RE: What, if anything, is _Apatosaurus_ (Was: Ceratops (was RE: Pterosaur diversity (was: Re: Waimanu)))

[Tim Williams <twilliams_alpha@hotmail.com>]

Mike Taylor wrote:

> Really?  What are the criteria that you allude to exactly?  That each
> valid genus should be diagnosed by at least one autapomorphy?  I would
> think that most sauropod genera have that.

Probably most sauropods do have autapomorphies - but by no means all 
(including some of those that are traditionally considered 'valid').  Like 
Mickey, I think the requirement that each genus/species have at least one 
autapomorphy in order to be deemed 'valid' is setting the bar way too high.  
IMHO, having a unique *combination* of characters should be enough for a 
diagnosis.  I'm certain _Apatosaurus_ has this.  For _Titanosaurus_, I think 
this is a little trickier.

OK Mike, take a deep breath - I'm going to use a non-sauropod example.  I've 
heard that autapomorphies are hard to find in _Archaeopteryx_.  But, 
_Archaeopteryx_ is valid, right?  It can be distinguished from every other 
theropod taxon, just as _Apatosaurus_ can be distinguished from any other 
sauropod taxon.  Therefore, the validity of these genera is beyond reproach.

Rauhut (2004) upheld the validity of _Genyodectes_ based solely on a unique 
combination of characters.  It had not a single autapomorphy.

Mickey Mortimer wrote:

> > This could be harder to prove.  After all, unless an explicit etymology 
> is provided for each clade, > it is open to interpretation whether it is 
> named *after* a given genus.  For example, we know
> > that when Titanosauria was first named (was it by Bonaparte and Coria, 
> 1993?) it was > ultimately named after _Titanosaurus_; but unless this is 
> explicitly stated in the paper, the > name could just be a descriptive term 
> in its own right ("titan lizards").
>
> That's really stretching plausibility there.

Yes, it's called "plausible deniability".  Everyone knows the truth, but no 
one can prove it.  :-)

I was really trying to illustrate the difficulties of *proving* that a 
higher-level taxon is derived from a genus name.

Cheers

Tim