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taphonomy (was Re: Tarbosaurus?)



Alan Blake Coulson wrote:

> " Show me how the speed or depth of the water"
> 
>         faster water can move larger skeletal elements, and move smaller
> elements more readily. Thus, faster water = smaller elements are harder to
> preserve in situ, or at all, as they're abraded during transport.

I've studied the effects of fluvial activity on objects at length, so at
last I'm qualified to say something useful! Most of the references I'm
familiar with deal with stone, but the same general principles should
apply to bone as well.

Faster water does have the potential to move larger elements, although
this will very much depend on the shape of the elements in question.
Elongated bones will be moved more readily than those with a smaller
surface area:volume ratio (ie. those that are rounder). Also, although
fast water can completely remove smaller elements in some instances, in
others it can actually help deposit sediment over them and thus preserve
their positions. Larger elements that stick out of the deposited
sediments may be moved or abraded more easily than smaller elements that
were completely covered. Of course if you're talking extremely fast
water there may be no deposition at all, and everything can end up being
swept away.

Water born sediments have to come to rest eventually, and such deposits
can have larger proportions of smaller objects since the water may not
have had enough energy to transport larger elements, or size/shape
sorting may have occured.


>  Thus, juvenile elements, which are smaller than adult elements, will be 
>  affected moreso than adult elements

Saying that fast water = no small objects is over simplifying the issue.
Field experiments have shown that there are various factors that
contribute to the resulting shape/size class distributions. Plus, even
the largest dinosaur has a few small bones in its skeleton. Dinosaur
Cove in south eastern Australia is a good example. The deposits seem to
have been formed by high energy streams or rivers depositing their load
of smaller (and often broken and abraded) elements, leaving
the larger bones somewhere up stream. There are a lot more smaller
dinosaur species represented in Dinosaur Cove. However larger animals
are still represented by the smaller bones in their bodies. The smaller
elements seem to contain a lot of limb bones, perhaps because their
elongated shape made them more susceptable to water movement.


GENERAL REFS:
Nash, D.T. and M.D.Petraglia 1987 The impact of fluvial processes on
experimental sites. In D.T.Nash and M.D.Petraglia (Eds) Natural
formation processes and the archaeological record. BAR International
series 352, Pp.108-130

Schick, K.D. 1986 Stone age sites in the making: experiments in the
formation and transformation of archaeological occurances. BAR
International series 319

Schick, K.D. 1987 Experimentally derived criteria for assessing
hydrological disturbance of archaeological sites. In D.T.Nash and
M.D.Petraglia (Eds) Natural formation processes and the archaeological
record. BAR international series 352, Pp.86-102 

Shackley, M.L. 1978 The behaviour of artefacts as sedimentary particles
in a fluvial 
environment. Archaeometry 20(1):55-61
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Dann Pigdon                   Australian Dinosaurs:
GIS Archaeologist           http://dannsdinosaurs.terrashare.com
Melbourne, Australia        http://home.alphalink.com.au/~dannj/
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