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Progress and future directions in archosaur phylogenetics
Journal of Paleontology, Nov 2001 by Brochu, Christopher A
Another potential problem with these studies is taxon sampling. Most analyses include a single turtle, one or two birds (usually a neognath), one or two lepidosaurs, and a single crocodylian. This last point is interesting because the sole crocodylian is usually Alligator mississippiensis. Mitochondrial rates in crocodylians seem to be higher than in other reptiles (Kumazawa and Nishida, 1995; Quinn and Mindell, 1996; Janke et al., 2001), and for at least some genes, rates are higher in alligatorids than in nonalligatorid crocodylians (Brochu, 1997). It would be interesting to reanalyze some of these sequences with some other crocodylian, or even with multiple crocodylians, as was done by Gorr et al. (1998). Switching to nuclear genes may not solve this problem entirely, as high rates and within-group rate variability have been reported for squamates (Hughes and Mouchiroud, 2001).
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It would be easy to dismiss the molecular analyses as being misled by sequence saturation, a long-branch attraction problem, inadequate ingroup sampling, or by any other standard retort. The turtle-archosaur hypothesis is unsettling whether the supporting data are osteological or molecular. But similar questions can be raised about the morphological data brought to bear on this question. Turtles and sauropterygians are very derived when they first appear in the fossil record, and selecting a taxon to polarize morphological character states for them is problematic. More importantly, we might regard the "turtle question" as a matter of taxon sampling. Turtles are outside Diapsida when euryapsids are not included, but diapsids when they are.
As with many in the systematics community, I am skeptical that turtles are derived diapsids. But resolution of this conflict rests with new fossil discoveries, more detailed anatomical descriptions, and more thorough sampling of both taxa and genes. Hypotheses should be overturned on the basis of new evidence and not the counterintuitive appearance of the hypothesis itself.
Pterosaurian relationships.-Pterosaurs are the earliest known flying vertebrates and first appear in the Norian (Benton, 1995). They radiated extensively throughout the Mesozoic and include the largest flying animals known (Kellner and Langston, 1996). The earliest pterosaurs had toothy jaws and long tails; by the Late Cretaceous, the derived pterosaurs (pterodactyloids) had reduced tails, toothless beaks, and often sported bizarre crests on the beak or cranium (Wellnhofer, 1991).
Most cladistic analyses regard pterosaurs as archosaurs closer to birds than to crocodylians, although they universally reject the notion that pterosaurs and birds both evolved from a common flying ancestor (Gauthier, 1986; Sereno, 1991). There has been some controversy whether pterosaurs are related to Scleromochlus taylori, a small archosaur from the Carnian of England (Padian, 1984; Gauthier, 1986; Sereno, 1991) or not (Benton, 1999), but the topological difference between these scenarios is rather modest.
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