Re: Thunder-Lizards

[Tim Williams <twilliams_alpha@hotmail.com>]

Mike Taylor wrote:

> There is a philosophic-taxonomic issue here: should judgements like generic 
> separation be based on morphology alone, or also take into account other 
> factors such as biogeography or age difference?  I've seen things done both 
> ways.

Another case in point is _Iguanodon_.  The type species (_I. 
bernissartensis_) is from Europe, but there are iguanodontid specimens from 
North America (_I. lakotaensis_) and eastern Asia (_I. orientalis_) that are 
virtually indistinguishable from _I. bernissartensis.  In fact, Norman 
(1996, 1998) suggests that both _I. lakotaensis_ and _I. orientalis_ should 
be junior subjective synonyms of _I. bernissartensis_.

> Curtice (2000) writes of the Cretaceous brachiosaurid _Sonorasaurus_, "Just 
> because it resembles a _Brachiosaurus_ does not it a _Brachiosaurus_ make" 
> (sounding rather like Yoda there :-) and recommends continued generic 
> separation of these morphologically identical taxa on age grounds alone.

Well, to extend the Jedi analogy... only the Sith deal in absolutes.  :-)  I 
reckon this issue of generic separation should be decided on a case-by-case 
basis.  If a genus can survive for tens of million years almost unchanged 
and/or disperse to more than one landmass, why split it up based on age or 
geography?  To play Devil's advocate here: if _B. altithorax_, _B. brancai_, 
and _Sonorasaurus thompsoni_ form a clade to the exclusion of all other 
known species AND the differences between each species are very minor, then 
maybe the genus _Brachiosaurus_ should include all three species. I know 
this depends upon how finely calibrated one's genus-meter is, but I don't 
understand why genera should have an expiration date.

Taking the above argument to its logical conclusion would probably make 
_altithorax_, _brancai_, and _thompsoni_ all species of _Pelorosaurus_, 
along with a half-dozen others!

(For _S. thompsoni_, Upchurch et al. [2004] actually treat it as a nomen 
dubium.)

> > The dorsals of _Tendaguria_ are certainly very weird
>
> This much I certainly do agree with!  :-)

Yeah, there's nothing like them anywhere else.  At least nothing that's been 
described.

Cheers

Tim