Crinoid or orthocone?

[Brian James Maguire]

Unsure of this one

[Tidgy's Dad]

Crinoid, I think. 

It looks more like a lumen than a siphuncle. 

[Brian James Maguire]

@Tidgy's Dad what was confusing me was the fact it is tapered , when ever i prep out crinoid stems i never came across a tapered one 

[Tidgy's Dad]

4 minutes ago, Brian James Maguire said:

@Tidgy's Dad what was confusing me was the fact it is tapered , when ever i prep out crinoid stems i never came across a tapered one 

That's a good point. 

But it seems to be at least a partial optical illusion as the white section does taper but that seems to be preservational as there seems to be more black as you go down. 

Also, the angle the piece is in the rock, perhaps?

Can you measure the width at both ends?  

[Brian James Maguire]

@Tidgy's Dad

[Tidgy's Dad]

Weird. 

Maybe it is a nautiloid of some kind, you do seem to have hit a hotspot for them. 

Or maybe the stem is widening as it nears the calyx as in this one:

 

[Brian James Maguire]

Thats a very good point, 

[westcoast]

Crinoid

[TqB]

I'm undecided on this one as there's a lack of crisp detail though I agree the fill looks like a lumen.

The top segment seems to be lot thicker than the others as there's no division going across the oblique cross section. That could maybe be a crinoid calyx basal or a calcite filled nautiloid living chamber.
There's no sign of serrated crinoid columnal articulation but that could be lost in the mineralisation.

 

What does the small end cross section look like?

[Brian James Maguire]

@TqB

[Kane]

Not seeing a lumen on the tapered end, I am leaning toward nautiloid. 

[TqB]

Zooming in on the first photo in Brian's next to last post, this seems to be a geopetal sediment fill with sparry calcite formed in the void above. There's a shell outer wall and chamber walls visible in the dark matrix fill in the bottom half. So it looks like nautiloid.

Edited September 16 by TqB

[TqB]

6 minutes ago, Brian James Maguire said:

@TqB

And this shows the geopetal fill again that I mentioned in my previous comment. Definite nautiloid. 

Edited September 16 by TqB

[Tidgy's Dad]

Not all crinoids have articulation 

3 minutes ago, TqB said:

There's no sign of serrated crinoid columnal articulation but that could be lost in the mineralisation.

Not all crinoids have articulation facets or a crunularium and therefore they don't have serrated areas where the columnals meet : 

Figures 1-6: Schematic diagram of the three morphologies of crinoid columnal articulation. Figures 1,
3, 5: Articular facets of synostosial, symplectial, and synarthrial articulations, respectively. Figures 2,
4: Lateral views of short lengths of column that articulate synostosially and symplectially, respectively.
Figure 6: Longitudinal section of a short length of column that articulates synarthrially. Position of axial
canal dashed in.

 

But I am also now leaning towards it being a nautiloid. 

 

[Brian James Maguire]

That was a challenging one guys! Thanks all for the input

[Tidgy's Dad]

4 minutes ago, Brian James Maguire said:

That was a challenging one guys! Thanks all for the input

Yes, that was a lot of fun.