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Re: Thunder-Lizards



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