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RE: A question about Mickey's non-theropod tree



Jean-Michel wrote-

I've been offlist quite long and surely missed some important points. I just
took a look at the non-theropod part from HP Mickey Mortimer's site, and I
saw that Mesosauridae were tentatively put within Diapsida along with
Claudiosauridae and that Anapsida is part of Achosauromorpha. Did I really
miss some important points or is there new evidence for these taxa?

Welcome back.

The position of Mesosauridae is based on Modesto's (1996) thesis. Based on his restudy of mesosaurs, he found-
|--Limnoscelis
`--+--+--Ophiacodontidae
| `--+--Varanopseidae
| `--+--Eothyrididae
| `--Caseidae
`--+--+--Rhiodenticulatus
| `--Captorhinus
`--+--+--Paleothyris
| `--Petrolacosaurus
`--+--+--Milleretta
| `--+--Procolophon
| `--Owenettidae
`--+--+--Stereosternum
| `--Mesosaurus
`--+--Claudiosaurus
`--+--Acerosodontosaurus
|--Younginia
`--+--Hovasaurus
`--Thadeosaurus


Modesto finds many characters supporting traditional nodes (Sauropsida, Romeriida, Diapsida, etc.) to be problematic, indicating the topology there isn't as certain as we usually take it to be. No surprise there, as I think that's true of just about any morphological cladogram. However, I do take issue with Modesto's method of objectifying quantitative characters. If he plots taxa on a line of potential ratios for a character (e.g. humerofemoral ratio) and sees no distinct gaps between where taxa plot, he drops the character. This does make it more objective in a way, but it also ignores real variation and does such when we need to use it most (i.e. when we have a lot of intermediate taxa that need to be sorted out).

As for Anapsida being inside Archosauromorpha, that's a consequence of molecular phylogenies finding turtles sister to Archosauria (Zardoya and Meyer, 1998; Hedges and Poling, 1999; Kumazawa and Nishida, 1999; Rest et al., 2003; Iwabe et al., 2005), and phylogenetic taxonomy defining Anapsida as turtles and taxa more closely related to them than to extant saurians. Ah, the consequences of poor definitions... when will they learn?

Mickey Mortimer