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RE: Turtles and Crocodylians are not Reptiles - no? What are they?
Robert Takata wrote:
> The number of *clades* is not so quite a limiting issue - and here, we
> are take into account just the extant taxa and their relative position
> on the tree.
I would say the number of clades is indeed a limiting issue. Squamata,
Sphenodontia, Crocodylia and Aves represent just a fraction of the total
sauropsid diversity that has ever existed. The fact that two of the surviving
clades (Squamata, Aves) are very speciose doesn't help very much. I think poor
taxon sampling (a fault of repeated extinctions, not the analyses themselves)
is throwing us a curveball. For example, I don't believe for one minute that
crocodiles are closer to turtles than to birds.
> It would be some problem of sequence alignment, but one
> could use just the parts that align well (and have informative sites).
Sequence alignment is not usually that big of a problem (unless you use
non-coding sequences, like the mitochondrial D-loop). But for any gene, how do
you know a priori which particular sites are informative? I'm not sure I
understand you here. I'm also not sure what you mean by "fossil DNA", which
you mention below.
> Of course that I'm not saying that when there is conflict between
> molecular and morphological data the molecular side always wins. But
> the turtle case is very similar to the cetacean case - the then
> available morphological/fossil data put the whales besides
> mesonychids, while the mitochondrial DNA analysis said that they would
> join with hyppos (Artiodactyla).
> Probably the turtle position issue will be settled down - as occurred
> in the Cetacea case - when a new informative fossil (or a collection
> of informative fossils) was found and/or descripted (or eventually
> fossil DNA).
The most recent morphological/fossil data place whales near the base of the
Artiodactyla. The analysis of Thewissen &c from late 2007 places Cetacea as
the sister taxon to the Raoellidae, a group of aquatic basal artiodactyls.
Thus, although the Cetacea come out as artiodactyls, they are not at all close
to hippopotamids, and are in fact outside the crown Artiodactyla.
Thewissen JGM, Cooper LN, Clementz MT, Bajpai S, and Tiwari BN (2007). Whales
originated from aquatic artiodactyls in the Eocene epoch of India. Nature 450:
So, whereas the molecular phylogenetic analyses recovered whales as the sister
group to hippos, and the morphological phylogenetic analyses previously found
whales and mesonychians to be sister taxa, the most recent morpho analysis
(including new fossil specimens) found the Cetacea to lie somewhere in between:
I reckon a similar situation will emerge with turtles (Testudinata): the
molecular analyses tend to place them inside or next to the Archosauria; older
morpho/fossil analyses put them in an anapsid clade that is outside the
Diapsida; but new fossil discoveries will consolidate a position for the
turtles somewhere in between, i.e. inside the Diapsida as either basal
lepidosauromorphs or as non-saurian neodiapsids.
> But there is a lack of less derived turtle fossil that show this
> affinity - i.e. fossil that is more similar to a putative common
> exclusive ancestral between Testudines and pareiasaurs.
Although there are no actual proto-turtles (yet) to back it up, Michael Lee
outlines a possible scenario here:
Lee MSY (1993). The origin of the turtle body plan: bridging a famous
morphological gap. Science 261: 1716-1720.
Lee MSY (1997). Pareiasaur phylogeny and the origin of turtles. Zool. J.
Linn. Soc. 120: 197–280.
I'm not actually advocating the pareiasaur-turtle hypothesis; and other studies
have found fault with these analyses (e.g., Rieppel & Reisz, 1999; Annu. Rev.
Ecol. Syst. 30: 1–22). But the pareiasaur-turtle hypothesis does provide a
series of intermediate stages, and can be tested against the fossil record
(e.g., such as when proto-turtles are discovered or recognized).
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