Re: Rapator,the giant alvarezsaur

["Tim Williams" <twilliams_alpha@hotmail.com>]

Dann Pigdon wrote:

>   I like to imagine giant alvarezsaurids like huge Maribu storks (or 
> Australia's Jabiru stork, with a confusingly similar name). Perhaps 
> alvarezsaurids started off large, mainly using sharp stabbing beaks (or 
> proto-beak structures), during which their forelimbs atrophied (similar to 
> what may have happened in tyrannosaurs).

Since the only material we have for _Rapator_ is a single phalanx (unless 
the _Walgettosuchus_ caudal belongs to it), we don't know how big the actual 
animal was.  Maybe it had relatively larger forelimbs compared to other 
alvarezsaurids?  _Rapator_ is of Aptian-Albian age, making it one of the 
earlier known members of the group.  (Based on phylogenies, the 
Alvarezsauridae should have been around since before the Late Jurassic.)

In tyrannosaurids the reduction of the forelimbs appears to have been the 
result of a specialized predation strategy: according to one hypothesis, the 
jaws grabbed the prey, and the forelimbs helped to hold the prey in place 
while the jaws went to work dispatching the prey.  In many ways, the 
tyrannosaurid strategy is an exagerration of the predation strategy inferred 
for many basal tetanurines (e.g., _Torvosaurus_), which also show short but 
very robust forelimbs.

By contrast, the pathway you're suggesting for alvarezsaurids is that the 
enlargement of the claw and strengthening of the forelimb musculature came 
*after* the forelimbs were downsized?  This hypothesis sounds similar to 
what transpired in the flightless bird _Titanis walleri_, which had tiny but 
apparently functional forelimbs tipped by a moveable claw.

>    Later, if conditions favoured smaller alvarezsaurs, their forelimbs may 
> have evolved into as functional a set of tools as was possible, given what 
> they had to work with.

But what was this "function"?  If the forelimbs did regain function after a 
period of atrophication, why did they remain so small?

>  People seem to place a lot of emphasis on their forelimbs alone, as if 
> their entire behavioural repatoir must have accounted for them. They had 
> other body parts as well...

True, but the forelimbs represent the most apomorphic part of the 
alvarezsaurid anatomy.  The stubby forelimbs tipped by a single claw are 
highly specialized, and contrast with the gracile, cursorial proportions of 
the hindlimbs.  I like to start from the premise that the unique forelimbs 
were somehow important to the behavioral repertoire of alvarezsaurids.  
Certain scenarios (e.g., scavenging; tearing open hives or termitaria; 
stripping bark) do at least try to tie together the function of the 
forelimbs and hindlimbs.


Tim

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