Your ammonite probably is...
Monophyllites
Suture: Ammonitic
Monophyllites was one of the first Phylloceratinans, an order that also includes Phylloceras. It had many fine, well-packed, slightly curved radial ribs. It had a simple leaf-shaped ammonitic suture. Its shell was round. Monophyllites lived worldwide in the Middle to Upper Triassic.
Image by Hectonichus, CC BY-SA 3.0 Unported and GFDL, from here
