Colonial Rugose Coral Identification Cheat Sheet

[andoran]

I've been working on a Petoskey Stone presentation for my rock and mineral club and have been collecting various colonial rugose corals that have been on sale on online and Etsy. I've been doing this because "Petoskey Stone" has become a popular name to use to sell fossilized coral and I want to be able to show our club how to differentiate Hexagonaria from other types of coral.

I've gone through this forum and gleaned some data from @TqB and @FossilDAWG while they were helping out on IDs and was wondering if anyone could help me fill out this cheat sheet with more information (and correct me if I've gotten anything wrong) on how to differentiate some of the fossils I often see for sale (named correctly or not).
 

Colonial Rugose Coral Identification Cheat Sheet

Acrocyathus

o   Carboniferous

o   Ceroid

o   Central lens-shaped columella

Actinocyathus

o   Carboniferous

o   Ceroid

o   Zone of bubbly dissepiments (londsdaleoid)

Arachnopyllum

o   Silurian

o   Astreoid

o   Septa extend to axial column

Hexagonaria

o   Devonian

o   Ceroid

Pachyphyllum

o   Devonian

o   Astreoid

 

Thanks for any help you can provide.

[FossilDAWG]

I hate to be a "wet blanket", but according to the Digital Atlas of Ancient Life, a web site maintained by the Paleontological Research Institute, there are 29 Families, 488 genera, and 1337 described species of rugosan corals.  No "cheat sheet" could come close to encompassing that level of diversity.  The risk of a "cheat sheet" like the one you propose is that it runs the risk of misleading people into thinking that there are only a small number of genera.

 

I think the goal of informing people that commercial Petoskey Stones could be a lot of different corals is a good one.  Maybe a slightly different way of approaching the subject would be to explain that "real" Petoskey stones are Hexagonaria, show how to recognize that genus, then show other examples that actually belong to other genera and explain why.  There is a lot of information you could use in the link I gave.  Ultimately the nature of your talk would be similar to what you suggested, but you could use the opportunity to introduce your audience to the idea that there is an astonishing diversity of rugose corals, not just one kind that happens to be known as Petoskey stone.

 

Don

[andoran]

Thank you. I am aware of that particular website and use it as one of my many sources for this presentation. I chose the five colonial rugose corals above because those are the ones I have seen most frequently for sale online (often misidentified). I did read about how many rugose corals have been identified so far, but the vast majority aren't available in commercially viable quantities and trying to narrow down a identification table for every single one is well beyond my ability or the scope of my presentation. 

I chose the Petoskey Stone because I went to high school in Petoskey, MI. I no longer live in the state and realized shortly after joining my rock and mineral group that if you haven't been raised with the Petoskey Stone in your backyard and in every shop you frequent, it's easy to be misled online. I can almost always recognize when a fossilized coral is not a Petoskey Stone, but it is very difficult to explain why.

 

My presentation starts out with general knowledge about rugose corals and a comparison with modern ones, moves into specifics about the Petoskey Stone, expands out to encompass all Hexagonaria, and finishes with comparing and contrasting the various rugose corals I've seen for sale online and some non-rugose corals that I've seen identified as Petoskey Stones.

Gleaning knowledge about these corals from online sources has been quite difficult. I often run into language I can't comprehend because I don't have the training in geology, biology, and paleontology to understand all of the papers I've read. I've ordered a couple of textbooks in the hope they aren't just a bunch of scientific papers and actually have information intended to teach rather than inform other scientists.

[andoran]

So, I bought a book called The Rugose Coral Genera by Geoffrey Cotton (1973) and it does an excellent job of giving a short description of what looks like thousands of different rugose corals (both colonial and solitary). Here is my original list of five filled out and corrected with information from the book. Let me know if you want to know what it says about any others. I don't mind copying it here.
 

Acrocyathus (synonym of Lithostrotionella) 

o  Permian, Carboniferous

o   Ceroid, corallites prismatic, long. Majors thin, nearly to axis; minors about 1/3 majors. Columella thin, lamelllar, in some joined to counter septum, or to counter septum and cardinal septum. Peripheral lonsdaleoid dissepimentarium bounded by inner wall; interseptal dissepimentarium very narrow. Tabulae usually complete, flat, or slightly wavy and incomplete.

 

Actinocyathus

o   Carboniferous

o   Ceroid, corallites polygonal, irregular, excavated, with irregular septa, brought together and as a net; columella convex, surrounded by a protuberance not continuous. Corallum hemispherical or discoid, up to at least 70 x 80 mm; corallites 5 to 7 sides, 13 to 17 mm diameter. Epitheca thin, crenate, Calice with gently sloping peripheral platform, deep axial pit, and well raised axial boss 2 to 3 mm diameter. Majors 24 to 28, not to axial structure; minors slightly into tabularium, of variable length. Axial structure irregularly polygonal or surrounded, of indistinct median plate, few septal lamellae and tabulae giving loose appearance. Tabulae complete, flat. Dissepiments londsdaleoid, of only one row, flat.

 

Arachnopyllum

o   Silurian

o   Astraeoid; corallum low, spreading; although no epithecal walls, boundaries or corallites well defined by sharp ridges. Septa to axis or nearly, normal only in tabularium; in dissepimentarium septa develop sporadically, thickened and contiguous, or each a network of small trabeculae standing vertically on dissepiments but not piercing more than 1 or 2 successive dissepimental floors. Tabularium small; tabulae steeply domed, incomplete. Dissepimentarium very wide; dissepiments numeroud, small, based horizontally.

 

Hexagonaria 

o   Devonian

o   Ceroid, corallites polygonal. Calices usually with axial pit and peripheral platform, but platform may be absent and calice funnel shaped. Septa radial, long, majors extending into tabularium, minors confined to dissepimentarium; septa may be dilated or lightly to heavily carinate; in some, meeting at axis where axial tabellae arranged in axial structure, otherwise withdrawn from axis when axial tabellae horizontal, and pariaxial inclined, in or out. Tabularium usually relatively narrow; tabulae closely set, complete or not. Dissepimentarium wide, of many rows of horizontal or inclined dissepiments.

 

Pachyphyllum (synonym of Phillipsastrea)

o   Devonian

o   Astraeoid, thamnasteriod, or aphroid axial pits with raised margins. Septa dilated at edge of tabularium, producing exsert rims on distal surface of corallum; majors extend towards axis becoming attenuate; minors only to edge of tabularium. Tabulae relatively flat, usually complete. Innermost dissepiments horseshoe, in a single row; peripheral dissepiments small, usually convex.

Edited January 19, 2022 by andoran

[Tidgy's Dad]

Acrocyathus also occurs in the Permian. 

At this point I gave up. 

[andoran]

1 minute ago, Tidgy's Dad said:

Acrocyathus also occurs in the Permian. 

You are totally right. Says it right there in the book. I just missed changing it on the post.

[LisaL]

Hi there!

 

This is going to be such a helpful and interesting presentation! It's a daunting thing to sort out all of these colonial coral genera, but you've got lots of information. I'd try to keep it as simple as possible, as FossilDawg says, highlighting differences among them with maybe a chart showing where each is found, age, septal arrangement/dissepementarium in transverse and vertical views (cerioid, astreoid/lonsdaleoid or not). I'm sure you've got that covered, already, though!

 

Just off the top of my head, here are a few notes that have helped me learn about these similar-appearing corals:

 

1) Arachnophyllum when viewed in rough form or when looking at the corallites straight down or in transverse section can be confused for a cerioid coral like Acrocyathus. This is because Arachnophyllum has sharp ridges around its corallites (as mentioned in your description) that can look like cerioid walls. A lot of times you need a vertical section to see whether the septa are astreoid or aphroid. Also, in Arachnophyllum, the dissepiments are not consistently lonsdaleoid as they are in Acrocyathus. 

The structure of Arachnophyllum is really neat in that it builds itself up on its layers of horizontal dissepiments--it laid down small, horizontal dissepimental plates and built septa on top of them and grew up that way. I'm not sure whether Acrocyathus has this same layered structure of horizontal dissepiments built on top of each other. In some rough chunks of Acrocyathus, I think it's easier to see columns rather than platforms of dissepiments. The Treatise on Invertebrate Paleontology only says that its dissepiments are lonsdaleoid (which Tarquin may have already explained means that they form a zone that the septa don't cross into.)

 

Here's a thread showing one of my finds -- I'm still not certain whether it's Acrocyathus or Arachnophyllum! 

 

 

2) Just noting that there is a genus called Prismatophyllum that as far as I can tell looks exactly like Hexagonaria. In fact, Prismatophyllum is one of the old genera that Hexagonaria was first known as. Acervularia and Cyathophyllum are a couple of other genera it previously was placed in. Sometimes contemporary sources still use the old genus names for Hexagonaria. (I see it called Prismatophyllum a lot), which adds to the confusion.) 

 

Anyway, apparently Prismatophyllum and Hexagonaria are distinct genera, both occurring in the Midwest (Ind-Ky-Ohio) (according to Dorothy Hill's 1981 volumes on Paleozoic corals in The Treatise of Invertebrate Paleontology). I think the differences in them come down to micro septal structure. I do note from The Treatise that they have been placed in separate families (Prismatophyllum is in Cylindrophyllinae while Hexagonaria has its own family, Hexagonariinae, which includes 3 genera. Hexagonaria is the most widespread, known from Europe, Asia, W. Australia and North America, while the other two are more limited in Europe and Asia. I think their placement in these families has to do with discreet differences in internal structure.

 

3) The genus Asterobillingsia is a fairly common rugose coral that turns up on our Great Lakes Rocks and Minerals group on Facebook. It's in the same subfamily as Prismatophyllum (Cylindrophyllinae), but its septa are astreoid (merging into each other instead of cerioid like Prismatophyllum's), and often times, there will be a raised zone around the axial pit in the centers of the corallites. Lots of information about this genus is still found using its old genus name, Billingsastraea. A new genus was erected in the 70s (I think) because of consusion over the source of the type specimen for Billingsastraea. 

 

Actinocyathus isn't found in our Mississippian rocks in Michigan, but I think it's found in Mississippian rocks in Alabama. I'm not sure about Missouri! I think it's pretty well known in the UK. 

 

I hope this is somewhat helpful! I can post some example photos if that would be helpful. A friend from the Facebook group I mentioned above says it's okay for me to share some of her excellent photos for your project if you'd like that. 

Good luck on your project!
Lisa

Edited January 26, 2022 by LisaL
clarification

[andoran]

Thank you. This is awesome. I really appreciate you taking the time to type all of that up. Any pictures you can send are always helpful and I definitely will make sure to credit you and whomever took the photos.

 

EDIT: @LisaL These are some pics of Arachnophyllum and Acrocyathus. The first is of Arachnophyllum that someone in The Petoskey Hunter Facebook group sent me. The fourth is the only one I own. I tried sanding it to see if the coral structure would stand out more and it did not. I've often noticed that the older the fossil is, the less distinct the structure looks. But silicification after fossilization always makes the structure more difficult to see. The other two are Acrocyathus I own (neither from Michigan). From what I understand, the wavy appearance is the defining characteristic of Arachnophyllum.

Edited January 26, 2022 by andoran

[LisaL]

Your Acrocyathus is beautiful!

 

I don't think your one on the bottom is an Arachnophyllum, though. I'm not seeing any patches of lonsdaleoid dissepiments or layers built on top of dissepiments in side view. I'm wondering if that last one is something closer to Asterobillingsia? It looks astreoid, with carinate septa (having little cross-bars ornamenting the septa perpendicularly, which I don't believe Arachnophyllum has.) Where did this one come from?

 

I'll grab some photos of Asterobillingsia and others for you to compare. Your photos are gorgeous! 

 

Back in a bit with the photos!

[andoran]

Yeah, I've been wondering whether it is or not. I got it from someone else in northern Michigan and I'm not 100% on the identification. It does not help that it is wrapped around itself. Here's a pic of the top and bottom.

Edited January 26, 2022 by andoran

[LisaL]

Oh my gosh, that's beautiful. I'm pretty confident that's Asterobillingsia. I think I'm making out "zigzag to subyardarm" carinae along the septa. It's kind of hard to highlight them, but like this:

 

Edited January 26, 2022 by LisaL

[andoran]

Alrighty then. One I didn't even have on my radar. Not only is it blue, it's unique.

[Wrangellian]

On 1/26/2022 at 8:52 AM, andoran said:

These are some pics of Arachnophyllum and Acrocyathus. The first is of Arachnophyllum that someone in The Petoskey Hunter Facebook group sent me. The fourth is the only one I own. I tried sanding it to see if the coral structure would stand out more and it did not. I've often noticed that the older the fossil is, the less distinct the structure looks. But silicification after fossilization always makes the structure more difficult to see. The other two are Acrocyathus I own (neither from Michigan). From what I understand, the wavy appearance is the defining characteristic of Arachnophyllum.

 

 

Hi, that piece is a dead ringer for something I picked up at a rock sale over here recently, but I have no idea what it is or where it's from. Do you mind having a look at my topic and see if it matches? Can you show my pics to others who know about things Petoskey and let me know? I'd appreciate it.