Wooster’s Fossil of the Week: A horn coral from the Upper Ordovician of Indiana

This week’s fossil is a very common one from the Whitewater Formation (Richmondian, Upper Ordovician) exposed near Richmond, Indiana. It was collected, along with hundreds of other specimens, during one of many Invertebrate Paleontology field trips to an outcrop along a highway. The fossil is Grewingkia canadensis (Billings, 1862), a species my students know well because many made acetate peels of cross-sections they cut through it.

Grewingkia canadensis belongs to the Order Rugosa, a group commonly called the “horn corals” because their solitary forms (as above) have a horn-like shape. Children often think they are dinosaur teeth! It is so common in Richmondian rocks that it is sometimes used to indicate current direction. Its robust skeleton provided attachment space to many encrusting organisms, and it often has multiple borings in its thick calcite theca.

We believe that the rugose corals lived much like corals today. They sat partially buried in the sediment with the wide end of the skeleton facing upwards. A polyp sat inside the cup-shaped opening, spreading its tentacles to catch small organisms swimming by.

Grewingkia canadensis has a complicated taxonomic history. It is likely also known as Streptelasma rusticum, Grewingkia rustica, Streptelasma vagans, Streptelasma insolitum, and Streptelasma dispandum. G. canadensis is characterized by cardinal and counter septa (the vertical partitions inside the coral skeleton) that are longer than the other major septa throughout ontogeny (growth).
The handsome man shown above is, of course, a paleontologist. This is Elkanah Billings (1820-1876), Canada’s first government paleontologist and the one who named Grewingkia canadensis. (He originally placed it in the genus Zaphrentis.) Billings was born on a farm near Ottawa. He went to law school and became a lawyer in 1845. But he loved fossils and in 1852 founded a journal called the Canadian Naturalist (and Geologist). In 1856, Billings left the law and joined the Geological Survey of Canada as its first paleontologist. He named over a thousand new species in his career, and is best known for describing the first fossil from the Ediacaran biota — a critical time in life’s early history. The Billings Medal is given annually by the Geological Association of Canada to the most outstanding of its paleontologists.


Billings, E. 1862. New species of fossils from different parts of the Lower, Middle, and Upper Silurian rocks of Canada. Paleozoic Fossils, Volume 1, Canadian Geological Survey, p. 96-168.

Elias, R.J. and Lee, D.J. 1993. Microborings and growth in Late Ordovician halysitids and other corals. Journal of Paleontology 67: 922-934.

Elias, R.J., McAuley, R.J. and Mattison, B.W. 1987. Directional orientations of solitary rugose corals. Canadian Journal of Earth Sciences 24: 806-812.

About Mark Wilson

Mark Wilson is a Professor of Geology at The College of Wooster. He specializes in invertebrate paleontology, carbonate sedimentology, and stratigraphy. He also is an expert on pseudoscience, especially creationism.
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11 Responses to Wooster’s Fossil of the Week: A horn coral from the Upper Ordovician of Indiana

  1. Katy Johnson says:

    I found one of these in my backyard.

  2. Tim brock says:

    We found 20 of these different sizes in a creek in sw ohio how old are they an are they collectable just curious we were looking for artifacts at the time

  3. Mark Wilson says:

    Likely Ordovician in age, Tim.

  4. K Kieser says:

    Numerous specimens found in Washington County, Indiana creek following flooding on 5/19/17.

  5. brenda says:

    Found a small cluster embedded in rock along with numerous shells in our backyard days ago… even though they are common, it’s rather neat to have something around that is hundreds of millions of years old.

  6. We found quite a few of these yesterday in a canyon just east of Taos, NM. It was at approximately 8600 ft. How interesting that three of the four of us went to COW and now find your page. Thank you for the information.

  7. Mark Wilson says:

    Glad you and your friends found our site, Frances!

  8. Dustin says:

    I have lots of these. Are they worth anything? My wife finds them all the time in the creeks in southeast indiana.

  9. Mark Wilson says:

    Hi Dustin. We can’t speculate on values of fossils. I can at least say these cool fossils are very common in some areas.

  10. Sterling Mullins says:

    Found my first while digging post holes on my property last fall. Took fossil to Hillsdale College Biology Department. BIG thank you to Dr. Anthony Swinehart, professor of Biology, for taking his time to educate me on what I had found. Dr. Swinehart gave me a tour of his lab, showed me samples of his field finds, told me about Paulding, Ohio, let me see my find thru his micro-scope, and generally lit a flame to continue hunting, which I will do. I am not a young student, just an older retiree with time to pursue interesting finds. Thank you for this site.

  11. Mark Wilson says:

    Well done, Sterling. You did all the right things with your find!

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