The seasonal highs for the year have been sticking around after our initial arctic bursts. So, on a sunny Sunday I was back out to my Amherstburg/Lucas Fm spot. Some rocks I had written off as not as viable have turned out to be much more productive than I thought. Case in point would be another larger piece of armour stone to break down. It seems that, for each trip, there is usually that one rock that gives up the most goodies. So the focus here is on a single rock this trip. No doubt about this rock being from the Amherstburg Fm, as here is a Mystrocephala stummi (albeit in a rather poor state). Several fragments of the proetids Crassiproetus and Pseudodechenella appear as well, as is usual with this material. This looks to be another genal spine (possibly belonging to Acanthopyge contusa), but it seems to be curving up and out. The two tubercles on the right appear to be on some kind of axial ring? Huh? I'll need to clean this one up a bit more to figure out where this fits. I initially picked it up thinking it might be a thoracic segment. But the spiny processes seem identical in their orientation to previous genal spine fragments. A closeup and greyscale image of a fairly mashed lichid cranidium, likely Acanthopyge contusa with the tell-tale scattering of tubercles. This was the trip-maker. Positive and negative. As is usual in these rocks, the preservation is not ideal. Also, the amount of force needed to split these rocks sometimes results in unavoidable damage in the field where parts of the rock are pulverized forever with no chance of retrieving much more than dust and flakes. The state of the positive (left) had me utter an expletive, but let's focus on what this is and not fixate on its aesthetics. This is not like the many small (~1-1.5 cm wide) Acanthopyge contusa pygidia. This whopper measures 3.5 cm wide. Notice the middle of the pygidial axis and that it has a kind of prominent "bit" that looks like an oversized tubercle. That is actually the base of a spinal process. I've grey-scaled and boosted the impression side to better show the pygidial outline. So what is it? We can likely rule out Acanthopyge contusa on account of a few factors (the robust width of the ovoid pygidium, the additional number of pygidial spines --three on each side plus two smaller ones on the postaxial border, the presence of pygidial rib annulations). The pygidial morphology seems very similar to that of Terataspis grandis, with the exception of the short, backward curving spines. . So it's not quite a match for either Acanthopyge or Terataspis -- and yet those are the only two Devonian lichids reported in Ontario. So I tapped the expertise of our Fossil Forum trilobite master to assist in finding the right fit for this oddball. According to him, it may be a match with Echinolichas eriopis, listed as Lichas (Conolichas) eriopis in Hall and Clarke (1888): (Image from: Hall, J. & Clarke, J.M. 1888
Palaeontology VII. Containing descriptions and figures of the trilobites and other crustacea of the Oriskany, upper Helderberg, Hamilton, Portage, Chemung and Catskill Groups. Geological Survey of New York, Natural History of New York, Palaeontology: Volume 7:1-236. The spinal process, if complete, would have been quite long. Hall & Clarke report it as being in the "corniferous limestones," which was later revised to the Onandaga Fm. The equivalent strata in southwestern Ontario would be the Amherstburg/Lucas Fms so it fits the geologic age of the material. J.A. Fagerstrom draws from E. eriopis as the type specimen from which he distinguishes E. parallelobatus in the Formosa Reef (Amherstburg Fm), but Rolf Ludvigsen (1987) distinguishes the Formosa Reef lichid as being Acanthopyge contusa. It is not unreasonable to consider E. eriopis as appearing in strata equivalent rocks in Ontario given a shared cratonic basin with New York, but it is not yet reported in the literature, making this perhaps the first of its kind on this side of the border as far as I know, and according to the literature. So, once again a lichid fragment has me all excited. It means there are still hidden surprises in these rocks after three months of diligent field work. As it looks like tomorrow will be warm and sunny -- perhaps the very last warm and sunny day left in 2019 -- I plan to return to the site and continue my exploration in the hopes of finding more. Comments are closed.
|
Kane Faucher
Archives
February 2024
|