A Most Unusual Coral, It Has An Operculum

[jkfoam]

Years ago I had an interest in fossil corals so I assembled a collection of fossil corals from all the time periods. Actually my interest was in the Phylum Coelenterata or if you prefer, Cnidaria, and I assembled represenatives of all the Classes I could. I guess I have about 80 species in the collection. Most of my specimens were purchased with a few collected locally here in Texas. One of the corals I acquired has always intrigued me because it had an operculum or a lid that could close and protect the animal from predators. I always wondered how the operculum functioned. Did it work like the operculum on snails or was it like the minor valve on a rudistid clam that raised up like a hat or was it hinged like the minor valve on a brachiopod like a Prorichthofenia?

The coral of my interest is the Devonian Rugose coral Calceola sandalina

Shown below are two pictures of C. sandalina. The one on the left has the operculum in place and the picture on the right is a specimen missing the operculum.

Recently I did some literature research and found a publication that answered my question about the operculum. In the Calceola sandalina the operculum swings open like a valve on a clam but unlike the clam there are no hinge teeth. The link to the publication ia as follows; http://www.mnhn.fr/museum/front/medias/publication/1311_g04n1a2.pdf.

There are a handful of silurian and devonian coral species and genus that have operculum and they all are part of the coral Family Calceolidae. Evidenty the presence of the operculum did not provide a great advantage to those corals as the operculum disappeared from coral features later in time.

JKFoam

[Auspex]

That begs many questions!

What was the selective pressure that spurred the development of the operculum in these corals? Was it defensive, and if so, was it in response to predation, or another environmental hazard? What changed that rendered it superfluous?

Cool stuff!

[Fossildude19]

Very interesting - I never knew that!

Thanks for the bit of enlightenment, today!

Regards,

[tracer]

That begs many questions!

What was the selective pressure that spurred the development of the operculum in these corals? Was it defensive, and if so, was it in response to predation, or another environmental hazard? What changed that rendered it superfluous?

Cool stuff!

it was a rumble seat. once they realized they weren't as social as they had thought, they dropped the option on future models to save...<drumroll, please> some clams...

p.s. - the .pdf link isn't available. guess they thought we were on to their little secret and went underground with it.

[squalicorax]

Tracer remove the period at the end...

Anyways this is an interesting evolutionary development.

[Earl]

Hi Jim,

This is a very interesting and informative post, with high quality and detailed photographs--such detail that I'm going to have to ask a couple of questions.

I need a little help in seeing what I'm supposed to be seeing in your two photographs. I've opened each photograph and put them side by side. (Great detail!) The one on the left (with the operculum) has the thin-looking and just barely detached layer on top; that, is the lid, the operculum, correct?

The one on the right (without operculum), I agree, doesn't have that slightly detached lid. But the depth and shape of the tops of both photographs look very similar. The one on the right has much the same texture and "growth rings" (for lack of a better word) as what I see on the one on the left.

The photographs in the plate on page 25 of the PDF that you referenced show quite a bit of depth descending into a lidless coral. Both of your photographs don't seem to have that depth--as if they both have lids.

On the next plate (page 27), when I look at specimen J (described with operculum present), it appears similar to both of your photographs.

So my question is: is it possible that both of your specimens have the operculum present? Perhaps the one on the right has mineralized with a bit of smoothing of the detachment that would distinguish the lid clearly from the body.

I'm probably just seeing things, knowing little about this. But perhaps you could walk me through the detail of, particularly, the photograph on the right. It sure looks like the same shape as Figure 6 (page 27), item J.

The Galle and Ficner article says "Unfortunately, the operculum is never preserved in situ on any of the specimens discussed." (page 22)

Maybe instead of one, you have two C. sandalina specimens with the rare operculum present!

Regards,

Earl

[jkfoam]

Earl,

Yes, on that specimen on the left, the thin looking layer on the top is the operculum. The specimen on the right does not have an operculum. I agree that the specimen on the right does not show as much depth as those in the reference but it does have depth. The lack of appearent depth is testiment to my poor photographic skills. I recommend you do a GOOGLE Images search of Caleola sandalina for better photos of this species. Also the lack of depth in my specimen may be due to it being partially filled with matrix.

I don't know why the reference article says that the operculum are not preserved in-situe, Maybe it is so at the locations they studied but it can't be so at all locations where the species is found, witness the GOOGLE Images photos.

Compare my specimen without the operculum to the specimen (J) on page 27 in the article and to the specimen (A) on page 27 in the article. Specimen (J) does have an operculum. Specimen (A) does not have an operculum. Note that my specimen and Specimen (A) both have a similar feature at the middle of the hingeline near the top that is absent in the specimen (J).

Also note another characteristic and oddity of this coral is how weak or diminutive the septa are. This may be why this species was once thought to be a brachiopod!.

JKFoam

[Shamalama]

Great pictures and write up, JK. I had a blog post last year on this same topic and used that same article as a reference: http://viewsofthemahantango.blogspot.com/2010/09/calceola.html

Earl - I have pics of some specimens in my collection on there too so you can see how shallow the depth of the corallite can get and also a few with the lid on.

[Earl]

Thanks, Jim, for walking me through the photographs, both yours and the ones in the Galle and Ficner article. The gist of my confusion, of course, can be attributed to the fact that I don't have the fossil in hand under a loupe. The next time I visit, you'll have to show me your cool Calceola collection.

Per you suggestion, I did do a Google Images search on Calceola sandalina, which resulted in an abundance of helpful photographs. That is a most interesting species.

Also, thanks, Shamalama, for the link to your excellent blog entry on C. sandalina. It had additional references that I look forward to reading. (I've already looked a bit on eBay, and have the feeling that a modest-priced Calceola will be making its way to my collection in the near future.)

One thing that is interesting is how pronounced is the convexity of the operculum of Shamalama's Calceola, while jkfoam's specimen has a flatter, slightly concave-looking operculum.

Would this be due to pressure and distortion during fossilization? Sexual morphism? Age differences? Locality differences?

Very interesting stuff!

Thanks for drawing me into such an interesting thread about a species I had never heard of!

Regards,

Earl