Below I’ve included a number of additional photographs that I took in 2012 of concentric structures in the rocks at Tackaberry’s quarry.
When I first looked at the fossils I did ask a few geologists for their suggestions. One suggested medusae. Another suggested poorly preserved Gastropods. To understand why their suggestions were reasonable, it helps to know that I showed them photographs of poorly preserved specimens from the large blocks surrounding the quarry, before I started looking at specimens revealed by fresh blasting and photographed the specimens shown above and in my September 13, 2013 posting. In addition, there are Gastropods in the March Formation and I’ve found broken specimens of Gastropods in loose blocks at Tackaberry’s quarry.
I now believe that most of the specimens I found were holdfasts, as the specimens are identical to Ediacaran fossils that have been identified as holdfasts. While some might be medusae or badly preserved Gastropods or cross-sections of burrows, these would be in the minority. Most appear to be Aspidella.
My photographs may not be representative of all of the circular structures that I found, as I took photos of the most circular and best preserved specimens that I found. There were many others that were poorly preserved, some that were more elliptical than circular, and some that were not complete circles, that did not get photographed. Admittedly, that does bias the identification, and if I were to go back I would photograph ones with different shapes. (I would also look for fronds.) Interestingly, there were literally hundreds of specimens that did not get photographed.
Since I published the blog posting a tremendous number of authors have written on concentric discoid fossils, and I’ve included a number of those papers below, plus others that I found interesting, and others that included photographs of concentric fossils that are similar to those found in Tackaberry’s quarry. The two Youtube videos -- Cope, 2018 and Burzynski et al. 2017a -- are worth watching.
What is interesting about the specimens that I photographed is that they are all loose specimens resulting from blasting at the quarry. Accordingly, it is as likely that I photographed a bed top as a bed sole. Another interesting point is that while everyone would likely concede that the top beds at the quarry belong to the March Formation and are of Ordovician age, it is not clear how old the lowermost beds are. The uppermost beds are variously coloured siltstones, mudstones and sandstones, and at least two uncomformities are visible. The bottom beds are a reddish sandstone, and at least one Lanark based geologist has suggested that the bottom beds are Potsdam sandstone (which is Cambrian in age). Interestingly, the second time I was at the quarry it was in the company of Dr. Bill Arnott and Dr. Al Donaldson. At that time I picked up a sample of the lowermost sandstone and suggested to Dr. Donaldson that it was Potsdam sandstone. He cautioned me that Precambrian sandstones can be as well preserved as Cambrian sandstones.
I did not find any concentric fossils in the red sandstone.
Christopher Brett
Ottawa
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