Re: gastroliths (was Re:Giant carnivorous geese)

["Steve Brusatte" <dinoland@lycos.com>]

On Mon, 26 Aug 2002 07:38:34  
 Stephen Wroe wrote:
>This is very interesting - a debate is ongoing as to whether gastroliths in 
>crocs and some pleisiosaurs can be explained as 'digestive aids' or part of 
>a bouyancy control system (or both) - but obviously this doesn't apply to 
>predatory birds (assuming their not semi-aquatic). 

Yep.  Forgot to mention that in my last post.  I seem to recall that the 
buoyancy hypothesis for plesiosaurs was under some fire, but I don't remember 
the details.  But, certainly, yes, perhaps gastroliths were/are used to control 
buoyancy, and hence diving behavior, in crocs and plesiosaurs.  Also, this 
might be explain their presence in otariids (fur seals, sea lions, and the 
like).

Oddly enough, I've read, otariids seem to be able to dive to greater depths 
than other pinnipeds, so perhaps gastroliths are used for this purpose?  
However, I've also read that otariid gastroliths are supposidely also used to 
break down fish bones.  Maybe the behavior of injesting gastroliths was 
originally evolved for one of these reasons (or another reason), and later 
co-opted for use in buoyancy control and/or feeding?

>If gastroliths are 
>present in any flying predatory birds, then to me this would suggest strong 
>selective pressure for them, rather than a presence based on ancestry and 
>maintained by neutral-selection. Anything maintained in a flying animal 
>that increases mass must surely be doing something that increases overall 
>fitness. If anyone has more info. on gastroliths in avian predators I'd 
>much appreciate a heads-up. Of course, in terrestrial birds such as 
>dromornithids selection pressure to minimise body-mass would have been 
>greatly relaxed. 

I don't know exactly how many flying predatory birds possess gastroliths.  
Maybe HP Naish has an idea?  But, yes, if they are common in flying birds then 
it indicates some sort of very strong evolutionary pressure.

>The allusion to gastroliths in Proteles is real cute and 
>another eye-opener. If true - they are converging on birds in their use of 
>gastroliths to circumvent constraints imposed by a degenerate dentition.

I find this interesting, too.  _Proteles_ (the aardwolf) is a member of the 
Family Hyaenidae, but is odd in that it doesn't eat meat (and crush bones), but 
is rather an insectivore.  Perhaps this might be a case in which ancestry has 
provided gastroliths?  Maybe they were originally developed in a bone-crushing 
ancestor as an aide to break bones, and then were retained in _Proteles_, where 
they are used to help crush insect exoskeletons??  Just a thought.  Do other 
hyaenas possess gastroliths?  What about fossil forms?

Here is the one reference (previously posted to the DML a few years back) I 
could find regarding _Proteles_ gastroliths:

Anderson, M.D., Richardson, P.R.K. & Woodall, P.F. (1992): Functional analysis 
of the feeding apparatus and digestive tract anatomy of the Aardwolf (Proteles 
cristatus).- J. Zool., 228, pp. 423-434.

Steve

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