Fossil ID Ammonite?

[Gracetk]

After walking 2+ years by the same spot, just noticed this, yesterday.  It sparked an interest, but I know absolutely nothing about fossils.  Is this a normal size for what I think is an ammonite?  Found in Northwest Austin, Texas near the bank of Walnut Creek.  I am assuming it’s limestone.  Grandfather’s hammer is 37 cm.

Edited May 6, 2018 by Gracetk

[Fossildude19]

Welcome to the Forum.  

There isn't a lot of detail in this imprint, but I could hazard a guess that it may be a Mortoniceras sp.

Perhaps some of our Texas locals can weigh in on this. 

Regards,

[Tidgy's Dad]

It's a good size! 

Hello, and a very warm welcome to TFF from Morocco! 

[caldigger]

Quick simple answer to your question. Yes, it is an ammonite impression.

 

It is likely the actual ammonite was hauled off in a 120 pound backpack through 9 miles of wild boar infested briars and 6 foot deep sticky mud holes in pitch black middle of the night conditions by a certain blonde wearing one boot. 

[Heteromorph]

I looked on the USGS Geological Map of Texas and that area of the creek exposes the Austin Chalk formation, which would match the kind of matrix shown in you picture, i.e. limestone/chalk. If it is from the Austin Chalk, the ammonite would probably be in the Texanitinae family. I am pretty sure Mortoniceras didn’t make it into the Austin Chalk, but a genus called Submortoniceras in the Texanitinae family did. That along with the genera Prionoclycloceras, Protexanites, and Texanites are good possibilities. They are all in the Texanitinae family.  

 

This is a pretty normal size for an ammonite from this time. My family has found an almost complete Protexanites sp. in the Austin Chalk from North Texas that has a diameter of 15 inches, so they can get big. I have even found two other Protexanites sp. fragments that were probably over 20 inches in diameter each when they were complete!  

 

Here is the geologic map I used. If you can determine the location where you found this on this map you can confirm whether or not it is in the Austin Chalk. 

 

USGS Geological Map of Texas

 

I suggest getting HGMS’s book Texas Cretaceous Ammonites and Nautiloids from their website. It will help you much more easily identify ammonites. 

 

@Uncle Siphuncle

 

EDIT: Because of how evolute this specimen is, another possibility is the genus Peroniceras. It is also known from the Austin Chalk. It is in the Collignoniceratidae family. 

[Gracetk]

Wow, thanks for the quick responses.  Need to work on my observational skills.  Hoping to find more fossils.  Didn’t think developed areas could still hold such surprises.

[Gracetk]

It is indeed located in Austin Chalk.

[Uncle Siphuncle]

Peroniceras