What kinds of fossils are these?

[allquieton]

I found these in the Huachuca Mountains of Arizona. Most are very small, about 1 cm each or less. A couple of them might be almost 2 cm across.

 

Can anyone tell me what these fossils might be?

[Sjfriend]

Pictures 1 and 4 appear to be crinoids.

[Rockwood]

I think they all are. The preservation seems a bit odd. It's as if they were case hardened by a pressure shadow.

[DPS Ammonite]

The fossils (mostly crinoid stems) are probably silicified since they stand out in strong 3-D relief.

[doushantuo]

agree wtith DPS/rockwood,probably incomplete chertification along/(parallel to) the cleavage plane

Edited June 26, 2022 by doushantuo

[doushantuo]

in life,something like

 

image below from :

Crinoids columnals (Echinodermata) of the Ererê Formation(late Eifelian early Givetian, Amazon Basin), State of Pará, Brazil
S.M. Scheffler , A.C.S.F. Fernandes , V.M.M. da Fonseca 

 

Journal of South American Earth Sciences 49 (2014) 

 

Edited June 26, 2022 by doushantuo

[IsaacTheFossilMan]

+1 for crinoids! Pretty neat preservation, I've not seen them like that before.

 

The disk shapes are often mistaken for vertebrae because of their discoid shape:

 

They are a sort of spinal column, but not a vertebrate! They belong to the class Crinoidea, a group of marine echinoderms, related to starfish and sea urchins. Lots of people think they're plants, but they're not, just a wiggly waggly prehistoric creature that still exist!

 

 

The discoid shapes are a part of the stem called a "columnal", which are small tileable shapes that allows them to move around and bend freely.

[doushantuo]

No, it's what is  BETWEEN the columnals that allows SOME freedom of movement.

[Rockwood]

Ah. But if it were not for columnals would there be a between ? 

[allquieton]

Thanks everyone for the wealth of information and explanations and wonderful photos--it's all very helpful to me. Much appreciated! I'm still reading about it all.

 

I'll be out looking for more!

[allquieton]

15 hours ago, Rockwood said:

I think they all are. The preservation seems a bit odd. It's as if they were case hardened by a pressure shadow.

 

Could you explain this a bit more? I'm not sure what you mean even after some googling.

[Lone Hunter]

You still may not understand even after he explains it, l probably won't either but let's see.  @Rockwood

[doushantuo]

Given there high original initial porosity, crinoid fossils are highly susceptible to:

1) Micritization, diagenesis (with subsequent dolomitization, and after that, possible dedolomitization). All of this might alter or destroy fine anatomical details.

2) The rocks the crinoid are in were stressed (probably compressional stress).

 

All of this causes an alteration of the original, biologically determined shape.They might become flatter, their outline (aspect ratio) may be changed.

The stresses might also have cause fluid movement, possibly hydrothermal silicic fluids, but that also depends on parameters like fluid/rock ratio, reaction kinetics, initial permeability AND porosity of the rock.

Rocks under stress may have parts where deformation is less, because they contain more rigid inclusion, less susceptible to deformation.

The "wake" of these rigid inclusions (e.g. mica "fish"), experience LESS strain than other areas: they are in a "pressure shadow".

Below -Outlined in blue squares: some examples of crinoid stereom, which is a unique structure, crystallographically speaking: it a SINGLE

clacite crystal. Petrographically speaking, crinoid parts are full of very tiny holes.

Below - in blue: parts of the stereom of a crinoid (from Sars (Sr, that is)/public domain

[doushantuo]

Black arrows: the arrowed parts point to what contributes to the "stepladder" morphology (cross-section of a stele), central canal clearly visible.

[DPS Ammonite]

21 hours ago, Rockwood said:

I think they all are. The preservation seems a bit odd. It's as if they were case hardened by a pressure shadow.

See photo below. A cross section of several columnals (disks) shows that only the outside of each disk was silicified. The original calcite in the center of each disk has dissolved away probably from near-surface exposure for thousands of years

Preservation is typical for Paleozoic fossils in Arizona, especially the Pennsylvanian Naco Formation. Not all fossils are fully silicified.

 

NB. Qtz = quartz, the “silicified” parts.

Edited June 27, 2022 by DPS Ammonite

[doushantuo]

This PDF is available on Researchgate, but contains a font error.

 

New information on crinoids (Echinodermata) from the Pennsylvanian Naco Formation of central Arizona
July 2004The Mountain Geologist 41(3):77-86

[DPS Ammonite]

14 minutes ago, doushantuo said:

This PDF is available on RG,but contains a font error

 

New information on crinoids (Echinodermata) from the Pennsylvanian Naco Formation of central Arizona
July 2004The Mountain Geologist 41(3):77-86

Is there something in this publication that helps to ID or help identify preservation of the fossils in the OP’s fossils?

[Rockwood]

7 hours ago, allquieton said:

 

Could you explain this a bit more? I'm not sure what you mean even after some googling.

Case hardening is a process used on steel surfaces which need to withstand heavy use. The outside surface being the hardened part while the inside is left softer, but less brittle. I suspect hot ground water under very high pressure and temperature, and containing dissolved silicates may have altered the outer layer of these fossils in a similar way. It's the  silicification process that @DPS Ammonite has been talking about as I understand it.