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Ralph Miller posted a useful summary of some thoughts on gastrolith 
distribution and utilisation in dinosaurs. Here are some additional 

Something we've mentioned on this list before is that gastroliths can 
apparently be distinguished from non-gastoliths by firing laser beams 
at them: the light-scattering effect of gastoliths differs from 
non-gastroliths and can be detected. The paper that first reported 
this (the technique was first used on moa gastroliths) was in 
_Journal of Paleontology_: I don't have the citation with me now, but 
it has been posted on the list. 

It may also be of interest that some work on bird gastroliths - it is 
cited in Anderson's book on moa but I cannot recall the author 
off hand - indicates that gastrolith replacement is actually very 
much rarer than was supposed. As acid etching and abrasion increases 
the number of microscopic worn faces on each gastrolith, it is 
apparently advantageous for the animal to be in possession of very 
old gastroliths. The more 'gastricised' the stone, the better. This 
only goes for those taxa that use gastroliths for digestion: don't 
forget that many aquatic animals - pinnipeds, plesiosaurs, crocs, 
etc. - use them as ballast. 

Exactly how gastroliths, and, indeed, gizzards, do function in many 
extant taxa is still controversial. Efforts to create an artificial 
goose gizzard and then test its function and abilities were published 
in _Journal of Zoology_ last year.

It is unfortunate that many of Bakker's (1986, 1987) statements are 
not supported by reference to primary literature. As Ralph noted, 
much of what he says about moa gastroliths seems to be speculation 
that is not based on published work (also, in attempting to 
demonstrate that the small sauropod head does not necessitate a 
non-endothermic physiology, Bakker (1986) makes out that dinornithid 
moa heads were smaller than they actually were). 

Bakker wrote that x-rays of a croc gizzard showed how the animal used 
gastroliths to help break down a dead mouse. But, in everything 
written about the function of gastroliths by Mike Taylor, it is 
emphasised that carnivorous animals do not use gastroliths for 
digestion: it simply would be maladaptively risky to break up bones 
in their stomachs. I will have to read up on crocodile digestion, 
this is something I cannot claim to know much about.

Finally, Bakker also mentioned that Ken Carpenter had found 
gastroliths in a nodosaur. Can't recall if this was ever written up.

"Right now, we don't know what it is. But that doesn't mean we 
*can't* know what it is"