Micro faecal pellets - What could have made them?

[Skatetom]

Hi people,

 

I have a query regarding the contents of some siderite nodules from a Duckmantian fossil forest site in N Wales for my PhD project.

I had thin sections made of parts of several nodules and they all seem to have an abundance of 'faecal pellets'; rounded, often curved (banana-ish) shaped, mostly stratified pellets. These pellets have been replaced by siderite (FeCO3) and sometimes include pyrite and seem to have been the locus for early sulphate reduction by bacteria before conditions arose for the siderite precipitation. I've attached SEM images of the thin sections showing some of these pellets.

We have only found a couple of fragments of crustaceans (Euproops I believe) and the host sediments were fine sands and silts.

 

All the images show individual pellets with scale bars but "pellet5" is a zoomed out image showing the abundance of the pellets. They are all replaced by an Fe-rich siderite and are all quartz free. "pellet6" is different with a mottled texture and some potential apatite mixed in.

 

Let me know if anyone has seen similar textures before and/or what could have made them! The palaeoenvironment was purely freshwater with meandering river systems, with a dynamic lycopod dominated fossil forest ecosystem.

pellet3.pdf pellet2.pdf pellet1.pdf pellet5.pdf pellet4.pdf

[Skatetom]

Forgot to add Pellet6...

pellet6.pdf

[Carl]

I'm having a hard time convincing myself these are fecal pellets. But if they are it will be extremely hard to connect them with any particular animal. One reason is that so many small invertebrate groups have similar feces. Another reason is that if these are truly fecal pellets they are too poorly preserved to offer critical morphology.

[Skatetom]

Thanks Carl.

 

Any ideas on what they could be? My mind went straight to fecal pellets once I saw them but I am no expert! Whatever they were, they are now replaced by siderite which cannot have formed at the surface as it requires anoxic conditions. Their relationship to the quartz grains suggests to me that they settled out of suspension/were deposited as 'solid' objects and the fact that they hold the only occurences of pyrite inside them suggests that they were the focus of the sulphate reducing bacteria (so something was 'eating' them).

A colleague of mine suggested they could be segments of worm burrows, lined with fecal matter, filled with muddy stuff which has since been replaced by siderite...

[Mark Kmiecik]

Are insects/larvae/nymphs a possibility?

 

 

 

Edited November 10, 2022 by Mark Kmiecik
added info

[Skatetom]

Anything apparently organic is a possibility really I think! Are you thinking insect poop or bits of actual insect? Some appear to be sort of stratified if you look at the images and I don't imagine body fossils would have such textures once replaced by minerals.

[Carl]

On 11/10/2022 at 9:28 AM, Skatetom said:

Thanks Carl.

 

Any ideas on what they could be? My mind went straight to fecal pellets once I saw them but I am no expert! Whatever they were, they are now replaced by siderite which cannot have formed at the surface as it requires anoxic conditions. Their relationship to the quartz grains suggests to me that they settled out of suspension/were deposited as 'solid' objects and the fact that they hold the only occurences of pyrite inside them suggests that they were the focus of the sulphate reducing bacteria (so something was 'eating' them).

A colleague of mine suggested they could be segments of worm burrows, lined with fecal matter, filled with muddy stuff which has since been replaced by siderite...

I'm pretty familiar with fossil marine invertebrate fecal pellets but not at all as well versed in sedimentology, which, I suspect, is where the answer lies. Sorry.

[fifbrindacier]

I'm sorry but i don't see where is what you want to speak about.