unknown heteromorph ammonite from Texas

[BobWill]

Visiting fossil collectors from the Odessa school district found the specimen in the first picture and I was reminded of one I found 7 years earlier at the same site shown in the next images. When they asked for the name I had to admit I didn't know. These came from Moss Creek, a nearby tributary of the North Sulphur River and exposing the same deposits, the Ozan Formation from the Taylor Group of the upper Cretaceous.

 

My example is 65 mm long, with a whorl 30 mm X 15 mm. The fragment has a slightly curved shaft, compressed whorl section with a rib index of 4 per whorl height. It has a sharp venter, rounded dorsum and ribs slanted in an adapical direction along the venter and interrupted on the dorsum. There are no visible sutures other than in an area to small to interpret. There appears to be a single row of tubercles in the center of the venter unless this is a result of being pinched out in preservation but both specimens show this feature.

 

The site produces some examples of Glyptoxoceras sp. which is the closest thing I know of but I don't believe those have the tubercles and they are curved a lot more. Glyptoxoceras ellisoni from the older Austin Group is less curved but also without the tubercles.

 

 

 

 

[historianmichael]

It reminds me a little bit of Nostoceras

[BobWill]

23 minutes ago, historianmichael said:

It reminds me a little bit of Nostoceras

I don't know that one so I looked it up. Like the lower Cretaceous heteromorphs I find at home those have a double row of tubercles but otherwise would be a good fit if they also appear in the Taylor Group.

[historianmichael]

45 minutes ago, BobWill said:

I don't know that one so I looked it up. Like the lower Cretaceous heteromorphs I find at home those have a double row of tubercles but otherwise would be a good fit if they also appear in the Taylor Group.

Although Nostoceras typically has a double row of tubercles, I have seen anomalies where a specimen only had a single row or no rows at all. I am not saying it is Nostoceras but it might be a possibility. I found pieces of Nostoceras hyatti in the Saratoga Chalk of Arkansas, which is a Taylor Group formation. 

[BobWill]

26 minutes ago, historianmichael said:

Although Nostoceras typically has a double row of tubercles, I have seen anomalies where a specimen only had a single row or no rows at all. I am not saying it is Nostoceras but it might be a possibility. I found pieces of Nostoceras hyatti in the Saratoga Chalk of Arkansas, which is a Taylor Group formation. 

Interesting.The problem with it being an anomaly is that we now have two identical from the same site. They could also be worn off.  Did the anomalies with a single row of tubercles have them on one of the ventrolateral margins or at the center of the venter?

 

Edited September 18, 2022 by BobWill

[historianmichael]

8 minutes ago, BobWill said:

Interesting.The problem with it being an anomaly is that we now have two identical from the same site. They could also be worn off.  Did the anomalies with a single row of tubercles have them on one of the ventrolateral margins or at the center of the venter?

 

Center of the venter.