Recently, Dilkes and Sues published a redescription of Doswellia in JVP. It is an attractively illustrated paper with a lot of informative text, but I noticed something that has bothered me for awhile...
+--Mesosuchus
`--+--Prolacerta
`--+--Proterosuchus
`--+--Euparkeria
`--+--Erythrosuchus
`--+--+--Doswellia
| `--Proterochampsidae
`--+--Turfanosuchus
`--+--Yonghesuchus
`--+--+--Marasuchus
| `--Scleromochlus
`--+--Gracilisuchus
`--+--Qianosuchus
`--+--Parasuchus
`--Stagonolepididae
Dilkes and Sues noted that the "relative positions of Parasuchus and Gracilisuchus differ from previous studies where phytosaurs are placed near the base of Crurotarsi, below Gracilisuchus, stagonolepidids, crocodylomorphs, and the various "rauisuchian" taxa. In this study, Gracilisuchus is at the base of Crurotarsi whereas Parasuchus is placed as the sister taxon to Stagonolepididae above Qianosuchus".
While their data set does place Gracilisuchus as outside of the "Parasuchus+Qianosuchus+Stagonolepididae" clade, it doesn't necessarily mean that Gracilisuchus is actually closer to the base of Crurotarsi than those taxa. I believe this is a taxon sampling artifact (possibly character sampling too). If some of those various "rauisuchian" taxa or crocodylomorphs were added to the matrix, Gracilisuchus may easily float back up. Remember, how you look at taxa are entirely relative. For example, I could do this:
+--Ornithischia
`--+--Sauropodomorpha
`--+--Tyrannosauridae
`--+--Ceratosaurus
`--Abelisauridae
And it doesn't mean that tyrannosaurids are necessarily basal theropods. ;)
I'm not implying that I think that phytosaurs and aetosaurs form a clade to the exclusion of all other crurotarsans, to be clear.
References cited:
Dilkes, D., and H.-D. Sues. 2009. Redescription and phylogenetic relationships of Doswellia kaltenbachi (Diapsida: Archosauriformes) from the Upper Triassic of Virginia. Journal of Vertebrate Paleontology 29:58-79.
Thursday, July 23, 2009
The relative phylogenetic position of Gracilisuchus in Dilkes and Sues (2009)
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2 comments:
I would strongly agree that phytosaurs and aetosaurs should not form a clade exclusive of other pseudosuchians. Two of the characters used to unite these taxa in Dilkes and Sues (2009) are problematic and if coded differently would change these relationships. 1) Character 67: The fibulae of aetosaurs and phytosaurs are very different in that phytosaurs lack the robust trochanter found in aetosaurs. 2)Character 85: The ventral armor in phytosaurs does not form a ventral carapace as in aetosaurs.
Those are some interesting things to take note of, and while I agree, I'm not sure it would necessarily alter the position of Gracilisuchus. I think it would just lead to:
`--+--Gracilisuchus
`--+--Qianosuchus
|--Parasuchus
`--Stagonolepididae
... at least in the current matrix.
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