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For nearly twenty years, I've collected some strange fossils from a unique Pennsylvanian deposit in northeast Kansas. I've been pondering them to this day, and I'm still drawing a blank. I first found this slab: The bold segments caught my eye. I then noticed that they have a branching habit. I assumed they were sponges, but then I found this: This one is also segmented and bifurcated, but it forms a nearly continuous surface. Perplexed, I looked at them up close. They seem to form thin (0.5 mm), leaf-like sheets (i.e. thalli). I nicknamed these 'pahoeids', because they resemble pahoehoe lava flows. Some appear to be featureless sheets. This one, with an attached Coelocladia sponge, was fractured. This may give a clue to its original consistency: Here, the tips of some 'pahoeids' seem to be present: (For scale, the squares on the couch cushion are 18 mm across. I took the photo after I found the rock back in October.) Here's the same piece from the side: This chunk demonstrates how the 'pahoeids' bind the sediment. They are so abundant, they actually form beds and lenses of limestone within a shaly matrix. The few 'pahoeids' I've found in situ were on the undersides of slabs. The top surfaces of the organisms are never cleanly exposed; they are always locked in matrix. Also, encrusting bryozoans and brachiopods are often attached to the surface. All of this tells me the 'pahoeid' thalli were once suspended above the substrate. Some more specimens.... Here's another bifurcated sheet: And another: These seem to be 'juveniles': These form long, parallel branches: This limestone nodule contains what are possibly undeformed 'pahoeid' thalli: Note how the orangish sediment seems to 'fill' the little 'cups'. An underlying limestone bed seems to contain fragmented 'pahoeids' as well as several Amblysiphonella sponges: As to the nature of 'pahoeids', the only thing I can think of is some form of red algae, perhaps similar to present-day Mesophyllum or Peyssonnelia. It's possible they could be something akin to the Pennsylvanian Archaeolithophyllum. So far, I've been unable to discern any fine internal structure. If they indeed are phylloid algae, this will be the only example I know of where they are preserved in shale, let alone left with readily observable morphology. For the sake of this post, I'm lumping all these thalli forms together as 'pahoeids'. They very well may represent a number of different organisms. Here's a stratigraphic chart based on my sketches and observations in the field: Going by the lithology, I've guessed that the 'pahoeids' lie in the Frisbie Limestone (which is, stratigraphically, a transgressive limestone). As far as I know, the beds could, instead, belong to the upper Liberty Memorial Shale. The 'pahoeid' beds are a complex bundle of shale and impure limestone beds and lenses. At the top, there is limestone made up of fine fossil debris. There is pyrite and glauconite present througout the unit. Below the Frisbie is the Liberty Memorial Formation, which is a typical shallow marine/non-marine shale. It is medium gray, sandy, and contains thin beds of sandstone in places. Trace fossils, including Conostichus, are present. Above the 'pahoeid' beds is the Quindaro, a deeper-water shale. Fossils include profusely abundant sponges (Heliospongia ramosa, Maeandrostia, and especially Fissispongia), as well as some small crinoids, bryozoans, and brachiopods. The thick limestone above the Quindaro appears to be typical Argentine (which is regressive, for those taking notes ), but the strata in the area are anything but routine. These sponge-'pahoeid' deposits are near the edge of a large algal reef build-up in the Wyandotte Formation. Less than a half mile away in either direction, fossils at this horizon are sparse to absent.