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RE: Turtles and Crocodylians are not Reptiles - no? What are they?
Mike Keesey wrote:
> Nick was obviously using "closer" in the sense of "sharing closer
> common ancestry with".
It is true that whales and hippos form a clade (Cetartiodactyla) to the
exclusion of mesonychians. In that sense, yes, whales and hippos share a more
recent common ancestor than either do to mesonychians. That's the one thing
that the Thewissen phylogeny and the Whippomorpha hypothesis agree on.
> Of course, that's not the only possible use of
> the word, but I think it was fairly clear in the context.
If it had been clear, I wouldn't have disagreed with it. :-) (Not Nick's
fault - it's just how I interpreted his message.) My original point was that,
in its overall topology, this new morpho/fossil phylogeny falls somewhere
between "old" morpho/fossil phylogenies which united cetaceans and mesonychians
as a clade (= Cete) and "new" molecular phylogenies which unite hippos and
whales as a clade (= Whippomorpha). What I was actually arguing is that the
whale-raoellid hypothesis of Thewissen &c is incompatible with either the Cete
or Whippomorpha hypotheses, which represent two separate extremes.
> Speaking of possible uses, which metric *are* you using? Generational
> distance? Chronological? Genetic? Morphological?
Relative phylogenetic position.
To use the example of birds and deinonychosaurs again... _Archaeopteryx_ and
_Passer_ are both birds, and Deinonychosauria is sister taxon to the bird
clade. Thus, _Archaeopteryx_ and _Passer_ share a more recent common ancestor
than either does to any deinonychosaur. But if you look at the overall
topology, the number of nodes separating _Archaeopteryx_ (a basal bird) from
_Deinonychus_ is far fewer than separate _Archaeopteryx_ from _Passer_. In
this sense, _Archaeopteryx_ is closer to _Deinonychus_ than it is to _Passer_.
Similarly, the number of nodes that separate mesonychians and whales is
certainly not greater than separates whales from hippos. Thus, the phylogeny
in question (that of Thewissen &c) cannot be used in support of the
Whippomorpha hypothesis. (This was the context in which Robert Takata brought
up the "whales are still closer to hippos" statement so many messages ago,
which I was arguing against.)
Anyway, this thread seems to have evolved (or devolved) into something that's
OT, so I'll sign off.
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