This beautifully polished fossil looks like half of an antique bowling ball. Normally I hate polished fossils because the external details have been erased, but in this case the smooth surface reveals details about the organisms and their relationship. We have here a large colonial rugose coral with a smaller tabulate coral embedded within it. The specimen is from the Devonian of Michigan. It may look familiar because it is a large “Petoskey Stone“, the state stone (not fossil!) of Michigan. The large rugose coral is Hexagonaria percarinata (Sloss, 1939).
In this closer view you can see the multiple star-like corallites of this coral. Each corallite held a tentacular feeding polyp in life. The radiating lines are thin vertical sheets of skeleton called septa. The corallites in this type of coral shared common walls and nestled up against each other as close as possible. In the lower center of the image you can see a very small corallite that represents a newly-budded polyp inserting itself as the colony grew. If rugose corals were like modern corals (and they probably were), the polyps were little sessile benthic carnivores catching small passing organisms with a set of tentacles. They may also have had photosymbionts to provide oxygen and carbohydrates through photosynthesis.
In the midst of the rugose coral is this irregular patch with another type of coral: a tabulate coral distinguished by numerous horizontal partitions in its corallites (and no septa). It was likely a favositid coral, sometimes called a “honeycomb coral”. It was clearly living in the rugosan skeleton and not pushed into it by later burial. Note, though, the ragged boundary between the two corals. The rugose coral has the worst of it with some corallites deeply eroded. What seems to have happened is that the rugose coral had an irregular opening in its corallum (colonial skeleton) after death and the tabulate grew within the space, eventually filling it. The tabulate likely stuck out far above the rugose perimeter, but the polishing shaved them down to the same level. This is thus not a symbiotic relationship but one that happened after the death of the rugose coral. [See comments, however!]
The rugose coral species, Hexagonaria percarinata, was named in 1939 by Laurence Sloss, a famous sedimentary geologist with an early start in paleontology, but it is best known through the research of Erwin Charles Stumm (1908-1969; pictured above). Stumm was at the end of his life a Professor of Geology and Mineralogy and the Curator of Paleozoic Invertebrates in the Museum of Paleontology at the University of Michigan. Stumm grew up in California and then moved east for his college (George Washington University, ’32) and graduate (PhD from Princeton in 1936) education. He taught geology at Oberlin College up the road for ten years, and then moved to Michigan to start as an Associate Curator and Assistant Professor. I knew his name because in 1967 he was President of the Paleontological Society. He is said to have been a dedicated teacher of undergraduates and effective graduate advisor. It is fitting that his name is connected to such a popular fossil as Hexagonaria percarinata.
Sloss, L. 1939. Devonian rugose corals from the Traverse Beds of Michigan. Journal of Paleontology 13: 52-73.
Stumm, E.C. 1967. Growth stages in the Middle Devonian rugose coral species Hexagonaria anna (Whitfield) from the Traverse Group of Michigan. Contributions from the Museum of Paleontology, The University of Michigan 21(5): 105-108.
Stumm, E.C. 1970. Corals of the Traverse Group of Michigan Part 13, Hexagonaria. Contributions from the Museum of Paleontology, The University of Michigan 23(5): 81-91.
Yet another fascinating fossil. I wonder whether the juxtaposition of favositid and rugose coral could be due to pressure solution? The irregularity and apparent presence of clay at the junction favour this alternative interpretation, although you would probably need to section the specimen perpendicular to its surface to find the right answer.
I like that idea, Paul. I would think there would be additional evidence elsewhere in the rugosan, but pressure solution is sometimes strange. Your hypothesis would explain the cut margins of the corallites. As you say, only a perpendicular cross-section will solve the problem, and that’s not likely to happen!
I like your fossil of the week web pages. I recently/presently went on a Tabulate coral collecting trip to Percy Priest Lake TN and found what I believe is a Roguse coral (horn coral) inside a chunk of Tabulate coral. While looking online I saw your “interlocking rugose and tabulate coral” Fossil of the week page. I was hoping you could confirm that this is really a horn coral inside a cave in the Tabulate coral.
Next week I’ll be in Mayfield Hts for the week visiting family.
I have a bunch of tabulates and what I believe are Acrocyathus from Kentucky, plus others. I would be interested in learning more about if you have the time.
I post on pinterest under “gregcohen_fse” or “Fossil Coral from Tampa Bay Florida”. In case you are interested in seeing some of them. There is a folder for Tennessee and Kentucky pieces.
I’d be interested in donating some if you are interested to thank you for any knowledge you can pass on to me about my collection.
Hello Greg: I love looking at fossils and rocks. I’m happy to assess any photos you send me by email (mwilson at wooster dot edu). I’m afraid I can’t have visitors next week because we’re in a crunch time with various academic projects, so email is the best way to communicate for now.