Re: Long-necked stegosaur coming out in Proceedings B

[Tim Williams <tijawi@yahoo.com>]

Ronald Orenstein wrote:

> Yes, of course it does.  If
> you are talking about evolutionary sequences, of course,
> that is one thing; but if you are talking about
> contemporaneous, sympatric, and reasonably closely related
> species, the question may change a bit. 


On this last point ("reasonably closely related"), we might have to be 
cautious.  _Miragaia_ and _Dacentrurus_ were assigned to a new clade 
(Dacentrurinae), as the sister taxon to _Stegosaurus_ (including 
_Hesperosaurus_ and _Wuerhosaurus_ here).  The two clades appear to have been 
quite different in their development of body armor.  

The dermal armor is fairly poorly known for _Dacentrurus_, although the plates 
appear to be quite modest in size.  For _Miragaia_, only the front half of the 
skeleton is known.  Nevertheless, it's clear that the plates of _Miragaia_ were 
relatively much smaller and lower than those of _Stegosaurus_ (as well as being 
paired in _Miragaia_, rather than alternating in _Stegosaurus_).  As in 
_Dacentrurus_, the arrangement of the tail spines is unknown (presuming 
_Miragaia_ had them).  Based on what is known, dacentrurines were less heavily 
armored than _Stegosaurus_, and we have no idea of how the tail-spikes were 
arranged.  (Did they even have a bona fide 'thagomizer'?)

So, when faced with a roving predator, perhaps _Miragaia_ and _Dacentrurus_ 
were more prone to take evasive action, whereas _Stegosaurus_ was more likely 
to stand its ground.  In this context, the longer neck may have helped 
_Miragaia_ spot approaching predators - not just by raising the height of the 
head relative to the trunk, but also by giving the head a wider arc of 
movement.  


> Looking at
> contemporary animals (eg antelopes, again) the differences
> that exist seem to relate more to foraging differences and
> niche separation (eg the gerenuk's neck or the long,
> splayed hooves of the sitatunga), or to sexual selection
> (horn shape and size) 


I can't disagree with you here.  However, Mateus &c are ambivalent on the idea 
of niche partitioning
mated browsing heights of _Miragaia_ and Lourinha _Stegosaurus_ overlapped to a 
considerable degree.  


> than to anti-predator defences, for
> which you might be more likely to predict convergence rather
> than divergence (eg in antelopes, increases in running
> speed) unless you postulate that each form was being hunted
> by a differnt predator, requiring a different response in
> each case -- a less likely proposition.


I would say that because of the morphological differences between _Stegosaurus_ 
and _Miragaia_ (especially in the armor), I'd say we can't rule out divergent 
defensive strategies in response to the same predator.  


Cheers

Tim