Re: Hanson 2006, Mortimer, Baeker response

[Tim Williams <twilliams_alpha@hotmail.com>]

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.

Cheers

Tim