Re: Hanson 2006, Mortimer, Baeker response

[Tim Williams <twilliams_alpha@hotmail.com>]

Mike Taylor wrote:

> I now find I don't need to reply to Tim because Mickey said _exactly_ what 
> I was going to say.  I think any disagreement between Tim and me here is 
> just cross-purposes.

And nothing a few pints couldn't solve.  :-)

> Of course in saying that _Tendaguruia_ and _Agustinia_ don't "fit at all 
> convincingly into any of the
> established clades less inclusive than Neosauropoda" I wasn't proposing 
> that there must be other Neosauropod clades besides Diplocodoidea and 
> Macronaria :-)  Just they they are morphologically
> so freaky that they're not "a good fit" anywhere.

Yep, that's how I interpreted it.  But what I was driving at is that 
"morphologically freaky" is usually not the same as "not a good fit 
anywhere".  Weird oddball taxa that are given their own "families" because 
they are so different (morphologically speaking) usually end up being sunk 
into a pre-existing family.

> So that if we accept that the notion of "families" is a useful one (on 
> which see below), then it does indeed make sense to accept Bonaparte's 
> erection of Tendaguriidae and Agustiniidae.

But if both _Agustinia_ and _Tendaguria_ turn out to be titanosaurids, then 
Tendaguriidae and Agustinidae cannot be used.  Otherwise, Titanosauridae 
becomes paraphyletic.  The idea behind phylogenetic taxonomy is that common 
ancestry is used to define clades, not characters or degrees of 
'different-ness', which was the basis for Linnaean ranks.

> Yes indeed.  But I don't have a problem with that.  Just because we 
> (mostly) all agree that families are not objectively real, it doesn't 
> follow that they have no utility.

The utility and validity of families should depend on the relationships of 
the taxon is question.  Agustiniidae and Tendaguriidae are only valid if 
_Agustinia_ and _Tendaguria_ are not contained within another "family-level" 
taxon, like Titanosauridae.  But if _Agustinia_ and _Tengaguria_ are each 
demonstrated to be nested within the Titanosauridae, then the family-level 
distinction is no longer necessary (and is also misleading, from a 
phylogenetic standpoint).

This kind of thing happens all the time - especially in the world of 
invertebrates.  There are some truly bizarre worms were once given in their 
own "phylum", because they appeared to be so weird and different that were 
considered to have "earned" their own phylum.  Many of these phyla have been 
shown to be nested within other clades.  Like the phyla Pogonophora and 
Vestimentifera, which evolved from within a group of annelids and so were 
subsequently sunk within the "family" Siboglinidae.  This reminds me of how 
birds used to be considered a "class" separate from theropods.  Keeping 
_Agustinia_ out of Titanosauridae because it's so "morphologically 
different" from other titanosaurids follows this same rationale.

> So to go back to my original point concerning _Tendaguruia_,_Agustinia_ and 
> the unnamed third example, rather than claiming that they fall into clades 
> that no-one knew existed, the thing that excites me is how morphologically 
> distinct they are from all the well-known taxa that you see on all 
> Upchurch's and Wilson's cladograms.  Afte all, if you're prepared to write 
> _Agustinia_ off as "just a
> titanosaur", then you also have write _T. rex_ off as "just a coelurosaur" 
> :-)

I think we can appreciate the distinctiveness of a genus (like _Agustinia_ 
or _Tendaguria_) without having to emphasize this distinctiveness by 
erecting (or maintaining) monotypic families.  The latter sounds rather 
Linnean and typological to me.  There's nothing bad about being a titanosaur 
- _Agustinia_ is weird AND it's a titanosaur.  Similarly, _T. rex_ is cool 
AND it's a tyrannosaurid AND it's a coelurosaur.

Cheers

Tim