The Biggest Ammonite Fossil Ever Found Was A Whopping 1.8 Meters
Rachel Funnel. IFL Science, June 9, 2023
[www.iflscience.com]
Parapuzosia seppenradensis
[en.wikipedia.org]

An open access paper is:

Ifrim, C., Stinnesbeck, W., González González, A.H., Schorndorf, N.
and Gale, A.S., 2021. Ontogeny, evolution and palaeogeographic
distribution of the world’s largest ammonite Parapuzosia (P.) seppenradensis
(Landois, 1895). Plos one, 16(11), p.e0258510.
[journals.plos.org]

Another paper is:

Scott, G. and Moore, M.H., 1928. Ammonites of enormous size
from the Texas Cretaceous. Journal of Paleontology, pp.273-279.
[www.jstor.org]

Yours,

Paul H.

"The past is never dead. It's not even past."
William Faulkner, Act 1, Scene III, Requiem for a Nun (1951)