Texas Austin Chalk Ammonite Id

[vellis]

Found this right at the top of the Austin chalk, where it meets the Ozan (upper cretaceous) in a Creek in Garland, Texas. It is about 5 or 6 inches in diameter. Anyone know what kind of Ammonite it is?

Thanks,

Van.

[Uncle Siphuncle]

possibly pachydiscus? hard to tell while viewing from my phone.

[BobWill]

Hard to tell without sutures but the age and closely spaced ribs suggest that Dan is right with pachydiscus. Does it have a rounded venter?

[vellis]

I think I would call it a slightly rounded venter. The venter is close to that of an Oxytropidoceras ammonite that I have. Altogether, as a whole, it looks fairly close to an Oxy ammonite, but the ribs are spaced closer and I know Oxy is found in lower Cretaceous. Help any?

[Uncle Siphuncle]

A shot of the keel would help, in an area where the ribs are best pronounced.

[BobWill]

Some oxys have closer rib spacing but the venter is usually high and with a strong keel and certainly shouldn't be in the Cenomanian as you said. The venter of a pachydiscus would be much rounder. There are some less common ammos with closely spaced ribs from the Eagle Ford, maybe someone who knows them will respond.

[JohnJ]

Based on the geology and size, you might consider one of the Parapuzosia species.

[Uncle Siphuncle]

Ahh yes, Parapuzosia is also a valid consideration. I'd check that closely if you are sure it is from the Austin, and look hard at Pachydiscus if you think there's a chance its Ozan. The two forms are pretty distinct.

[vertman]

Good find. How certain are you that the Ozan was exposed above the layer where you found this? If it is the creek I think it might be I have always somewhat questioned the geology there. It may be the same creek I dug a 48 inch diameter ammononite out of several years ago. It does not seem like Austin. It may be one of the chalks from the lower part of the Taylor Group. But if that is true I don't know what happened to the Ozan in the eastern part of Garland.