Any ideas on this one?

[DrGrip]

All 4 pics are of the same rock. There are 2 distinct patterns on here and I've never encountered this before. These pics are from my phone, I'll get better ones later if needed for ID assistance.

I found this in the creek bed in Collin County, TX.

 

[Mosasaurfiend]

Looks like the septa (inside of the ammonite wall).

 

[Harry Pristis]

Looks like the septa (inside of the ammonite wall).

That's exactly what these represent. The fluted surfaces are the evidence (an internal cast) of the walls (septa) of a large ammonoid shell. These walls separate chambers (camerae) in the shell. Each wall was laid down sequentially at the rear of the living chamber as the animal grew in size.