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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.