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In a message dated 9/26/00 6:16:54 AM EST, darren.naish@port.ac.uk writes:

<< Sorry, this is untrue - thanks to recently published reviews (see Turner  
and Anton) their skulls can be reliably distinguished: in the postcrania 
tigers are generally more robust.>>

I have no idea what "generally more robust" might mean here; a mature lion 
would be "generally more robust" than a young tiger. It's interesting to hear 
that they are after all distinguishable cranially, but I don't think this 
undermines the import of my post. With sufficient study, it might be possible 
to distinguish, say, four different species of tyrannosaurines from the Late 
Maastrichtian of North America, but there is no incentive to do so when 
everyone simply starts from the premise that it's a big tyrannosaurid from 
North America, so it must be Tyrannosaurus rex, and then ascribes all the 
observed variation among the known specimens to the usual three intraspecific 
causes (individual, ontogenetic, sexual).

Since we cannot induce fossils to interbreed, we will, of course, never be 
able to settle the question of what species there are in the fossil record. 
Ordinarily, this would not be bothersome to me, but I've recently read 
several articles in which it is categorically asserted that species diversity 
has increasedon earth over time, and that modern species diversity is as 
great as it has ever been in all of earth's history. Balderdash. There is 
simply no basis for this kind of assertion whatsoever; the horribly 
depauperate fossil record cannot be compared to the rich modern world in 
species diversity.

<< Incidentally, most cat workers do not think that lions and tigers are 
particularly closely related: lions are part of a 'spotted clade' that 
includes jaguars and leopards - tigers are outside of this group. >>

So if they're not closely related (not that that matters to their skeletal 
morphologies, which could be homoplastic rather than apomorphic), how come 
they produce viable hybrids? Have viable hybrids been produced between other 
species of these two groups?