Re: Limusaurus Inextricabilis

[Tim Williams <tijawi@yahoo.com>]

Mike Taylor wrote:

> Dave Hone's blog has a nice, comprehensible write-up:
>     
> http://archosaurmusings.wordpress.com/2009/06/18/limusaurus-?-four-fingers-and-a-whole-mess-of-homologies/


It does have a nice summary of the I-II-III / II-III-IV homology issue.  

I also liked the accompanying piece on herbivory in _Limusaurus_.  The blog 
mentions other non-avian theropod groups that were probably either herbivorous 
or omnivorous, such as ornithomimosaurs, therizinosaurs, and oviraptorosaurs.  
Omnivory has also been proposed for troodontids - the derived ones, anyway, 
some of which (interestingly) also lost the sickle-shaped slashing claw.  We 
also have theropods such as spinosaurids and _Masiakasaurus_, both of which may 
have been specialized for eating fish (piscivory); although insectivorous, or 
even frugivorous, habits have also been proposed for _Masiakasaurus_.  
_Poekilopleuron_ also shows evidence of piscivory by its stomach contents.  
Alvarezsaurs might have been insectivorous, but even if they weren't, they 
certainly weren't hypercarnivorous.

Collectively, these taxa highlight the trophic diversity of non-avian 
theropods.  _Limusaurus_ is direct evidence that putative herbivory appeared 
much deeper in theropod evolution than previously thought.  Given this trophic 
diversity, especially the shifts to herbivory, you have to wonder how many 
theropod taxa for which the skull is unknown or fragmentary might also have 
been herbivorous - especially if the teeth are unknown (or dubiously referred). 
 

The _Limusaurus_ paper argues that the presence of gastroliths is an indication 
of herbivory.  As pointed out on the DML and elsewhere, gastroliths do not 
automatically indicate herbivorous habits, and there are other indicators of 
herbivory too (like the beak/rhamphotheca); and the gastroliths of _Limusaurus_ 
are ornithomimid-like.  _Baryonyx_ and _Poekilopleuron_ are reported to have 
gastroliths, and these tetanurans were clearly not herbivores.  (In _Baryonyx_, 
the gastroliths may have helped to provide ballas
tic habits, as in modern crocs.)

Nevertheless, some of the theropods that have gastroliths also lack skull 
material, such as _Nqwebasaurus_ and _Lourinhanosaurus_.  The thing is, with 
critters like _Limusaurus_ turning up, we can no longer be safe in assuming 
that *any* theropod was carnivorous - unless positive evidence turns up in 
support of carnivory (especially tooth morphology).  The prospect of 
_Elaphrosaurus_ being a herbivore has been raised, using _Limusaurus_ as a 
templete.  But what about bigger ceratosaurs like _Deltadromeus_?  
_Deltadromeus_ has very slender limbs for a theropod of its size (similar 
length/diamter ratios to _Ornithomimus_) and the skull is so far unknown.  If 
the ornithomimosaurs and oviraptorosaurs could come up with big-ass 
non-carnivores like _Deinocheirus_ and _Gigantoraptor_, why not the ceratosaurs 
too?

_Limusaurus_ is direct evidence that herbivory could arise in theropods other 
than coelurosaurs.  So some of the coelophysoids, ceratosaurs, and carnosaurs 
that have little or no preserved skull material may ultimately turn out to be 
herbivores as well.  Or at least something other than 'typical' 
hypercarnivorous theropods.

Sorry for the stream of consciousness.  I'd have liked to have edited this to 
make this shorter and/or more coherent, but lunchtime is over and I gotta rush.

Cheers

Tim




 Although there are sometimes indicators of dietary preference in the 
postcranium, the proof



> 
> 
> _/|_     ___________________________________________________________________
> /o ) \/  Mike Taylor    <mike@indexdata.com> 
>   http://www.miketaylor.org.uk
> )_v__/\  "Can't talk.  Eating" -- Homer Simpson.
> 
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