The coiled-and-ribbed fossil mystery deepens on Hiiumaa

KÄINA, ESTONIA–It has been a rainy day on the Estonian island of Hiiumaa. The Wooster geologists stayed inside most of the day to work on their Geological Society of America abstracts. Bill Ausich and his Ohio State University team, though, returned to the Hilliste Quarry and continued to collect fossils. To our great surprise, they picked up two more specimens of that strange planispirally-coiled shell that Richa found on July 13. One is shown above, and the “venter” view is shown below. All specimens were found in a yellowish unit that matches the matrix in the fossils (although we’ll check in the lab to make certain). These new finds reduce the chances that the fossils are a product of “site contamination” in which a visitor discards specimens from a previous trip, often to make room for new ones. That is still a possibility, but an increasingly remote one.

So what are these? They look very much like Mesozoic ammonites, all the way down to deflections of ribs along the periphery as you might be able to see above. (Specimen photography in a hotel room has its challenges.) The earliest ammonoids, the larger group that contains ammonites, appear in the Devonian (the period after the Silurian), so it is unlikely we are looking at that group. They are certainly mollusks, though, so most likely gastropods (snails) or nautiloids. Coincidentally enough, Wooster alumnus James St. John has a webpage with a photograph of a coiled, ribbed nautiloid known as Graftonoceras, which you will note has many similarities with our mystery critter. The specimen he photographed is in the museum at, of all places, Ohio State!