How Do I Know What Kind Of Orthocone?

[TMNH]

I've been collecting fossils from the Mimico Creek (Georgian Bay Formation, 450 myo) for years and I often find nautiloid shells. I know the biggest ones are Endoceras, but I'm not sure about some of my smaller specimens.

My question is what other genera/species of orthocones can be found in this formation and how can I differentiate between them?

Some typical specimens (compared to a penny)

Edited August 21, 2011 by TMNH

[JimB88]

without seeing the siphuncle it can be difficult to tell. There were many types in the middle Ordovician period. They could be actinoceras or maybe dawsonoceras. Again, its hard to tell when the siphuncle isn't visible.

[TMNH]

Here is the siphuncle of the specimen on the bottom left:

Here is another specimen which shows a cross section of the siphuncle:

I hope these are helpful, I can provide more if needed.

Edited August 21, 2011 by TMNH

[piranha]

Hi TMNH,

There are quite a few similar types from the Ordovician of Canada listed in Index Fossils of North America. A few Michelinoceras look close enough to be convincing but to be certain try to track down a bulletin that describes the faunal assemblage from Georgian Bay. Perhaps one of the local universities can point you in the right direction? The last photo you posted is superb.

One of the best!

[TMNH]

Thanks for the suggestions, guys!

I have so many nautiloids that I think I should try to figure out what species there are.

I'll keep an eye out for bulletins

[JimB88]

Thanks for the suggestions, guys!

I have so many nautiloids that I think I should try to figure out what species there are.

I'll keep an eye out for bulletins

The bottom pic is an actinocerid of some kind, there were many.

[TMNH]

The bottom pic is an actinocerid of some kind, there were many.

Thank you for the ID! That does look like it.

[FossilDAWG]

I used to collect in that area, and all my specimens that resemble yours are a species of Treptoceras. There are several species in the Cincinnati area, but I recall that there was only one known in the Georgian Bay Formation; it might have been Treptoceras duseri but I'll have to check my notes on that. Here is a picture of T. duseri, taken from this excellent web site:

Treptoceras was a member of the order Michelinoceratida, characterized by a nummuloid (bead-like)siphuncle and absence of cameral deposits. Cameral deposits were organic material deposited in the cameral chambers (the chambers that make up the phragmocone, which is all of the shell except the living chamber. Actinocerids had much larger siphuncles in relation to the size of the shell, and they had more-or-less extensive development of cameral deposits. Also, actinocerids tend to be flattened on one side; it is thought that these cephalopods lay on the sea floor and fed on prey also on the sea floor, such as (possibly) trilobites. Michelinocerids were for the most part lighter, with shells that are circular in cross section and often very long and slender, indicative of an active swimmer in open water that preyed on free-swimming animals.

In the top photo you can clearly see the bead-like siphuncle, which closely resembles your specimen. You can also see the camerae (chambers); the calcite that lines the camerae was deposited by ground water infiltrating the hollow chambers, like a geode. Cameral deposits have a different appearance, massive and not crystalline, and they tend to be concentrated on specific surfaces rather than being evenly distributed over the whole chamber.

Don

[TMNH]

OK, you may well be right.

To complicate matters further, I found this site:My link

Edit: I also just found info. on the same species (Actinoceras crebriseptum) mentioned in the site in one of my books. Could that be it?

Edited August 23, 2011 by TMNH

[TMNH]

That Treptoceras does look realy similar to at least some of my specimens though...

[FossilDAWG]

"Actinoceras" crebriseptum is an old name for Treptoceras crebriseptum. This is the most abundant nautiloid species in the Georgian Bay Formation in the Toronto area. The name "Actinoceras" used to be applied to almost any Ordovician nautiloid with a more-or-less straight shell, and even some with noticeably curved shells such as Oncoceras. There is also an actinocerid nautiloid in the fauna, Cameroceras inaequabile, but it is much rarer and can be distinguished by its much larger siphuncle relative to the shell size. I only ever found one of those, compared to dozens of the Treptoceras. There are a few other genera that might be found there, including Michelinoceras, Oncoceras, and Schuchertoceras, but those are even more scarce.

Don

[TMNH]

"Actinoceras" crebriseptum is an old name for Treptoceras crebriseptum. This is the most abundant nautiloid species in the Georgian Bay Formation in the Toronto area. The name "Actinoceras" used to be applied to almost any Ordovician nautiloid with a more-or-less straight shell, and even some with noticeably curved shells such as Oncoceras. There is also an actinocerid nautiloid in the fauna, Cameroceras inaequabile, but it is much rarer and can be distinguished by its much larger siphuncle relative to the shell size. I only ever found one of those, compared to dozens of the Treptoceras. There are a few other genera that might be found there, including Michelinoceras, Oncoceras, and Schuchertoceras, but those are even more scarce.

Don

(Sorry I didn't respond sooner- I was at my cottage)

Thank you for the clarification Don! That's very helpful. I've found so many specimens that I thought I should be able to tell which one's which!

I'd better look over all my specimens again...and make new labels