Your ammonite probably is...
Austrauliceras
Australiceras began its growth normally, but then made one massive whorl that did not come close to touching the previous coils. It had a rounded venter and many faint radial ribs along its original coil that eventually became well-spaced, large ones along the part closer to where the animal was. Australiceras lived in the Lower Cretaceous. It normally is found in fragments.
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