New paper on turtle relationships

["David Marjanovic" <David.Marjanovic@gmx.at>]

Sorry if you're tired of this topic which surfaces regularly every few
months... I have mentioned this paper last time but was unable to give the
citation and to summarize it. I'll try to make it short:

Rafael Zardoya, Axel Meyer: The evolutionary position of turtles revised,
Naturwissenschaften (2001) 88, 193 -- 200, published online 17 May 2001

Paleontologists favor placing turtles next to procolophonids (Laurin & Reisz
1995) or (newer and with better support) dwarf pareiasaurs (Lee 1995, 1996,
1997, 2001). Molecular data are sparse and have for a long time hardly
produced any results. (In 1990 someone found Haematothermia with 18S rRNA!!!
Reanalyses of this have meanwhile put mammals back into the position as the
basalmost living amniotes.) Analyses of mtDNA find turtles next to
archosaurs with high support. Some nucleus-encoded proteins put turtles
either next to archosaurs or next to crocodiles alone -- but "[b]oth
crocodiles and turtles show significantly long branches which might
introduce biases into the phylogenetic analyses (see also Lee 2001 for other
problems that may affect phylogenetic analyses of reptile molecular data)".
Some molecular analyses have found turtles next to or even within
Lepidosauria, though always with rather low support. "On the other hand, new
morphological evidence links turtles to lepidosaurs (tuatara, lizards, and
snakes) (deBraga and Rieppel 1997). This evidence depends on the relative
position of Sauropterygia (Rieppel and Reisz 1999), which have recently been
proposed to be closely related to archosaurs (Merck 1997). However, it is
important to note that a recent reanalysis of deBraga and Rieppel's (1997)
data seems to favor the anapsid condition of turtles (as sister group of
pareiasaurs), and statistically rejects the putative diapsid affinities of
turtles (Lee 2001). This result is dependent on the inclusion of fossil taxa
in the phylogenetic analysis. Moreover, a total evidence analysis, including
mitochondrial and morphological data, seems also to group turtles with
anapsid parareptiles [that's double :-) ] (Lee 2001). The molecular and
morphological combined data set can statistically reject a
turtle+lepidosaurian grouping but not a turtle+archosaurian sister group
relationship (Lee 2001)." When fossils are excluded from Lee's 2001
morphological analysis all three positions for turtles are equally
parsimonious. However, the authors want to have turtles next to archosaurs,
pointing out that the temporal regions of turtle skulls might indicate that
the quadratojugal closed a fenestra (deBraga & Rieppel 1997).

Lessons (IMHO):
- It is hard to tell plesiomorphies and apomorphies apart in molecules, even
more so than in fossils.
- If you do a morphological analysis and have the chance to include fossils,
always do it, otherwise the outcome is likely to be seriously flawed.
- Early molecular phylogenies were largely junk, but the methods are
steadily improving.
- I was wrong when I said that only one fragmentary sequence of one turtle
is used in molecular systematics: Fig. 5A shows "Side-necked turtle",
"Painted turtle" and "Green turtle" in a mtDNA tree.

Refs:
M. deBraga, O. Rieppel (1997): Reptile phylogeny and the interrelationships
of turtles. Zoological Journal of the Linnean Society 120, 281 -- 354
M. Laurin, R. R. Reisz (1995): A reevaluation of early amniote phylogeny.
Zool J Linn Soc 113, 165 -- 223
M. S. Y. Lee (1995): Historical burden in systematics and the
interrelationships of 'parareptiles'. "Biol Rev" [Biological Review?] 70,
459 -- 547
same (1996): Correlated progression and the origin of turtles. Nature 379,
812 -- 815
same (1997): Pareiasaur phylogeny and the origin of turtles: Zool J Linn Soc
120: 197 -- 280
same (2001): Molecules, morphology, and the monophyly of diapsid reptiles.
Contributions to Zoology 70, in press [can't check whether it has appeared
meanwhile]
J. Merck: A phylogenetic analysis of the Euryapsid reptiles. JVP Abstracts
1997 (Suppl. to volume 17), 65A [at last an accessible journal!]
O. Rieppel, R. R. Reisz (1999): The origin and early evolution of turtles.
"Annu Rev Ecol Syst" [Annual Review of Ecology and Systematics? Or what is
Ecol?] 30, 1 -- 22

Back to dinosaurs, I won't bring up this topic again in the foreseeable
future... :-)