Re: gastroliths (was Re:Giant carnivorous geese)

[Stephen Wroe <swroe@bio.usyd.edu.au>]

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). 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. 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.

Cheers,

Steve

>***as I wrote, I've heard use of gastroliths has been reported for crocs (in
>that case they were talking about Nile crocs) ...and they're certainly
>carnivorous
Yeah, but up until your post I hadn't heard about that. But, after 
searching through the archives, I found a post by HP Darren Naish that 
mentioned, among other carnivorous animals, some crocodylians, otariids, 
and predatory birds possess gastroliths. In addition, the ant/termite eater 
_Proteles_ also does. Interesting. Not widespread...but some carnivores do 
indeed use them.
>phillidor11@snet.net commented:
><<The implicit question is whether use of gastroliths could develop
>in a carnivore or might it be an indication that the animal was
>a herbivore that became an omnivore or carnivore?
>No axe, or stone, to grind, but I'm wondering if use of gastroliths
>has anything to say about ancestry.>>
>
>***** I _think_ erbivorous diets have never been really common amongst crocs
>; therefore I assume the use of gastrolithgs _by crocs_ has always had to
>do with their carnivorous diets (i.e. evolved in animals that were already
>eating flesh, as it more likely for extant crocs to have had meat-eating
>ancestors than plant-eating ones if what i said before is correct )
Certainly herbivorous crocodylians are rare. _Simosuchus_, the pug-nosed 
Malagasy form, immediately comes to mind (it's so crazy looking), but it's 
long extinct. AFAIK, no modern crocs are exclusively herbivorous. So, in 
the Nile crocodile, I would tend to say that gastrolith possession 
represents a unique adaptation or behavior unrelated to ancestry, but I 
could be wrong.
>Since we've seen how those stones have been found in association with
>presumed strictly carnivorous and erbivorous forms (fossil or living ones),
>i think their presence may have little ,if anything, to say about the
>ancestry of the animal those pebbles are found in; it's, however,
>interesting to note how solutions to different feeding "problems" evolved
>convergently.
It certainly is interesting. I'm also unsure about ancestry...especially 
regarding long-term trends in the fossil record. What I do know is that 
various theropods (from quite different clades) possessed gastroliths: 
_Poekilopleuron_, _Lourinhanosaurus_, _Baryonyx_, and apparently some sort 
of tyrannosaurid. This behavior appears to be something that might indeed 
have been widespread, but perhaps isn't well represented in the fossil 
record. Or, perhaps, in the past paleontologists have dismissed 
gastrolith-like stones found near theropod dinosaurs, relying on the a 
priori assumption that carnivorous animals don't use gastroliths, and hence 
the stones couldn't possibly be gastroliths.
But, I think the new discoveries of theropod gastroliths will certainly 
make paleontologists keep their eyes open (or so I hope).
Steve


Stephen Wroe - Homepage - http://www.bio.usyd.edu.au/staff/swroe/swroe.htm