Re: Hanson 2006, Mortimer, Baeker response

[Tim Williams <twilliams_alpha@hotmail.com>]

Mickey Mortimer wrote:

> Tendaguria seems to have gotten its own family largely because Bonaparte 
> was being more phyletic than cladistic.

I would say more "phenetic" than "phyletic", but I get what you mean.

> In regard to the theme of this thread, finding 'new' categories of 
> dinosaurs is impossible in a way.

I don't know what DP meant (his emails have a tendency to make my head 
spin), but I was not so much referring to new categories of dinosaurs as 
filling in the phylogenetic gaps.  The gaps in dinosaur phylogeny are not so 
nearly as large (or as numerous) as they once appeared to be.  I gave two 
examples in a previous message: segnosaurs (therizinosaurs) and 
_Opisthocoelicaudia_.  Back when segnosaurs were regarded as some sort of 
ornithischian-prosauropod transitional group, this required an enormous 
ghost lineage stretching back from the Late Cretaceous to the Late Triassic 
or Early Jurassic.  When _Opisthocoelicaudia_ was regarded as a weird Late 
Cretaceous camarasaur, this also required a lengthy ghost lineage (though 
not as long as the segnosaur one).  Now that segnosaurs and _O_ are 
comfortable in the Maniraptora and Titanosauridae, respectively, this 
drastically reduces two big gaps in dinosaur evolution.

What tended to happen in the past is that oddball dinosaurs were assumed to 
represent the culmination of very long, unknown lineages.  Nowadays, these 
oddballs are usually recovered as autapomorphy-rich taxa that have a closely 
related known sister taxon.

>  And the gaps are sometimes huge.  So there's still plenty of work to do.

That's true.  But cladistics has helped sort out some of these weird taxa, 
by showing that the 'weirdness' is usually concentrated in a relative few 
anatomical characters, and the rest of the anatomy does a better job in 
resolving the relationships of these 'weird' taxa.  _Agustinia_ looks weird 
courtesy of its strange body armor, and that's why Bonaparte gave it it's 
own family.  _Opsithocoelicaudia_ seems to be an atypical titanosaur, 
because of the forked neural spines and opisthocoelous caudals; but the 
majority of its anatomical characters redaily identify it as a titanosaur, 
fairly close to _Saltasaurus_.

Cheers

Tim