Lower Ordovician fossil needs ID.

[AJ58]

I am guessing Coral Sponge or Bryozoan I do not know.So I need an ID.Found in creek gravel,Northeast Arkansas,Lower Ordovician.Thanks. 

[Tidgy's Dad]

It would appear to be a solitary rugose 'horn' coral. 

But they didn't appear til the Middle Ordovician, I don't think. 

[FossilDAWG]

I agree, it is certainly a solitary rugosan coral.  The earliest such corals in North America appeared in the Middle Ordovician (the Blackriverian stage) but those corals (Lambeophyllum, and a little later Streptelasma) had a simple structure.  Your specimen is more complex, and seems more consistent with corals.from the Late Ordovician or younger.  Stream gravels can contains a sampling of all the formations in the area.

 

Don

[AJ58]

I wonder if this creek that I find a lot of my fossils produce different age rocks?The map says lower ordovician but it is close to the border.I find Ceratopa that is only found in upper part of the lower ordovician.

[ynot]

1 hour ago, AJ58 said:

I wonder if this creek that I find a lot of my fossils produce different age rocks?

Creeks can carry rocks for long distances. The harder the rock the further it can go.

Yours looks like it has traveled some distance before You found it.

[AJ58]

Thanks.This makes it harder to ID fossils from this creek. When asking for ID I will leave the time out..

[ynot]

11 minutes ago, AJ58 said:

Thanks.This makes it harder to ID fossils from this creek. When asking for ID I will leave the time out..

Better to check the maps and see what is upstream from You, and give a broader time range.

After a while You will get to a point where You can better tell the age by the amount of wear on the pieces You find.

[Johannes]

Its a bryozoan colony.

[AJ58]

The pattern I see on this fossil looks like bryozoan colony but I do not know?

[ClearLake]

I'm not sure what folks are seeing that looks bryozoan?  I see nothing but coral here.  You can see the tabulae and on the end view (last picture) you can see the septa.

[AJ58]

I do not know the makeup of coral or bryozoan fossils but looking at it with a magnifying glass it appears that the whole fossil is made up of lines that are broke up into small rectangles and they all the way through .

[caldigger]

That's definately looks to be a rugose. I have a smoother surface one that looks exactly like the patterns you have on yours.

[FossilDAWG]

18 minutes ago, AJ58 said:

I do not know the makeup of coral or bryozoan fossils but looking at it with a magnifying glass it appears that the whole fossil is made up of lines that are broke up into small rectangles and they all the way through .

The lines that are oriented with the long axis of the fossil are septa that originate on the coral wall and extend towards the center.  The lines at right angles to the septa are tabulae and dissepiments.  You can see them because the outer layer of the coral wall (the epitheca) has been eroded away.  

 

Don

[AJ58]

Thanks for the explanation now I know

[Johannes]

19 hours ago, ClearLake said:

 

 

 

 

Here are some ordovician bryozoan thin sections to compare with:

 

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2

 

3

 

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[Johannes]

Sorry for only short answers, unfortunately I have 20hr-Working days at the moment and only limited internet access.

 

Why bryozoan? The "Septa" of the UFO shown above interpreted as feature for coral, are not "walls" but a lot of single polygonal branching columns, clearly pointing against the "coral"-interpretation.

 

The tabulae are also a common feature of branched bryozoan colonies from the ordovician to the devonian, so not an absolute argument "pro corallia". An argument for this position are septa inside of the polygons, but those are not visible in the pictures above, for that we need polished section in adequate magnification.

 

Personal experience from sections of a lot palaeozoic fossils of this kind show, that most of the "tabulate coralls" with a diameter of the columns below 1mm are bryozoans instead (in general, exeptions prove the rule...)

 

But just my two cents, I didn't want to snub the ordovician professionals here, who have more experience with bryozoan and coral morphology than I have.

[ClearLake]

@JohannesThose are beautiful thin sections of bryozoans and they show very nicely that bryozoans have a similarity in form to certain corals.  I believe what is missing is the very important feature of scale.  One of the distinguishing features that set bryozoan fossils apart from corals is the size of the "pores" (living chamber of the animal).  I'm guessing (but don't know for sure as I don't see a scale) the ones in your pictures are significantly smaller than the hand sample shown in the original post.  What I guess we really need is a nice thin section of the specimen posted at the same scale as your pictures to see any differences or similarities.

 

I am no expert, but all I do know is the original pictures posted look like dozens of samples of Ordovician corals that I have seen/collected and do not look like the equally plentiful Ordovician bryozoans that I have seen and/or collected.

[Johannes]

@ClearLake Unfortuntely that are not my pictures. Bryozoan polygonal columns are generally under 1mm in diameter. I have some fist-sized colonys of lower middle ordovician bryozoans with sections in different planar angles(right term?) (which a lot of people would assign to "Favosites"). I will try to post them within the next days, if they are from interest for this discussion.

 

To be honest, for a clear identification you need a polished section crosswise to the orientation of the polygonal columns. But my point for the ufo above is still bryozoan colony.

[ClearLake]

4 minutes ago, Johannes said:

To be honest, for a clear identification you need a polished section crosswise to the orientation of the polygonal columns

Agreed

 

I guess we were both typing a response at the same time   

 

I agree with your reference to bryozoan "individuals" being under 1 mm in size, that usually seems to work.

[Johannes]

unfortunately, I havn't found the scans of my large "favositid" and the "coralliform" ordovician bryozoa (yet), but one or the "ball-shaped" colonies from the ordovician of sweden, where you can see the tabulae and the polygonal zooids:  Trepostomata

 

A good introduction about anatomy of those bryozoans can be read here.

[Mark Kmiecik]

Longitudinal and transverse cross-sections of this specimen would be quite stunning and more conducive to the study of its structure. I would be tempted to make it so if I had multiple specimens.

[AJ58]

Only having one specimen I am not ready to slice it up at this time.I do want to thank everyone that got involved in this ID process I learn more when the process becomes a good discussion than just a simple answer.

I do agree that it is a solitary rugosan coral. even though Johannes has a good argument for bryozoan colony.

 

 Sometimes this old country boy needs an explanation of the fossil. thanks again.