Silurian trilobite

[aek]

Hi, I recently found this in the Wenlockian Sugar Run formation in Illinois. I'm guessing it is a Dalmanites pygidium, but not sure. It has some interesting ornamentation. Any ideas?

[Kane]

Not sure if this is a dalmanitid. I know @piranha will be of help here.  

[FossilDAWG]

Not a dalmanitid.  It is an encrinurid, possibly Encrinurus nereus which has been reported from the Sugar Run Dolomite.

 

Don

[Kane]

1 minute ago, FossilDAWG said:

Not a dalmanitid.  It is an encrinurid, possibly Encrinurus ornatus.

 

Don

Kind of what I was thinking, too, judging by the pleural arrangement but didn't feel confident enough to make that call. Kudos, Don.  I don't see nearly enough Silurian trilobites.

[FossilDAWG]

Here is a figure of pygidia from Encrinurus nereus, and the source paper.

 

Don

[piranha]

According to Mikulic 1999 Encrinurus egani is the only encrinurid from Sugar Run:

 

Mikulic, D.G. (1999)

Silurian trilobite associations in North America.

In: Boucot, A.J., & Lawson, J.D. (eds.)

Paleocommunities - a case study from the Silurian and Lower Devonian.

World and Regional Geology, 11:793-798

 

 

[aek]

Thanks for the help! Greatly appreciated

[piranha]

14 hours ago, FossilDAWG said:

It is an encrinurid, possibly Encrinurus nereus which has been reported from the Sugar Run Dolomite.

 

 

Gass et al. 1992 mentions Sugar Run in the description of E. nereus, but only as a comparison to E. egani from Sugar Run.  

I checked all the literature and apparently, E. nereus is only found in Racine County, Wisconsin:

 

 

"Occurrence. - The type specimens from Racine were collected by J. Hall or were purchased from T.J. Hale, according to Hall's collection catalogues; the precise locality was not recorded. However, either of the following two limited areas near Racine are likely: Ives, in quarries operated by S. Ives or W. Cooley in SE 1/4, sec. 29, and NE 1/4, sec. 32, T4N, R23E, Caledonia Township, Racine County; or Horlicksville, in quarries operated by J. Horlick or W. Beswick, and natural outcrop along the Root River, E 1/2, sec. 6, T3N, R23E, Mount Pleasant Township, Racine County. Prior to 1867, when the type specimens were described, a number of small, shallow quarries operated at these site. All quarries and outcrops were located in the Racine Dolomite, either in reef or reef-infuenced strata, of late Wenlock Ludlow age. Encrinurus nereus is not common at Racine localities, although specimens have been collected from both Ives and Horlicksville. This species occurs in distal flank beds that are thought to have been deposited under low-energy conditions below wave base. Communities found within these Racine Dolomite reefs are assigned to Benthic Assemblage 3 (Mikulic, 1979)."

 

Gass, K.C., Edgecombe, G.D., Ramsköld, L., Mikulic, D.G., & Watkins, R. (1992)

Silurian Encrinurinae (Trilobita) from the central United States.

Journal of Paleontology, 66(1):75-89   PDF LINK

[FossilDAWG]

Thanks Scott.  I apparently scanned the paper a little too quickly and mixed up the reference to the Sugar Run locality.  Encrinurus egani it is.

 

Don

[Shamalama]

That is a really nice looking pygidium. Very decorated!