I actually didn't anticipate posting again until after the upcoming dig at Penn Dixie, but two consecutive days of collecting at my riprap hill nearby has brought some excitement into the collection.
For some many years of collecting from this place, I knew a lot of that material was shipped in. My assumption was that it was more locally sourced (and thus mostly Dundee Formation). Many of the fossils I was pulling from there are reported as part of the Formation. Until I found one that wasn't. But also peculiar were the fossils I was NOT finding that are typical of the Dundee. Not even typical of the Hamilton Group (Arkona, Widder, etc.). I guess I was blinded by what I assumed to be true as opposed to facing the cold, objective truth as evidenced by the biota.
Yesterday I found this:
And a closer view:
So, fairly eroded away, leaving only a cast/mold of a part of the cephalon of this trilobite. But it is a whopper. I don't really find Devonian trilobites approaching this size (Ordovician ones, yes). So I posted it on the fossil forum, and our resident trilobite expert Scott weighed in and said it was either Trypaulites calypso or Coronura aspectans, both of which are recorded in the Dundee Formation. There isn't a lot of diagnostic details to go on, but you can make out the distinctive occipital ring and the deep furrow on the lower left of the cephalon. So, we know it could be one of these two dalmanatid trilobites, right? Hold on...
I spent another four hours today carefully taking on any rock on the hill, splitting it, and seeing if anything interesting might come up. As I've pulled Basidechenella from there in the past, I was hoping for more than just another pygidium. There was this one greyish, round rock about the size of a softball lying there. I wasn't going to bother with it as I thought it would just be another blank, or just one of those dense balls of largely erosion-resistant rocks where you have to bring out the big guns to split it and find only a tiny smattering of brachiopods. But I was already sitting in the gully breaking rocks, not in a hurry to get back up, so I thought, "why not while I'm here?" So glad I did, because this was the rock that became the key for determining the origin of many of the rocks on this hill - and thus explains the biota.
Without double-checking, I thought, "ok, another Trypaulites specimen. That's pretty lucky to find two in less than 24 hours." I assumed from my patchy memory of looking at a Trypaulites in the Ludvigsen volume. I really should have looked more closely to confirm that assumption. I was still tickled pink to find another specimen. But when I posted it, Scott chimed in again by saying it was NOT a Trypaulites. It was instead a Anchiopsis anchiops, and those only occur in the Bois Blanc Formation.
The only significant outcrops of Bois Blanc are hundreds of kilometres away, in the Niagara escarpment. So what the heck was it doing here in London, Ontario? There is no doubt on the ID of this one. I checked the literature and, Scott was bang on as usual. Scott then revised his earlier ID on the big cephalon above and said it was also Anchiopsis, but missing the distinctive occipital spine. Here are the images Scott presented for comparison with what I found:
So I went back and took a longer look at the several finds I've made in this location over the past four years. I compared those to what was listed in the Bois Blanc, and there they all were. What didn't tip me off is that there is a great deal of overlap of species between the Bois Blanc, Dundee, and other formations in the Devonian. For example, the trilobites Eldredgeops rana and Basidechenella sp. appear in both formations. I had reasonably assumed that the materials delivered and dumped to make this hill would be more locally sourced, as that would seem more economical. And thus I stuck with my first assumption that everything I pulled out of the hill was likely Dundee Formation or thereabouts. But the presence of the Anchiopsis throws all of that out, as it ONLY appears in the Bois Blanc. So, in sum, I am collecting from the Niagara escarpment pretty much in my backyard. The stratigraphy is as follows in descending order of age:
The Bois Blanc is the second oldest Devonian strata, underlain by the Oriskany. According to Armstrong and Dodge (2007):
"The Oriskany Formation is disconformably overlain by the grey to brown, very cherty, fossiliferous, argillaceous limestones and dolostones of the Bois Blanc Formation (Uyeno et al. 1982). Thin beds of glauconitic quartz sandstone that occur near the base of the Bois Blanc are assigned to the Springvale Member (this member is not shown on OGS maps). The Bois Blanc Formation outcrops and subcrops in a narrow belt from Fort Erie on the Niagara River to MacGregor Point on Lake Huron. North of Norfolk County outcrops of this unit are sparse."
According to Ludvigsen (1979), there are five major genera of trilobites in the Bois Blanc: Phacops [Eldredgeops] rana, Basidechenella sp, Crassiproetus sp, Anchiopsis anchiops, and Terataspis grandis. I have so far collected three of the five.
I could dare to dream in finding, say, one of the most famous of all trilobites - a monster at over 50 cm or more, for which only fragments have been collected, Terataspis grandis:
I'm not saying I will find one, only that it is now possible. The probability of finding one is entirely different story.
So I've now had to reassess my assumptions that have been engrained for years, and I am delighted to be wrong and to welcome yet another new species into my collection - quite a few with the season just beginning. I'll sign off with a pic of some other finds of the day: