letting the pictures tell the story this time having already handled the traits in a prior series beginning here.

The large reptile tree recovered Youngina basal to Prolacerta and Proterosuchus.

Figure 2. The large reptile tree recovered Youngina basal to Prolacerta and Proterosuchus. Youngina had a generalized shape and generalized teeth but was excluded by Nesbitt (2011).  Note: Some Youngina specimens precede Prolacerta in the large reptile tree. Others succeed it. Some specimens had a weakness in the rostrum that presaged the antorbital fenestra. So the genus Youngina is in need of revision. 

Why Youngina is a better root taxon than Mesosuchus
Taxa at the base of a clade generally have a generalized anatomy, but Mesosuchus does not. It has a very derived plant-eating dentition. Nesbitt (2011) did not test Mesosuchus against other rhynchocephalians, but the large reptile tree does test and demonstrate this relationship. Nesbitt (2011) could have used an even more generalized taxon, like Thadeosaurus, but did not.

Taxon Exclusion and Inappropriate Inclusion
I know I harp on this, but taxon exclusion (in this case, Youngina) and inappropriate taxon inclusion (in this case, Mesosuchus) is the prime cause for all the trouble Nesbitt (2011) and other similar restricted studies have. Missing from the professional literature is a large reptile study that encompasses a large gamut of clades, like the large reptile tree. From such a large study, smaller more focused studies can proceed with the confidence that they are minimizing the inclusion of inappropriate taxa.

Why Criticize a Better Solution?
Darren Naish and others have criticized the methods and results of the large reptile tree (without getting specific), but these tests and these results demonstrate the relative validity of the large reptile tree – at least in comparison to the latest “gold standard,” the Nesbitt (2011) tree. The recovered Nesbitt (2011) relationships (you’ll see them in all their glory over the next few days) also demonstrate how ridiculous a tree can become when taxa are included that should not be there. One look at Nesbitt’s resulting “closest kin” brings immediate understanding that there’s something wrong with a matrix that produces such strange bedfellows and untenable sisters. So where is the professional criticism of results like these?

The professionals may be busy with their own projects, which may be why they have left it to the enthusiasts to point out the errors. Or like sharks and lawyers, it may be professional courtesy.

As always, I encourage readers to see specimens, make observations and come to your own conclusions. Test. Test. And test again. If anyone can defend the placement of Mesosuchus closer to Prolacerta and Proterosuchus than Youngina, I’d be glad to publish that report.

Evidence and support in the form of nexus, pdf and jpeg files will be sent to all who request additional data.

Nesbitt SJ 2011.
 The early evolution of archosaurs: relationships and the origin of major clades. Bulletin of the American Museum of Natural History 352: 292 pp.


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