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South Texas Upper Cretaceous Coprolite


CraigHyatt

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These finds are from Eagle Pass, TX, Upper Cretaceous, marine, found lying on the ground on a layer of sandstone at a construction site. I see a lot of these doodads, but never picked one up until today when I sat down on a rock for a rest. I noticed it had a bunch of little cubic crystals. I thought maybe iron pyrite, but some of the dark brown crystals almost have an organic look to them, and aren't quite as regular in size and proportion as I'd expect crystals to be.

The other thing is most of the objects are roughly the same size, proportions, and have similar rounding. I thought there was a chance they might be coprolites and the bits were chewed up chitin. If they are, then clearly the host wasn't getting enough fiber in its diet.

I'll see whether Lori agrees or just poopoos my theory.

Edit: You know, the more I look at the magnified images, I think they are just inorganic crystals. Stuff that had been chewed up wouldn't be that regularly shaped. Also, the crystals have that "fractal" self-similar look to them. Not sure why they got clumped together in similar fashion for each specimen though. I probably need to break them open and see what's inside.

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Info: Craig Hyatt, retired software/electrical engineer

Experience: Beginner, fossil hunting less than a year

Location: Eagle Pass, TX USA on the border with Mexico, hot dry desert

Formation: Escondido, Marine, Upper Cretaceous

Materials: Sandstone, Mudstone, Shale, Chert, Chalk

Typical: Thalassinoides, Sphenodiscus, Exogyra, Inoceramus

Reference: http://txfossils.com/Txfossils.html

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They do not look anything like the coprolites I find. On that premise I do not think they are. Very cool looking whatever they are. I think they are geologic.

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They do not look anything like the coprolites I find. On that premise I do not think they are. Very cool looking whatever they are. I think they are geologic.

Yeah, you are probably right. I broke open all 4 of them. Three of them look like iron concretions. The "yellow" one looks like a conglomerate.

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Info: Craig Hyatt, retired software/electrical engineer

Experience: Beginner, fossil hunting less than a year

Location: Eagle Pass, TX USA on the border with Mexico, hot dry desert

Formation: Escondido, Marine, Upper Cretaceous

Materials: Sandstone, Mudstone, Shale, Chert, Chalk

Typical: Thalassinoides, Sphenodiscus, Exogyra, Inoceramus

Reference: http://txfossils.com/Txfossils.html

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I am no expert but I am thinking they look similar to some marcasite I have seen before

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How heavy are these?

This chunk weighs about 15 g (using a stack of pennies to compare). The volume is roughly 8 cm^3 so the density is roughly 2 g/cm^3.

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Info: Craig Hyatt, retired software/electrical engineer

Experience: Beginner, fossil hunting less than a year

Location: Eagle Pass, TX USA on the border with Mexico, hot dry desert

Formation: Escondido, Marine, Upper Cretaceous

Materials: Sandstone, Mudstone, Shale, Chert, Chalk

Typical: Thalassinoides, Sphenodiscus, Exogyra, Inoceramus

Reference: http://txfossils.com/Txfossils.html

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Definitely geologic in nature.

Some form of crystal .

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Thanks. The magnified marcasite images I found look pretty similar. Mix of yellow-ish and flat brown crystals.

Info: Craig Hyatt, retired software/electrical engineer

Experience: Beginner, fossil hunting less than a year

Location: Eagle Pass, TX USA on the border with Mexico, hot dry desert

Formation: Escondido, Marine, Upper Cretaceous

Materials: Sandstone, Mudstone, Shale, Chert, Chalk

Typical: Thalassinoides, Sphenodiscus, Exogyra, Inoceramus

Reference: http://txfossils.com/Txfossils.html

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I second iron sulfide crystals (pyrite). They are cubic, like your photo, and the color is right.

https://en.wikipedia.org/wiki/Pyrite

I don't know anything about poop, but I understand every does. =-)

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I agree that they are some sort of mineral. However they are still a very interesting find.

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It is almost impossible to tell if little pyritized nuggets like these are coprolites unless you can see bone/scale inclusions. If it is any consolation, in a sense they were probably formed by the waste of sulfate-reducing (anaerobic) bacteria. If it had fecal origins it could have happened something like this:

The poo (or other decaying organic matter) floats to the bottom of the sea and is covered in iron-rich sediment (or there is a heavy iron content in the water). It is not exposed to oxygen, so it doesn't decompose in a traditional sense. However, the poo contains sulfates (the stinky stuff) on which sulfate-reducing bacteria thrive. The bacteria takes in sulfates and produces sulfide waste (aka anaerobic respiration) which combines with the iron. After thousands/millions of years, voila: poo-l's gold (iron pyrite).

Did yours start out as fecal matter? Only Mother Nature knows for sure!

:)

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Thanks all!

Info: Craig Hyatt, retired software/electrical engineer

Experience: Beginner, fossil hunting less than a year

Location: Eagle Pass, TX USA on the border with Mexico, hot dry desert

Formation: Escondido, Marine, Upper Cretaceous

Materials: Sandstone, Mudstone, Shale, Chert, Chalk

Typical: Thalassinoides, Sphenodiscus, Exogyra, Inoceramus

Reference: http://txfossils.com/Txfossils.html

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Pseudomorphs of pyrite are common. Pyrite is unstable and reacts with water and air (rusts) to form different minerals. Al Dente is correct... your pyrite is changing into other iron bearing minerals such as limonite, hematite and possibly maghemite which is magnetic. Are the pyrite pseudomorphs magnetic?

For pictures of pyrite pseudomorphs see Mindat: http://www.mindat.org/mesg-82-202977.html

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Pseudomorphs of pyrite are common. Pyrite is unstable and reacts with water and air (rusts) to form different minerals. Al Dente is correct... your pyrite is changing into other iron bearing minerals such as limonite, hematite and possibly maghemite which is magnetic. Are the pyrite pseudomorphs magnetic?

For pictures of pyrite pseudomorphs see Mindat: http://www.mindat.org/mesg-82-202977.html

Dang. Left my magnet in my other pants. ;-)

Edit: very handy link, going in my bookmarks!

Info: Craig Hyatt, retired software/electrical engineer

Experience: Beginner, fossil hunting less than a year

Location: Eagle Pass, TX USA on the border with Mexico, hot dry desert

Formation: Escondido, Marine, Upper Cretaceous

Materials: Sandstone, Mudstone, Shale, Chert, Chalk

Typical: Thalassinoides, Sphenodiscus, Exogyra, Inoceramus

Reference: http://txfossils.com/Txfossils.html

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