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Re: Hanson 2006, Mortimer, Baeker response
Mike Taylor wrote:
I don't buy this, at least for dinosaurs. In the sauropods alone, you
have _Tendaguria_, _Agustinia_ and at least one more as-yet unnamed
taxon, none of which fit at all convincingly into any of the
established clades less inclusive than Neosauropoda.
Every time I query Mike on matters sauropodan, I end up with egg all over my
face... oh well, here goes.
It is possible that both _Tendaguria_ and _Agustinia_ will be easily
accommodated in known groups if and when we find better material.
_Tendaguria_ could be a highly aberrant basal titanosaurian - if (and I mean
IF) it is the same as _Janenschia_. Bonaparte et al. (2000) split the
_Janenschia_ material three ways: limb bones (retained in _Janenschia_);
presacral vertebrae (assigned to a new sauropod, named _Tendaguria_); and
caudal vertebrae (unnamed). But the limb bones and presacral vertebrae both
come from a VERY heavily built sauropod; the caudals are strongly
procoelous; and the limb bones also show titanosaur characters. The dorsals
of _Tendaguria_ are just plain weird (puny neural spines, massive transverse
processes), and this makes _Tendaguria_ hard to place. If _Janenschia_ and
_Tendaguria_ prove to be the same after all, then this sauropod is a
titanosaur with highly autapomorphic dorsals.
As for _Agustinia_, the problem here is that this sauropod is too poorly
known to confidently ascertain its relationships. The presence of body
armor suggest titanosaur affinities, but I admit this doesn't seal the deal.
I think there is_a lot_ still to be discovered -- not just minor tweaks on
the taxa we
know and love already, but dramatic departures.
Although I don't agree with David Peters's phylogenetic shoehorning, the
Dinosauria has a long history of 'oddball' forms finding a home in
pre-existing clades, thereby paring back many previous ghost lineages.
The most famous examples is the Therizinosauria (Segnosauria), which at one
stage were considered a surviving relic of a basal sauropodomorph radiation
(requiring a huge ghost lineage stretching from the Late Triassic to Late
Cretaceous). They were even given their own 'order': Segnosaurischia.
Therizinosaurs eventually ended up inside the Maniraptora, where they seem
very content to stay.
Another example: _Opisthocoelicaudia_ is quite clearly a titanosaurid,
despite its weird (highly autapomorphic) tail and a superficially
camarasaur-like body plan. Other highly autapomorphic sauropod taxa (e.g.,
_Bonitasaura_, _Nigersaurus_, _Brachytrachelopan_) can likewise be assigned
to known families.
I think the problem with many 'hard-to-place' taxa is that we lack
transitional forms that may tie these more confidently to known groups
and/or they are tantalizingly incompletely known. While I think we have a
lot to learn in terms of the morphological diversity of dinosaurs, I fear we
may be approaching the limits of phylogenetic diversity. Although the known
morphological diversity of dinosaurs is expanding year by year, phylogenetic
diversity isn't keeping pace. Most hard-to-place taxa turn out to be more
closely related to known taxa than initially thought.