Gull River Formation <not Shadow Lake> Trilobites

[clfossils]

Gull River Formation Trilobite Parts

By Corey Lablans

A few weeks ago I came across an excavation site where they are building a new road. Examining the layers, there is one in particular layer that proves to be quite fascinating, a dark shale layer. The downside is that this layer is very limited in exposure, although I have found some unique stuff. Here is what I found the other day:

Bathyurus (Trilobite) - Gull River Formation of the Black River Group, Kingston, Ontario

Bathyurus or Raymondites (Trilobite) - Gull River Formation of the Black River Group, Kingston, Ontario

Raymondites? (Trilobite) - Gull River Formation of the Black River Group, Kingston, Ontario

(5 mm x 7 mm)

Raymondites? (Trilobite) - Gull River Formation of the Black River Group, Kingston, Ontario

(5 mm x 7 mm)

Bathyurus or Raymondites (Trilobite) - Gull River Formation of the Black River Group, Kingston, Ontario

(3 cm x 2 cm)

Bathyurus or Raymondites (Trilobite) - Gull River Formation of the Black River Group, Kingston, Ontario

Link to the past fossils from the same site (cephalopod, brachiopod and conulariid): http://www.thefossilforum.com/index.php?/topic/36307-shadow-lake-formation-cephelopod-and-others/

THANK YOU

Edited April 8, 2013 by clfossils

[Nandomas]

THANK YOU for sharing this page. i didn't know anything about this place/formation

You did great photos of the trilo pygidium details

Edited April 8, 2013 by Nandomas

[Northern Sharks]

How sure are you that it is Shadow Lake? I may be wrong, but I thought it was pretty barren (it is around here anyway, Ottawa might be different) Could it be Gull River instead?

[clfossils]

I will admit, I am hesitant but it is quite far down in the Kingston limestone. A few people have said that the Shadow Lake Formation is prety scarce, so I am having second guesses. I have a few papers to read about the Shadow Lake Formation that I will read once I get some time. It is possible that it is the Gull Formation.
The layer that I am collecting from is only about 5 inches and everything else is barren limestone above and below. Trying to figure out exactly where the barrier is drawn between the Gull and Shadow Lake.

[piranha]

Hi Corey,

Checking over the charts and tables there are at least 17 localities for Shadow Lake (Lowville equivalent NY) in Ontario and only a 3-4 of those contact the Gull River Fm. The 1943 paper lists approximately 36 Ontario Black River exposures that are certainly worth investigating further. One correction for your database is the last trilobite you posted is an Isotelus. An important key for differentiating Bathyurus and Raymondites is the eye lobe position is farther back on Raymondites. Whittington placed less importance on occipital and pygidial spines as both genera have species with those morphologies.

Whittington, H.B. (1953)

North American Bathyuridae and Leiostegiidae (Trilobita).

Journal of Paleontology 27(5):647-678

[FossilDAWG]

I think the last specimen is an Isotelus, not Bathyurus/Raymondites.

I also have doubts about the Shadow Lake correlation. These specimens seem pretty typical of the Gull River. The Shadow Lake is a trangressive deposit, a shaley unit deposited in a shallow high energy environment as the sea advanced over the land. It is very poorly fossiliferous, and the material that does occur is usually fragmented. Articulated trilobites suggest a deeper water, less energetic environment.

You definitely seem to have hit on a productive site, well worth keeping tabs on as long as it is available.

Don

[clfossils]

Thank you Don and piranha. I really need to get around to reading some stuff on the Shadow Lake Formation, although being in the middle of exams I should not distract myself, ha ha ha, but this is so much more interesting. I would like to thank both of you for all the information that you have been providing me. It is fascinating going through the various species and trying to pin point exactly what formation I am collecting in, use to never think these things when I was collecting.

Also I find it very difficult to pin down exactly what species of trilobite I'm looking at: Ceraurus, Bathyurus, Raymondites or Isotelus. I guess it seems like Bathyurus and Raymondites are very similar, but it is all in the eyes that separates each species. Ceraurus and Isotelus seem pretty distinctly different.
I was kind of hesitant to call the last one Isotelus because of the pygidium not being overly smooth.

Any recommendation on the best way to tell the species?

[piranha]

I can send you some additional reference material to assist with the IDs. It's only a matter of collecting time and research before the different trilobites become easily recognizable.

[clfossils]

Thank you piranha.

I did end up getting sidetrack and did some research into the Shadow Lake Formation. The fossils I have been posting are from the Gull Formation. I read a report on the Shadow Lake Formation by the Geological Survey of Canada 1998. As Don mentioned, the Shadow Lake Formation was deposited during a time of a shallow marine environment, with periodic episodes of high energy. Coarser grained material should be present. Located below the shale is distinct limestone of a moderate to deep marine environment and no quartz grains are present, eliminating any coastal influence.

The lithology for the Shadow Lake Formation should be: medium to coarse grained glauconitic sandstones deposited in a low energy shallow marine system, dark green to black laminated siltstone and fossiliferous medium to coarse sandstone broken up by periodic high energy event. I guess there are a few outcrops along HWY 35, as well as up at Marmora Mine.

THANK YOU

[clfossils]

Does anyone know how to change your post topics?

[Auspex]

Does anyone know how to change your post topics?

Edited, but I left a Shadow Lake reference to avoid confusion, given the ensuing discussion.

[clfossils]

Awesome, thank you Auspex.

[Northern Sharks]

Not to nit-pick, but the formation is Gull River, not just Gull if anyone wants to update the title again

[piranha]

For anyone interested please send a PM as I just received this PDF:

Brookfield, M.E., & Elgadi, M. (1998)

Sedimentology and paleocommunities of the Black River and

Trenton limestone groups (Ordovician), Lake Simcoe area, Ontario.

Canada Geological Survey - Field Trip Guide 6:1-35

Additionally, the author has recommended this unpublished thesis:

McFarlane, R.B. (1992)

Stratigraphy, paleoenvironmental interpretation, and sequences of

the middle Ordovician Black River Group, Kingston, Ontario, Canada.

Queen's University, Kingston, Ontario PhD Thesis 165 pp.

[piranha]

Hi again Corey,

Check your email as I just sent you the Black River field trip guide and thesis referenced above. Two corrections as its actually a MSc thesis and over 300 pages with many correlation charts / tables. Also included is a must-have photo atlas of Ontario trilobites to assist with the IDs. Good luck on all of your fantastic future finds!

[Shamalama]

Pretty cool finds for a road cut.

[FossilDAWG]

Corey,

Something to consider when IDing trilobites (or anything else) is the difference in appearance between the internal and external aspects of the shell. In your Isotelus specimen, you'll notice that the actual shell material is mostly attached to the right-hand specimen in your photos above. That means the left hand specimen is showing a mold of the inner surface. In Isotelus, the outer surface of the pygidium (the "tail" segment) is smooth, though if you look closely in some species you can see a faint raised area along the axis. However, the axis, some faint evidence of the segmentation, and the marginal area where the carapace folded under can all be seen more clearly on the underside of the trilobite, or (as in your specimen) an impression of the underside. Internal molds can differ a lot in appearance from the exterior of the trilobite, and learning the difference is all part of the fun of learning to know your trilobites.

Don

[clfossils]

Thank you Don, I'm thinking that it will be a matter of collecting more, trying to figure out what species I'm looking at. Over time the various differences in details will sink into my head and I will learn. Truthfully, before I even started trying to figure out these fossils I didn't know many trilobite species, yet along many species of other classes. This site and well knowledgeable individuals such as yourself are making me love paleontology more and more.

[piranha]

I found the faunal lists for the Gull River and Shadow Lake trilobites. Only 3 bathyurids are listed for Shadow Lake with B. extans found in both Gull River and Shadow Lake. It may be possible that you collected at one of the contact points but the Isotelus is definitely a Gull River trilobite. If by chance the Bathyurus is B. superbus that might indicate Shadow Lake but we need to see a cranidium and genal spines to tell them apart. According to Whittington another difference between B. extans and B. superbus is an almost smooth pygidial border but that feature is somewhat confusing as it looks similar on the examples of each that I looked at. Because of the bewildering variety of interbedded Black River lithologies they are divided into a basic minimum of ideal lithotypes (Brookfield, 1998). Hopefully you will collect additional material and attempt to corroborate if this is one of the GR-SL contact points. This list of fauna is likely incomplete as Raymondites sp. (cf. longispinus?) has also been reported from Gull River exposures.

Brett, K., & Rudkin, D.M. (1997)

Ordovician stratigraphy and trilobite faunas of south central Ontario.

2nd International Trilobite Conference, Brock University, St. Catharines, Ontario

Field Trip 3, Guidebook 34 pp.

Gull River

Basiliella barrandei

Bathyurus extans

Bumastoides billings

Calyptaulax callicephalus

Failleana indeterminata

Isotelus gigas

Nanillaenus latiaxiatus

Raymondites spiniger

Thaleops ovata

Shadow Lake

Bathyurus acutus

Bathyurus extans

Bathyurus superbus

[clfossils]

Wow, breaking it down much much further then I know for sure. I would very likely say that it is the Gull River Formation, because directly below the shale layer is a solid thick layer of limestone. Unfortunately I will likely not have the chance to dig anymore of the layer, as by the end of the week I believe it will be covered in soil and I have dug all the small layers that were exposed.

[pleecan]

Excellent finds Corey!

PL