Graf, Iowa Orthocone Shell remnant or epibiont?

[Crusty_Crab]

This is Isorthoceras sociale (Hall, 1877) from the Ordovician Elgin member of the Maquoketa from the well known Graf, Iowa location.

 

They exhibit unusual preservation, including nacre and hollow camerae. According to Invertebrate Fossils (1952) by Moore, et al,, shells of nautiloids have an inner annulus layer, middle nacreous layer and outer porcelaneous layer. Is the circular pitted texture on the outside then the remnant of the porcelaneous layer of the shell or some sort of epibiont like a bryozoan? 

 

Plate 220 of Shimer and Shrock's Index Fossils of North America (1944) has orthocone shells illustrated, but the texture appears wavy. 

 

 

 

[Tidgy's Dad]

I think it's an encrusting bryozoan. 

Possibly Spatiopora iowensis that doesn't always seem to show the clear, elongate, shell aligned monticules that other species of this genus exhibit.  

Edited March 21, 2021 by Tidgy's Dad

[FossilNerd]

3 hours ago, Tidgy's Dad said:

I think it's an encrusting bryozoan. 

Possibly Spatiopora iowensis that doesn't always seem to show the clear, elongate, shell aligned monticules that other species of this genus exhibit.  

I agree with Adam. Looks more like an encrusting bryozoan to me, which is typical of many Ordovician fossils. 

[fossilcrazy]

Hello Crusty_Crab,  I found that some layers at Graf, IA have poor preservation and the Isorthoceras have much of the Calcium minerals leached away leaving that net-like pattern on the shell. It is my guess the pattern is the fabric of the Cephalopod test. There are no necrea color patterns at that level. 

[Crusty_Crab]

@Tidgy's Dad and @FossilNerd Thanks for the response!

 

On 3/21/2021 at 5:40 AM, fossilcrazy said:

Hello Crusty_Crab,  I found that some layers at Graf, IA have poor preservation and the Isorthoceras have much of the Calcium minerals leached away leaving that net-like pattern on the shell. It is my guess the pattern is the fabric of the Cephalopod test. There are no necrea color patterns at that level. 

@fossilcrazy That pattern does look like this closeup of another specimen:

 

There is iridescence below that which makes me think the nacreous layer has been preserved: