[Rockhounds] Coprolites

[Thomas Yancey]

This comment brings up another feature of the coprolite-shaped 
concretions at Salmon Creek, Washington that allows argument over 
origin to continue. The coprolite concretions range in shape and 
detail from ones with a perfect coprolite morphology to ones that are 
extensively cracked with deep 'breadcrust' type fractures, wider at 
the surface anddecreasing width inward. With a little search a 
complete gradation can be gathered from one type to another.

When mineral composition is determined for these two types, the ones 
with perfect coprolite form are composed of siderite and the ones 
with deep fractures are composed of iron oxides. Intermediate forms 
often have outer layers of iron oxide and a core of siderite. 
Incomplete oxidation. The oxidation of siderite to iron oxides causes 
an increase in volume and the earliest oxidized layers on the outside 
are broken into breadcrust fractures as the inner parts oxidize and 
expand. There is an excellent correlation with degree of oxidation 
from siderite to iron oxide and quality of fossil.

The coprolite concretions from Salmon Creek that I have purchased 
have a yellowish-brown crust that is soft and iron oxides rub off 
easily when they are handled.

The "sharp edges" are a result of surface weathering and is not 
related to concretion formation.

Tom Yancey



>I and others have talked to Ms. Skinner about the paper. She admits 
>that neither of them have been anywhere near Salmon Creek, WA. Once 
>again, I find it preposterous that anything, including "guts" of 
>vertebrates could be perfectly preserved while the rest of the 
>animal left absolutely no trace. I invite her to watch a dog poop 
>the fresh, un-eroded concretions found in such abundance at Salmon 
>Creek. Said dog would have a very bad feeling in its hindquarters 
>from the almost fractal, sharp edges all over the surface of the 
>concretions as they are present in the Wilkes clay, before they are 
>weathered by the stream. There no concretions that look anything 
>like those in the paper present in the un-eroded clay of the Wilkes 
>Formation. All of their samples were purchased at rock and mineral 
>shows or online. All have been severely weathered by the stream. I 
>liken their paper to the blind men describing the elephant. It 
>simply does not hold water when confronted with the whole picture at 
>Salmon Creek.
>
>At 04:44 PM 4/8/2008, you wrote:
>>At the Tucson show we bought a bunch of 'crorolites' from Madagascar and
>>because we had so many customers ask about what they were, I did a bit of
>>research on the net and the best thing I found written about them was 'A
>>Fresh Look at Sideritic 'Coprolites' by Adolf Seilacher, Cynthia Marshall,
>>H. Catherine Skinner and Takanobu Tsuihiji in the Journal of Paleobiology,
>>Vol 27, No.1 (Winter 2001) pages 7-13. I even paid $12 to down load the
>>article. The authors believe that 'coprolites' are the fosilized remains of
>>the intestinal tracts of dead amimals. The authors observe  that fresh
>>animal dropings are commonly pointed at one end only and invite any doubters
>>to watch a dog deficate. They also point out that the striations observed on
>>the 'coprolites' are very much like those one might expect would be fromed
>>by the mussel bands observed in the intesteines of many animals. The authors
>>conclude:
snip
-- 
Thomas Yancey