Very large whorl, from what and where?

[Lone Hunter]

In the few trips made to Trinity river in Tarrant county I've spotted several peices of these large whorls, I'm wondering if these are from Mariella ammonites or a heteromorph?  I have found some pretty big Mariellas in Grayson marl but nothing close to this size. Is that maybe where they originate?, is the Trinity cutting through that formation?  I'd love to figure that out and try to find a more complete one. 

[JamieLynn]

Definitely Mariella. They can get quite large in the Grayson.  This was not the biggest I found and it's only a partial. Also, Mariellas are heteromorphs. Just means they are not curled in like "normal" ammonites.