Texas Upper Cretaceous Ammonite

[BobWill]

This ammonite from Choctaw Creek west of Sherman Texas was near the Eagle Ford Group and the Austin Group contact zone, possibly closer to the Eagle Ford. This is the Upper Cretaceous, I believe Turonian Stage.

The specimen is approximately 1/5th of a whorl showing a portion of the siphuncle and with part of the outer surface broken away to reveal edges of two septa. Evolute coiling, with whorl cross-section higher than wide and oval profile (70mm by 54mm).

Weak ribs, maybe 10 to 14 per whorl. Strong ventro-lateral tubercles, no umbilical tubercles. Ventral keel, some sutures showing moderate folding. The septa have two thick layers with two more very thin layers in between.

[PFOOLEY]

Prionocyclus or Collignoniceras?

Edited August 15, 2014 by PFOOLEY

[BobWill]

I tried to make that ID fit but I think the ribs are too far apart. Also the smaller Prionocyclus species are 120mm and this would have been at least 150 without the body chamber and the larger ones had ventral groves flanking the keel. I'm sorry the photos didn't show that well but no groves are present. This is all based on what little information I could find so you could still be right.

Edited August 15, 2014 by BobWill

[vertman]

Prionocycloceras maybe? It is really hard to tell from a fragment of that size. I have a very large complete example of this genus that I got in the Austin Chalk last summer.

[BobWill]

Would you mind breaking yours open to see if it has those multi-layered septa?

Prionocycloceras was just suggested by someone else and is probably right. I had eliminated it because of the rib spacing and quadrate whorl cross-section but there may have been enough variation among them to account for that. The ventro-lateral tubercles are about as far apart as the whorl is high, and that is before the body chamber. My guess of 10 to 14 ribs per whorl was based on the gradual widening of the spacing on typical ammonites but like you say I need to find the rest of this guy and maybe some more complete sutures.

Edited August 16, 2014 by BobWill

[vertman]

Bob, I feel like we may be talking about two different genus? The earlier suggestion I saw in this thread was Prionocyclus. I am mentioning Prionocycloceras. Attached is an image of the one I found last summer.

Edited August 16, 2014 by vertman

[BobWill]

I actually did catch the distinction. My qualifier just happened to be the same for both...the question of rib spacing. But now with this new photo I can see you're right. It seems like the spacing seems to widen exponentially toward the aperture in an adult.

When I said Prionocycloceras was suggested by some else I was talking about a personal email response from Mark McKinsey who had been along when I found it . His ID was the same as yours and that new pic has me convinced now. Thanks for the reply.

[vertman]

I actually did catch the distinction. My qualifier just happened to be the same for both...the question of rib spacing. But now with this new photo I can see you're right. It seems like the spacing seems to widen exponentially toward the aperture in an adult.

When I said Prionocycloceras was suggested by some else I was talking about a personal email response from Mark McKinsey who had been along when I found it . His ID was the same as yours and that new pic has me convinced now. Thanks for the reply.

Bob, I went to a concert with Mark Saturday night and he told me you guys had spoken about the specimen. I am glad he and I said the same thing. I have found that if Mark says it, I believe it. He is very good.

[BobWill]

I realized there was a layer of material that is easily broken away between the calcite in the chambers and the outer surface of the internal mold. It's between 2 and 3mm thick and covers the ventral and lateral surfaces. This is about the same thickness as the adoral layer of the septa whereas the adapical layer is closer to 4mm. There are also several other very thin layers on each septum.

I broke away some of the outer layer to reveal more of the calcite-filled chamber and this was inside. I am thinking about trying to free or at least expose all but one surface of the outermost chamber. It would be great if it was one solid piece of calcite. Would anybody else consider doing this or should I leave it as is?

Edited April 17, 2015 by BobWill

[BobWill]

I just heard from Dallas geologist Tom Dill that the "septa appear so thick because they have been coated on both sides by a layer of radially-arranged calcite crystals" different from the coarser calcite filling the rest of the void. Sort of like some septarian nodules. This makes good sense to me. I still think I will remove half of the outer septum to reveal part of the inner crystal. I'll post a picture.