RATITES REVISITED

[darren.naish@port.ac.uk]

Regarding giantism/gigantism in certain flightless birds, Matt 
Troutman wrote..

> Another explanation for this is that since many of the larger 
> flightless birds are folivores and or nearly so, a larger size would 
> be needed to accomadate the larger gut and to edge out competition 
> for the food source.

Clearly this applies to herbivorous flightless ornithurines like moa 
where, yes, it would have been advantageous to be big. Same goes for 
turtle-jaw geese, _Cnemiornis_ and herbivorous columbiforms too (I 
will not use the word Raphidae as I realised they can't possibly be 
monophyletic), and others. But let me make it blindingly clear that 
Matt is not saying that the evolution of herbivory or folivory 
_explains_ avian giantism, because there are clearly many reasons for 
becoming, or not becoming, a giant: e.g. to become a macropredator, 
to increase high-browsing efficiency, to produce bigger clutches of 
well developed young?, to be more offensive in combat and defence?, 
because you can, all of the above... etc. etc.

And on the issue of whether or not paedomorphosis explains 
flghtlessness in birds, Matt said..

> The feather features can be explained by paedomorphosis as well as 
> hypermorphosis, but the other features are more iffy.

OR.. 'The feather features can be explained by hypermorphosis as well 
as by paedomorphosis'. I am not coming down hard on either side of 
the debate: what I am objecting to is the Feduccia and Olson approach 
that paedomorphosis is _the_ one explanation. 

As for what is 'iffy' and what is not, well, again, interpretation of 
these things as hypermorphic is no more iffy than are paedomorphic 
interpretations PLUS I have a hard time with some of the paedomorphic 
features because they are just plain wrong. For example, it is stated 
that unfused skull sutures in adult palaeognaths etc prove 
paedomorphy. This may or not be true (when bones fuse is a 
complicated factor that does not always equate with ontogeny), but, 
in my experience, adult ratites actually have fully fused skull 
sutures. Whereas ironically, in tinamous, there are at least two 
skull sutures that _never_ fuse. If ratites are supposed to be big 
paedomorphic palaeognaths, with tinamous as an outgroup, then the 
fused ratite skull hardly equates with logical paedomorphosis.

Matt also said...

> I find that the chicks of flightless birds are similiar to their 
> parents in most all of their features evindence that paedomorphosis 
> is the most likely explanation for avian flightlessness. 

For palaeognaths, I don't think this has anything to do with 
paedomorphosis: it could just as parsimoniously be used as evidence 
indicated the relatively primitive status of palaeognaths in the 
ornithurine tree. Adult palaeognaths are as much like their juveniles 
as lizards, crocodiles and non-avian dinosaurs were like theirs, so 
far as we know. Furthermore, the fact that adult palaeognaths lack 
the globose porportionally inflated cranium and huge eyes of their 
juveniles **can**, again, be used as counter-evidence for 
paedomorphosis in this taxon: in paedomorphy that has been described 
for hominin hominids, phocoenid odontocetes and alligatoroid 
crocodylians, adults have 'juvenile-like' skulls. I can't see this in 
palaeognaths.

Finally, I cannot see that flightless carinates particularly resemble 
their juveniles more than do volant taxa.

"You smell like a fox in October"

DARREN NAISH
darren.naish@port.ac.uk