Unusual Michigan Coral Fossil

[John Joldersma]

I have been cutting Petoskey Stone and other Michigan corals for a long time as a business. Last week I cut into a piece that I thought was what we call "fish egg coral", it turned out to be something entirely new to me. We had a local rock show last week in Grand Rapids and I took the fossil. Everyone including 2 fossil dealers agreed it was a coral fossil, being from Northern Michigan probably Devonian. Nobody could ID it. 2 people did have the same reaction to it that I had at first look, that it looked like Louisianna palm wood. Other than that I am at a loss. Any help in ID would be appreciated. I scanned the actual stone and adjusted the color It is 4" by 2.5": Edited April 15, 2014 by John Joldersma

[4circle]

This isn't quite a coral (although, don't worry, for centuries they've been mistaken as tabulate corals, even by experts). This is a transverse section through a chaetetid sponge. These are a bizarre and obscure group of demosponge which usually lacks most sponge-like features (fossil varieties have no spicules). We assign it to the sponges following recent discovery and study of a living relative, Acanthochaetetes, which also shows this structure, but possesses tell-tale sponge signs in its soft parts, such as bona fide spicules.

Because these are an obscure group, the internet is quite lacking on good references I'm afraid. If you want a good, entry-level guide to these showing this deviously coral-like structure, see AAPG Mem77 Pa106 Fig2-3.

[fossilized6s]

Am i safe to assume that the faint circular surface pattern is caused by your cutting it and not growth?

[John Joldersma]

I use a MK 303 10" blade which cuts pretty smoothly. I wet the face before scanning. I can't see any saw marks in the scan. The variations of color are accurate if that is what you mean.

If you see marks that I can't then they would fit the 10" diameter blade.

[John Joldersma]

At second look, that is saw marks mostly on the lower left side.

[FossilDAWG]

Any chance of getting a much closer in photo? The chaetetid sponge hypothesis is reasonable, but I'd like to see a high resolution close-up before ruling out other possibilities.

Don

[John Joldersma]

I tried to add a blow up but seem to have killed the original image instead.

I'm a newbie so I don't want to repost without advice, Sorry about that.

[fossilcrazy]

I'm not even seeing a photo.

[Auspex]

I tried to add a blow up but seem to have killed the original image instead.

I'm a newbie so I don't want to repost without advice, Sorry about that.

Just use Full Editor to reattach the original image.

[John Joldersma]

This time the full edit worked. Could not add another picture so will post the blow up seperately.

[John Joldersma]

Here is a blow up of the scan, camera work later.

[4circle]

Yes, this is Chaetetes sp. The way we tell it from a coral is this:

- if this were a tabulate coral, we would expect inter-corallite structure in the (pale) regions between the 'holes' (which would be corallites), unless of course they were cateniform in which case there'd be a coenchyme between corallites, or syringoporiform in which case the corallites would be isolated and thick walled but wiht lateral stolons connecting them; and in both cases unwanted sediment between corallites.

- in chaetetids the structure is within the 'holes' (non-septate living chambers) so here doesn't show, and there's little between columns

To summarise that in vaguely comprehensible English, in a coral you have structure between the living chambers which almost always has a visible geometry. Here there's no such luck.

[4circle]

It's a nice specimen though, don't let go of it!

[John Joldersma]

The piece weighs in just over a kilo. I knew it was special the day I cut into it. It is waterworn through beach action but in Michigan that is also true of a lot of gravel pit stones as well as Great Lake beaches. I either found it in a gravel pit last year or got it off the pile at my partners house. That pile has both found and bought stones going back decades. So I am just sure it is from Michigan. It would be fairly easy to polish since it is fairly smooth. The sponge structure is visible from all sides. This has to be the rarest thing I ever found.

[FossilDAWG]

I'm comfortable with Chaetetes sp. From the first photo I was wondering about something such as Acanthohalysites encrustans, but that is ruled out by the new photo.

Don

[Wrangellian]

In the first photo I can see both faint saw marks, which are almost vertical, and a faint, concentric pattern in the distribution of 'corallites' of broader arcs centered on the upper left, which I assume Charlie was referring to. Might this be a manifestation of colony growth and might it have any bearing on the ID? In any case I will take Don's word for it on 'Chaetetes'.