On a warm Sunday, I made it up north to the Formosa reef road cut. It was likely my last major trip of the season, and it certainly did not disappoint. I had read and re-read the research papers on this site, which is the type locality for this massive biohermal feature situated in the Amherstburg Formation. The two main sources would be these: Fagerstrom J.A. (1961) The Fauna of the Middle Devonian Formosa Reef Limestone of Southwestern Ontario. Journal of Paleontology 35.1: 1-48. Ludvigsen, R. (1987) Reef trilobites from the Formosa Limestone (Lower Devonian) of southern Ontario. Can. J. Earth Sci 24 The road cut itself is quite sizeable, and represents a massive knoll. My focus was on the windward side of the reef where skeletal debris would have been swept in by currents. The wackestones and stromatoporoidal boundstones contain a staggering amount of faunal diversity. There were certainly corals, both tabulate and rugose, but just about everything else including gastropods, brachiopods, bivalves, crinoids, and a profusion of different kinds of nautiloid. Of course, there are trilobites as well, but none have been reported complete. The cut runs along both sides of the road, with additional access from behind the reef as well. And it is fairly tall, too. There really is an abundance of material, much of it more fossil than matrix. What we collected in 2.5 hours is hardly representative of what this material has to offer. This was the main focus of the visit: the windward side of the reef. Every single rock was full of fossils -- and not just coral. The weathering and oxidation made for some interesting steinkerns. Although the material looks quite dense as it weathers a dark grey (inside is a creamy off-white with orange oxidation), it breaks apart easily. Unlike a shale that will split in sheets, this material breaks apart in angular chunks, a little like the material at Hungry Hollow (but much clearer to see what's inside). Pictured above are some usual surface features including a nautiloid cast and a chunk of tabulate coral. The material was quite easy to work with. It took just a matter of minutes to start piling up stuff to bring home. Every rock and every split had something new and interesting. Of course, I wasn't too picky about what I was collecting as this was my first visit. It is a little like someone's first visit to Arkona when the tendency is to fill buckets with horn coral! Nautiloids were abundant. Weathering made for some very neat looking cross-sections, as pictured here with the exposed siphuncle. This would be my first cyrticonic nautiloid. Quite neat! I found a number of these small rostroconchs, too. Of course, I'm pretty full up on rostroconchs from my local imported fill site. Plenty of gastropod steinkerns, some flat and others high-spired. Some also contained large crystals inside. There were also plenty of brachiopods of varying types, but I didn't manage to collect them this round. Deb found this large, double-valved bivalve that popped free of the matrix. I had found a smaller more beat up one, so this example was the clear winner to be taken home. Corals, corals, everywhere. Some had very nice crystallized cross-sections among the rugose variety, but we left those in the field. Despite this being a reef, the corals were less "in the way" compared to the Hungry Hollow Member at Arkona; many more types of fossils were given equal billing. Also not pictured were the crinoidal hash. So what about the mud bugs? Ludvigsen reported that trilobites were rare in this material, and that is not something I could confirm from experience; just about every rock had some kind of trilobite fragment, be it a pygidium, cranidium, or librigena. By far the most abundant species would be Crassiproetus crassimarginatus (subspecies brevispinosus). Pictured above is just one (exfoliated) example of about ten or more I brought home, leaving about as many or more in the field! Ludvigsen's text reports 222 fragments found over a matter of a few years; I could probably acquire that many in a full day there. Of course, no complete example has ever been reported, and it is unlikely due to the nature of the depositional environment. One has to be content with partials. If Crassiproetus is the most abundant trilobite represented there, the second most would be Mannopyge halli. Pictured here is a rather beat up version, and it was odd I only found the one -- but then I may have missed a few in the haste to make the most of our short time at the site. You can see the pygidial nodes like a necklace along the right edge. In third place for relative abundance would be Mystrocephala stummi. I was pleased to find this fairly nice example which looks a bit more well articulated and robust than the more flattened examples I've found at my local spot (yes, I found a second fragment a day before this trip!). No traces of the other two most rare trilobites in this material (Acanthopyge contusa, and Harpidella sp. of which the literature reports a single cranidia).
Overall, this was an incredible trip filled with new surprises. Although I had to miss out on Bowmanville this autumn, this certainly was a significant event of the season. I do plan on returning there again, either later this year or more likely in the spring. I hardly did this site justice as there is still so much to explore. Comments are closed.
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Kane Faucher
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February 2024
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