Trilobite ID

[Kane]

I found this in my usual rip rap spot, which has a maddening mix of formations (from Bois Blanc, Amherstburg, Dundee, and even some Hamilton Gp formations, too - yargh). 

 

My first instinct with this one was Anchiops anchiopsis, but there is something not quite right about this one. It is tough to make out if there is a pygidial spike for one, I cannot make out any incised axial rings, and it does not have the same acute angle of pleural tapering I usually see with this species. There appears to be a border on the lower left of the pygidium, and the pleura seem to stretch out quite widely while the axial part is quite thin (reminiscent of Coronura aspectans).

 

Is this something else, or just another Anchiops that is simply freakish?

 

I made a minor attempt to reveal more of it. Trilobites in this matrix do not preserve particularly well.

[Kane]

This one is turning out to be a real stumper! 

 

I am reasonably certain that this is a Devonian dalmanatid. I'm not satisfied in calling this one Anchiopsis anchiops, but neither does it seem to share the exact same properties as Coronura aspectans.  

Anchiopsis anchiops from Ludvigsen, 1979.

 

Coronura aspectans from Ludvigsen, 1979

 

 

My specimen, cropped and adjusted.

 

What leads me to suspect it is more a Coronura than Anchiopsis is the large number of ribs (something like over 15 pairs in just this pygidium alone!). The more flattened/truncated base of the pygidium doesn't make it a perfect match, however. 

[FossilDAWG]

Can you take a photo with lighting from an angle to use shadows to bring out the surface relief?  I can't see much structure on the pygidium, such as axial rings.  Also the whole thing looks very flat.  Or is that just the nature of the specimen?

 

Don

[Kane]

Can do!

 

Taken with light angled from the top and bottom. It is a little flat, so attempts to adjust the angle of the light while picturing the specimen head on made very little difference. 

 

The detail is not great on this one, but most of the trilobite fragments I find in this almost sandy, larger-grained matrix do not preserve terribly well. The Leptaena shells come out very nicely, however. 

 

Hopefully this helps! I can make out the border on the pygidium (bottom left) much clearer when I position the specimen and light obliquely. 

[Kane]

And as close as I can get to revealing the pygidium using the iPad and zoom (which accounts for some pixel density loss, sadly). I can almost make out a concave area at the end that would either have continued as a single pygidial spike, or the two pronged type of Coronura.

[FossilDAWG]

It's a tough call due to the condition of the specimen, but I'd go with Coronura, due to the fineness/greater number of segments as you noted.  Also Anchiopsis seems to have a pair of broad nodes on each pygidial segment and I don't see those on your specimen. 

 

Don

[Kane]

Thanks, Don! I'll put this tentatively down as Coronura (and perform a small, if restrained, happy dance as these are not common and it is a new example for me). 

[piranha]

8 hours ago, Kane said:

What leads me to suspect it is more a Coronura than Anchiopsis is the large number of ribs (something like over 15 pairs in just this pygidium alone!). 

 

 

Check that again.  I count approximately 12 ribs.

 

Also, there is no evidence of pygidial spines, so Coronura seems unlikely.  Instead, Trypaulites erinus, reported from Bois Blanc, appears to be a better possibility.

 

[Kane]

Thanks, @piranha - I was having some trouble counting the ribs given the condition, so may have double counted . The T. erinus looks like the right match (also, from the plates I've seen, most Coronura species seem to have prominent pustules/nodes all over their pleura). I think this mystery is solved! 

 

I am *still* going to do a small happy dance as this is a new species find for me, although not a new genus (I had found a Trypaulites calypso in the Dundee Fm back in March).

 

Any chance you could tell me the source of the image? I'll see if I can track down a paper for this one in the mean time.

[piranha]

14 minutes ago, Kane said:

...Any chance you could tell me the source of the image? I'll see if I can track down a paper for this one in the mean time.

 

 

Lesperance, P.J. (1975)

Stratigraphy and paleontology of the Synphoriidae (Lower and Middle Devonian dalmanitacean trilobites).

Journal of Paleontology, 49(1):91-137

 

Check your email, paper sent.

 

text from:

 

Whiteley, T.E., Kloc, G.J., & Brett, C.E. (2002)
The Trilobites of New York.
Cornell University Press, 380 pp.

 

Trypaulites erinus (Hall, 1861) Plate 127A
Holotype NYSM 4320 - From the Middle Devonian Onondaga Group of western New York in Ontario, Genesee, and Erie Counties, this trilobite is known only from the pygidium.  It has 12 (13?) slightly furrowed pleural ribs and 16 axial rings with a postaxial ridge.  There is a border.  The surface is covered with granules and the termination is slightly upturned.  Pygidia of this species have been collected from the Bois Blanc near LeRoy, Genesee County, New York.

[Kane]

Scott strikes again! Fantastic. 

 

I had no luck in my search (only pulled up a reference to the genus mostly in Stumm 1954). 

 

My thanks again, good sir.