Mystery Invertebrate
Identified as a stromatoporoid
Aulacera plummeri (1843)

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I recently purchased an invertebrate fossil that I can't identify. It's from Southeastern Indiana and was collected between Elrod and Dillsboro so that places it in Ripley or Dearborn County which is divided down the middle by a Silurian-Ordovician boundary going west to east.

The Silurian deposition near Elrod is the Osgood Member and the Ordovician fossiliferous zone closer to Dillsboro is the Maquoketa Group.

I've gone through the Treatise, Index Fossils of North America, and done exhaustive searching on the net but I can't seem to find anything close.

There has been recent speculation that it may be a mineralized concretion from a prominent member of the Yale Geology and Geophysics Department, although he had to concede that he could not explain the "starburst" pattern, only offering that the starburst does not seem to match well with any known skeletonizing invertebrate.

If you look closely at photo 5 you can see the reticulated surface, additionally photo 6 of the cross section seems to exhibit a calcified layer of integument. I'm still leaning toward invertebrate at this point.

The cross section reminds me of a crinoid columnal, other than that my best guess is some strange crinoid sp., it measures 9" x 2.5" x 1.5" another aspect that has me completely stumped.

My profound gratitude for any reasonable conclusions.

Best regards,
Scott Morrison


Looks like an ironstone nodule. Perhaps it is from a trace fossil. I don't see any skeletal structure found in typical invertebrates from that time period.

This could be the in-filling of a burrow structure but I would definitely say it is of animal origin. You're right about the surface texture and also note the impressed collar. Are there 8 ridges? Legs.....a.k.a. cephalopod soft body mold?

There are starfish burrows (resting places) similar to this in KS Penn.

This specimen would be a great candidate for a newsletter of the Paleontology Society (Palaeos?) that has an "Unknown Fossil" section. Lots of experts view it. I'll share the link once I find it. They would want it placed in context of formation it was from though.

This is an upper Ordovician stromatoporoid or [sponge] if you prefer. In the older literature there were several genera, and varieties or sub-species depending on the presence and or absence of ridges and bumps. One "variety" had both - Beatricia nodulifera var intermedia.

You have acquired a really fine example.

Goggle the Genus I have provided for an up dated pedigree.

It looks entirely concretion. The concentric layers look just right to be an ironstone or siderite concretion, but the elongate shape and ridges are really strange. It could well have formed around a burrow. If it were possible to look at it directly and under magnification, it would be much easier to make some decisions.

Mystery Fossil Identified
April 29, 2009

My sincere thanks to everyone who took the time to evaluate the "Mystery Invertebrate". Special mention to Tom Kammer who supported my initial suggestion of poriferan and to Charlie Oldham who has unraveled the riddle at long last. Thanks to Charlie's astute attribution of stromatoporoid I promptly contacted Barry Webby who is the coordinating author for the upcoming Treatise Volume 4 Part E Porifera (revised) chiefly Stromatoporoids, Chaetetids, and Archaeocyathids. I have been informed by one of the editors at KSU that this massive volume may actually release in as many as two or three parts. Respectfully to all who suggested ironstone nodule, perhaps we can now exhale a sigh of relief and defer to the preeminent scholar of Stromatoporoidea.

Scott Morrison

I appreciate seeing those fine photos of the fragmentary remains of the some large, tree trunk-like, skeletons of cylindrical stromatoporoid sponges that you have collected from the Late Ordovician of southeastern Indiana. These forms are probably best currently assigned to the genus Aulacera of the Family Aulaceratidae (Order Labechiida), and I suggest they are from the Ordovician side of the boundary because in their many other occurrences in North America, Siberia and Novaya Zembla (Arctic Russia), China and Tasmania, at least where the best stratigraphic records exist (especially Anticosti Island in Eastern Canada) they are never found in the overlying Silurian; indeed they are widely believed to have become extinct during the the major Ordovician-Silurian mass-extinction event. Use of the generic name Aulacera Plummeri 1843 is preferred on grounds of priority over Beatricea Billings 1857. Even Schuchert's (1919) sketch of Plummer's original specimen (that came from Indiana) shows the prominent spiraling longitudinal ridges that your specimens display - and they can be seen in the type species A. plummeri Galloway & St Jean 1957 (see illustration in J.J. Galloway's 1957 paper entitled "Structure and classification of the Stromatoporoidea" - see the Bulletin of American Paleontology vol 37 (No.164), p. 470 (Plate 37, fig. 1a). There are some other species like A. radiata, and A. undulata that also exhibit the prominent longitudinal ridges.

More work needs to be done on the taxonomy of the cylindrical Aulaceras. Paul Copper (Laurentian University, Sudbury, Canada) has the best collections and plans still to describe the taxonomy of the Anticosti faunas - in places on Anticosti they form "fossil forests". He wrote a useful paper in 1994 entitled "Paleoecology of giant Late Ordovician cylindrical sponges from Anticosti, E Canada" with D Cameron as first author, in "Sponges in Time and Space" edited by RWM van Soest & others published by AA Balkema (Rotterdam and Brookfield, VT), pp.13-21.

I hope that helps. The Treatise, part E (sponge) volume that includes the section on the Class Stromatoporoidea is now being edited and will hopefully be published in a year or so.

Barry Webby

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